Static VAr compensator (STATCOM) based on single- phase chain circuit converters

J.D.Ainsworth M. Davies PJ.Fitz K.E.Owen D. R. Tra i n er

Indexing terms: Chain circuit converter, Voltuge-sourced converter, FACTS, SVC, STA TCOM, AC system, GTO

Abstract: Voltage-sourced converter circuits using gate turn-off (GTO) thyristors have been proposed as an alternative means of providing static VAr compensation (SVC). Such ‘advanced SVCs’ are classified under the nomenclature STATCOM (STATic COMpensator). The published converter circuits suitable for power system application include the classical Graetz bridge and the multilevel converter. An alternative circuit arrangement has been developed with significant advantages in terms of cost and performance; this is in the form of a number of GTO converter links connected in series to form a chain and is referred to as a chain circuit converter. The paper discusses the chain circuit converter and its application to var compensation, followed by the design and simulated performance of a 3-phase STATCOM under both steady state and transient operating conditions. The technical advantages of the chain circuit converter compared with other known arrangements of STATCOM are given. These include good harmonic performance, fast response and improved output at low voltage compared with conventional SVCs. The paper includes comments on the successful testing of two prototype chain links for both leading and lagging reactive currents up to 2000A peak and concludes with a description of the equipment now being designed for commercial service.

1 introduction

Shunt connected static VAr compensators (SVC) are being used extensively to control the AC voltage in transmission networks. Modern power electronic based equipment, such as thyristor controlled reactors (TCR) and thyristor switched capacitors (TSC) have gained a significant market, primarily because of their fast speed of response, low maintenance requirements and low cost [ 1, 21. With the advent of high power gate turn-off

0 IEE 1998 IEE Proceedings online no. 19982032 Paper first received 24th July 1997 and In revlsed form 9th February 1998 The authors are with GEC ALSTHOM T&D Power Electromc Systems Lmited, Stafford, UK

(GTO) thyristors a new generation of power electronic equipment, the STATCOM is now poised to take a sig- nificant proportion of the SVC market. The STAT- COM is also an SVC but takes advantage of the GTO’s ability to turn current off as well as on.

I. I STATCOM operating principles Fig. 1 illustrates the basic principle of a STATCOM based on a voltage-sourced converter. A simplified rep- resentation of the AC system is a Thkvenin equivalent of EMF Es behind reactance X,. A STATCOM volt- age-sourced converter can be considered to generate a ‘back-EMF’ represented by Ec (the fundamental fre- quency component of converter voltage) connected via buffer reactance X , to the AC system.

Fig. 1 system

Single line equivalent of a ST14TCOM connected to an AC

With a suitable closed-loop control system, the STATCOM back-EMF is controlled to be in phase with the AC system EMF, the current drawn I is then almost purely reactive and given by:

(1) Es - Ec

J(XS + Xc) I =

The back-EMF of the converter is directly related to the DC side voltage which can be changed by closed- loop control to enable the converter to draw either pos- itive (leading) or negative (lagging) values of reactive current in a similar manner to a synchronous compen- sator, but much more rapidly [3].

2 Voltage-sourced converter circuits

Simple single-phase or 3-phase GTO bridge arrange- ments are not directly applicable as STATCOM cir- cuits because of their poor harmonic performance and limited rating [ 3 ] . The total VAr rating could be increased by using GTOs connected either in series or parallel, however, this would not improve the harmonic performance and introduces problems associated with volinge and/or current sharing.

Increased VAr rating can also be achieved by using multiple 3-phase bridges and phase-shifting transform- ers [3, 41. The 3-phase bridges can be connected in parallel on the DC side and the converters may incor-

38 1 IEE Pro<.-Gener. Trunsm. Distrib., Vol. 145. No. 4, July I998

porate series-connected GTOs and diodes to achieve higher ratings. The converter transformers are usually arranged to make the bridges appear in series when viewed from the AC side [5] . By arranging phase shifts between the bridge transformer windings, selected har- monics can be nulled to give a multipulse arrangement, e.g. 24 or 48-pulse, under balanced conditions. With this method it is not practicable to connect the series- connected transformer primaries directly to the EHV system, and a further fully rated stepdown transformer is required for this purpose.

Pulse width modulation (PWM) can also be applied to null selected harmonics. However, increasing the number of GTO switching operations per cycle increases switching and snubber losses, increases high frequency harmonics, and reduces the fundamental var rating.

An alternative method of increasing rating and reducing harmonics is to use a ‘multilevel’ converter [6]; which produces a ‘multistepped’ output voltage wave- form. In this circuit the single energy storage capacitor of the conventional 3-phase bridge circuit [4] is replaced by several capacitors stacked directly in series, and the basic switch per bridge arm (one GTO plus one reverse diode) is changed to several switches in series, coupled to the capacitor stack via auxiliary diodes. A stack of N capacitors produces an ( N + 1) level sym- metrical voltage waveform per phase, which gives increased var rating and a better approximation to a sine wave than the basic 3-phase bridge circuit. The main disadvantage with this circuit is that the total number of auxiliary diodes increases rapidly as the number of levels increase, and a maximum of nine lev- els (eight capacitors) is probably the economic limit. This gives a relatively low maximum 3-phase bridge rating of the order of 220MVAr.

3 The chain circuit converter

In this arrangement each phase comprises a separate ‘chain’, each having a number of ‘links’ which are con- nected in series on their AC sides. Each link is a single- phase bridge voltage-sourced converter as shown in Fig. 2. A 3-phase circuit can be made by a star (Y) or delta (A) connection of three such chains. An important feature of the chain circuit is that, although it has the normal antiparallel diodes across each GTO, it does not require the auxiliary diodes associated with the multilevel capacitor stack circuit.

I n u Fig.2 Single-phase chain circuit

Each of the GTO-diode pairs operates as a two-way switch as illustrated in the circuit analogy of Fig. 3. At any instant the switches can be connected to the posi- tive or negative terminal of its associated capacitor, therefore each link can contribute a voltage of + V, zero (capacitor bypassed), or -V. With N links connected in series the circuit can synthesise a voltage waveform with (2N + 1) voltage levels, which can give a good approximation to a sinewave.

382

AC

Fig. 3 Switch analogy of three-link chain converter

The total fundamental frequency voltage of the chain is the sum of the individual link AC voltages, and simi- larly for the total chain VAr rating. A good harmonic performance can be achieved if a different switching angle is used for each link as shown in Fig. 4, for a three-link chain. The three individual link voltages are still three-level waveforms, but they have different pulse widths, which combine to give an effective seven- level waveform; this is achieved by switching each GTO ‘on’ and ‘off only once per cycle of fundamental fre- quency. The switching angles al, a2, and a3 of the respective chain links are chosen so that the summed voltage is a good approximation to a sine wave.

link ’ 2

voltages

+ 2 ” t i total chain

voltages

Fig.4 Voltage waveforms for a 3-link (7-level) chain converter

The general arrangement for a STATCOM for power system application is shown in Fig. 5. A stepdown transformer provides coupling from the EHV busbars to the point of connection with the STATCOM. The connection reactance X, can be typically 0.2p.u. (based on nominal system voltage and rated current) and can be an external reactor as shown, or in some cases could be designed into the effective transformer reactance.

Fig. 5 General arrangement of a STATCOM

There are no special design requirements for the step- down transformer because the chain link STATCOM draws very little harmonic current. Existing EHV trans- mission transformers with tertiary windings of suitable rating could be used. An optional fixed capacitor (or reactor) can be connected to the LV or HV busbar to provide an economical extension to the range of the basic STATCOM in the leading (or lagging) direction.

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A significant number of chain links per phase will be needed for a typical transmission STATCOM, which gives the opportunity to significantly reduce harmonic generation compared with TCR-type SVCs. By utilising harmonic cancellation techniques within the chain cir- cuit, the size of AC harmonic filters can be greatly reduced or they may even be eliminated.

4 Chain circuit STATCOM performance

4.1 Static characteristics A comparison between the steady state characteristics of a STATCOM and a conventional SVC using a TCR and a TSC is shown in Fig. 6.

system voltge (P.u.) I ,'

capacitive current 6 inductive current (leading) (lagging)

Fig. 6 Comparison of static characteristics

Owing to its principle of operation, the STATCOM can maintain rated leading (or lagging) current at low busbar voltages. This is a significant improvement over the performance of conventional SVCs, which are essentially fixed impedance systems where the current reduces in proportion to system voltage.

4.2 Chain link ratings Chain link rating is fixed by the capabilities of the GTOs employed. Readily available commercial devices have a peak voltage rating of 4.5kV and peak turn-off current capability of 4kA. In practice the steady state voltage and current rating of the link has to be lower than these values to withstand the transients introduced during device switching and to give adequate safety margins for the stresses encountered during faults and disturbances.

Since the power circuits of individual chain links are independent of each other, increasing converter VAr rating requires only the addition of more chain links in series to raise the converter voltage; the current rating remains unaltered. Ratings of the order of +100MVAr or more in one 3-phase unit are possible using the same basic link design as building blocks.

4.3 Harmonic performance A STATCOM with N chain links per phase produces a converter waveform with N voltage steps (transitions) per quarter cycle and in principle it is therefore possible to eliminate N harmonic voltages from the output waveform by the appropriate choice of GTO switching angles. For example, a relatively small STATCOM with eight chain links per phase can theoretically elimi-

operating conditions, where triplen harmonics can be ignored, or up to the 17th for unbalanced applications which additionally require the cancellation of the tri- plens. In practice the effects of capacitor ripple limit

nate harmonic voltages up to the 25th fwr bulanocd

IEE Proc-Gener. Transm. Distrih., Vol. 145, No. 4, July I998

the possible number of perfectly nulled harmonics to somewhat less than the theoretical. For installations of higher rating the number of links and therefore voltage steps increases, giving further improvements in the har- monic performance.

4.4 Losses Power losses are usually capitalised for evaluation and hence are an important factor in determining the eco- nomic viability of specific SVC designs, particularly in transmission systems. For all types of STATCOM the most significant contributions to the total losses are incurred in the main transformer(s) and the power elec- tronics equipment; the latter includes the losses associ- ated with the GTO conduction, switching and the snubber circuit.

The chain circuit converter gives a low loss design as illustrated in Fig. 7. Switching losses are minimised by requiring only one switching operation per GTO per cycle of fundamental frequency, and snubber circuit losses are minimised by the use of a low loss design. In addition, the chain circuit allows the possibility of recovering energy from the snubber circuit to reduce losses further as generally described in [7].

-1 .o -0.5 0 0.5 1 .o lagging STATCOM leading

current, (P.u.) Fig. 7 STATCOM converter losses as a percentage of MVAr rating

Fig. 7 gives typical calculated converter losses as a percentage of the STATCOM var rating over the range 1p.u. lagging to 1p.u. leading current. These figures exclude the stepdown transformer and any recovery of snubber losses.

4.5 Redundancy Redundancy is designed into the chain circuit converter by adding an extra link per phase; then, in the event of a GTO failure, the affected chain link operates continu- ously in the bypass mode (short circuit), until the next planned maintenance outage. Since all of the links in the chain are identical, the control system is arranged to automatically re-optimise the switching pattern on the remaining links to minimise the generated harmonic voltage.

4.6 Control and protection The principal functions of the control and protection qyctems are-

(1) to provide synchronised turn-on and turn-off gating instructions to each GTO to control the magnitude, frequency and phase angle of the voltage generated by each chain circuit converter;

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(2) to limit converter currents and voltages in response to system faults and other disturbances; (3) to provide an optimised GTO switching pattern for harmonic voltage control. In implementing these requirements, it is recognised that the principle of operation of the STATCOM cir- cuit differs fundamentally from conventional thyristor SVCs. The use of the chain circuit converter, with many GTO links and many switching events during one cycle of fundamental frequency, adds flexibility to the control and protection strategy that may be adopted. This increased flexibility is harnessed using modern digital control methods.

The converter voltage is controlled in a closed loop manner to stabilise the voltage of the AC system. A sloping characteristic (Fig. 6) is provided to regulate reactive current with respect to AC system voltage and is typically adjustable over the range 0-10%, generally as for conventional SVCs. In steady state conditions the converter voltage is controlled by varying the mag- nitude of the voltage step contributed by each link. This is achieved by control action to change the voltage on all chain link capacitors equally in the required direction [8]. The link switching angles are then main- tained individually at values to give optimum harmonic performance.

The steady state and dynamic performance of the STATCOM has been studied using digital simulation methods. Fig. 8 shows the performance o f the STAT- COM for a step increase in AC system load at time t = 0.045s. V,, IC and V, are as defined in Fig. 5. The con- verter current changes from the lagging to the leading direction to compensate for the increase in load. At time t = 0.225s the process is reversed. Note that both changes involve a fast change of the STATCOM out- put voltage, giving a symmetrical response of less than one cycle of the fundamental frequency. This represents a speed of response improvement of about 2:l over a conventional SVC.

I 1

‘C

1 0.0 0.1 0.2 0.3

time,s STATCOM response to step increase in A C system load Fig.8

4.7 Protect ive control action The control system acts as the first level o f protection in limiting overcurrents and overvoltages within the converter, using both ‘soft’ and ‘hard’ limits. When operating within the normal steady state range of sys- tem voltages a soft current limit is used to define an ‘end-stop’ to the control range, within the safe thermal ratings of the semiconductors. For major system distur- bances the control system acts to provide maximum reactive power support to the AC system within equip- ment ratings, which may invoke current and voltage limits.

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4.8 Abnormal operating conditions In the event of major AC system faults which elevate or depress the primary voltage significantly, the STAT- COM will naturally tend to oppose the change in volt- age, however, the control systems must keep the response to within equipment ratings. During a major voltage depression this entails bypassing a number of chain links to rapidly reduce the converter voltage to limit the current to approximately 1p.u. leading to give maximum voltage support. When the AC system volt- age recovers following clearance of the fault, reconnec- tion of bypassed links follows immediately, giving a step increase of the STATCOM output voltage to ena- ble operation to continue uninterrupted.

A chain link STATCOM with separate converters per phase enables independent control of each phase during unbalanced faults. Fig. 9 gives an example of a remote AC system fault between phase A and ground. In this example as the LV transformer winding is delta connected then ab phase of the STATCOM generates 1 p.u. leading current in response to the fault, thus pro- viding maximum voltage support, while the other two phases remain near full lagging current. Therefore, the STATCOM uses its full capability to reduce the unbal- ance (negative phase sequence components).

I

0.05 0.25 time,*

STATCOM response to a single phuse-ground A C system fault Fig. 9 Initial voltage 1 p-u., current 1p.u. lagging

4.9 Advantages of the chain circuit STATCOM The chain circuit converter has a number of advantages in comparison to other GTO based circuit topologies and conventional thyristor SVCs [9]; these include:

VAr rating can be increased simply by adding chain links in series, the converter cost being in proportion to total rating.

IEE Proc -Cener Transm Dtstrlb I Vol 145, No 4, July 1998

Chain circuit converters are essentially single phase which offers the potential for AC system phase balanc- ing.

There is only one GTO turn-on and turn-off switch- ing operation per cycle, giving a low loss design.

Low loss snubber circuits and snubber energy recov- ery can be implemented to minimise losses.

Redundancy against a chain link failure can be built- in.

Because chain links switch in sequence, the maxi- mum instantaneous voltage excursion of the converter waveform is approximately 2kV. Therefore radio inter- ference is minimised.

Only one transformer of conventional design is required to step down from transmission voltage to the chosen STATCOM connection voltage.

Good harmonic performance can be achieved with small or no harmonic filters. With a suitable choice of switching angles the generation of low-order harmonics (particularly triplens) can be prevented during unbal- anced AC system conditions, which is impracticable with circuits based on 3-phase bridges.

A fast response is inherent with STATCOM technol- ogy. The transient performance is enhanced by the abil- ity to instantaneously change the output voltage by the independent control of the switching angle of each chain link and by inserting or bypassing links.

The problems and limitations of the direct series connection of GTOs are avoided.

Due to its constant current characteristic, a chain link STATCOM can operate down to low AC system voltage and maintain full rated leading current to sup- port the AC system during faults.

Space saving of around 50% relative to conventional thyristor-based SVCs of the same rating, hence the STATCOM can be a relocatable design. Although beyond the scope of this paper, a chain link converter of this type has other applications.

The reservoir capacitors in each link can be replaced with a battery or other suitable energy storage compo- nent giving the equipment the capability of providing real power compensation to the AC system.

This could have application in frequency control, peak lopping, replacement of spinning reserve and starting an unenergised AC system (i.e. black start).

A chain circuit STATCOM could be connected in series with each phase of an AC system to provide a controllable positive or negative reactance to control power flow. This could have application in the control of power flow in parallel circuits of an AC system.

The ability of the STATCOM converter to control self-generated harmonics could be utilised to control pre-existing harmonics on AC systems. This would give the converter an active filtering capability up to the rat- ing of the equipment.

5 Prototype chain link tests

A development programme of STATCOM technology is well under way with advances being made on both the design and testing of chain link modules. Digital circuit simulations of the associated control and protec-

IEE Proc-Gener. Trunsm. Distrib , Vol. 145. No. 4, July I998

tion systems have been validated by practical test results taken from a ‘real time’ hardware simulator complete with digital control systems.

To validate the chain link power circuit design, two links each with a steady state rating of nominally 2.25kV peak and 2kA peak have been constructed for evaluation tests up to full rating. Fig. I O shows one of these prototype chain links prepared for test. To achieve a compact design, the GTOs and power diodes are liquid cooled with the damping components and snubber diodes sharing the same heatsinks. The chain link utilises a low loss snubber circuit where surplus energy is recovered and returned to the DC capacitor. The GTO gate unit power supply is derived from the link DC voltage by means of a relatively low power IGBT auxiliary inverter module. The steady state per- formance of each chain link has been demonstrated up to full rated output, with the GTOs switching 2.25kV peak and 2 kA peak.

Fig. 10 Prototype chain lid

6 Design for production

The electrical, thermal and mechanical design of the converter equipment is currently being optimised. It will be mounted in one or more transportable cabins which enables the equipment to be fully tested before shipment to site and makes future relocation possible. One such application is the 275MVAr STATCOM ordered by the National Grid Company as part of a 0- 225MVAr relocatable SVC for initial installation at its East Claydon substation. This installation is planned to enter service in the autumn of 1999 and will be the first application of STATCOM in the UK.

7 conclusions

There is considerable economic incentive in continuing to improve the efficiency of power transmission net- works on a global basis. High power electronic systems

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such as SVCs have already been demonstrated to bene- fit power transmission systems. The technical advan- tages of the chain circuit STATCOM over the thyristor-based SVC technologies present unique opportunities for both performance and cost improve- ments for the end user.

The first phase of STATCOM development has pro- duced detailed designs for chain link modules and con- trol systems, which have been built and subjected to an extensive test programme confirming the initial design predictions.

Following successful completion of these design proving tests, work has now progressed to the imple- mentation phase with manufacture in progress for the U K s first STATCOM at East Claydon - a pioneering application to address the system needs of the 21st cen- tury.

8 Acknowledgments

The authors wish to acknowledge the permission of GEC ALSTHOM T&D Power Electronic Systems Limited to publish this paper and to thank the National Grid Company for allowing reference to their intentions at East Claydon.

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