Seediscussions,stats,andauthorprofilesforthispublicationat:https://www.researchgate.net/publication/272685099

GeneratorCircuit-Breakers–ApplicationGuide-Edition2

TECHNICALREPORT·JANUARY2012

READS

198

4AUTHORS,INCLUDING:

MirkoPalazzo

ABB

22PUBLICATIONS24CITATIONS

SEEPROFILE

Availablefrom:MirkoPalazzo

Retrievedon:08February2016

Product Brochure

Generator Circuit-BreakersApplication Guide

Edited by

ABB Switzerland LtdHigh Voltage ProductsDepartment: High Current SystemsBrown Boveri Strasse 5CH-8050 Zurich / Switzerland

Text: Dieter Braun, Giosafat Cavaliere,Kurt Dahinden, Mirko Palazzo

4 | ABB

Table of contents

1 Introduction 6

3 Design of generator circuit-breakers 93.1 Interrupting chamber 9

3.2 Hydraulic spring operating mechanism 11

3.3 SF6-gas density monitoring system 11

3.4 Disconnector 12

3.5 Earthing switch 12

3.6 Starting switch (for gas turbine power plants) 12

3.7 Short-circuiting connection 13

3.8 Current transformer 13

3.9 Voltage transformer 13

3.10 Ferroresonance damping device 13

3.11 Surge capacitor 14

3.12 Surge arrester 14

3.13 Connecting zone 15

3.14 Phase enclosure 16

3.15 Control and supervision 16

2 History of the development of generator circuit-breakers 7

5 Selection of generator circuit-breakers 185.1 Duties of generator circuit-breakers 18

5.2 Requirements for generator circuit-breakers 18

5.3 Selection of generator circuit-breakers 18

5.3.1 Rated maximum voltage 19

5.3.2 Power frequency 19

5.3.3 Rated continuous current 19

5.3.4 Rated dielectric strength 20

5.3.5 Rated short-circuit duty cycle 20

5.3.6 Rated interrupting time 20

5.3.7 Rated closing time 20

5.3.8 Short-circuit current rating 20

5.3.8.1 System-source short-circuit current 20

5.3.8.2 Generator-source short-circuit current 23

5.3.8.3 Required closing, latching, and carrying capabilities 27

5.3.8.4 Required short-time current-carrying capability 27

5.3.9 Transient recovery voltage rating 27

5.3.9.1 First-pole-to-clear factor 28

5.3.9.2 Amplitude factor 28

5.3.9.3 Power frequency recovery voltage 28

5.3.9.4 Rated inherent transient recovery voltage 29

5.3.9.5 System-source faults 30

5.3.9.6 Generator-source faults 30

5.3.9.7 Calculation of TRV in case of terminal faults 30

5.3.10 Rated load current switching capability 32

5.3.11 Capacitance current switching capability 32

5.3.12 Out-of-phase current switching capability 32

5.3.13 Excitation current switching capability 34

5.3.14 Rated control voltage 34

5.3.15 Rated mechanism fluid operating pressure 34

4 Standard for generator circuit-breakers 17

ABB | 5

6 Application of generator circuit-breakers 356.1 Power plant layouts 35

6.1.1 Thermal power plants 35

6.1.2 Gas turbine power plants 35

6.1.3 Hydro power plants 35

6.1.4 Pumped storage power plants 36

6.2 Advantages of generator circuit-breakers 38

6.2.1 Simplified operational procedures 38

6.2.2 Improved protection of the generator and the main and unit transformers 38

6.2.3 Increased security and higher power plant availability 38

6.2.3.1 Transformer failures 39

6.2.3.2 Short-time unbalanced load condition 41

6.2.3.3 Generator motoring 42

6.2.3.4 Synchronizing under out-of-phase conditions 42

6.2.4 Economic benefit 43

Table of contents

8 Case study 1: Impact of the method of connecting a generator to the high-voltage grid on the availability of a power plant 458.1 Power plant layout 45

8.1.1 Layout of extra high-voltage substation 47

8.1.2 Layout of high-voltage substation 48

8.1.3 Generator circuit-breaker 48

8.1.4 Station transformer 48

8.2 Data for availability calculations 48

8.3 Simulations 48

8.4 Simulation results 49

8.5 Economic evaluation 50

7 Maintenance of generator circuit-breakers 44

References 57

9 Case study 2: Interrupting capability of generator circuit-breakers in case of delayed current zeros 529.1 Generator circuit-breaker model adopted for the simulations 52

9.2 Generator terminal faults 53

9.3 Out-of-phase synchronising 55

9.4 Conclusions 56

6 | ABB

AUXG

MT

EHV HV

UT STGCB

AUX

G

MT

EHV HV

UT ST

1 Introduction

A major objective of all power plant operating companies is the achievement of the highest possible plant availability at the lowest possible cost. Obviously, how a generator is con-nected to the high-voltage grid and how the power supply to the unit auxiliaries is secured has a decisive influence on the availability of a power plant.Two basically different ways of connecting a generator to the high-voltage transmission network are in use today, namely the connection without a circuit-breaker between the genera-tor and the low-voltage terminals of the main transformer (i.e. the "unit connection") and the connection with a generator circuit-breaker (Figure 1). The layout with a generator circuit-

breaker has several advantages over the unit connection, e.g.:

– simplified operational procedures

– improved protection of the generator and the main and unit transformers

– increased security and higher power plant availability

– economic benefit

ABB generator circuit-breakers are suitable for application in all kinds of new power plants such as fossil-fired, nuclear, gas turbine, combined cycle, hydro and pumped storage power plants as well as for replacement or retrofit in existing power stations when they are modernized and/or extended.

Figure 1: Layout of a thermal power plant without generator circuit-breaker a) and with generator circuit-breaker b)

a) b)

LegendMT Main transformer

UT Unit transformer

ST Station transformer

GCB Generator circuit-breaker

EHV Transmission system

HV Sub-transmission system

AUX Unit auxiliaries

ABB | 7

2 History of the development of generator circuit-breakers

During the sixties, when there was a trend towards higher unit ratings and, consequently, increased use of phase- segregated generator busducts, ABB developed a circuit-breaker which could meet these new requirements. This was the first circuit-breaker designed to be installed in the run of generator busducts (Figure 2). Since the delivery of the first specific purpose generator circuit-breaker in 1970, there has been a continuous develop-ment of this piece of power plant equipment. At the beginning the circuit-breakers consisted of three metal-enclosed, phase

segregated units using compressed air as operating and arc-extinguishing medium.In the 1980’s SF6 generator circuit-breakers were successfully introduced into the market. The design of these circuit-break-ers was a three-phase system in single-phase enclosures, supplied fully assembled on a common frame with operating mechanism and control equipment. Mainly the economical aspect and reasons of reliability and maintainability convinced customers of this modern arc-extinguishing medium.

Figure 2: Air blast generator circuit-breaker type DR mounted in the run of an isolated phase bus

Originally conventional distribution circuit-breakers were used to connect the generator to the step-up transformer.With the increasing output of the genera-tors, the required ratings exceeded the load currents and short-circuit levels of the switchgear available. Therefore the unit connection became the accepted standard power plant layout.

8 | ABB

Figure 4: Generator circuit-breaker type HEC 7 based on SF6 technology and self-blast principle

Figure 3: SF6 generator circuit-breaker type HECS-130R for open installation

Current transformer

Voltage transformer

Interrupting chamber

Disconnector

Surge arrester

Figure 5: View into one pole of a generator circuit-breaker system

In the 1990’s SF6 generator circuit-breakers were specifically developed for open installation, i.e. without enclosure. This solution was introduced to allow quick and easy installation even for projects with very small space requirements (Figure 3). Today SF6 generator circuit-breakers with rated currents up to 24’000 A with natural cooling and up to 57’000 A with forced air cooling, respectively, and with short-circuit breaking

Another development has been the integration of all the associated items of switchgear into the generator circuit-breaker housing. Series disconnectors, earthing switches, short-circuiting connection, current transformers, single-bushing voltage transformers, protective capacitors and surge arresters can be mounted in the enclosure of the generator circuit-breaker (Figure 5). Depending on the type of power plant additional items like starting switches (for gas turbine

currents up to 210 kA are available. This breaking capacity corresponds to the highest short-circuit breaking current ever achieved with a single SF6 interrupting unit. The development was made possible by using the most advanced SF6 self-blast principle. With this achievement modern SF6 generator circuit-breakers can now be delivered for generating units with ratings up to 2’000 MVA (Figure 4).

and hydro power plants) can also be fitted in the generator circuit-breaker housing. This greatly improved functionality allows simpler and more economic power plant layouts. Beside a substantial reduction of the first costs this new solution - being fully factory assembled and tested - also makes possible considerable savings in time and expenditures for erection and commissioning.

ABB | 9

3 Design of generator circuit-breakers

ABB generator circuit-breaker systems are three-phase sys-tems with a SF6 circuit-breaker and a disconnector in single-phase enclosures, supplied fully assembled on a common frame, with operating mechanisms and control equipment.In addition to the circuit-breaker and disconnector, the generator circuit-breaker systems are available with earthing switch, starting switch, short-circuiting connection, current and voltage transformers, surge capacitor and surge arrester. The single line diagram of a generator circuit-breaker system is depicted in Figure 6.

All the components are integrated and mounted in the phase enclosures (Figure 5). The generator circuit-breaker system is designed for welded connections to the isolated phase bus enclosures. Each enclosure is made of aluminium and capable of carrying the induced return current.The phase distance can be selected to suit the busbar spacing in the power plant.

Figure 6: Typical single line diagram of a generator circuit-breaker system

1 Generator circuit-breaker 2 Line disconnect switch 3 Earthing switch 4 Starting switch for SFC connection 5 Manual short-circuiting connection

(only with generator side earthing switch)(by removal of cover)

G7

8

6

3

4

1 2 7

5

3

6 9

8

10

6 Surge capacitor 7 Current transformers 8 Voltage transformers 9 Surge arrester10 Motor-operated short circuiting link

3.1 Interrupting chamberWithin the interrupting chamber SF6 gas is used for both arc extinguishing and internal insulation. The external insulation is air. For current interruption the self-blast principle is used which represents an optimised design to achieve a signifi-cant reduction in operating energy. The main advantages of employing SF6 gas as interrupting medium with self-blast principle can be summarised as follows:

– the arc-voltage of the circuit-breaker is high enough to ensure current zeros in case of fault currents with delayed current zeros without the need of delaying the tripping

– the pressure of SF6 which is needed for interruption de-pends on the magnitude of the current

– an efficient operation can be achieved with a smaller oper-ating mechanism due to lower energy consumption during contact movement

– a gentle interruption of small inductive currents can be obtained thus reducing the risk of chopping the arc and generating subsequent overvoltages

– SF6 gas can be monitored

10 | ABB

The interrupting chamber of a generator circuit-breaker is depicted in Figure 7. On the left side the terminal is visible. The contacts are oper-ated by a shaft passing through the vertical support insulator.

The design of SF6 generator circuit-breakers consists of two separate contact systems, one for current carrying and one for arc interruption (Figure 8).

During the interruption process the current has to commutate from the nominal contact system to the arcing contact system. This avoids wear and erosion of the current carrying contacts and ensures trouble-free current carrying even after a large number of operations.

Figure 7: Interrupting chamber of a generator circuit-breaker

a Circuit-breaker “CLOSED”

b Initiation of opening movement (transfer of current from the main contacts to the arcing contacts)

c Separation of arcing contacts with interruption of small currents supported by puffer action

Separation of arcing contacts with interruption of large currents supported by the thermal effect of the current arc itself to build up the pressure in the heating volume

d Circuit-breaker “OPEN”

Arc Extinguishing Technology:Mode of operation of the interrupting chamber of the type HECS circuit-breaker systems

a

b

c

d

7 6 8 3 1

5 4 2

Figure 8: Contact systems of an SF6 generator circuit-breaker and description of a current interruption procedure

1 Terminals 2 Cylindrical coil 3 Fixed arcing contact 4 Moving arcing contact

5 Fixed main contacts 6 Moving main contact 7 Puffer 8 Heating volume

ABB | 11

3.2 Hydraulic spring operating mechanismThe hydraulic spring operating mechanism combines the advantages of a hydraulic operating mechanism with those of a spring energy storage system (Figure 9). Energy storage is accomplished with the aid of a disk spring assembly, with the advantages of high long-term stability, reli-ability and non-influence of temperature changes. Tripping of the operating mechanism and energy output are based on proven design elements of the hydraulic operating technique, such as control valves and hydraulic cylinders.The operating mechanism is based on the so-called differen-tial piston principle.For the closing operation the piston head side is isolated from the low pressure and simultaneously connected to the high pressure oil volume. As long as the pressure is maintained, the piston remains in the “closed” position. A pressure controlled mechanical

interlock prevents movement of the piston to the “open” posi-tion in case of a pressure drop.For the opening operation, the piston head side is isolated from the high pressure and simultaneously connected to the low pressure oil volume.The charging state of the spring disk assembly is controlled by switching elements, actuating the pump motor to immedi-ately maintain the oil pressure. A non-return valve between pump and high-pressure oil volume prevents pressure loss in the event of a pump outage. The hydraulic system is hermetically sealed against atmo-sphere. The mechanically operated position indicator provides reliable indication of the circuit-breaker position.The drive operates all three circuit-breaker poles simultane-ously by mechanical linkages, thus keeping the switching time difference between the poles to a minimum.

High pressure 1 Breaker operating rodLow pressure 2 Energy storage device

Figure 9: Hydro-mechanical spring operating mechanism a) and its schematic diagram b)

3.3 SF6-gas density monitoring systemThe breaking capacity of an SF6 circuit-breaker and the dielectric withstand level across its open contacts is depen-dent upon the density of the SF6-gas. Under the condition of constant volume the gas density is independent of the gas temperature, while the pressure varies with the temperature. It is therefore more practical to measure and use the gas density rather than the pressure for circuit-breaker supervision purposes. The density monitor operates according to the reference-

volume-density principle. The density of the gas in the circuit-breaker chamber is compared with the density of the gas in a sealed reference gas volume. When the gas density drops below the specified value, the density monitor signals the loss of SF6-gas in several steps.Since the gas volumes of the three circuit-breaker poles are connected via the refilling pipe only one SF6-gas density monitor per circuit-breaker is required to supervise the gas density.

a) b)

12 | ABB

3.4 DisconnectorThe switchgear concept provides a disconnector fitted in series with the circuit-breaker. It is placed on the transformer-side of the circuit-breaker and within the same enclosure. The disconnector is a tubular telescopic unit and it is equipped with a drive which operates through a mechanical linkage all three poles. This layout provides easy access and simplifies maintenance. In the open position of the disconnector the iso-

lating air distance can be clearly seen through an inspection window. The moving contact is motor driven. A locking feature prevents motor operation while the disconnector is being manually operated. A mechanically driven position indicator is provided in a visible position and a crank handle is provided for manual operation. The view of a disconnector being in the open position is depicted in Figure 10.

Figure 10: View of a disconnector in the open position

3.5 Earthing switchThe earthing switch can be provided on either one or both sides of the system. The switch and its connections are designed for protective earthing purposes, i.e. it is rated for the full fault current but has no current making or continuous carrying capacity.The design of blade type (for generator circuit-breaker

systems type HECS and HEC 7/8 up to 160 kA) or of tubular telescopic type (for generator circuit-breaker systems type HEC 7/8 up to 210 kA) is depicted in Figure 11.The earthing connection is made via the system enclosure. The moving contact is motor driven.

Figure 11: Blade type a) and tubular telescopic type b) earthing switches

a) b)

3.6 Starting switch (for gas turbine power plants)A starting switch can be provided on the generator-side of the system. It is designed for being employed for the start-up

of the machine from a static frequency converter (SFC). The moving contact is motor driven.

ABB | 13

3.7 Short-circuiting connectionThe short-circuiting connection helps to expedite the test-ing and adjustment of the power plant protection system. It can be provided manually mounted for the use between the circuit-breaker and the disconnector of the system or motor operated. In the former case the cover of each phase

enclosure has to be removed to allow the fitting of the short-circuiting bar. In the latter case the short-circuiting link is used in conjunction with the earthing switch installed on the generator-side of the circuit-breaker (for generator circuit-breaker systems type HECS).

Figure 12: Ring core current transformer

3.9 Voltage transformerSingle-phase voltage transformers can be provided on either one or both sides of the circuit-breaker system. Up to three voltage transformers can be fitted at each side and each volt-age transformer can be supplied with one or two secondary windings, depending on the class and output power required (Figure 13). The secondary windings are permanently wired back to terminal blocks in the control cubicle.

3.10 Ferroresonance damping deviceIn order to prevent the occurrence of ferroresonance ABB generator circuit-breaker systems are equipped with a damping device installed in the open delta formed by the tertiary windings of the three voltage transformers on the transformer-side of the generator circuit-breaker (see Figure 14 and Figure 15).Ferroresonance is characterised by a periodic displacement of the potential of the system neutral in a three-phase system with an isolated neutral. These so-called relaxation oscillations are caused by discharging and recharging the capacitances to ground via magnetising inductances of e.g. single-pole insulated voltage transformers and the periodic repetition of this process. The magnetic core is temporarily subjected to saturation during these phenomena. As a consequence of saturation high currents are flowing through the primary windings of the voltage transformers that heat up these windings and often lead to the destruction of the voltage transformer. In practice the ferroresonant oscillations may be

initiated by momentary saturation the core of the inductive element resulting from e.g. switching operations or other type of events leading to an unbalance in the system. The insertion of a ferroresonance damping device in the open delta of the residual voltage windings (tertiary windings) of a set of voltage transformers is a very efficient solution for the damping of second subharmonic relaxation oscillations. This device basically consists of a saturable coil (damping coil) paralleled by a group of resistors with a relatively high resis-tance. For power frequency voltages, i.e. in case of persistent single-phase-to-ground faults, the saturable inductance works in the linear range of the magnetising characteristic and car-ries only a small current thus avoiding any thermal overloading of the voltage transformer as well as of the inductance itself. For second subharmonic voltages however the inductance saturates and absorbs active power sufficient to damp out the relaxation oscillations. This power is dissipated in the resis-tance associated with the damping coil.

3.8 Current transformerA ring core current transformer can be provided on either one or both sides of the circuit-breaker system (Figure 12). Depending on the class up to three cores per current trans-former can be accommodated. The secondary windings are permanently wired back to terminal blocks in the control cubicle.

Figure 13: Single-phase voltage transformers

14 | ABB

3.11 Surge capacitorSurge capacitors are fitted on both sides of the generator circuit-breaker system to provide additional protection against overvoltages and to support arc extinction in the circuit-breaker by transient recovery voltage limitation (Figure 16). The surge capacitors are used to reduce the rate-of-rise of the transient recovery voltage from the very high prospec-tive values (and at the same time to increase the time delay from the very low prospective values) to values the generator circuit-breaker can cope with. The capacitors are therefore to be considered as an integral part of the generator circuit-breaker.

Figure 14: Insertion of a ferroresonance damping device (DE6) in the open delta of the residual voltage windings (tertiary windings) of a set of voltage transformers (voltage transformer with one secondary winding)

3.12 Surge arresterSurge arresters can be fitted on the transformer-side of the generator circuit-breaker system, to provide protection for the equipment connected to the generator busbar against overvoltages. Metal-oxide surge arresters with silicon housing are installed in ABB generator circuit-breaker systems (Figure 17). Metal-oxide surge arresters have a highly non-linear resistance characteristic. At service voltage a predominantly capacitive low current flows. Any voltage increase leads to a rapid increase of the current, thereby limiting any further rise in the voltage. When the voltage decreases, the condition reverts to its essential non-conducting state.

Figure 15: Insertion of a ferroresonance damping device (DE6) in the open delta of the residual voltage windings (tertiary windings) of a set of voltage transformers (voltage transformer with two secondary windings)

Y-connection of the primary windings

open delta connection of the tertiary windings

R S T

DE 6

Earth Fault Protection Relay

Y-connection of the primary windings

open delta connection of the tertiary windings

R S T

DE 6

Earth Fault Protection Relay

Y-connection of the secondary windings

Figure 16: Surge capacitor Figure 17: Metal-oxide surge arrester

ABB | 15

a)

b)

c)

d)

3.13 Connecting zoneThe connecting zone is designed to provide a detachable (bolted) connection between the generator circuit-breaker life parts and the conductors of the adjacent isolated phase bus (IPB) or busduct. The main components of the connect-ing zone are depicted in Figure 18 and Figure 19. The flexible connections shall be designed for:

1) carrying the rated continuous current and the rated short-time withstand current without exceeding the maximum permissible temperatures

2) ensuring that the dielectric strength requirements are met

3) compensating expansion and contraction of the conductor due to temperature changes

4) compensating vibrations and withstanding the stress caused during switching operations

5) withstanding the mechanical stress resulting from electro-dynamic forces in case of short-circuit currents

6) providing a low resistance, safe and stable electrical connection

ABB recommended type and arrangement of flexible copper straps responds to these requirements as follows:

1), 2) & 5) Fully type tested together with the generator circuit-breaker to prove that the stringent requirements imposed by the relevant IEC and IEEE standards with regard to dielectric strength, hottest spot temperature and mechanical stress are fully met. The special shape easily adapts to different distances between terminals ensuring that dielectric strength require-ments are always met.

3), 4) & 5) Flexible type employing laminates with pressure-welded contact ends designed and tested for high mechanical stress.

6) Silver plated contact ends with high requirements on contact surface evenness and material properties.

Figure 19: Connection between one pole of a generator circuit-breaker and the associated phase bus

Figure 18: Main components of the connecting zone

a) Flexible copper straps b) Fastening and securing bolts & nuts c) Terminal with silvered contact surfaces for welded connection to the conductor of the IPB or busduct d) Support ring for withstanding the mechanical stress and to reduce the contraction of the connectors resulting from electro-dynamic forces in case of short-circuit currents

16 | ABB

3.14 Phase enclosureThe magnetic field in the neighborhood of the connection be-tween generator and transformer may have adverse effects on equipment and building steel if the current exceeds a certain value. The values of magnetic fields outside of the generator circuit-breaker housing could induce voltages and currents which in turn might produce undesired heating effects.For this reason, and to avoid electromagnetic forces between the current-carrying busbars the generator circuit-breaker system is designed for welded connections to the isolated phase bus enclosures.Each single phase enclosure is made of aluminium and

capable of carrying the induced return current thus minimising the impact of the magnetic field. In order to avoid pollution due to ingress of dust and moisture, the generator switchgear enclosure is designed to allow air tightness and to withstand a slight internal overpressure.Inspection windows are provided in the phase enclosures near to the disconnector, earthing switch and starting switch, to allow visually checking of the position of each of them.Occasionally, the busbars in power plants are not enclosed and in general, effects of magnetic fields for small generator continuous current is usually of no concern.

3.15 Control and supervisionAll control and supervisory apparatuses are mounted in the control cubicle. An active mimic diagram is provided with position indications and the integrated local control of the circuit-breaker and all other switching apparatuses. In the control cubicle there is also installed equipment for local/remote changeover facilities and counters for CO operations of the circuit-breaker and pump starts of the circuit-breaker drive.

ABB | 17

4 Standard for generator circuit-breakers

IEEE Std C37.013-1997 (R2008) “IEEE Standard for AC High-Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis” covers the requirements applicable for generator circuit-breakers [1]. It is the only standard worldwide specifically relating to generator circuit-breakers. Therefore generator circuit-breakers have to be designed and tested in accordance with [1] and its amendment IEEE Std C37.013a-2007 “IEEE Standard for AC High Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis - Amendment 1: Supplement for Use with Generators Rated 10–100 MVA” [2]. Since no other national or inter-national standard on generator circuit-breakers exists, this standard is used worldwide. Specifically, IEC publication 62271-100 “High-voltage switchgear and controlgear – Part 100: Alternating-current circuit-break-ers” does not apply to generator circuit-breakers as it explicitly excludes generator circuit-breakers from its scope [3]. Circuit-breakers that have been designed and tested in accordance with IEC 62271-100 do not meet the stringent requirements imposed on generator circuit-breakers and therefore are not suitable for the use as generator circuit-breakers.Contrary to general purpose circuit-breakers covered by IEC 62271-100 generator circuit-breakers have two fault ratings, i.e. the system-source short-circuit current interrupting capability (in case of a fault between the circuit-breaker and the generator) and the generator-source short-circuit current interrupting capability (in case of a fault between the circuit-breaker and the transformer).

The stresses imposed on generator circuit- breakers differ from the stresses imposed on gen-eral purpose circuit-breakers mainly in the following respects:

1. The degree of asymmetry of the system-source short-circuit current is in the order of 60% to 80%.

2. The degree of asymmetry of the generator-source short-circuit current is in the order of 90% to 150%, i.e. the generator-source short-circuit current may exhibit delayed current zeros (degree of asymmetry > 100%).

3. The rate-of-rise of the transient recovery voltage after the interruption of a system-source short-circuit current may be as high as 6.0 kV/µs.

4. The rate-of-rise of the transient recovery voltage after the interruption of a generator-source short-circuit current may be as high as 2.2 kV/µs and the corresponding time delay may be extremely short (< 0.5 µs).

The test quantities given in IEC 62271-100 for the short-circuit tests do not adequately cover the above requirements. The only standard which cov-ers the requirements for generator circuit-breakers is IEEE Std C37.013-1997 (R2008). This standard in particular covers the requirements imposed on generator circuit-breakers regarding the d.c. component and the degree of asymmetry of the fault currents (including the case of fault currents with delayed current zeros) and the characteristics of the transient recovery voltages (rate-of-rise, time delay and peak value).In order to cover the stringent requirements which are imposed on generator circuit-breakers the type tests listed in Table I have to be performed on generator circuit-breakers in accordance with IEEE Std C37.013-1997 (R2008).

Description of Test Standard Clause

Rated continuous current carrying tests IEEE Std C37.013 Cl. 6.2.1

Rated dielectric strength IEEE Std C37.013 Cl. 6.2.2

Short-time current-carrying capability IEEE Std C37.013 Cl. 6.2.3

Short-circuit current rating IEEE Std C37.013 Cl. 6.2.3

Rated transient recovery voltage IEEE Std C37.013 Cl. 6.2.4

Rated standard operating duty IEEE Std C37.013 Cl. 6.2.5

Rated interrupting time IEEE Std C37.013 Cl. 6.2.6

Short-circuit current with delayed current zeros IEEE Std C37.013 Cl. 6.2.7

Load current switching tests IEEE Std C37.013 Cl. 6.2.8

Out-of-phase current switching tests IEEE Std C37.013 Cl. 6.2.9

Mechanical endurance life IEEE Std C37.013 Cl. 6.2.10

Excitation current switching tests IEEE Std C37.013 Cl. 6.2.11

Sound level tests IEEE Std C37.013 Cl. 6.2.12

EMC tests IEEE Std C37.013 Cl. 6.2.13

TABLE I: LIST OF TYPE TESTS FOR GENERATOR CIRCUIT-BREAKERS ACCORDING TO

IEEE Std C37.013-1997 (R2008)

18 | ABB

5 Selection of generator circuit-breakers

5.1 Duties of generator circuit-breakersThe main duties of a generator circuit-breaker are as follows:

– synchronise the generator with the main system

– separate the generator from the main system (switching off the unloaded/lightly loaded generator)

– carry and interrupt load currents (up to the full load current of the generator)

– interrupt system-source short-circuit currents

– interrupt generator-source short-circuit currents

– interrupt fault currents due to out-of-phase conditions up to out-of-phase angles of 180°

5.2 Requirements for generator circuit-breakersThe requirements imposed on generator circuit-breakers greatly differentiate from the requirements imposed on general purpose transmission and distribution circuit-breakers.Due to the location of installation of a generator circuit- breaker high technical requirements are imposed on the circuit-breaker with respect to:

– rated current

– short-circuit currents (system-source and generator-source)

– fault currents due to out-of-phase conditions

– degree of asymmetry of fault currents, fault currents with delayed current zeros

– rate-of-rise of the recovery voltages

Circuit-breakers are only capable of providing satisfactory service when they are capable of fully meeting these require-ments.Specifications must therefore fully reflect the technical and reliability requirements and equipment, confirming to such specifications, must be designed and tested in full accor-dance with recognized and relevant standards.

5.3 Selection of generator circuit-breakersAccording to IEEE Std C37.013-1997 (R2008) the ratings and required capabilities of a generator circuit-breaker are the fol-lowing ones:

– rated maximum voltage – power frequency – rated continuous current – rated dielectric strength – rated short-circuit duty cycle – rated interrupting time – rated closing time – short-circuit current rating – transient recovery voltage rating – rated load current switching capability – capacitance current switching capability – out-of-phase current switching capability – excitation current switching capability – rated control voltage – rated mechanism fluid operating pressure

ABB | 19

5.3.1 Rated maximum voltageThe rated maximum voltage is the generator circuit-breaker’s upper limit for operation and it is selected so that it is higher than or equal to the maximum operating voltage of the generator.

5.3.3 Rated continuous currentThe rated continuous current of a generator circuit-breaker is the designated upper limit of current in r.m.s. amperes at power frequency, which it shall be required to carry continu-ously without exceeding any of the limitations designated in IEEE Std C37.013-1997 (R2008). Due to their installation location generator circuit-breakers have to be able to carry continuously load currents of very high magnitude. These currents place a severe stress on the conductors, connec-tions and contacts. In order to guarantee that the switches have a high degree of reliability and a long service life, they must be so designed that limits for temperature increase are not exceeded. In order to make optimal use of the conductor material employed in the circuit-breaker, the power losses have to be minimized and the transfer of heat from the conductor path to the environment must be intensified.The maximum value of load current which the circuit-breaker shall be able to carry continuously can be calculated by using the following formula:

The current carrying capability of a generator circuit-breaker depends on the operating condition at the specific location. When assessing the current carrying capability of a generator circuit-breaker special attention shall be paid to the following items:

– power frequency

– design temperature of the isolated phase bus to which the terminals of the generator circuit-breaker are connected (normally these temperatures are 105 °C (or 90 °C) for main conductor and 80 °C (or 70 °C) for enclosure)

– ambient temperature

– installation location (indoor or outdoor)

– colour of the enclosure of the generator circuit-breaker

In special cases the isolated phase bus is equipped with its own forced cooling system. In such a case also the technical parameters of this cooling system shall be taken into account in the assessment of the current carrying capability of the generator circuit-breaker.In Figure 20 the current carrying capability of the generator circuit-breaker type HECS-100L is displayed.

Imax is the maximum r.m.s. value of the current which the generator circuit-breaker shall be able to carry continuously

Figure 20: Current carrying capability curves for generator circuit-breaker type HECS-100L operating at a power frequency of 50 Hz and isolated phase bus temperatures of 90 °C / 70 °C (conductor / enclosure respectively)

-25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 Ambient Temperature (°C)

11000 12000 13000 14000 15000 16000 17000 18000 19000 20000

Cur

rent

(Arm

s)

indoor outdoor (RAL 9010)

minmax

3 V

SI n=

5.3.2 Power frequencyThe rated frequency for generator circuit-breakers is 50 Hz or 60 Hz, depending on the system power frequency in which the generator circuit-breaker is installed.

Sn is the rated power of the generatorVmin is the minimum operating voltage of the generator

20 | ABB

5.3.4 Rated dielectric strengthThe rated dielectric strength of a generator circuit-breaker is selected in accordance with the Table II depending on its rated maximum voltage.

Rated maximum voltage [kVrms]

Power frequency withstand voltage

[kVrms]

Lightning impulse withstand voltage

[kVpeak]

5 20 60

8.25 28 75

8.25 / 15 38 95

15.5 50 110

27 60 125

38 80 150

TABLE II: RATED DIELECTRIC STRENGTH OF GENERATOR CIRCUIT-BREAKERS IN

ACCORDANCE WITH IEEE Std C37.013a-2007

5.3.5 Rated short-circuit duty cycleThe rated short-circuit duty cycle of a generator circuit-breaker is two unit operations with a 30 min interval between operations (CO–30 min–CO) [1].

5.3.6 Rated interrupting timeAccording to IEEE Std C37.013-1997 (R2008) the rated inter-rupting time of the generator circuit-breaker is the maximum permissible interval between the energizing of the trip circuit at rated control voltage and rated fluid pressure of the operat-ing mechanism and the interruption of the main circuit in all poles on an opening operation.A typical interrupting time for ABB generator circuit-breakers is about 3 cycles.The interrupting time is the sum of the opening time (i.e. the time interval between the energizing of the opening circuit and the mechanical separation of the arcing contacts) and the arcing time (i.e. the time interval between the contact separation in the first pole and the final arc extinction in all poles).

5.3.7 Rated closing timeAccording to IEEE Std C37.013-1997 (R2008) the rated closing time of the generator circuit-breaker is the interval between energizing of the close circuit at rated control voltage and rated fluid pressure of the operating mechanism and the closing of the main circuit.

5.3.8 Short-circuit current rating

5.3.8.1 System-source short-circuit currentAccording to IEEE Std C37.013-1997 (R2008) the system-source short-circuit current of a generator circuit-breaker is the highest r.m.s. value of the symmetrical component of the three-phase short-circuit current. It is measured from the envelope of the current wave at the instant of primary arcing contact separation and is the current that the generator

circuit-breaker is required to interrupt at the rated maximum voltage and rated duty cycle when the source of the short-circuit current is from the power system through at least one transformation.The a.c. component of the system-source short-circuit current can be calculated by using the following formula:

eqac

X

VI =

3max

Iac is the a.c. component of the fault currentXeq is the equivalent reactance of the circuit referred to the LV-side of the step-up transformer

Vmax is the maximum r.m.s. value of the applied voltage prior to fault (it can generally be considered equal to the maximum service voltage of the HV-system re- ferred to the LV-side of the step-up transformer)

ABB | 21

The required asymmetrical system-source interrupting capa-bility of a generator circuit-breaker is composed of the r.m.s. symmetrical current and the percentage d.c. component. The values of the d.c. component are expressed in percent of the peak value of the symmetrical short-circuit current and are measured at the primary arcing contact parting time. The pri-mary arcing contact parting time can be generally considered

equal to the sum of 1/2 cycle (protection system tripping delay) plus the minimum opening time of the particular generator circuit-breaker.The standard value for the time constant of the decay of the d.c. component is 133 ms. For time constants different than 133 ms, the following formula can be used:

Idc is the d.c. component of the fault currentIac is the a.c. component of the fault current

τcpt

acdc eII−

= 2

eq

eq

R

X=ω

τ

Xeq is the equivalent reactance of the circuit referred to the LV-side of the step-up transformerω is equal to 2 π f with f being the power frequency

When the fault current is asymmetrical it is characterized by a degree of asymmetry which is defined as follows:

a is the degree of asymmetry.

ac

dc

I

Ia

2=

The typical course of the system-source short-circuit current and of its degree of asymmetry are shown in Figure 21 and Figure 22, respectively. It is understood that the degree of asymmetry of the system-source short-circuit current is gen-erally monotonically decreasing with time as the a.c. compo-

nent of the fault current is usually constant. Its value depends on the opening time of the circuit-breaker and on the relay time of the protection system and it assumes a typical value of 75% at the primary arcing contact parting time.

Time (ms)

Cur

rent

(pu)

Iac

Idc

tcp

2

-1

-0.5

0

0.5

1

1.5

2

0 10 20 30 40 50 60 70 80 90 1000.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Time (ms)

Deg

ree

of a

sym

met

ry (%

)

0 10 20 30 40 50 60 70 80 90 100

Figure 21: Prospective system-source short-circuit current Figure 22: Degree of asymmetry of the system-source short-circuit current

tcp is the primary arcing contact parting timeτ is the time constant of the decay of the d.c. component and it can be calculated by using the fol lowing formula:

Req is the is the equivalent resistance of the circuit referred to the LV-side of the step-up transformer

22 | ABB

The system-source short-circuit current is generally fed by the HV-system and by the motors connected to the LV-side of the unit auxiliary transformer (see Figure 23). The a.c. component of the system-source short-circuit current for the power plant layout depicted in Figure 23 can be calculated by using the following formula:

G

Step-UpTransformer

GeneratorCircuit-Breaker

M

HV-System

Unit AuxiliaryTransformer

Motors GeneratorFigure 23: (right side) Typical power plant layout with a step-up transformer and a unit auxiliary transformer

( )+

++

=

AUXTLVAUXT

HVAUXT

rM

rM

LR

rMGSUTsys

ac

XV

V

S

V

I

IXX

VI

2

_

_2

max 11

3

Vmax is the maximum r.m.s. value of the applied voltage prior to fault (it can be considered equal to the maximum service voltage of the HV-system referred to the LV-side of the step-up transformer)Xsys is the equivalent reactance of the HV-system referred to the LV-side of the step-up transformerXGSUT is the short-circuit reactance of the step-up transformer referred to the LV-side of the step-up transformerVrM is the rated voltage of the motors connected to the LV-side of the unit auxiliary transformer

SrM is the rated apparent power of the motors connected to the LV-side of the unit auxiliary transformerILR / IrM is the ratio of the locked-rotor current to the rated current of the motorXAUXT is the short-circuit reactance of the unit auxiliary transformer referred to the HV-side of the unit auxiliary transformer

is the transformation ratio of the unit auxiliary transformer

The d.c. component of the system-source short-circuit current for the power plant layout depicted in Figure 23 can be calculated by using the following formula:

( )( )

+

++

= ++

−−

AUXTM

cp

GSUTsys

cp t

AUXTLVAUXT

HVAUXT

rM

rM

LR

rM

t

GSUTsys

cpdc e

XV

V

SV

II

eXX

VtI ττ

2

_

_2

max 11

32

Rsys the equivalent resistance of the HV-system referred to the LV-side of the step-up transformerRGSUT the resistive component of the short-circuit impedance of the step-up transformer referred to the LV-side of the step-up transformerXM / RM the X/R ratio of the motors connected to the LV-side of the unit auxiliary transformer

( )( )GSUTsys

GSUTsyssys + GSUT

RR

XX

+

+=ω

τ

+

+

=+

AUXTLVAUXT

HVAUXT

rM

rM

LR

rM

M

M

AUXTLVAUXT

HVAUXT

rM

rM

LR

rM

AUXTM

RV

V

S

V

I

I

X

R

XV

V

S

V

I

I

2

_

_2

2

_

_2

ω

τ

RAUXT the resistive component of the short-circuit impedance of the unit auxiliary transformer referred to the HV-side of the unit auxiliary transformerω is equal to 2 π f with f being the power frequency

VAUXT_HV

VAUXT_LV

ABB | 23

The degree of asymmetry of the fault current measured at the contact parting time is:

In some cases a three-winding transformer is used to connect two generators to the HV-system (see Figure 24). In this case the system-source short-circuit current has three contribu-tions, i.e. it is fed by the HV-system, by the motors connected to the LV-side of the unit auxiliary transformer and by the other generator through the step-up transformer. Special attention shall be paid to this scheme because the degree of asymmetry of the system-source short-circuit current can be very high depending on the reactances and time constants of the generator. In some cases the current wave-shape might show delayed current zeros (i.e. degree of asymmetry higher than 100%).

( ) ( )ac

cpdccp

I

tIta =

2

G

Step-UpTransformer

GeneratorCircuit-Breakers

M

HV-System

Generator

G

Generator Motors

Unit AuxiliaryTransformer

Figure 24: Power plant layout with a three-winding step-up transformer and a two-winding unit auxiliary transformer

5.3.8.2 Generator-source short-circuit currentAccording to IEEE Std C37.013-1997 (R2008) the generator-source short-circuit current of a generator circuit-breaker is the highest r.m.s. value of the symmetrical component of the three-phase short-circuit current. It is measured from the envelope of the current wave at the instant of primary arcing contact separation that the generator circuit-breaker shall be required to interrupt, at rated maximum voltage and rated duty cycle when the source of the short-circuit current

is entirely from a generator through no transformations. The generator-source symmetrical short-circuit current is usually lower than the system-source symmetrical short-circuit current.

The generator-source symmetrical short-circuit current can be calculated using the following simplified formula for no-load conditions:

+−+−= −

ddd

t

ddrG

rGmGsymgen xxx

exxV

SVI d

11'1

'1

''1

3''

2τ −te d'τ

Igen sym is the a.c. component of the generator-source short- circuit currentVmG is the maximum generator line-to-line voltageSrG is the rated power of the generatorVrG is the rated voltage of the generator xd is the pu value of the direct-axis synchronous reactance

x'd is the pu value of the direct-axis transient reactancex"d is the pu value of the direct-axis subtransient reactanceτ 'd is the direct-axis transient short-circuit time constantτ "d is the direct-axis subtransient short-circuit time constant

If the fault initiation takes place when the voltage in one phase passes through zero the resulting fault current in that phase exhibits the maximum degree of asymmetry. The a.c. compo-nent decays with the subtransient and transient time

constants of the generator; the d.c. component decays with the armature time constant τa. The armature time constant can be calculated with the following formula:

aa R

Xπ

τ2

2=f

τa is the armature time constantX2 is the negative-sequence reactance of the generator

f is the power frequencyRa is the d.c. armature resistance

24 | ABB

The value of X2 can be approximated by:

The generator-source asymmetrical short-circuit current for the phase with the highest asymmetry, the generator being in the no-load mode, can be calculated by the following simplified formula:

If the a.c. component of the fault current decays faster than the d.c. component, it can happen that for a certain period of time following the initiation of the fault the magnitude of the d.c. component of the fault current is bigger than the peak value of its a.c. component. In such a case the degree of

asymmetry of the fault current is higher than 100% thus lead-ing to delayed current zeros. The typical course of the degree of asymmetry of the generator-source short-circuit current is shown in Figure 25.

2

''''

2qd XX

X+

=

X"d is the direct-axis subtransient reactance of the generator

X”q is the quadrature-axis subtransient reactance of the generator

( ) ( )

−++−+−+−= −−−− texx

exx

tx

exx

exxV

SVI aadd t

qd

t

qdd

t

dd

t

ddrG

rGGmasymgen ωω ττττ 2cos

''1

''1

21

''1

''1

21

cos11

'1

'1

''1

3

2 '''2

ω is equal to 2 π f with f being the power frequency x”q is the quadrature-axis subtransient reactance in pu

Since x"d is approximately equal to x"q for turbo generators, the equation can be written as follows:

( )

−+−+−= −−− ex

tx

exx

exxV

SVI add t

dd

t

dd

t

ddrG

rGGmasymgen ω τττ

''1

cos11

'1

'1

''1

3

2 '''2

00.02 0.04 0.06 0.08 0.10 0.12 0.140

time [s]

100

50

Deg

ree

of A

sym

met

rie [%

]

150

Figure 25: Typical course of the degree of asymmetry of the generator-source short-circuit current

ABB | 25

In addition the a.c. component of the generator-source short-circuit current and its degree of asymmetry can vary if the generator is unloaded or delivering power with lagging power factor (i.e. working in the over-excited mode) or leading power factor (i.e. working in the under-excited mode) prior to fault. Typical fault current wave-shapes are depicted in Figures 26, 27 and 28.The magnitude of the a.c. component of the fault current which is fed by the generator is typically about 80% of the magnitude of the a.c. component of the system-source short-

circuit current and typically shows a degree of asymmetry measured at the primary arcing contact parting times are in the order of 130% (see Figure 25). Special attention should be paid if the generator is loaded with leading power factor before fault initiation. In such a case the degree of asymmetry of the fault current can reach very high values and exceed 130%.In order to accurately simulate the behaviour of the genera-tor in case it is loaded prior to fault computer simulations are necessary.

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

Figure 26: Prospective generator-source short-circuit current (generator unloaded prior to fault initiation) - fault initiation at UA = 0

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

Figure 27: Prospective generator-source short-circuit current (generator delivering power with lagging power factor prior to fault initiation) - fault initiation at UA = 0

Figure 28: Prospective generator-source short-circuit current (generator delivering power with leading power factor prior to fault initiation) - fault initiation at UA = 0

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

Additional resistance in series with the armature resistance forces the d.c. component of the short-circuit current to decay faster. Such additional resistance may be the connec-tion from the generator to the fault location, but especially the circuit-breaker arc resistance after contact separation. If there is an arc at the fault location, this arc resistance further reduces the time constant of the d.c. component from the initiation of the fault. The values of these additional series resistances are normally high enough to force a fast decay of the d.c. component of the short-circuit current so that current zeros are produced.

In case of a short-circuit current with delayed current zeros the capability of a circuit-breaker to interrupt a given short-circuit current can be considered as being demonstrated if the generator circuit-breaker is capable of forcing the current to zero within the time interval in which it is able to interrupt a current (i.e. within the maximum permissible arcing time). According to IEEE Std C37.013-1997 (R2008) demonstrating the capability of a generator circuit-breaker to interrupt short-circuit currents with delayed current zeros may be difficult and limited in high power testing stations. Since various designs

26 | ABB

of generators behave differently, it may not be possible to simulate the required current shape in the testing station. Therefore the capability of a circuit-breaker to interrupt a short-circuit current with delayed current zeros can be ascer-tained by calculations that take into account the effect of the arc-voltage of the circuit-breaker on the prospective short- circuit current. The arc-voltage model used for this purpose has to be derived from tests (IEEE Std C37.013-1997 (R2008), Clause 6.2.7). The technical data of the actual generator shall be used for these computations.

According to IEEE Std C37.013-1997 (R2008), Clause 7.3.5.3.5.3 the following two cases shall be investigated:

1) Generator at no-load with the generator circuit-breaker closing into a three-phase fault. In the computation the arc-voltage of the generator circuit-breaker starting at contact separation shall be taken into account.

2) Generator in service with leading power factor. An arcing fault is assumed in at least two phases. In the computation the arc-voltage at the fault location starting at the initiation of the fault and the arc-voltage of the generator circuit-breaker starting at contact separation shall be taken into account.

Further the following two situations shall be considered for a particular generator-source short-circuit current in case of a three-phase fault (IEEE Std C37.013-1997 (R2008), Clause 6.2.7.2):

1) Fault initiation at voltage zero in one phase which implies that the current in the corresponding phase exhibits the maximum degree of asymmetry.

2) Fault initiation at voltage maximum in one phase which implies that the current in the corresponding phase is symmetrical.

The arc-voltage of a circuit-breaker depends on the instanta-neous value of the current and on the type of the extinguish-ing medium, its pressure, the intensity of its flow and the length of the arc. The uarc-i characteristic of one break of the circuit-breaker has to be derived from short-circuit current interrupting tests. To be able to investigate the behaviour of the circuit-breaker during the interruption of short-circuit currents with delayed current zeros the arc-voltage versus current characteristic has to be transferred into a mathematical model. From the arc-voltage uarc(i,t) and the current i(t) the arc resistance Rarc(i,t) can be obtained. In order to model the behaviour of the SF6 circuit-breaker a non-linear time-varying resistance of the value Rarc(i,t) has to be inserted into the simulation at the time of the separation of the contacts of the circuit-breaker.

Figures 29 to 32 show examples of the corresponding cal-culation results. Figures 29 and 30 represent the case of the generator being under no-load condition with the generator circuit-breaker closing into a three-phase fault. In the compu-tation the arc-voltage of the generator circuit-breaker starting at contact separation is taken into account. Figure 29 repre-sents the case with fault initiation at voltage zero and Figure 30 represents the case with fault initiation at voltage maxi-mum in one phase. Figures 31 and 32 represent the case of the generator being in service with a leading power factor. In the computation the arc-voltage at the fault location starting at the initiation of the fault and the arc-voltage of the genera-tor circuit-breaker starting at contact separation is taken into account. Figure 31 represents the case with fault initiation at voltage zero and Figure 32 represents the case with fault initiation at voltage maximum in one phase.As the maximum calculated arcing time (i.e. 20.9 ms, see Figure 29) is shorter than the maximum arcing time of the generator circuit-breaker of concern it can be concluded that the circuit-breaker is capable of interrupting these fault currents showing delayed current zeros.

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

tcp

Figure 29: Interruption of generator-source short-circuit current with a SF6 generator circuit-breaker – generator unloaded prior to fault initiation – fault initiation at UA = 0 – contact parting time tcp = 39 ms – arcing time = 20.9 ms

Figure 30: Interruption of generator-source short-circuit current with a SF6 generator circuit-breaker – generator unloaded prior to fault initiation – fault initiation at UA = max – contact parting time tcp = 39 ms – arcing time = 20.7 ms

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

tcp

ABB | 27

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

tcp

Figure 31: Interruption of generator-source short-circuit current with a SF6 generator circuit-breaker – generator delivering power with leading power factor prior to fault

initiation – fault initiation at UA = 0 – contact parting time tcp = 39 ms – arcing time = 18.2 ms

Figure 32: Interruption of generator-source short-circuit current with a SF6 generator circuit-breaker – generator delivering power with leading power factor prior to fault

initiation – fault initiation at UA = max – contact parting time tcp = 39 ms – arcing time = 18.6 ms

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

tcp

In some cases the arc-voltage of the generator circuit-breaker is not high enough to force current zeros within the maximum permissible arcing time of the circuit-breaker (this can hap-pen for example if a vacuum interrupter is employed as a generator circuit-breaker). In such a case a solution which is sometimes adopted is to delay the tripping signal to the generator circuit-breaker. It has to be noted that this solution

is not recommendable because the longer fault arcing time might lead to severe damages to power plant equipment with consequent long downtime for repair. A better approach con-sists in choosing a generator circuit-breaker which is proven to be able to interrupt the fault current without the aid of any intentional tripping delay.

5.3.8.3 Required closing, latching, and carrying capabilitiesThe short-circuit current into which the generator circuit-breaker must close is determined by the higher value of either the system-source short-circuit current or the generator-source short-circuit current. In the majority of applications the system-source short-circuit current is higher than the generator-source short-circuit current.According to IEEE Std C37.013-1997 (R2008) the generator circuit-breaker shall be capable of the following:

a) Closing and latching any power frequency-making current (50 Hz or 60 Hz) whose maximum crest (peak making current) does not exceed 2.74 times the rated symmetrical short-circuit current or the maximum crest (peak making current) of the generator-source short-circuit current, whichever is higher.

b) Carrying the short-circuit current for a time of 0.25 s.

5.3.8.4 Required short-time current-carrying capabilityAccording to IEEE Std C37.013-1997 (R2008) the generator circuit-breaker shall be capable of carrying for a time equal to 1 s, any short-circuit current, whose peak value does not exceed 2.74 times the rated short-circuit current, as deter-

mined from the envelope of the current wave, at the time of the maximum peak, and whose r.m.s. value determined over the complete 1s period, does not exceed the rated short-circuit current considered above.

5.3.9 Transient recovery voltage ratingThe transient recovery voltage is the voltage appearing across the open contacts of the generator circuit-breaker immediately after current interruption. The characteristics

of the generator and of the associated step-up transformer dictate the wave-shape of the inherent TRV for various duties.

28 | ABB

A three-phase fault is the most severe case and gives the maximum short-circuit current and the maximum TRV rate.The neutral of the generator is not solidly grounded, thus the phase-to-ground fault current is not significant.The typical power plant layout is shown in Figure 23, where the generator and the step-up transformer have essentially the same rating. For TRV calculations the contribution of auxiliary transformer to the fault current can be neglected as it is a minor source of short-circuit current.

The TRV shall be calculated after the interruption of a sym-metrical current as any asymmetry in the current wave-shape would lead to a less severe TRV. At the interruption of the short-circuit current with maximum asymmetry, the transient oscillation of the recovery voltage will be very small or even non-existent since at the moment of short-circuit current interruption, the normal frequency voltage value may be very small or zero.

5.3.9.1 First-pole-to-clear factorWhen interrupting any symmetrical three-phase current the first-pole-to-clear factor kpp is the ratio of the power frequency voltage across the first interrupting pole before current inter-ruption in the other poles, to the power frequency voltage

occurring across the pole or the poles after interruption in all three poles [3]. Standard value for generator circuit-breakers is 1.5. The first-pole-to-clear factor can be calculated by using the following formula:

10

0

2

3

ZZ

Zkpp +

=

kpp is the first-pole-to-clear factorZ0 is the equivalent zero-sequence impedance of the three-phase circuit

Z1 is the equivalent positive-sequence impedance of the three-phase circuit

In practical applications the step-up transformer is Ynd connected and the star point of the stator winding of the generator is usually grounded via a high resistance.

The zero-sequence impedance of such a system is much higher than Z1 thus leading to

5.12

3

2

3

0

0

10

0 =≈+

=Z

Z

ZZ

Zkpp

5.3.9.2 Amplitude factorThe amplitude factor kaf is ratio between the maximum excursion of the transient recovery voltage to the peak value of the power frequency recovery voltage [3].

Standard value for generator circuit-breakers is 1.5 without considering any capacitance connected at the terminals of the generator circuit-breaker.

5.3.9.3 Power frequency recovery voltageThe power frequency recovery voltage is the recovery voltage after the transient voltage phenomena have subsided [3].The magnitude of the power frequency recovery voltage which is imposed on the first pole which clears the current is 1.5

higher then the power frequency voltage. The second and third poles clear the current at the same time and the power frequency recovery voltage which is imposed on each of them is √3/2 times the power frequency voltage.

ABB | 29

5.3.9.4 Rated inherent transient recovery voltageThe rated inherent transient recovery voltage is the reference voltage that constitutes the limit of the inherent transient recovery voltage of circuits that the generator circuit-breaker shall be capable of withstanding under fault conditions and shall be defined by an oscillatory wave-shape having a TRV rate-of-rise, time delay (td) and peak voltage (E2) [1].The waveform of transient recovery voltages approximates to a damped oscillation.

The TRV curve is bounded by three lines:

a) one line starts at the origin of time axis and is tangent to the TRV curve with a slope equal to the TRV rate-of-rise

b) one line is horizontal and is tangent to the TRV curve at the time of TRV peak T2.

c) one line starts on the time axis at the rated time delay (td) and runs parallel to the first reference line

An example of a transient recovery voltage wave-shape is depicted in Figure 33

T2 is the time to reach the peak voltage E2E2 is the peak value of the TRVt3 is the intersection point of the tangent to the transient recovery voltage which starts at the origin of the time axis and to the horizontal tangent to the TRV curve at the time of TRV peak T2

E2

t d t 3 T2

TRVTRV rate-of-rise

Figure 33: Inherent TRV curve for first-pole-to-clear for required symmetrical interrupting capability for three-phase faults

The standard value of E2 can be calculated with following formula:

VkkVE ppaf == 84.13

22

V is the rated maximum voltage of the generator circuit-breaker.

The rated TRV is the inherent value assuming an ideal generator circuit-breaker. These values may be modified by the generator circuit-breaker characteristics or by the asymmetry of the current.A system with a TRV that exceeds the rated values of the generator circuit-breaker must be modified in such a way as to lower the TRV. This is generally achieved by connecting capacitors phase-to-ground on both sides of the generator circuit-breaker.The additional capacitance has three effects:

– it decreases the oscillation frequency and the RRRV of the TRV

– it increases the time delay of the TRV

– it increases the peak value of the TRV

If the circuit-breaker requires that the inherent TRV be modified by the addition of capacitors, then the amount of equivalent capacitance required has to be given in the test report and on the nameplate [1].

30 | ABB

5.3.9.5 System-source faultsFor system-source faults the maximum value of short-circuit current is obtained for a given transformer when Xsys is minimum or assumed to be zero. It is assumed that the contribution of the auxiliary system to the fault current is negligible. The natural frequency of the transformer is much higher than the natural frequency of the HV-system. The TRV first oscillates at the prospective value of 1.5 √2 XGSUT Iac, where Iac is the

r.m.s. value of the symmetrical short-circuit current until the second and third poles open. The voltage drop in the transformer is equal to the total power frequency recovery voltage for Xsys = 0. Therefore, the TRV rate is maximum when the short-circuit current is maximum [1].

5.3.9.6 Generator-source faultsFor generator-source faults the short-circuit current is generally lower than for system-source faults because of the higher reactance of the generator windings. Although the short-circuit current is lower for generator-source faults than for system-source faults, generator-source faults cannot be ignored because of the short time delay specified in IEEE Std C37.013-1997 (R2008).For a generator-source fed fault occurring at the HV-side of the step-up transformer the short-circuit current is lower when compared to a fault

at the LV-side of the step-up transformer. The TRV results from transformer and generator voltage oscillations. The magnitude of each oscillation is approximately proportional to the transformer and generator reactances, respectively. This fault loca-tion can usually be ignored because the resulting stresses on the generator circuit-breaker are much lower than for faults occurring at the LV-side of the step-up transformer [1].

5.3.9.7 Calculation of TRV in case of terminal faultsTRV calculations need to be performed with computer simulations which allow to model power plant equipment with distributed parameters. Anyway a simplified single-phase circuit for calculating the TRV in case of interruption of terminal fault currents is depicted in Figure 34 where Veq is the r.m.s. value of the voltage source and Req, Leq and Ceq are

respectively the values of equivalent resistance, in-ductance and capacitance to ground of the circuit for assessing the TRV across the first pole to clear (lumped parameters).

GCB

Ceq

LeqReq

Veq

Figure 34: Single-phase circuit for TRV calculation in case of terminal faults

Veq can be calculated by using the following expression:

max10

0max 5.1

23

VZZ

ZVVeq ≈

+=

Vmax is the maximum r.m.s. value of the applied voltage prior to fault (it can generally be considered equal to the maximum phase-to-ground service voltage of the HV-system referred to the LV-side of the step-up transformer and to the maximum phase-to-ground operating voltage of the generator

in case of system-source short-circuit currents and generator-source short-circuit currents, respectively)Z0 is the equivalent zero-sequence impedance of the three-phase circuitZ1 is the equivalent positive-sequence impedance of the three-phase circuit

ABB | 31

Leq can be considered equal to 1.5 L1 where L1 is the equiva-lent positive-sequence inductance of the three-phase circuit.Following the same procedure Req is equal to 1.5 R1 where R1

is the positive-sequence resistance of the three-phase circuit.

Ceq can be calculated by using the following expression:

thus leading to an underdamped wave-shape of the TRV.The TRV will appear as the superposition of sinusoidal curves oscillating at different frequencies, i.e. one oscillating at power

C0 is the zero-sequence capacitance of the three-phase circuit

C1 is the positive-sequence capacitance of the three- phase circuit

In all practical applications the following expression is valid:

2

2

4

1

eq

eq

eqeq L

R

CL>

frequency and one oscillating at the frequency imposed by the circuit parameters:

( ) ( )−=−

)coscos2 2 tetVtTRVut

L

R

eqeq

eq

νω )

where ν can be calculated with following formula:

eqeqeq

eq

eqeq CLL

R

CL≈−=

1

4

12

2

ν2

2

4

1

eq

eq

eqeq L

R

CL>>assuming

Assuming that cos(ω t) ~ 1 at the time of TRV peak (being ν >> ω) the transient recovery voltage can be finally

−−

)cos12 2 teVt

L

R

eqeq

eq

ν)( ) =tTRVu The TRV peak value occurs at time eqeq CLT = π2

The corresponding peak value of the TRV is

+=−=−−

eq

eqeq

eq

eq

L

CR

eq

TL

R

eq 2 eVTeVEπ

ν 222 12)cos12

2 )

32 10 CC

Ceq+

= If C0 = C1 then Ceq = C1 = C0

represented by the following expression:

32 | ABB

5.3.10 Rated load current switching capabilityDuring normal service of the generator, the load current is reduced to zero before an opening operation of the genera-tor circuit-breaker is initiated. However, the interruption of full load current may be required occasionally for emergency circumstances or when the synchronous machine is working

in the motor mode in pumped storage power plants. The gen-erator circuit-breaker shall be capable of interrupting those currents and withstanding the TRV appearing across the open contacts immediately after the interruption of the current.

5.3.11 Capacitance current switching capabilityIEEE Std C37.013-1997 (R2008) considers this as a special case where the line or bus capacitance is separated from the generator circuit-breaker through transformation.

The generator circuit-breaker normally is not called on to switch purely capacitive currents.

5.3.12 Out-of-phase current switching capabilityThis capability applies to a generator circuit-breaker used for switching the connection between two parts of a three-phase system during out-of-phase conditions. The assigned out-of-phase current switching rating is the maximum out-of-phase current that the generator circuit-breaker shall be capable of switching at an out-of-phase recovery voltage.Out-of-phase synchronising occasionally occurs in power plants [4]. The main reasons for out-of-phase synchronis-ing are wiring errors made during commissioning or during maintenance when connecting voltage transformers and synchronising equipment. These wiring errors lead to particu-lar out-of-phase angles, i.e. multiples of 60°el.. E.g. polarity errors at a voltage transformer cause synchronising at 180°el. out-of-phase angle; phase connection errors lead to 60°el. and 120°el. out-of-phase angles. Besides these particular out-of phase angles any value may be caused by inadequate settings of the synchronising equipment, e.g. due to an incor-rect value of the closing time of the circuit-breaker.The TRV appearing immediately after the interruption of fault currents resulting from out-of-phase synchronising is very severe with respect to both peak value and rate-of-rise and time delay. Even though it is recognized that synchronising with out-of-phase angle up to 180° might occur, IEEE Std C37.013-1997 (R2008) covers only requirements for a maximum out-of-phase angle of 90°. The current resulting from out-of-phase synchronizing might show delayed current zeros whose causes are totally different compared to generator terminal faults. The rapid movement of the rotor from initial out-of-phase angle δ0 to δ = 0 results in a very small a.c. component of the fault current and a domi-nant d.c. component when the condition of δ = 0 is reached. The current resulting from out-of-phase synchronizing has to be assessed by the aid of computer simulations which allow to model with high level of accuracy power plants equipment and especially the synchronous machine. As the instant when the δ = 0 condition is reached is determined by the movement of the rotor, the inertia constants of turbine, rotor and excita-tion equipment of the generator are of special importance. As the fault current to be interrupted by the generator circuit-breaker is characterized by delayed current zeros it extremely important to prove that the circuit-breaker by means of its arc-voltage is capable of forcing current to zero within its maximum arcing time.

The most important parameters which influence the wave-shape of the fault current resulting from out-of-phase synchronizing and the occurrence of delayed current zeros are power plant equipment parameters, out-of-phase angle δ0, power frequency of the system and instant when the synchro-nization is initiated.The wave-shape of the out-of-phase current is depicted in Figures 35 to 40 for different values of δ0. It is evident that at the time when δ = 0 the fault current is dominated by a d.c. component. In modern power systems the protection sys-tems sends the tripping signal to the generator circuit-breaker before the δ = 0 condition is reached, thus leading to a less severe tripping operation. If the tripping is delayed this might lead to extremely severe interrupting conditions and even unsuccessful interruption. It is shown in published literature that circuit-breakers installed at the HV-side of the step-up transformer may not be suitable for interrupting fault currents resulting from out-of-phase synchronizing [5]. Although the arc-voltage of the HV circuit-breaker is of the same order of magnitude of the arc-voltage of the generator circuit-breaker, its value referred to the LV-side of the step-up transformer is reduced by the transformation ratio and has practically no effect on the time constant of the decay of the d.c. component of the fault current.

ABB | 33

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

Figure 36: Prospective out-of-phase fault current – out-of-phase

angle δ0 = 60°

Figure 37: Prospective out-of-phase fault current – out-of-phase

angle δ0 = 90°

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

Figure 38: Prospective out-of-phase fault current – out-of-phase angle

δ0 = 120°

Figure 39: Prospective out-of-phase fault current – out-of-phase angle

δ0 = 150°

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

Figure 40: Prospective out-of-phase fault current – out-of-phase angle

δ0 = 180°

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

Figure 35: Prospective out-of-phase fault current – out-of-phase

angle δ0 = 30°

34 | ABB

5.3.13 Excitation current switching capabilityIEEE Std C37.013-1997 (R2008) defines the excitation cur-rent switching capability as the highest magnetizing current that a generator circuit-breaker shall be required to switch at any voltage up to rated maximum voltage at power frequency without causing an overvoltage exceeding the levels agreed upon between the user and the manufacturer.During normal operation, a generator step-up transformer is rarely switched in an unloaded condition. Anyway, consider-ation should be given to switching of transformer excitation current. Excitation current switching is not so much a matter of the generator circuit-breaker capability, but a question of whether overvoltages are produced due to current chopping.Due to instabilities of the arc between the circuit-breaker contacts premature current zeros at high frequencies occur frequently when switching small inductive currents, leading to current chopping. The chopped current flowing in the no-load inductance charges the capacitances of the transformer windings and the capacitances of the connection between the step-up transformer and the generator circuit-breaker (e.g.

buses or cables). This might results in voltage oscillations of high amplitudes. Modern transformers have a low no-load current value compared to older designs, and their magnetic characteristics are such that a relatively low amount of energy is released when current chopping occurs during switching, leading to moderate chopping overvoltages [1]. Chopping overvoltages are produced only on the transformer side of the generator circuit-breaker. No overvoltages occur on the generator side because the inductance of the generator is much lower than the magnetizing impedance of the trans-former, and the energy content is low and not of sufficient magnitude to produce overvoltages [1].The overvoltage generated by current chopping can be esti-mated with the following formula where it has been assumed that the energy stored in the magnetizing inductance of the step-up transformer is transferred to the equivalent capaci-tance without losses. In addition the magnetizing character-istics and the hysteresis loop of the step-up transformer have been neglected.

= 22

21

21

vCiL eqmageq

mag

C

Liv =⇒

v is the voltage generated by current choppingi is the chopped currentLmag is the magnetizing inductance of the step-up transformer

Ceq is the equivalent capacitance to ground of the step- up transformer windings and the connection of the step-up transformer to the generator circuit-breaker terminals

The value of chopped current, and consequently the overvol-tages produced, are mainly dependent on the type of gen-erator circuit-breaker. Experience indicates that the current chopping level of SF6 self-blast generator circuit-breakers is low and no overvoltages of concern are expected. Further-more, the transformer LV-side is usually protected by surge arresters which reduce these overvoltages. The energy to be absorbed by the arresters is usually extremely small. In

addition the generator circuit-breaker systems are generally equipped with capacitors which help to mitigate the transient recovery voltage appearing after current interruptions. Those capacitors are also very effective in reducing the overvoltages produced by current chopping. It has to be mentioned that the capacitors installed at the generator circuit-breaker termi-nals increase the chopping current level but on the other hand they help reducing the generated overvoltage.

5.3.14 Rated control voltageAccording to IEEE Std C37.013-1997 (R2008) the rated con-trol voltage of a generator circuit-breaker is the designated voltage that is to be applied to the closing or tripping devices to close or open the generator circuit-breaker. Rated voltages and their permissible ranges for the control power supply of

generator circuit-breakers are shown in Table 10 of IEEE Std C37.013-1997 (R2008). Other control voltages may be speci-fied according to other national or international standards depending on the point of original installation.

5.3.15 Rated mechanism fluid operating pressureAccording to IEEE Std C37.013-1997 (R2008) the rated mechanism fluid operating pressure of a generator circuit-breaker is the pressure at which a gas- or liquid-operated

mechanism is designed to operate. The pressure is allowed to vary above and below its rated value within a specified range.

ABB | 35

The major demands on the electrical layout of power plants can be summarised as follows:

– transfer the generated electric energy from the generator to the HV-transmission system considering operation require-ments as well as availability, reliability, and economical aspects

– supply of electric power for auxiliary and station service systems to ensure a safe and reliable power plant operation

The criteria which are used to evaluate and assess the generator circuit-breaker’s application exert considerable influence on the electrical layout of the power plant.

6 Application of generator circuit-breakers

6.1 Power plant layoutsExamples of power station layouts which employ a generator circuit-breaker to connect the generator to the main trans-former are shown in Figure 41.

6.1.1 Thermal power plantsThe typical layout of a thermal power plant is depicted in Figure 41a. During normal operation the generator supplies power to the auxiliary system. The main net is the backup source for the auxiliaries; i.e. whenever a unit is shutdown the power is drawn from the main net through the main transformer. The station transformer is generally rated as an emergency shut-down transformer.

An interesting alternative is represented by the layout of Figure 41b. In this case no station transformer is available and the backup source for the auxiliaries are the auxiliary busbars of another unit. This solution is very attractive because it allows to reduce the investment costs as well as the operation and maintenance expenses by omitting the station transformer and the associated MV and HV equipment.

6.1.2 Gas turbine power plantsThe typical layout of a gas turbine power plant is depicted in Figure 41c. When a gas-turbine generator is started-up, its rotor must be accelerated by external means to about 60% of the rated speed before the start-up process becomes self-sustaining, i.e. before the turbine can generate suffi-cient power to continue process independently. The energy required for this purpose can be provided for instance by a pony motor or a static frequency converter (SFC). Starting-up with the help of a pony motor is suitable for smaller machines but has several disadvantages when applied to larger ma-chines and especially to single shaft units in combined-cycle

power plants. For this reason the use of SFC starting equipment is becoming more and more widespread. ABB generator circuit-breakers also contain the switching functions required for SFC starting within its enclosure. The output of the SFC (voltage of variable amplitude and frequency) is fed to the generator terminals via the starting switch that is designed for the voltage, current and current duration occurring during the SFC start-up period of the gas turbine. Its rated voltage is chosen according to the rated voltage of the SFC which in general is considerably lower than the generator rated voltage.

6.1.3 Hydro power plantsThe typical layout of a hydro power plant is depicted in Figure 41d. Due to the low power consumption of unit auxiliaries the electrical layout of hydro power plants generally employs one unit transformer to supply power to station auxiliaries.Three-winding transformers are sometimes used in hydro power plants (Figure 41e). Making a justifiable decision for applying three-winding transformers requires detailed technical and economical evaluations.Some of the items which should be considered for selecting transformer arrangement in a power plant are the following ones:

Installation: The accommodation of a three-winding transformer is generally preferred in power plants where the available space is at a premium.Power delivery to the high-voltage substation: The loss of a three-winding transformer would result in the loss of two paths for electrical energy. For instance a failure in one three-winding step-up transformer in Figure 41e would cause the outage of the power output of two units. Besides the cases in which economic or space issues are important, connect-ing two units to a three-winding transformer is not generally adopted in power plants.

36 | ABB

Protection: The differential protection is used as the main protection for a step-up transformer. The block differential protection is applied as a back-up protection. Due to a higher number of windings involved, the protection of a three-winding transformer is relatively more complicated compared to the case when two-winding transformers are employed. Except for the number of windings, no additional noticeable differences can be found between protection functions of two- and three-winding transformers.

Operation and maintenance: Preventive and especially corrective maintenance of a three-winding transformer might be an issue as this occurrence would lead to the outage of two units. A solution which can be adopted to reduce the downtime is to buy a spare transformer. In order to reduce the cost of the spare equipment sometimes three single-phase three-winding transformers are employed and only one spare single-phase machine is considered. In addition to the above mentioned items, availability and commercial feasibility have to be considered too in order to make a more thorough comparison between two- and three-winding transformers.

6.1.4 Pumped storage power plantsPumped storage power plants are of great importance for the economical operation of a power system as they make use of electrical energy during off-peak hours to pump water from a lower reservoir to a higher reservoir for the use in the genera-tion of electrical energy during system peak periods. Due to space limitations, especially in the case of power plants located in caverns, compact and therefore simple layouts are highly desirable in the case of pumped storage power plants. Simple layouts also bring about operational advantages and lead to an increased power plant reliability. Due to high number of transitions from one operating mode to another possible in a pumped storage power plant the requirements on the mechanical and electrical endurance of the switchgear used are very high. A circuit-breaker for instance may have to carry out up to 10 operations per day.When a machine is started-up in the motor mode, it has to be accelerated to its rated speed before it can be connected to the system (unless asynchronous starting is used). The two most common methods used for starting-up machines in the motor mode are the static frequency converter (SFC) and the “back-to-back” starting arrangements. Both are synchronous starting modes. In the SFC starting arrangement the machine is connected to a converter (line side constant voltage and frequency, machine side variable voltage and frequency), and by increasing the frequency and voltage of the converter the machine is accelerated [6]. At about 95% of the synchronous speed, the synchronizing equipment will take over control of the SFC and after reaching the conditions necessary for syn-chronization it will give a closing command to the generator circuit-breaker and block the SFC impulses. Therefore there is no current flow caused by the SFC after synchronization. When the generator circuit-breaker has been closed, the SFC will be disconnected. In the “back-to-back” starting arrange-ment another machine in the station, acting as a generator, is employed. The generator and the motor to be started-up are connected together electrically. The wicket gate of the gen-erator turbine is opened and both machines are accelerated to synchronous speed. In asynchronous starting the unexcited machine is connected to full or reduced voltage at system frequency and is acting as an asynchronous motor during the starting period.Another important issue in all hydro power plants and there-fore also in pumped storage power plants is the braking of the machine after it has been disconnected from the system.

Electrical braking by short-circuiting the stator and re-excitation of the rotor is attractive because it assists hydraulic braking at lower speed and allows to substantially reduce the wear of the mechanical braking system and hence to increase the maintenance intervals and to decrease the associated costs [7]. Hydraulic braking is most effective only down to about 50 to 60% of the rated speed of the machine, because its effectiveness decreases approximately with the 3rd power of the speed. Mechanical braking is then applied only at 10% or less of the rated speed until to full stop. The current used for the electrical braking typically is in the range of 1.0...1.3-times the rated current of the machine. The current will remain at this value nearly to full standstill of the unit, because both the voltage and the reactance decrease proportional with the speed of the unit.

ABB | 37

AUXG

MT

EHV HV

UT STGCB

AUXG

MT

EHV

UTGCB

AUXG

MT

EHV

GCBUTSS

SFC

AUXG

MT

EHV

UTGCB

G

MT

GCB

BSBSAUXG

MT

EHV

UTGCB

G

GCB

BSBS

AUX

G

MT

EHV

UT

GCB

G

MT

GCB

BSSSBS SS

SS SS

PRD PRD

SFC

Figure 41: Different power plant layouts which employ generator circuit-breakers

a) b) c)

d) e) f)

Generator circuit-breakers are widely used in pumped storage power plants because the use of such circuit-breakers allows the electrical scheme (Figure 41f) to be greatly simplified.

Because of their ability to interrupt fault current also at frequencies below 50/60 Hz ABB generator circuit-breakers ensure an adequate protection of power plant equipment.

LegendMT Main transformer

UT Unit transformer

ST Station transformer

GCB Generator circuit-breaker

EHV Transmission system

HV Sub-transmission system

AUX Unit auxiliaries

SS Starting switch

BS Braking switch

SFC Static frequency converter

PRD Phase-reversal disconnector

38 | ABB

6.2 Advantages of generator circuit-breakersThe use of generator circuit-breakers for the switching of gen-erators at their terminal voltage offers many advantages when compared to the unit connection, e.g.:

– simplified operational procedures

– improved protection of the generator and the main and unit transformers

– increased security and higher power plant availability

– economic benefit

6.2.1 Simplified operational procedures – The installation of the generator circuit-breaker directly in the connection between the generator and the main trans-former provides a clear and logical plant arrangement.

– During the starting-up or shutting-down of the generator only one circuit-breaker must be operated thus reducing the number of switching operations necessary. A comparison of operation procedures between layouts which employ a gen-erator circuit-breaker and power plant electrical schemes without generator circuit-breaker is displayed in Figure 42.

– The automatic rapid changeover switching equipment required to transfer the supply of the unit auxiliaries from the station to the unit transformer (and vice versa) can be avoided, thus eliminating stresses with possible subsequent damages to the drive motors of pumps, fans, etc.

– The division of responsibility for the operation of the power plant and the switching of the high-voltage transmission network is clearly defined.

As a disadvantage of this solution in comparison with the unit connection especially in the case of large generating units the high costs of the generator circuit-breaker are sometimes mentioned. This argument however refers to air-blast genera-tor circuit-breakers and with the appearance of modern SF6 generator circuit-breakers for units with ratings of up to 2’000 MVA is no longer valid.

6.2.2 Improved protection of the generator and the main and unit transformers – The differential protection zones of the generator, the main and unit transformers can be arranged to achieve maximum selectivity.

– Generator-fed short-circuit currents are interrupted within a maximum of 4 cycles whereas the reduction of the fault current by the de-excitation equipment requires a number of seconds.

6.2.3 Increased security and higher power plant availability – Simplified operational procedures and clearly defined responsibilities reduce the likelihood of operational errors.

– The synchronisation of a generator with the high-voltage grid can be carried more reliably with a generator circuit-breaker than with a high-voltage circuit-breaker. During synchronization the voltage across the open contacts of the circuit-breaker varies from zero (i.e. in phase) to twice the normal voltage (i.e. 180° out-of-phase condition). In the latter case high stresses will be imposed on the exter-nal insulation of the circuit-breaker which might result in a flashover of the circuit-breaker insulator. This is especially true when the circuit-breaker operates in heavily polluted atmosphere. A generator circuit-breaker even in case it is installed outdoor is protected by its enclosure and operates under indoor conditions. Therefore these stresses will not be imposed on it.

– The use of a generator circuit-breaker allows the unit aux-iliaries supplies to be drawn directly from the high-voltage transmission network at all times. Supply from this source is considerably more reliable than that from a local sub-transmission network.

– The avoidance of changeover switching operations required to transfer the supply of the unit auxiliaries from the station to the unit transformer (and vice versa) eliminates stresses with possible subsequent damages to the drive motors of pumps, fans, etc.

– The rapid and selective clearance of all types of faults helps to avoid expensive secondary damage and the conse-quently long down times for repair. Examples of serious secondary damage being caused by the delayed clearance of a fault are:

– bursting of the transformer tank following an internal fault in the main or unit transformer

– thermal destruction of the generator damper winding due to short-time unbalanced load conditions

– mechanical destruction of a turbine-generator set due to generator motoring

– thermal/dynamic stress caused to the generator by synchronising under “out-of-phase” conditions

ABB | 39

Such incidents have a detrimental effect on the availability of a power plant. It should be noted that the generator rapid de-excitation equipment is in most cases too slow to avoid this kind of damage.In many cases equipment damage can be reduced or even prevented by using a generator circuit-breaker in conjunction with an adequate generator protection.

Compared to high-voltage circuit-breakers modern SF6 gen-erator circuit-breakers exhibit higher maintenance intervals as they are especially designed for a high mechanical and electri-cal endurance. Depending on the application the down-time of a unit due to circuit-breaker maintenance can therefore be significantly reduced when a generator circuit-breaker is used.

Layout without generator circuit-breaker Layout with generator circuit-breaker

Figure 42: Description of operation procedures for layouts with and without generator circuit-breaker

6.2.3.1 Transformer failuresPower transformers normally show high reliability. However in some cases internal dielectric flashovers occur resulting into a fault arc inside the transformer causing significant internal pressure rise by production of large quantities of gas (mainly hydrogen) due to the dissociation of the oil. Common causes of transformer internal failures are the flashover of a bushing, winding interturn faults, failures of the tap-changer and carbonisation and/or excessive moisture content of the transformer oil. Depending of the fault arc energy (which is a function of arc current, arc duration and arc-voltage) the pres-sure rise can be so high to crack the transformer tank or to blow out one or more of the bushings. Especially arc-current and arc-voltage strongly depend on the fault location inside the transformer. In most cases severe damages occur inside the transformer but also other equipment of the power plant is jeopardized by the burning oil or hydrogen outside the trans-former. Unacceptable outage of the power plant or at least parts of it is generally the consequence. Steep pressure rise inside the transformer tank can be gener-ated by faults occurring in different locations, e.g.:

– flashover across the winding (portion of full winding)

– flashover across the bushings

– flashover between winding and tank

– flashover across two positions of the tap changer

Initially the fault arc current is delivered both by the HV system and by the generator. Even if the system-fed component of the fault current is interrupted by the high-voltage circuit-breaker within approximately 3 to 4 cycles, in a layout without generator circuit-breaker the generator continues to supply a fault current throughout the de-excitation time interval which can take up to several seconds (see Figure 43a). The energy dissipated in the fault arc leads to a vaporisation of the trans-former oil and hence to a pressure rise inside the transformer. This pressure stresses the transformer tank, and, if it rises above a certain value, will cause the tank to rupture, with a resulting oil spillage and possibly an oil fire. Typical times to tank rupture are 4.5 to 5 cycles [8]. Nowadays it is common practice to install a generator circuit-breaker between the generator and the step-up transformer (Figure 43b) which allows a rapid clearance also of the generator-fed component of the fault current and can therefore make up the difference between a repairable damage and a catastrophic event with severe environmental pollution and possible personnel jeopardy [9], [10]. The full lines in Figure 44 show the course of pressure rise inside the transformer tank for different fault locations. After the system-fed component of the fault current is interrupted by the high-voltage circuit-breaker the pressure rise is less

Unit start-up:1) Run-up unit on station transformer (start-up supply) and synchronise generator with high-voltage grid by means of high-voltage circuit-breaker2) Parallel unit auxiliaries supplies3) Separate unit auxiliaries from station transformer (start- up supply)

Unit routine shut-down:1) Parallel unit auxiliaries supplies2) Separate unit auxiliaries from unit transformer3) Trip high-voltage circuit-breaker and shut-down unit on station transformer

Unit emergency shut-down:1) Trip high-voltage circuit-breaker, unit auxiliaries are isolated2) Automatic transfer of unit auxiliaries from unit trans- former to station transformer (approx. 4...5 cycles)3) Shut-down unit on station transformer

Unit start-up:1) Run-up unit on unit transformer and synchronise generator with high-voltage grid by means of generator circuit-breaker

Unit routine shut-down:1) Trip generator circuit-breaker and shut-down unit on unit transformer

Unit emergency shut-down:1) Trip generator circuit-breaker and shut-down unit on unit transformer

40 | ABB

steep because the fault current is only fed by the generator. Anyway the pressure can quickly reach the tank withstand pressure thus leading to the explosion of the transformer. The dotted lines represent the course of the pressure rise if no generator circuit-breaker is available.When the generator-fed component of the fault current is interrupted by the generator circuit-breaker the pressure rise is generally stopped quickly enough to prevent the transformer tank explosion. Anyway depending on the fault arc energy and on the transformer tank withstand pressure the pressure rise inside the transformer tank can be so steep that the tank withstand pressure of the transformer is reached even before the high-voltage circuit-breaker can be operated for contri-buting to transformer protection.

Investigations have shown that a generator circuit-breaker can prevent tank rupture in about 80% of all cases of main transformer internal failures [11]. The use of a generator circuit-breaker can greatly reduce the probability of a trans-former tank rupture during internal fault. This will reduce the related downtime of the power plant thus leading to a higher availability.In Figure 45 the sequence of events which have led to a transformer explosion is shown. It is interesting to see how fast a ground fault developed into a three-phase fault and consequently into an explosion after approx 150 ms. In such a case the presence of a generator circuit-breaker would have allowed the interruption of the generator-fed fault current in less than 4 cycles thus preventing such a damage.

Fault Current

Time

Is+Ig

Ig

Inte

rrup

tion

of H

V

Circ

uit-

Bre

aker

tens of ms seconds

IsIgGridG

Fault Current

Time

Is+Ig

Ig

Inte

rrup

tion

of H

V

Circ

uit-

Bre

aker

tens of ms seconds

Inte

rrup

tion

of G

ener

ator

C

ircui

t-B

reak

er

IsIgGridG

Figure 43: Interruption of generator-fed fault currents with and without generator circuit-breaker

b) Case with generator circuit-breakera) Case without generator circuit-breaker (unit connection)

-Bre

aker

HV

Time

Gen

erat

or C

ircui

t

Circ

uit-

Bre

aker

Pressure

tank withstand pressure

flashover between winding and tank

flashover across a portion of winding

flashover across two positions of tap changer

flashover across bushing

flashover across full winding

Figure 44: Pressure rise inside transformer tank for different fault locations

ABB | 41

Sequence of events

t =t =t =t ≈

0 ms:45 ms:95 ms:

150 ms:

earth fault at HV-side of transformer2-phase short-circuit3-phase short-circuitexplosion of transformer

Figure 45: Consequence of a transformer failure

6.2.3.2 Short-time unbalanced load conditionAccording to theory of symmetrical components an unbal-anced load can be split into 3 systems, i.e. a positive- sequence system rotating with the same speed and direction as the rotor, a negative-sequence system rotating with the same speed but with opposite direction to the rotor and a zero-sequence system consisting of a set of phasors of equal magnitude and always in phase. It is the negative-sequence system which causes harm to the generator. The rotor rotates

in opposite direction to the negative-sequence system which results in a double frequency current flowing in the field wind-ing, in the damper windings and in the rotor. Single- and two-phase faults represent a short-time unbalanced load condition with critical mechanical and thermal stresses for a generator [12]. Such conditions can arise due to single- or two-phase faults within the main transformer or on its connections to the high-voltage circuit-breaker (Figure 46).

HV circuit-breaker: 1 phase does not close Transformer LV terminals: two phase fault

HV circuit-breaker: 1 phase does not open Transformer HV windings: various types of faults

Transformer HV bushings: single phase earth fault HV circuit-breaker: two phase flashover

Transformer HV bushings: two phase fault

Figure 46: Examples of failures which can lead to unbalanced load conditions

42 | ABB

The thermal stresses result from the negative sequence com-ponent of the fault current that interacts with the generator damper windings. Unbalanced load conditions can give rise, within a very short time, to dangerously high temperatures in the damper windings. These temperatures are particularly critical for turbo generators and in the worst case may cause the rotor to jam in the stator. If a generator circuit-breaker is present it will separate the generator from the fault within 4 cycles and thus effectively prevent damage to the generator. If no generator circuit-breaker is fitted, the generator will con-tinue to supply a negative sequence current until de-excitation is completed. The de-excitation may take several seconds, during which time the generator may suffer severe damage. Unbalanced load cases might lead to severe damage as depicted in Figure 47. In this case the rotor’s touching of the stator destroyed the generator completely.

Figure 47: Damage resulting from unbalanced load conditions (source: Allianz Insurance Company)

6.2.3.3 Generator motoringGenerator motoring can occur when the supply of energy (e.g. steam, gas or water) to the turbine is removed and the field is still excited. If such a condition occurs the generator will act as a synchronous motor absorbing electric active energy from the network and converting it into mechanical energy to drive into rotation its rotor and turbine. Generator motoring can lead to severe damages especially to steam turbines because of overheating and damage to steam turbine blades. The layout of a power plant in which a case of generator motoring occurred is depicted in Figure 48. In that case after the high-voltage circuit-breaker was tripped the turbine-generator set

was running down normally. As a consequence of an internal breakdown which occurred in one pole of the high-voltage circuit-breaker the generator absorbed power from the grid and started working as a motor. Due to the increased speed of rotation the turbine rotated for too long time in the range of its critical speed, i.e. the natural frequency of the turbine shaft material. When the forcing frequency is close to its natural frequency, machine causes noise and high vibrations because of resonance due to matching of frequency. As a conse-quence of generator motoring the generator was lifted out of foundations and the shaft was destroyed.

Overhead LineGS3~

Coupling

GeneratorPn = 500 MW

Main Transformer Overhead Line(Transmission)

HV Circuit-Breaker

Figure 48: Layout of a power plant in which a case of generator motoring occurred

6.2.3.4 Synchronizing under out-of-phase conditionsThe out-of-phase conditions are abnormal circuit conditions due to loss or lack of synchronism between generator and power system at the instant of the synchronizing operation of the circuit-breaker. The phase angle difference between phasors representing the generated voltages on each side of the circuit-breaker may exceed the normal value and may

be as much as 180°. The out-of-phase current resulting from this condition is dependent on this phase angle and attains its maximum value at 180° (phase opposition). The resulting thermal and electro-dynamic overstress might lead to severe consequences for the generator windings.

ABB | 43

6.2.4 Economic benefitSeveral economic advantages are brought about by the employment of a power plant layout with generator circuit-breaker compared to the unit connection:

– The possible integration of all the associated items of switchgear into the enclosure of the generator circuit-breaker allows simpler and more economic power plant layouts. This solution hence allows savings in time and expenditures for erection and commissioning.

– When a layout with a generator circuit-breaker is used it is possible to omit the station transformer and the associated high-voltage and medium-voltage switchgear. If the station transformer cannot be dispensed with, the use of a trans-former with reduced rating (rated as a “shut-down trans-former“) is usually sufficient.

– Should the high-voltage substation be erected at some distance from the power plant the generator circuit-breaker can be used to protect the overhead line linking the power plant to the substation. No separate high-voltage circuit-breaker at the power plant is required for this purpose.

– The through-fault capability required of the unit transformers is substantially reduced.

– The higher availability of the power plant leads to an in-creased number of the operating hours and therefore to a higher profit for the operator of the power plant. Substantial surplus of receipts can be achieved in this way and the pay-back time for the expenditures of a generator circuit-breaker is generally very low.

44 | ABB

Overhaul of generator circuit-breakers is scheduled based on the criteria of service time, number of mechanical CO opera-tions and number of current interruptions whichever occurs first.Specifically the electrical endurance of the generator circuit-breaker depends on the magnitude of the current which is switched. To each opening operation, an ablation coefficient k is assigned. The ablation coefficient depends on the r.m.s. value of the switched current. Within a range up to 150% of the rated continuous current the ablation coefficient is proportional to the ablation of the arcing contacts. With the

interruption of currents far above the rated continuous current longer arcing times are expected. Therefore, the ablation of contact material increases disproportionately. In addition, high electro-dynamical forces arise which reduce the mechanical lifetime of the circuit-breaker.

For typical applications generator circuit-breakers do not have to switch load currents and therefore overhaul is generally scheduled based on service time. Depending on the type of generator circuit-breaker typical overhaul intervals are 15 to 20 years.

7 Maintenance of generator circuit-breakers

ABB | 45

In order to quantify the impact of how a generator is con-nected to the high-voltage grid on the availability of a power plant, the contribution of the connection of the generator to the high-voltage transmission network and of the supply to the unit auxiliaries to the unavailability of one generating unit has been determined with the help of computer program based on the Monte Carlo method [13]. Three different ways of connecting the generator to the high-voltage transmission

network were considered, namely:

– a layout without a circuit-breaker between the generator and the low-voltage terminals of the main transformer (“unit connection”), refer to Figure 49)

– a layout with a generator circuit-breaker and a station trans-former (rated as “shut-down transformer”), refer to Figure 50)

– a layout with a generator circuit-breaker, refer to Figure 51)

8 Case study 1: Impact of the method of connecting a generator to the high-voltage grid on the availability of a power plant

8.1 Power plant layoutThe layout of a typical power plant is shown in Figure 49. It consists of two 360 MW steam turbines. Each unit is directly connected to two sets of three winding unit transformers (UT). The generator is directly connected to the generator step-up transformer (GSUT) as well. The extra high-voltage substation (rated voltage 345 kV) consists of an air insulated one and a half circuit-breaker arrangement. The number of outgoing

lines is two. The three winding station transformers (ST) are connected to the 138 kV substation by an air insulated double busbar with single circuit-breaker arrangement. The reserve net is the backup source for the station auxiliaries; i. e. whenever a unit is shutdown, the reserve net supplies power to the auxiliary busbars through the station transformer.

G G

345 kV

138 kV

ST STUT UTUT UT

GSUT GSUT

GEN 22 kV GEN 22 kV

Figure 49: Layout of a 2 x 360 MW thermal power plant with two station transformers and no generator circuit-breaker

46 | ABB

G G

345 kV

138 kV

STUT UTUT UT

GSUT GSUT

GEN 22 kV GEN 22 kV

Figure 51: Layout of a 2 x 360 MW thermal power plant with generator circuit-breakers and no station transformer

Figure 50: Layout of a 2 x 360 MW thermal power plant with one station transformer and generator circuit-breakers

G G

345 kV

UT UTUT UT

GSUT GSUT

GEN 22 kV GEN 22 kV

ABB | 47

8.1.1 Layout of extra high-voltage substationThe secure operation of extra high-voltage substations is greatly influenced by their layout. In order to assure the conti-nuity of the supply, the links between incoming and outgoing feeders of a substation have to remain intact, even in spite of a number of connecting elements not being available. Obvi-ously every effort is made to attain this goal with a minimum capital outlay.The following substation schemes have been investigated:

– double busbar with single circuit-breaker (Figure 52a)

– one and a half circuit-breaker (Figure 52b)

– double busbar with double circuit-breaker (Figure 52c)

– ring (Figure 52d)

– crossed-ring (Figure 52e)

For large installations the double busbar with single circuit-breaker arrangement (Figure 52a) is preferred. The presence of two busbars makes maintenance possible without inter-rupting the supply. On the other hand a circuit-breaker failure leads to the loss of all feeders connected to that busbar and the busbar protection may cause the loss of the substation if all feeders are connected to the same busbar.A scheme representing a mixture of equipment and structural redundancy is the “one and a half” circuit-breaker arrange-ment (Figure 52b). It is often used for very important substa-tions because of its high availability and good operational flexibility. In this case three circuit-breakers are employed for two outgoing feeders. All circuit-breakers are normally closed. Uninterrupted supply is thus maintained even if one busbar fails.

A feature of the double busbar with double circuit-breaker arrangement (Figure 52c) is that each outgoing feeder is con-nected to the rest of the installation by two parallel circuit-breakers, i.e. this scheme uses circuit-breaker redundancy to secure operation under disturbed conditions.Since each line has two circuit-breakers, one circuit-breaker can be taken out of service at any time without interrupting the operation.A more economic kind of redundancy is achieved with the ring arrangement (Figure 52d) which is considered as an appropriate solution for substations with only a few feed-ers. Each feeder requires only one circuit-breaker and each circuit-breaker can be isolated without interrupting the supply. Starting from this scheme, new concepts were developed to increase structural redundancy.In the normal state of the crossed-ring substation arrange-ment (Figure 52e) the circuit-breakers of the basic ring (BR) are closed while those of the cross-links (CL) are open. If one circuit-breaker in the basic ring fails, another ring can be formed so that the original availability is maintained. It can be seen that even in the case of non-availability of two adjacent circuit-breakers, the respective node can be fed via the remaining circuit-breaker. With any of the other topologies introduced above, this situation would automatically lead to the loss of the node. The impact of the use of gas insulated switchgear (GIS) instead of air insulated switchgear (AIS) has also been investigated. The GIS solution leads to a lower failure rate and to a higher MTTR and, even though it is more expensive, it is to be preferred when problems of space or pollution are present.

BR CL CL BR

BR

BR

CL

CL

BR

BR

BR

BR

Figure 52: Schemes of extra high-voltage substations

a) b) c)

d) e)

48 | ABB

8.1.2 Layout of high-voltage substationThe layout of the high-voltage substation used for the investi-gation is a double busbar with single circuit-breaker arrange-ment (Figure 52a) and uses air insulated switchgear.

8.1.3 Generator circuit-breakerFor each layout the possible use of a generator circuit-breaker has been investigated (see Figure 50 and Figure 51).The presence of a generator circuit-breaker located between the generator and the main transformer allows the plant aux-iliaries to be fed directly from the extra high-voltage transmis-sion system (main net). In addition the rapid interruption of

generator-fed short-circuit currents reduces the extent of fault damage and the related down-time, contributing to increased power plant availability. The presence of a generator circuit-breaker can thus lead to a reduction of the MTTR of power plant equipment.

8.1.4 Station transformerThe power plant layout used for the investigation has two station transformers. In the cases with a generator circuit-breaker, only one station transformer (one station transformer per two units) rated as an emergency shut-down transformer has been considered. The influence of using no station

transformer has also been investigated for the cases with a generator circuit-breaker installed. When no station trans-former is available, the backup source for the auxiliaries are the auxiliary busbars of another unit (see Figure 51).

8.2 Data for availability calculationsFor each component of the power plant the following data is needed:

– number of failures

– downtime

– maintenance frequency

– maintenance duration

Reliability parameters have been taken from published literature [14], [15], [16], [17], [18], [19]. The circuit-breaker fail-to-close and fail-to-open probabilities have also been taken into account [16].Moreover, information about switching times, maintenance frequency and maintenance duration was obtained from published literature [14].

8.3 SimulationsThe simulations have been carried out with the help of a computer program based on the Monte Carlo method [20]. This is a very powerful technique to quantitatively estimate the reliability of complex systems like power plants; furthermore it allows to quantify the impact of the connection scheme of a generator to the extra high-voltage network on the availability of the plant.Monte Carlo methods estimate the reliability of a system by simulating the process and its random behaviour. The simula-tion consists in a repeated process of generating deterministic solutions to a given problem with each solution correspond-ing to a set of deterministic values of the underlying random variables. The main element of Monte Carlo simulation is therefore the generation of random numbers from probability distributions describing the random variables of interest, e.g. the failure and repair rates of different items of power plant equipment.

During a simulation run, when a failure occurs it is treated by tripping the circuit-breakers forming the protection group of the failed component immediately after the occurrence of the failure. After the time necessary to isolate the failed compo-nent (i. e. the switching time) the circuit-breakers are closed again. When the repair of the component is completed (or a spare part has become available), the above procedure is repeated. Also the transfer of the auxiliaries between different sources during the starting-up and the shutting-down of the unit (or when a failure occurs) is modelled. The operational state of a unit further depends on the state of its auxiliaries, as the number of auxiliaries available influences the level of possible power production.One of the results obtained from the simulations is the power throughput of the power plant.

ABB | 49

8.4 Simulation resultsThe simulations have been carried out assuming that the power plant supplies base load. The availability of the unit (turbine and generator) has been set to 86.67%. This value takes into account forced and scheduled outages of the unit. The results of the simulations are summarized in Table III and Table IV.The difference in the throughput power directly reflects the contribution of the different schemes used to connect the generators to the extra high-voltage transmission network on the availability of the power plant. The results show that the use of a layout with a generator circuit-breaker positively affects the availability. Figure 53 depicts the possible avail-ability improvements when a layout with a generator circuit-

breaker is used. This improvement is in the order of 0.4%. The ring scheme seems to be very interesting: in this case the availability improvement is in the order of 0.44%. The results clearly indicate that, from a point of view of power plant availability, a layout with a generator circuit-breaker offers a distinct advantage over the unit connection.With respect to the design of the extra high-voltage substa-tion, it can be seen that in case of a layout with generator circuit-breaker, the number of station transformers has a negligible influence on the power plant availability.On the other side, the difference in the throughput power be-tween a gas insulated substation and an air insulated substa-tion is generally very small (see Table IV).

EHV substation (refer to Figure 52)

HV substation (refer to Figure 52)

Generator circuit-breaker

Station transformer

Power throughput

SCHEME AIS GIS SCHEME AIS GIS YES/NO No. MW

a) x a) x no 2 619.38

b) x a) x no 2 619.50

c) x a) x no 2 619.62

d) x a) x no 2 619.35

e) x a) x no 2 619.45

a) x a) x yes 1 621.93

b) x a) x yes 1 622.08

c) x a) x yes 1 622.09

d) x a) x yes 1 622.06

e) x a) x yes 1 622.04

a) x - - - yes 0 621.97

b) x - - - yes 0 622.07

c) x - - - yes 0 622.09

d) x - - - yes 0 622.07

e) x - - - yes 0 622.00

TABLE III: RESULTS OF SIMULATIONS: INFLUENCE OF THE PRESENCE OF A GENERATOR CIRCUIT-BREAKER

Figure 53: Relative availability improvement for a layout with generator circuit-breaker (related to the basic scheme without a generator circuit-breaker)

0.42% 0.42%

0.40%

0.44%

0.42%

a) b) c) d) e)

Layout of EHV substations (refer to Figure 52)

50 | ABB

TABLE IV: RESULTS OF SIMULATIONS: COMPARISON BETWEEN AIS AND GIS

8.5 Economic evaluationIn order to make a more thorough comparison between the different options, an economic analysis has also been carried out. For the economic evaluation the following issues have been considered:

– life cycle costs for selectable equipment (extra high-voltage

and high-voltage switchgear, station transformers, generator circuit-breakers, medium voltage switchgear)

– power delivered to the grid

– energy selling price

The life cycle cost of a piece of equipment comprises besides

the initial capital outlay also all those expenses arising from its installation, operation, maintenance and, at the end of its service life, its disposal.The power delivered to the grid is given by the power throughput minus the power consumed by the auxiliaries.Monetary values are time dependent, and can only be summed or subtracted when they are referred to the same point in time. For this reason a cash value corresponding to each cost term must be calculated to allow for interest and inflation rates:

i

i

pv Cr

C+

=11

Ci is the expense payable in i years in “zero-year” currency valueCpv is the present value of cost Ci

r is the effective discount rate (interest rate - inflation rate)i is the number of years at the end of which the expense Ci is paid.

PV_FM is the present value of the figure of meritN is the number of years of service (service life)r is the effective discount rateli is the income in year iOCi are the operation costs in year i

MCi are the maintenance costs of selectable equipment in year iAC are the acquisition costs of selectable equipmentCWC are the civil works costs of selectable equipmentIC are the installation costs of selectable equipment

( )∑ −−−−−+

==

N

iiii

i

ICCWCACMCOCIr

FMPV1 1

1_

EHV substation (refer to Figure 52)

HV substation (refer to Figure 52)

Generator circuit-breaker

Station transformer

Power throughput

SCHEME AIS GIS SCHEME AIS GIS YES/NO No. MW

b) x a) x no 2 619.50

b) x a) x yes 1 622.08

b) x - - - yes 0 622.07

b) x a) x no 2 619.47

b) x a) x yes 1 622.10

b) x - - - yes 0 622.07

ABB | 51

100.50%

100.40%

100.30%

100.20%

100.10%

100.00%

99.90%

99.80%

99.90%a) b) c) d) e)

Layout of EHV substations (refer to Figure 52)

With no GCB

With GCB and onestation transformer

The differences in the figure of merit of different power plant layouts are depicted in Figure 54. It can be noticed that lay-

outs with a generator circuit-breaker generally have a higher figure of merit than layouts without a generator circuit-breaker.

Additional calculations have shown that layouts with a generator circuit-breaker and without a station transformer may even have somewhat higher figures of merit, especially in cases with low downtimes (e. g. power stations which supply base load) where the losses during the time when the unit is shut down do not matter very much.

Figure 54: Differences in figure of merit of different power station layouts (related to the layout depicted in Figure 49)

Moreover, the use of a generator circuit-breaker makes the ring scheme (Figure 52d) without station transformer one of the best options; such a conclusion is due to the fact that this scheme is very cheap (low number of components) and shows a similar reliability as the other schemes when a generator circuit-breaker is installed.

52 | ABB

The requirements imposed on generator circuit-breakers greatly differ from the requirements imposed on general purpose transmission and distribution circuit-breakers. Due to the location of installation between the generator and the associated step-up transformer a generator circuit-breaker must meet high technical requirements with respect to the interruption of fault currents. In addition to their generally high magnitude, these currents can be characterized by delayed current zeros.

The capability of the generator circuit-breaker to interrupt fault currents which show delayed current zeros can be ascer-tained by calculations that take into account the effect of the arc-voltage of the circuit-breaker on the prospective fault current. In order to carry out a thorough investigation on the interrupting capability of generator circuit-breakers, a com-parison between SF6 and vacuum extinguishing technologies is provided.

9 Case study 2: Interrupting capability of generator circuit- breakers in case of delayed current zeros

9.1 Generator circuit-breaker model adopted for the simulations

According to [1], [2], demonstrating the capability of a gen-erator circuit-breaker to interrupt short-circuit currents with delayed current zeros may be difficult and limited in high power testing stations. Considering that various designs of generators behave differently and that the pre-load of the generator influences the course of the fault current, it can be impossible to simulate the required current wave-shape in the testing station [21]. Therefore the capability of a circuit-breaker to interrupt a short-circuit current with delayed cur-rent zeros has to be ascertained by calculations that take into account the effect of the arc-voltage of the generator circuit-breaker on the prospective short-circuit current. The genera-tor circuit-breaker’s arc-resistance is an additional resistance which forces the d.c. component of the short-circuit current to decay faster. It is of utmost importance that the magnitude of the arc-voltage is high enough to force a fast decay of the d.c. component of the short-circuit current, so that current zeros are produced within the maximum permissible arcing time of the generator circuit-breaker. In order to investigate the behaviour of the generator circuit-breaker during the interruption of short-circuit currents with delayed current zeros, the arc-voltage versus current characteristic has to be transferred into a mathematical model. From the arc-voltage and the current the arc-resistance is obtained. A non-linear time-varying resistance is inserted into the simulation at the time of the separation of the contacts of the circuit-breaker to model the behaviour of the generator circuit-breaker.According to [1] the following two cases shall be investigated:

– fault initiation at voltage zero in one phase which implies that the current in the corresponding phase exhibits the maximum degree of asymmetry

– fault initiation at voltage maximum in one phase which implies that the current in the corresponding phase is sym-metrical

The capability of a generator circuit-breaker to interrupt a given short-circuit current can be considered as being de-monstrated when the following conditions are met:

– the maximum operating voltage is less than or equal to the power frequency recovery voltage during the short-circuit test with the corresponding symmetrical short-circuit cur-rent

– the making current is less than or equal to the making cur-rent demonstrated by a short-circuit test

– the symmetrical short-circuit current is less than or equal to the symmetrical short-circuit breaking current demonstrated by a short-circuit test

– the asymmetrical short-circuit current is less than or equal to the asymmetrical short-circuit breaking current demon-strated by a short-circuit test

– the rate-of-rise and the peak value of the transient recovery voltage are less or equal to the rate-of-rise and the peak value of the transient recovery voltage during the short-circuit test with the corresponding symmetrical short-circuit current

– the time delay of the transient recovery voltage is longer than or equal to the time delay of the transient recovery voltage during the short-circuit test with the corresponding symmetrical short-circuit current

– in case of a short-circuit current with delayed current zeros the generator circuit-breaker is capable of forcing the current to zero within the time interval in which it is able to interrupt a current (i.e. within the maximum permissible arc-ing time)

In order to correctly simulate the behaviour of the generator circuit-breaker, the arc-voltage model used for this investiga-tion has to be derived from tests [1].

ABB | 53

9.2 Generator terminal faultsThe current to be interrupted by the generator circuit-breaker in case of faults between the terminals of the generator circuit-breaker and the LV-windings of the step-up trans-former is called generator-source short-circuit current. If the fault initiation takes place when the voltage in one phase passes through zero the resulting fault current in that phase exhibits the maximum degree of asymmetry. The symmetrical component decays with the subtransient and transient time constants of the generator; the d.c. component decays with the armature time constant. If the symmetrical component of the fault current decays faster than the d.c. component, it can happen that, for a certain period of time following the initia-

tion of the fault, the magnitude of the d.c. component of the fault current is bigger than the peak value of its symmetrical component. In such a case the degree of asymmetry of the fault current is higher than 100%, thus leading to delayed cur-rent zeros. The degree of asymmetry of the generator-fed fault current is typically about 130%. The course of the generator-source short-circuit current is depicted in Figure 55. Fault initiation takes place at 100 ms and a bolted fault has been assumed (i.e. that there is no arc-voltage at the fault location). The fault initiation occurs at voltage zero in phase A.

-150

-100

-50

0

50

100

150

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

]

Figure 55: Prospective generator-source short-circuit current (generator unloaded prior to fault initiation, fault initiation at UA = 0)

Figure 56 and Figure 57 show the corresponding calcula-tion results with the generator circuit-breaker closing into a three-phase fault. In the computation the arc-voltage of a SF6 generator circuit-breaker starting at contact separation is

taken into account. Figure 56 represents the case with fault initiation at voltage zero and Figure 57 shows the case with fault initiation at voltage maximum in one phase.

-150

-100

-50

0

50

100

150

[kA]

0.08 0.10 0.12 0.14 0.16 0.18 0.20[s]

tcp

Figure 56: Interruption of generator-source short-circuit current with a SF6 generator circuit-breaker (generator unloaded prior to fault initiation, fault initiation at UA = 0, arcing time = 17.6 ms)

-150

-100

-50

0

50

100

150

[kA]

0.08 0.10 0.12 0.14 0.16 0.18 0.20[s]

tcp

Figure 57: Interruption of generator-source short-circuit current with a SF6 generator circuit-breaker (generator unloaded prior to fault initiation, fault initiation at UA = max, arcing time = 20.2 ms)

54 | ABB

For comparison purposes the interrupting capability of a gen-erator circuit-breaker employing vacuum extinguishing tech-nology is also analysed. Figure 58 and Figure 59 show the

corresponding calculation results. Figure 58 represents the case with fault initiation at voltage zero and Figure 59 shows the case with fault initiation at voltage maximum in one phase.

tcp

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24[s]-150

-100

-50

0

50

100

150

[kA]

Figure 58: Interruption of generator-source short-circuit current with a vacuum generator circuit-breaker (generator unloaded prior to fault initiation, fault initiation at UA = 0, arcing time = 39.0 ms)

A method sometimes adopted to reduce the arcing time of the circuit-breaker is to introduce an intentional tripping delay. A value in the range of 100 ms – 200 ms is usually sufficient to limit the degree of asymmetry of the fault current at contact separation to values the generator circuit-breaker can cope with. It has to be noted that this solution would lead to longer fault duration and consequently to severe damages to power station equipment with consequent long downtime for repair. Fault durations exceeding 100 ms are usually sufficient to let

the step-up transformer explode in case of internal failures. For this reason many power station operators consider the solution of intentionally delaying the tripping as not recom-mendable. Therefore the preferred method to handle the delayed current zeros phenomena is to choose a generator circuit-breaker having an arc-voltage magnitude sufficiently high to force current to zero without the aid of any intentional tripping delay [22].

tcp

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24[s]-150

-100

-50

0

50

100

150

[kA]

Figure 59: Interruption of generator-source short-circuit current with a vacuum generator circuit-breaker (generator unloaded prior to fault initiation, fault initiation at UA = max, arcing time = 80.9 ms)

ABB | 55

9.3 Out-of-phase synchronisingOut-of-phase synchronising occasionally occurs in power plants [4]. The main reasons for out-of-phase synchronising are wiring errors made during commissioning or during main-tenance when connecting voltage transformers and synchro-nising equipment. The current resulting from out-of-phase synchronising may show delayed current zeros; their causes are totally different compared to generator terminal faults. The rapid movement of the rotor from initial out-of-phase angle δ0 to δ = 0 results in a very small symmetrical component of the fault current and a dominant d.c. component when the condi-tion of δ = 0 is reached. As the instant when the δ = 0 con-dition is reached is determined by the movement of the rotor,

the inertia constants of turbine, rotor and excitation equip-ment of the generator are of special importance. Because the fault current to be interrupted by the generator circuit-breaker is characterized by delayed current zeros it is extremely im-portant to prove that the circuit-breaker by means of its arc-voltage is capable of forcing current to zero within its maxi-mum permissible arcing time. Even though it is recognized that synchronising with out-of-phase angle up to 180° might occur, [1], [2] cover only requirements for a maximum of 90°. Therefore for the present study simulations referring to such a fault conditions have been performed. The wave-shape of the out-of-phase current in case of δ0 = 90° is depicted in Figure 60.

-150

-100

-50

0

50

100

150

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

]

Figure 60: Prospective out-of-phase current (out-of-phase angle δ0 = 90°, fault initiation at UA = 0)

The simulation results are depicted in Figures 61, 62, 63 and 64. Figures 61 and 62 show the course of the fault current in case a SF6 generator circuit-breaker is employed. Figures 63 and 64 show the corresponding results in case of use of a vacuum generator circuit-breaker. Figures 61 and 63 repre-

sent the case of synchronisation occurring when the voltage across the open contacts of pole A (UA) of the generator circuit-breaker is zero, while Figures 62 and 64 show the case when UA is at its maximum value.

tcp-150

-100

-50

0

50

100

150

[kA]

0.08 0.10 0.12 0.14 0.16 0.18 0.20[s]

Figure 61: Interruption of out-of-phase current with a SF6 generator circuit-breaker (out-of-phase angle δ0 = 90°, fault initiation at UA = 0, arcing time = 16.5 ms)

tcp-150

-100

-50

0

50

100

150

[kA]

0.08 0.10 0.12 0.14 0.16 0.18 0.20[s]

Figure 62: Interruption of out-of-phase current with a SF6 generator circuit-breaker (out-of-phase angle δ0 = 90°, fault initiation at UA = max, arcing time = 18.9 ms)

56 | ABB

tcp-150

-100

-50

0

50

100

150

[kA]

0.08 0.10 0.12 0.14 0.16 0.18 0.20[s]

Figure 63: Interruption of out-of-phase current with a vacuum generator circuit-breaker (out-of-phase angle δ0 = 90°, fault initiation at UA = 0, arcing time = 18.2 ms)

The results show that the fault current resulting from out-of-phase synchronising can impose extremely severe interrupt-ing conditions if the generator circuit-breaker closes when the voltage across its contacts in one pole is at its maximum

value and the arc-voltage of the circuit-breaker is not high enough to force current to zero before the condition of δ = 0 is reached.

.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]

]

0-150

-100

-50

0

50

100

150

[kA

tcp

Figure 64: Interruption of out-of-phase current with a vacuum generator circuit-breaker (out-of-phase angle δ0 = 90°, fault initiation at UA = max, arcing time = 206.8 ms)

9.4 ConclusionsThe possible occurrence of fault currents in power stations which show delayed current zeros has been investigated. In addition to the delayed current zeros phenomena associated with generator terminal faults the case of synchronisation under out-of-phase conditions has also been analysed. The capability of the generator circuit-breaker to interrupt fault currents which show delayed current zeros has been investi-gated by calculations that take into account the effect of the arc-voltage of the generator circuit-breaker on the prospective fault current. In order to carry out a more thorough investiga-tion on the interrupting capability of generator circuit-breakers a comparison between SF6 and vacuum extinguishing tech-nologies has been made.In all the cases analyzed the application of a vacuum circuit-breaker results in longer arcing times compared to the SF6 device. Furthermore the fault occurring at voltage maximum in

one phase leads to a longer arcing time compared to the case of a fault occurring at voltage zero.The generator circuit-breaker employing SF6 as extinguishing medium is suitable for the application as the calculated arcing time lies well below the maximum permissible value.The cases of generator terminal faults and 90° out-of-phase synchronisation occurring at voltage maximum in one phase lead to severe stress for the vacuum circuit-breaker. The vacuum circuit-breaker is not suitable for the application be-cause it is not capable of forcing the current to zero within the permissible arcing time.Therefore the preferred method to cope with currents exhibit-ing delayed current zeros is to choose a generator circuit-breaker having an arc-voltage magnitude sufficiently high to force the current to zero within the maximum permissible arcing time.

ABB | 57

[1] IEEE Std C37.013-1997 (R2008) “IEEE Standard for AC High-Voltage Generator Circuit Breakers Rated on a Sym-metrical Current Basis”.

[2] IEEE Std C37.013a-2007 “IEEE Standard for AC High Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis - Amendment 1: Supplement for Use with Gen-erators Rated 10–100 MVA”.

[3] IEC 62271-100 “High-voltage switchgear and con-trolgear – Part 100: Alternating-current circuit-breakers”.

[4] D. Braun and G. S. Köppl, “Transient Recovery Volt-ages During the Switching Under Out-of-Phase Conditions”, International Conference on Power Systems Transients, New Orleans, 2003.

[5] I. M. Canay, D. Braun and G. S. Köppl, “Delayed cur-rent zeros due to out-of-phase synchronizing”, IEEE Transac-tions on Energy Conversion, Vol. 13, No. 2, June 1998.

[6] Terens, L.; Neudörfler, W.: “Application Aspects of the Static Frequency Converter System in Pumped Storage Power Plants”, Waterpower ’95, Proceedings of the Interna-tional Conference on Hydropower, San Francisco, July 25-28, 1995.

[7] Trnka, R.: “Die elektrische Bremsung grosser Maschi-nensätze”, Elin-Zeitschrift, 1979, pp 2…9.

[8] Electric Power Research Institute: “Power Transformer Tank Rupture: Risk Assessment and Mitigation”, EPRI Report TR-104994, 1995.

[9] D. Braun, L. Widenhorn and J. Ischi, “Impact of the electrical layout on the availability of a power plant,” Proceed-ings of the 11th CEPSI, Kuala Lumpur, 1996, pp. 330-336.

[10] L. Widenhorn; K. Froehlich; B. Culver: “Minimised Outage Time of Power Plant Units after Step-up Transformer Failure”, Conference Proceedings of POWER GEN Asia ‘94, Hong Kong, 1994, pp.145-150.

[11] B. Culver, K. Froelich and L. Widenhorn, “Prevention of Tank Rupture of Faulted Power Transformers by Generator Circuit Breakers”, ETEP, Vol 6, January/February 1996, pp 39-45.

[12] I. M. Canay; L. Werren: “Unbalanced Load Stresses in Generators due to Switching Failures, Faults in Power Transformers, Instrument Transformers and Lightning Arrest-ers”, ABB Technical Report ASB 88/200, 1988.

[13] A. Dubi, “Monte Carlo Applications in Systems Engi-neering”, John Wiley & Sons Ltd, 2000.

[14] D. Braun, F. Granata, M. Delfanti, M. Palazzo and M. Caletti , “Reliability and Economic Analysis of Different Power Station Layouts”, Conference Proceedings of IEEE Power Tech, Bologna, 2003.

[15] IEEE Power Engineering Society, “Survey of Genera-tor Step-Up (GSU) Transformer Failures”, Special Publication of the IEEE Power Engineering Society Transformers Commit-tee, 1998.

[16] CIGRE Working Group 13.06, “Final Report of the Second International Enquiry on High Voltage Circuit-Breaker Failures and Defects in Service”, CIGRE Publication No. 83, 1994.

[17] M. H. J. Bollen, “Literature Search for Reliability Data of Components in Electric Distribution Networks”, Eindhoven University of Technology, 1993.

[18] CIGRE Working Group 12.05, “An International Survey on Failures in Large Power Transformers in Service”, Electra, No. 88, 1983, pp 21-42.

[19] CIGRE Working Group 23.02, “Report on the Second International Survey on High Voltage Gas Insulated Substa-tions (GIS) Service Experience”, Cigre Publication No. 150, 2000.

[20] CLOCKWORK GROUP, “A User’s Guide to Power Plant Workbench Version 1.1”, Austin , 1999-2000.

[21] I. M. Canay, “Comparison of Generator Circuit-Breaker Stresses in Test Laboratory and Real Service Condition”, IEEE Transactions on Power Delivery, Vol. 16, No. 3, July 2001.

[22] M. Palazzo, D. Braun and M. Delfanti, “Investigation on the Occurrence of Delayed Current Zeros Phenomena in Power Stations and Related Stress Imposed on Generato r Circuit-Breakers”, International Conference on Power Systems Transients, Delft, 2011.

References

58 | ABB

Notes

ABB Switzerland LtdHigh Voltage ProductsBrown Boveri Strasse 5CH-8050 Zurich / SwitzerlandPhone: +41 58 588 34 24Email: sales.gcb@ch.abb.com

www.abb.com/gcb

Contact us

© C

opyr

ight

AB

B.

1HC

0079

219

E02

-S /

AC

12Note

We reserve the right to make technical changes or

modify the contents of this document without prior

notice. With regard to purchase orders, the agreed

particulars shall prevail. ABB does not accept any

responsibility whatsoever for potential errors or

possible lack of information in this document.

We reserve all rights in this document and in the

subject matter and illustrations contained therein.

Any reproduction, disclosure to third parties or

utilization of its contents in whole or in parts – is

forbidden without prior written consent of ABB.

Copyright© 2012 ABB

All rights reserved