Power Flow Control

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Power Flow Control on the Italian network by means of phase-shiftingtransformers

Enrico Maria Carlini*, Gabriele Manduzio*, Dietrich Bonmann**

* TERNA Italy** ABB Germany

SUMMARY

This paper presents the Italian experience in the installation of PSTs in the 380 kV Rondissone

substation, with particular regard to the characteristics of these devices, to the single line diagram and

lay-out adopted, to the operational strategy and maintenance considerations.

Design restraints such as maximum switching power of on-load tap changers, short circuit withstand

and transportation are briefly discussed. The special challenges in performing dielectric and thermal

factory testing on such large units are presented. Insulation coordination of the PST with the

surrounding network deserves special attention, since a PST is a series connected network element.

The selection of the station surge arresters protecting the PST is highlighted. The operational strategy

of a PST needs to determine criteria for the control of the phase angle. The automatic tap changer

controller allows different control modes: maintaining a remotely set power flow or remote control of

the tap position. The insertion and de-insertion of the PST requires special switching sequences.

A description of the operational impact of the PSTs is also provided as well as elements on the co-

ordination with other PSTs already installed. The paper finally describes solutions adopted by other

utilities in the world in similar PSTs projects.

KEYWORDS

PST, Phase shifting Transformers, power flow control, NTC, maintenance, operation experience.

21, rue dArtois, F-75008 PARIS A2 206 CIGRE 2006http : //www.cigre.org


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1 General Considerations About Import and Load Flows Between Italy, France and

Switzerland

The exploitation of the deregulated electricity market implies the utilization of the interconnection

transmission capacities wherever available under security conditions and this to happen within a

global European market, in the perspective of a pan-European real political/economic entity.

Considering the present and near future structural situation of power generation facilities in Italy (by

far, expectedly more expensive than in EU countries), the already high import capacity could be

further increased, both by better use of existing interconnections and also by the construction of new

interconnectors.

The northern Italian grid is interconnected to the neighboring countries with 17 circuits (8 circuits at

380 kV, 8 circuits at 220 kV and 1 circuit at 132 kV). These interconnections can be ideally grouped

in 3 corridors connecting Italy to France, Switzerland and Austria-Slovenia. In 2004, the net yearly

import to Italy through the northern interconnection has been equal to about 44 TWh. Each of

mentioned corridors is influenced by the protection scheme and by the dispatching criteria of the TSOs

of the neighbor countries.

Fig. 1: Power flow (GWh) exchange between Italy and other countries. Year 2004.

Compared to a total theoretical gross transfer capacity during winter period, equal to more than 12000

MW, the net transfer capacity (NTC) available today in import direction to Italy is actually equal to

7150 MW in winter peak hours and 6050 MW in summer peak hours. These NTC values are

calculated on the basis of the ETSO (European Transmission System Operators) endorsed

methodologies and are assessed for different typical time frame (peak and off-peak hours) and

different grid scenarios (in terms of statistical distribution of power flows on the interconnection). The

grid constraints are firstly determined by the electrical characteristics of the lines and the n-1 securityoperation of the meshed interconnection operation; loop flows, transient overloads, as well as the

transmission reliability margin (TRM) are also taken into consideration.

During the process of NTC assessment, the availability of operative countermeasures to solve grid

congestions, occurring in case of normal operating condition or consequently to the loss of grid

elements, are checked as well.

These strategies usually include:

automatic or manual tripping for the breakers of parallels of the bus bars, where possible;

automatic or manual connecting of power flow control devices as Phase shifting Transformers

(PSTs);

automatic or manual opening of internal lines a 132-220 kV for local congestions resolutions;

automatic or manual disconnection by using short time acting devices for generators placed on key-locations for congestions, introduced by occurring disturbances or system contingencies;

modulation of the generation reserve for every national transmission grid.


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In this framework, therefore, PSTs actually represent one of the available tools to relieve congestion

and, in general, to reduce the effects of the parallel flows and optimize the flows distribution on the

interconnection. This can significantly increase the level of operational security, within long and

medium-term system planning as well as in day-ahead operational planning and real-time operation

level.

2 Technical Characteristics of PST2.1 Functional SpecificationThe load flow and contingency calculations and feasibility studies regarding manufacture and

transportation of the PSTs led to the following nominal characteristics.

Nominal frequency: 50 Hz

Nominal PST voltages: 400 kV / 400 kV

Nominal line current: 2350 A

Nominal throughput power: 1630 MVA

Voltage shift type: Asymmetrical (quad-booster)

Off-load regulation angle: 18 (Advance)

On-load regulation angle12, with line current = 70% of In

Max commutation time over full regulating range: 180 s

Busbar short circuit current: 40 kA

Maximum sound pressure level at nominal voltageand frequency

78 dB(A)

Table I: Main items of the functional specification for the phase-shifting transformers

Limited space available in the substation led to the requirement of one three-phase unit for each of the

two transmission lines. The units had to meet the dimensional limits of the Italian railway since road

transportation had to be limited to an absolute minimum.

The regulation angle was specified in a functional manner as 12, under a line current of 70% of

rated current. This allowed the potential suppliers to find the best design compromise betweenminimum short circuit impedance needed for short circuit withstand of windings and tap changer and a

minimum no-load regulation angle for smallest possible physical size.

The N-1 criterion was applied to the operation of the coolers, and in addition an emergency operation

at 120% rated load with N-1 coolers as well as operation at rated load without coolers for 20 minutes

each were specified. The asymmetric quad booster concept for the PST was selected because it

allows maximizing the utilization of the on-load tap changers switching capacity. The moderate

variation of output voltage with angle regulation in the asymmetric quad booster concept could be

tolerated by the system in this case.

Fig. 1a: Exciter and booster transformers, winding connection. Voltage Phasor diagram.

Since the asymmetric quad booster concept does not need a 400 kV centre tap in the serieswindings, the connections between series and shunt transformer are greatly simplified.

Series unit

Main (shunt,

exciting) unit

V1S

V2S

V3S

U1reg

V

u1inj

V1SV1LV

u1inj

U1reg


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As pointed out in the general considerations, the PSTs have to be manoeuvred from a stand-by

operation to maximum advance phase shift in the shortest possible time in order to relieve overloads

on transmission lines in case of certain contingencies.

3 Realization of the Phase Shifting Transformer

The optimisation of the electrical design led to the following results:

Each PST consists of a three-phase series transformer with six 400 kV oil/air bushings and a three-phase shunt transformer with three 400 kV oil/air bushings. The connections between the regulation

windings of the shunt transformer and the excited windings of the series transformer are made via

oil/oil bushings in an oil filled duct. This solution allows separate handling and factory testing of the

transformers with well-defined interfaces and minimum exposure of the windings to the atmosphere

during transportation and on-site assembly. Electromagnetic forces due to external fault currents were

dimensioning for the minimum achievable short circuit impedance of the series transformer. The short

circuit impedance of the shunt transformer was designed as low as possible in order to minimize the

no-load phase angle. Short circuit forces in the shunt transformer were not a challenge.

Transient oscillations could be well controlled by proper winding design, such that no internal surge

arresters were necessary. The on-load tap changer consists of three single-phase units with a common

motor drive with coarse/fine-regulation and 33 positions. It fulfils the requirements regarding speed of

operation, short circuit withstand and overload ability.

Series transformer

Rated voltage induced in series winding 74875 V

Rated current through series windings 2353 A

Two-winding rated power 528.5 MVA

Shunt transformer

Rated voltage 400000 V

Rated current through series windings 725.8 A

Two-winding rated power 502.8 MVA

Complete PST

Short circuit impedance rel. to 1630 MVA at 0 angle regulation 11%Short circuit impedance rel. to 1630 MVA at 18 angle regulation 13.5%

Load losses 2596 kW

No-load losses 269 kW

Total weight 734000 kg

Dimensions L x W x H 18.2 x 12.6 x 10.25 m

Table II: Main technical data of phase-shifting transformers in Rondissone substation

Fig. 2: First of the two PSTs during assembly on site.


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4 Factory Testing

Where applicable the type and special tests according to IEC 60076 were applied.

Lightning and switching impulse tests were applied to the individual units. The switching impulse on

the series transformer could only be performed as a potential test, i.e. with source and load side

bushings connected together, as described in IEEE C57.135, since the transformation ratio of the

series transformer would have led to very high voltages in the excited windings. In normal operationthe voltage that can occur across the excited windings is limited by the connection to the regulating

windings of the shunt transformer.

Power frequency voltage withstand tests and sound level tests were performed on the individual units

as well as on the complete PST. The test winding in the exciter transformer has permanent bushings

suitable for induced voltage and partial discharge testing of the complete PST as well. Under normal

operation a metallic cover protects these bushings.

Load loss and temperature rise tests up to 120% of rated load could be performed on the individual

transformers. Due to the very high currents through the HV bushings of the series transformer this test

is often a challenge for transformer test rooms.

Measurements of phase angle regulation and zero sequence impedance voltages were performed on the

complete PST.

5 Verification of Insulation Coordination

At the end of the electrical design stage a transient model of the PST for implementation in ATP was

created. Various single-phase and three-phase switching events as well as lightning strokes at various

locations of the system were simulated in order to verify the dimensioning of the station surge

arresters located on both sides of the PSTs. Special consideration was given to the fact while a

lightning hits one end of a series winding the opposite end of the series winding may be at the peak of

opposite polarity of the operating AC voltage.

6 Single Line Diagram Adopted

The scheme adopted for the phase shifter, installed on the 380 kV double line Rondissone

Albertville, is shown in the figure below.

Fig. 3: Single line diagram adopted for the PST installation on the 380 KV double circuit line

Rondissone Albertville

This type of configuration allows to have the following advantages.

1. When the PST is out of order, every 380 kV connection with Albertville is on service.


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2. The normal operation of connection with the open by - pass and PST in stand - by is assured.3. It is possible:

to insertthe PST when the line is in service, without interruptions of grid connection;

the return of PST in stand by, when the line is in service and without interruptions ofgrid connection;

to put the PST out of order, when the line is in service.

With regard to the PST and by pass control system, the following conditions can be obtained:

PST in stand by connected on the busbar A and by pass closed on busbarA;

PST in stand by connected on the busbar B and by pass closed on busbar B;

PST closed on busbar A and open by pass;

PST closed on busbar B and open by pass;

open PST;

open by pass.

7. Testing, Operation and Maintenance Aspects

As can be seen in table II the PST has a short circuit impedance voltage at rated load of 11 to 13.5%. If

the PST is inserted into the line the PSTs impedance tends to reduce the power flow, which is the

opposite of what is necessary in a post-contingency situation when the power flow in the line shall be

increased as fast as possible in order to avoid overloading of parallel lines. This can be avoided by

bringing the PST into an equivalent zero tap position before opening the bypass. Equivalent zero

position means that the voltage drop caused by the load current is approximately balanced by the

voltage injected by the series windings of the PST. The switching sequence would be:

PST in stand-by, connected to the busbar; by-pass closed, carrying load current.

Close PST branch, tap changer in 0 or in another parking position. Load current remains

in the by-pass.

Move tap changer to equivalent zero position. The voltage injected by the series windings

of the PST superposes a circulating current through by-pass and PST over the load current.As a result the current in the by-pass becomes almost zero, the load current commutates into

the PST branch.

Open by-pass, load current flows through PST.

Normal power flow control mode of the automatic power flow controller can be enabled, if desired.

This switching sequence can be performed manually or as an automated command sequence. The

equivalent zero tap positions as a function of load current are given to the SCADA system in the

form of a table. The parking position of the tap changer can be set to a value that allows to minimize

the time for reaching the equivalent zero position.

The automatic power flow controller allows running the power lines at a constant power flow or at a

fixed tap position. The set values for power flow (in MW) or tap position can be set locally or via the

SCADA system.

With regard to maintenance each PST corresponds to two large 400 kV transformers in a strategically

important location and is treated like that according to TERNA standard procedures. In order to

support the maintenance planning, and in order to provide more early indicators of incipient faults all

four transformers are equipped with TEC (Transformer electronic control) system and a bushing

monitoring system. The data from the four TECs are transmitted via optical fiber to a common server

PC located in a substation building. The data from the four TECs and the four bushing monitoring

systems can be accessed from remote locations via Intranet using standard web browsers.

Figures 4a 4c show some of the data that was downloaded after approximately one year of operation.

However, this huge amount of data may be of interest in case of detailed

investigations after a fault has occurred, but for the operators at the dispatch center it is of little value

during normal operation. Therefore the TEC system continuously supervises whether the oil, winding

and tap changer temperatures are consistent with the actual load, actual ambient temperature andnumber of coolers in operation. In case of discrepancies the TECs provide alarm signals to the

SCADA system for:


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- Hot Spot temperatures- Moisture in oil- Thermal balance tank- Thermal balance for the on-load tap-changers (OLTC)- Top oil temperature in the OLTC- OLTC needs maintenance- OLTC contacts need to be replaced

The bushing monitoring system measures voltages, leakage currents and their phase at the nine 400 kV

bushings of each PST. The system calculates capacitances and loss factors of the bushings. Based on

an internal expert system software the bushing monitoring system issues alarm signals of three levels

of urgency for the SCADA system. Error codes at the local display of the monitoring system give

further indications of the type of bushing problem.

Further alarms are generated by the gas-in-oil sensor (Hydran): Gas-in-oil concentration high and Gas-

in-oil concentration high-high; In case of a condition monitoring alarm signal the remote operator has

the option to view detailed information and recommendations for action via intranet access to the TEC

and bushing monitoring systems.

Fig. 4a: Main parameters recorded by TEC for the PST 2 series unit

Fig. 4b: Main parameters recorded by TEC for the PST 2 exciter unit


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Fig. 4c: Main parameters recorded by TEC for the on-load tap changer. The tap changer consists ofthree single phase units.

In order to support diagnostic measurements in case of need the PSTs design allows access to the

connections between exciting and regulating windings. After draining just very few 1000 liters of oil

from the short oil duct between series and exciter transformers the connections between exciting and

regulating windings can be opened. The terminals are then accessible for low voltage tests of the

individual transformers: ratio, impedance, and resistance.

The 15 kV test winding in the exciter transformer has permanent bushings suitable for induced voltage

and partial discharge testing of the complete PST. In normal operation a metallic cover protects these

bushings.

8 Experience in PSTs utilization, operational impact on the system and co-ordinationTwo main modalities of Rondissone PSTs utilization are currently applied by the National Control

Centre (NCC) of the Italian TSO:

Preventive action on unbalanced distribution of power flowsPSTs act to redistribute power flows on the northern interconnection under fully meshed grid

conditions, aiming at keeping secure operating conditions.

This modality is presently the most frequently adopted and allows increasing the firmness of

the cross-border exchange programs while guaranteeing adequate standard of security.

Corrective action on post-fault situationsPSTs are activated in case of tripping of one of the 380 kV interconnectors between Italy and

Switzerland or in case of tripping of the 380 kV Venaus-Villarodin interconnector between

Italy and France. In the first case, the contribution of PST operation is the increase of the

power flows from France to Italy on the 380 kV double-circuit Rondissone Albertville to

restore in due time security conditions on the rest of the northern interconnection. In the

second case, Rondissone PSTs operation contributes to timely relieve security violations on

the northwestern Italian 220 kV grid.

This modality of utilization allows an increase of the levels of operational security at real-time

level. It has been actually less used in the recent past, in particular due to the introduction in

January 2005 of the new CH-IT double circuit interconnector named S.Fiorano-Robbia.

Other modalities of PSTs utilization are however allowed in operation. It is worthwhile noting thepossibility to adjust flows on the interconnection under conditions of major tie-lines/internal lines

unavailability, for scheduled or unscheduled maintenance activities. In this case, also, PSTs provide a


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further degree of flexibility in achieving the interconnection security, without recurring to reduction of

exchange programs or curative redispatching actions.

Fig. 5: Re-distribution of the power flows in import direction to the Italian system, as determined by

the operation at extreme taps of the two Rondissone PSTs (PSTs in stand-by position, PSTs tap 33)

The experience of the possible impact of Rondissone PSTs operation on neighboring systems have

suggested the need of an appropriate and deep co-ordination of PSTs operation between the TSOs

involved. Co-ordination covers a crucial role to optimize the efficiency of PSTs utilization and

guarantee the best standards of operational security.

With this purpose, the Italian and French System Operators have set-up a joint procedure for the co-

ordinated operation of the PSTs installed in 380 kV Rondissone substation and La Praz (F). In fact,due to the grid topology, one of the circuits of the 380 kV Rondissone-Albertville interconnector is

electrically close to the 380 kV La Praz-Villarodin-Venaus corridor, on which the La Praz PST is

installed. Therefore, the uncontrolled variation of the Rondissone PST taps in question could result in

a direct and opposite effect on the La Praz corridor and vice versa.

The procedure allows the optimized utilization of the available means for congestion while increasing

the level of operational security of the northern interconnection and, in particular, of the F-I border.

The common objectives and the tasks and actions of the NCCs during the possible scenarios of

operation are specified. The co-ordination results also in reducing the risk of tap-change hunting by

TSOs when operating conditions change and in the increase reliability of PSTs operation in case of

unavailability of one of the transformers.

In normal operation, two of the three PSTs are operated to manage flows on F-I interconnection. In

case of very unbalanced flows on the Northern Italian grid and specifically of very low flows on F-Iborder, the simultaneous operation of the three PSTs is undertaken to push flows on that border.

Co-ordination in PSTs handling is implemented at three stages:

| Long and medium-term system planning, to perform joint security analyses on the impact of

all the PSTs operation on the interconnected grid.

| Day-ahead operational planning, to optimise the PSTs utilisation based on the day-ahead load-

flow calaculations and n-1 security analyses

| Real-time operation, to relieve security violations through a co-ordinated congestion

management strategy. This modality can be undertaken under fully meshed grid conditions

(preventive way) or following the loss of one grid element (corrective way).

The co-ordination in PSTs operation is based on the mutual exchange of information regarding thesystem state and the objectives of the TSOs in optimising PSTs taps. The real-time exchange of PSTs

selected taps represents an effective mean to monitor the switching margins available for relieving

ITALIA

Villarodin

Venaus

S. Fiorano

RobbiaLienz

Soverzene

Redipuglia

Divaca

Padriciano

Soazza

Bulciago

Gorduno

Mese

Airolo

Ponte

MusignanoPallanzeno

MoerelRiddes

AviseValpelline

Albertville

Rondissone

CamporossoLe Broc-Carros

Lavorgo

4499 MW4499 MW 4053 MW4053 MW

SWITZERLANDSWITZERLAND

1597 MW1597 MW

2087 MW2087 MW

FRANCEFRANCE

856 MW856 MW

824 MW824 MW

SLOVENIASLOVENIA

198 MW198 MW 189 MW189 MW

AUSTRIAAUSTRIA

ITALIA

Villarodin

Venaus

S. Fiorano

RobbiaLienz

Soverzene

Redipuglia

Divaca

Padriciano

Soazza

Bulciago

Gorduno

Mese

Airolo

Ponte

MusignanoPallanzeno

MoerelRiddes

AviseValpelline

Albertville

Rondissone

CamporossoLe Broc-Carros

Lavorgo

4499 MW4499 MW 4053 MW4053 MW


4499 MW4499 MW 4053 MW4053 MW


1597 MW1597 MW

2087 MW2087 MW

FRANCEFRANCE

1597 MW1597 MW

2087 MW2087 MW

FRANCEFRANCE

856 MW856 MW

824 MW824 MW

SLOVENIASLOVENIA

856 MW856 MW

824 MW824 MW

SLOVENIASLOVENIA

198 MW198 MW 189 MW189 MW

AUSTRIAAUSTRIA

198 MW198 MW 189 MW189 MW

AUSTRIAAUSTRIA


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congestion and to increase the accuracy of the security analyses as far as the PST simulation is

concerned.

Such level of co-ordination between TSOs represents at the moment a unique example in the UCTE

community.

9 Single line diagrams adopted from other utilities in the world in similar projectPSTs devices can be adopted to fit the need of liberalized electricity market requiring ever more global

interconnected electricity networks. Interesting feasibility studies by introducing PST in strategic

European electric nodes to optimize\increase the power flow among different countries, are going on.

To point out :

Netherlands-Germany Interconnection (Meeden substation)

France-Italy interconnection (La Praz substation)

UK National Grid (Keadby substation)

9.1 PST Meeden substation. [1]

In recent years the liberalization of the European market has pointed out the need to strengthen the

interconnection between Netherlands and Germany. TenneT, the Dutch Transmission System Operator

(TSO), has been faced therefore with the desire of the market to increase the available cross-border

capacity. The installation of PSTs at the Dutch 380 kV substation Meeden in series with the two

interconnections to the grid of E.ON Netz, allows to upgrade the NTC of the cross-border line

Meeden-Diele/Conneforde with about 1100 MVA. The Netherlands has five interconnections,

comprising 10 circuits, with two neighboring countries. Because of the geographical and electro

technical location of the country, main interconnection flows are directed from Germany to the

Netherlands. The loading of the circuits in the South border, is often causing a transfer limitation.

Installing phase shifters in the 380 kV substation Meeden at the German border in the North would be

most beneficial regarding import capacity.

Fig. 6: Single line diagram adopted for the PST installation at the Meeden substation

9.2 PST La Praz substation. [2]

As mentioned beforehand, the 400 kV interconnection between France and Italy consists of a high

capacity two circuit 400 kV line (north Albertville Rondissone line) and a weaker one circuit 400 kV

line (south Venaus Villarodin). The south line has a far lower capacity than the north line, which

raises two problems, It does not allow optimum use of the water resources in the Alps region and does

not enable to increase the European exchanges.


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Fig. 7: Single line diagram adopted for the PST installation at the La Praz substation

The PST has been thought with a rating of 1181 MVA and the maximum phase shift has beenspecified to be 15, so as to enable the power system to remain stable after the loss of the double north

line. An automatic controller will alter the phase shift according to the situation and the kind of event

occurring: in normal operating conditions, the phase angle will be set near to zero so as to avoid

disturbances for the Italian TSO. The phase angle will be shifted to its maximum for the most severe

contingency (loss of the double line). The scheme in figure 7, consisting of a PST with a by pass

disconnector, allows:

use of tap changer in real time;

easy utilizations of the PST in the different conditions;

a reasonable flexibility of operation.

9.3 PST Keadby substation. [2]

The main difference between French and United Kingdom scheme is use of a circuit breaker at the

bottom of circuit.

Fig. 8 Single line diagram adopted for PST installation at the Keadby substation

The largest quadrature boosters in the UK are connected in the 400 kV system and have a throughput

rating of 2750MVA and the ability to shift the angle by up to 11.3 degrees in either direction by meansof 19 voltage taps in each direction. The Quadrature Booster in question is a 2750 MVA, 400/400 kV

+/-j80 kV unit to be installed at the National Grid Transco (NGT) 400 kV substation at Keadby, near

Scunthorpe on Humberside.


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10 Conclusion

In the deregulated European electricity market the trading of electrical power over larger distances as

well as the installation of new generation, especially wind power, create new load flow patterns that

often challenge the capacity or security of existing transmission lines. PSTs can be inserted into a line

with just two extra disconnectors for less demanding applications or with a bypass and two or three

extra circuit breakers for the highest demands of flexibility and protection. Different modes of anglecontrol can be chosen. The design and installation of two PSTs at Rondissone substation near Torino

is an example of an application in a strategically important network node and is described in some

detail. The special considerations given to maintenance and condition monitoring are described. The

modality of PSTs operation is described as well as its impact and sensitivity on the redistribution of

flows across the northern Italian interconnection; elements on the co-ordination with La Praz PST are

also provided. Three further applications of PSTs are briefly highlighted. Phase-shifting transformers

are a demanding yet well-proven means of controlling quasi-stationary load flows.

BIBLIOGRAPHY

[1] W.L. Kling, D.A.M. Klaar, J.H. Schuld, TenneT bv A.J.L.M. Kanters, C.G.A. Koreman, TenneTbv H.F. Reijnders, C.J.G. Spoorenberg, Smith Transformatoren B.V.

Cigr paper C2-207, 2004

[2] M.H. Baker, Reason to prefer a Phase-shifting transformer (PST) and which studies are required.Cigr SC 14, Paris session, 2002

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