RE A CTIVE COMPENSA TION

Shunt-connected reactors and capacitorscontrolled by thyristors

S. Torseng, Dr. Tech.

Indexing terms: Power systems and plant, Power system protection, Controllers

Abstract: Two basic schemes for thyristor-controlled static compensators are described, namely, thyristor-switched capacitors (TSC) and thyristor-controlled reactors (TCR). A more advanced scheme using acombination of TSC and TCR is presented. It is shown that this combination gives a greater degree offlexibility in the designing of a compensator. The paper also briefly describes the control system in acompensator comprising both thyristor-controlled reactors, thyristor-switched capacitors and reactors (TSR).One method to damp power oscillations, using TSC with a certain control strategy, is presented. The problemof unbalanced loads and load balancing methods using TSC and TCR are discussed. It is shown that thecombined system (TSC/TCR) generates very low harmonics, because the TSC does not generate anyharmonics. Finally, some pictures from the first installations comprising both TSC and TCR are shown.

1 Introduction

There are many reasons for applying shunt compensation inpower networks. Ordinary shunt reactors and shunt capacitorsare, to date, extensively used for this purpose. The powernetwork can be utilised more effectively if the reactive shuntpower can be controlled in an intelligent way.

The development of thyristor valves capable of handlinglarge currents, as well as the technique of using them to switchcapacitors in and out and control the current through areactor, have provided the power-system engineer with a newtool to meet reactive power generation and absorptiondemands. This new thyristor technique is today used in severalcompensators for industrial networks. A 60 MVAR compen-sator comprising thyristor-switched capacitors was installed asfar back as 1972, in Sweden, for compensation of arc furnaces.Growing interest is also being" shown in this technique fortransmission systems, and a few compensators of this type arenow in operation.

2 Thyristor-controlled static compensators

Two basic schemes for thyristor-controlled static compen-sators are now being used to provide a variable reactive power.In one scheme, reactive power is varied by the synchronous

mX X

1

yn steps

F ig. 1 Static compensator of thyristor-switched capacitor type

Paper 1612C ( P l l , P9), presented in original form at the IEE TechnicalSeminar on Control of Reactive Compensation for AC Power Systems,September 1980The author is with the Power System Consulting Department, ASEAAB, S-721 83 Vasteras, Sweden

switching of capacitor banks. In the other scheme, a fixedcapacitor bank is connected in parallel with a thyristor-controlled Variable' reactor. In most cases, a combination ofthese two schemes will be the best solution.

2.1 Thyristor-switched capacitorsFig. 1 shows the basic scheme of a static compensator of thethyristor-switched capacitor (TSC) type. The shunt-capacitorbank is split up into appropriately small steps, which areindividually switched in and out using bidirectional thyristorswitches. Each single-phase branch consists of two major parts,the capacitor C and the thyristor switch TY, see Fig. 2. Inaddition, there is a minor component, the reactor L, thepurpose of which is to limit the rate of rise of the currentthrough the thyristors and to prevent resonance with thenetwork.

Fig. 2 shows the switching of a capacitor at differentcapacitor voltages. A capacitor bank is switched in by applyinga gate trigger pulse to the thyristor switch, at the peak valueof the voltage U. This means that an inrush current will ariseif the capacitor is not fully charged. The highest current peakoccurs for a discharged capacitor. The series reactor limits theinrush current to a small value. Trigger pulses at the peakvalues of the voltage keep the capacitor switched in.

The capacitor is switched out through the suppression ofthe gate trigger pulses of the thyristors. This ensures that thethyristor carrying current will block as soon as the currentbecomes zero, i.e. at the peak voltage.

As the capacitors are provided with discharge resistors R,the capacitor in the stand-by state slowly loses its voltage.However, it is immediately ready for a new connection, even ifit has not been completely discharged.

Static compensators of the TSC type are characterised byhaving the following properties:

(a) stepwise control(b) average delay of one half-cycle (maximum one cycle) in

the execution of a command from the regulator, as seen for asingle phase

(c) very low inrush transients(d) no generation of harmonics(e) low losses at low-compensator reactive-power output.

The control system will order the capacitor bank to beswitched out when the voltage increases over a certain value.Unlike a breaker switched bank, the TSC can be switched inagain immediately, even though the capacitor is charged.Furthermore, the TSC valve can be switched an unlimitednumber of times without special maintenance. The TSC will

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not be exposed at all to such high-current transients, onswitching, as the breaker-switched capacitor. Therefore, thelife of the capacitor cans is expected to be longer than fornormal shunt capacitors. Experience so far, in this respect, hasbeen very good.

Fig. 3 shows the voltage at a busbar close to arc furnaces,without and with TSC. These recordings have been made atSvenskt Stal AB's Steelworks in Borlange, where a 60 MVARTSC has been in successful operation since 1972.

2.2 Thyristor-controlled reactorsFig. 4 shows the basic scheme of a static compensator of thethyristor (phase-angle) controlled reactor type (TCR). In most

Ic L

-0A5puOpu

Fig. 2 Operating principle of a thyristor-switched capacitor

20kv bus voltage

I unstabilised

stabilised

Is-/.

0 1 2 3 A 5 6

t ime, s

Fig. 3 Voltage on 20kV busbar feeding arc furnaces without andwith static compensator connected to busbar

cases, the compensator also includes a fixed capacitor. This isnot shown on the diagram. Each of the three phase branchesincludes an inductor L and a bidirectional thyristor switch TY,see Fig. 5.

Fig. 5 illustrates the operating principle. The current, andconsequently also the fundamental-frequency current compo-nent, is controlled by delaying the closing of the thyristorswitch with respect to the natural current zero passages.

The adjustment of the equivalent impedance, and thus thatof the reactor current, can only take place at discrete instantsof time, i.e. an adjustment cannot be made more often thanonce in each half cycle.

A^A^ 00

Fig. 4 Static compensator of thyristor-controlled reactor type

wjfiringpulses

/ \

\y \y

c

timeFig. 5 Operating principle of thyristor-controlled reactor

The technique of controlling the conduction intervals ofthe thyristor switch generates harmonic current components.

Static compensators of the TCR type are characterised byhaving:

(a) continuous control(b) maximum delay of one half cycle in the execution of a

command from the regulator, as seen for a single phase(c) practically no transients(d) generation of harmonics

2.3 Combined TSC and TCR systemIrrespective of the reactive control range required, any staticcompensator can be built up from one or both of these devicesand, if required, in combination with an ordinary shunt-capacitor bank or shunt reactor, fixed or breaker-switched.The optimum solution is a technical and an economic choice,which strongly depends on the cost evaluation of the losses.

In the switched-capacitor scheme the total reactive power issplit into an appropriate number of parallel-capacitor banks.The reactive power from the compensator follows the load orterminal voltage variations in a step-like manner. A continu-ously variable reactive power can be achieved by using athyristor-controlled reactor in combination with switchedcapacitor banks.

Fig. 6 shows a schematic diagram of a static compensator,with continuous control of the reactive power from — 1 p.u. to

IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981 367

+ 3 p.u. The losses are very small, especially at no load and atlow absorption of reactive power. The harmonic generationwill be low, because the controlled reactor is small comparedwith the total controlled power.

Fig. 6 Static compensator of combined TSC and TCR type

reactor current

^capacitor current

I total current

lagging current ' lead ing current

Fig. 7 Operating performance of compensator according to Fig. 6 forcontinuous change in control order from fully lagging to fully leadingcurrent

Fig. 7 illustrates the operating performance of the compen-sator according to Fig. 6, for a continuous change in thecontrol order from fully lagging to fully leading current. Theoperation of the controlled reactor is in perfect co-ordinationwith the switched-capacitor banks.

Static compensators of the combined TSC and TCR typeare characterised by:

(a) continuous control(b) practically no transients(c) low generation of harmonics(d) low losses(e) flexibility in control and operation

3 Control system

The control system in a thyristor-controlled static compen-sator offers excellent possibilities to influence and controlvarious parameters in a power system. Some of these possi-bilities are discussed below.

3.1 Voltage controlThe most common parameter to be controlled is thevoltage. Fig. 8 shows a block diagram of an automatic voltageregulator (AVR). The Figure represents the American electricpower (AEP) compensator in Beaver Creek, Kentucky; withthe exception of the slope arrangement, which is of differentdesign.

The main input to the regulator is the voltage U on thebusbar. The voltages in the three phases are fed, via PTs, to ameasuring device (MD), where they are converted to a low-voltage DC signal V. The measuring device contains, basically,a voltage adaptation unit, a six-pulse rectifer and a low-passfilter with a time constant of 1.5 ms to reduce fast transientsin the busbar voltage.

The voltage response V is fed to the summing junction ofthe regulator, together with the voltage reference Vref. If thevoltage response deviates from the reference, an error signalAV will appear. This signal is fed to an integration amplifier,with an output that will increase or decrease depending on thesign of AV, until AV is zero and V= Vref. The output from

i

|AVR

Fig. 8 Block diagram of control system

the integrator is the susceptance reference Bref for thecompensator, or static VAR system (SVS) as it is also called.Bref is decomposed in the distribution unit into three digitalsignals ('On' orders for TSC and TSR), and one analogue signal(control signal for TCR), in such a way that the effectivesusceptance of the SVS will correspond to the susceptancereference.

In most applications, it is also desirable to have a certainslope in the U/I characteristic, to prevent the compensatorfrom reaching the end positions too frequently. This can beachieved in different ways. One way, which is shown in theFig. 8, is to measure the SVS current and add a signal Vt inthe summing junction of the regulator proportional to thiscurrent.

Vj has different signs for leading and lagging current. It isadded to the summing junction of the regulator in such a waythat it corresponds to a decrease or an increase of thereference voltage for leading or lagging current, respectively.

The control system also contains a synchronising unit,which ensures the exact timing of the trigger pulses to thethyristors, as well as the sampling of regulator signals.

It is very important to be able to change both the referencevoltage and the slope of the U/I characteristic. From theabove, it is clear that these values can easily be changed to suitaltered conditions in the network.

Fig. 9 shows an example of the U/I characteristics. Thechosen curves both have a slope of 5 per cent. This is changedby just adjusting the gain in the measuring device (see Fig. 8).

Fig. 10 shows the behaviour on a voltage change within thecontrol range. This change was caused by the switching in of a

368 IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981

100 MVAR reactor to the busbar. It can be seen that the SVSresponds by switching in the second capacitor bank (62.5MVAR) at the same time as the current in the TCR isdecreased (37.5 MVAR). It should be noted that the currentscales in the three current oscillograms are not the same. TheFigure also shows that the total settling time, in this case, wasapproximately 2 cycles.

The notation in Fig. 10 is according to Fig. 8, wherephase A denotes the high-voltage side of the transformer andphase a the low-voltage side.

U.p.u.

1 . 0 1.0

Fig. 9 Example of U/l characteristics for S VS according to Fig. 8uref - °-90 pu , Uref = 1.05 pu , slope = 5%

U(phaseA)

Bref

1.0 2.0 3.0 4.0 5.0

x10 ms

Fig. 10 SVS response to 'large'voltage change

The response time depends on the gain setting in thecontrol system, and also on the network itself. The gain settingmust be matched with the short-circuit levels in the network.This can be seen from eqn. 1, which shows the voltage changeAU caused by a change of the reactive shunt power AQ.

AUU

AQ(1)

where Ssc is the short-circuit power (fault level) at the con-nection point. When setting the gain one must take intoaccount the lowest value of S8C that may exist. Otherwise,voltage instability may occur. The response time will, there-fore, be longer for a higher short-circuit power in the network.

As is well known, a line-to-earth fault will cause over-voltages in the healthy phases during the fault. Fixed orbreaker-switched capacitor banks will further increase thevoltage during the fault, thereby aggravating the situation. Ifundervoltage detection is introduced, an additional signal canbe added to the summing junction in the AVR in such a waythat Bref is forced to become fully lagging or forced down tozero, when the voltage in one or more phases falls below a

certain preset value. In this way, the overvoltages in thehealthy phases during the fault will be reduced, and there willalso be a smoother return to normal voltage after faultclearing. Because the system offers great flexibility, theundervoltage control strategy can be individually designed foreach installation, taking its specific features into account.

3.2 Damping of power oscillationsStability is one of the basic factors to be considered when onedetermines the power transfer capability of an AC trans-mission system. In many cases, the transient stability limit,accounting for both transitory and permanent primary faultson lines and in stations, is the critical factor. This then auto-matically results in sufficient margins to the steady-state andoscillatory stability limits.

However, in large power systems with long transmissiondistances, and especially systems also containing tie lines, thedamping of oscillations, due to disconnection of heavilyloaded lines after permanent faults or on tripping of largegenerating units, may be the determining factor for thepower-transfer capability, of certain networks sections.. Thyristor-switched shunt-capacitor banks and thyristor-

controlled shunt reactors will primarily influence the voltagein a local area. This means that loads connected close to suchan element and the power transfer between machines, can bechanged immediately. Through proper control of the reactivepower generated or absorbed by a shunt element, it is possibleto improve both the transient stability and the damping ofoscillating machines.

The capacitor/reactor should be controlled in such amanner that it always introduces damping power in the oscil-lating machines. This requires an appropriate control signalfrom the oscillation that is to be damped out. In most cases,line power transfers or busbar voltages can be used. Thecontrol system must be able to analyse the signal and orderswitching at the right oscillation instants. One method isdescribed below.

The hypothetical diagram in Fig. 11 illustrates the basicidea, where a simple two-machine network with intercon-nection is considered. A thyristor-controlied static compen-sator, comprising thyristor-switched capacitor banks TSC, isinstalled at busbar B. The control signal is the transfer power

When a power swing starts, i.e. the derivative dP/dt ispositive and exceeds a certain value, the TSC are switched inone step at a time every 0.04 s.

During the following oscillations one or more TSC aredisconnected after maximum P is reached and reconnectedafter minimum P. The number of banks which will be recon-

f A with TSC

\v.without TSC

TSC TSC TSCout in out

t ime

Fig. 11 Control method for damping of power oscillations

IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981 369

nected depends on the magnitude of the oscillation, i.e. thelatest peak-to-peak value. This control technique will improvethe damping. Studies have shown that the transient stabilitycan be slightly improved also if TSC are used.

3.3 Other control possibilitiesA thyristor-controlled static compensator can also be used toimprove the power factor in distribution systems. This meansthat the reactive power does not need to be transported overlong distances. The current will thus be lower with decreasinglosses as a result. Furthermore, this also means that the activepower transmission can be increased on existing distributionlines.

Because each phase of the SVS can be controlled individ-ually, it is very easy to use the thyristor-controlled compensatorto balance the voltage in a system where unbalanced loadsoccur. Such a load may be, for example, a train. This isdiscussed in Section 4.

It is very easy to make changes in the control system anddesign it to suit each individual installation. Other controlstrategies, not mentioned above, can also be used.

4 Balancing of unbalanced loads

4.1 GeneralSingle-phase loads connected between two phases are relativelycommon. Typical examples are arc furnaces, electric trains,welding machines, etc. This unbalance causes negative phase-sequence voltages in the network. The voltage unbalance isoften expressed as the ratio between the negative and thepositive phase-sequence voltage, giving

U- =\U.\ \Z\ \Z\

\U+\ \Z + ZL\ \ZL\(2)

if the negative and positive phase-sequence impedance areequal (Z_ = Z+ = Z), where ZL is the load impedance. Theabove relation can normally also be written as

(3)

where Ssc is the short-circuit power at the connecting pointand SL is the rated power of the load.

There are several disadvantages with the voltage unbalance.The occurrence of negative phase-sequence current means thatthe losses in the network are doubled, and that certain phaseand phase-to-phase voltages are changed more on load con-nection than if the load had been balanced on all three phases.

There will be an increased and uneven heating of theconductors in the network and of motors and apparatusconnected to it. Induction motors are particularly sensitive.

In IEC Publication 34-1, 'Rotating electrical machines', it ismentioned in paragraph 12.1 that motors shall be capable ofwithstanding an unbalance of

Now, if it is desired to connect a certain single-phase loadto the network, there must be a certain fault level on thebusbar, as described above. If this is not sufficient, the loadmust be balanced.

4.2 Load balancingAccording to the above, it is sometimes necessary to balance asingle-phase load. This can be performed with reactors and/orcapacitors. The reactive loads shall be connected between thephases so that the generated negative phase-sequence voltagewill compensate the negative phase-sequence voltage caused bythe single-phase load.

Fig. \2a shows a resistive load between two phases (S andT) and the corresponding positive and negative phase-sequencecurrents. With a capacitor between R and T and a reactorbetween R and S, the corresponding current components areobtained according to Figs. 12b and c. It is clear from theseFigures that if the reactor and the capacitor are equal, andtheir impedances are equal to \f$R, then the negative phase-sequence currents are eliminated, see Fig. \2d. The balancedload in Fig. lid can, of course, be balanced in many differentways by adding or deleting capacitative or inductive load inall three phases symmetrically. It is thus possible to balancethe system with only capacitors or only reactors. If onlycapacitors are used, there will also be a contribution to thephase compensation in the system.

If the single-phase load consists not only of active powerbut also of reactive power, the reactive part can be compen-sated with a capacitor connected in parallel with the load.

Because many loads are varying, it is of great interest tobe able to vary and control the balancing reactive elements.

load positive phasesequence currents

negative phasesequence currents

l S

\U-\..= 0.02

An unbalance of 0.02 is achieved if SL = 0.02 x Ssc.According to British recommendations [6] the following

applies: : .

SL < 0.013 Ssc for voltages below 33 kV

SL < 0.01 Ssc for voltages of 33 kV and above

These lower factors have been selected, because severalindependent loads may be accumulated. In addition, harmonicsgenerally occur at the same time and these also increase thelosses in induction motors.

T R S

Fig. 12 Balancing of single-phase load Xc = XL = y/3R for a bal-anced system (d)

370 IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981

By using a thyristor-controlled static compensator withindividual control of each phase we can balance a varyingload. Fig. 13 shows the single-line diagram for a compensatorused for this purpose.

This compensator is connected to a 132 kV system in SouthAfrica. It belongs to ESCOM, and has been in successful

132kV

5MVAR 5MVAR 5MVAR 5MVAR 5MVAR 5MVAR

Fig. 13 Single-line diagram ofFerrum SVS

The main components are the 20 MVA step-down transformer, the twoS MVAR TCR-branches (TCR = thyristor controlled reactor) and thefour 5 MVAR TSC-branches (TSC = thyristor switched capacitor) Thethyristor valves VI and V2, V3 and V4 and V5 and V6 are located inthe same convertor cubicles

1 .0

Fig. 14

100 120 140control angle , deg

Harmonic currents generated by TCR

160 180

fundamental5 th harmonic

3rd harmonic7th harmonic

TSC TSC

Fig. 15 Static compensator with 12-pulse control

1EEPR0C, Vol. 128, Pt. C, No. 6, NOVEMBER 1981

T

in «:• IJ&V j . i s •'

... ^^-mm

Fig. 16 ESCOM compensator for balancing of unbalanced loads

operation since May 1979. The single-phase loads, in this case,are trains connected between two phases.

The regulator, in this case, is a voltage regulator for single-phase control. The phase-to-phase voltages are measuredindividually and compared with a reference voltage. Equationsfor the needed reactive power between the phases, can becalculated as a function of AURS, AUST and AUTR, whereAURS

= Uref — URS etc. This means that it is possible tomeasure the phase-to-phase voltages during balancing of thesingle-phase load.

5 Harmonics

A thyristor-switched capacitor bank (TSC) will not generateany harmonics, because it is not phase angle controlled, butonly switched in or out. A thyristor-controlled reactor (TCR)will generate harmonics of odd order, if it is phase anglecontrolled. Fig. 14 shows the harmonics, in p.u., of themaximum reactor current (90° control angle) against thecontrol angle for the thyristors. The generated harmonics 3rd,5th, 7th, 9th, 11th and 13th have the following maximumamplitudes: 13.8%, 5%, 2.5%, 1.6%, 1.0% and 0.7%. Theharmonics of zero-sequence character, i.e., 3rd, 9th, etc., areeliminated with a delta connection and three-phase sym-metrical control. The amplitude of the harmonics is propor-tional to the size of the reactor.

A compensator comprising a TCR (with 6-pulse control)often implies that a filter has to be installed. The need forfilters should be studied for each case.

Let us look at the compensator in Fig. 8. This has aregulation range from - 125 to + 125 MVAR, i.e. 250 MVARcontrolled range. The technique of using both TSC and TCR(ASEA design) means that, in this case, only one fourth (onereactor) of the total regulation range of 250 MVAR is phaseangle controlled. Therefore, the harmonic currents generatedby this reactor amount to only one fourth of what is generatedif no TSC is used, but the whole control is in one 250 MVARreactor.

One way of reducing the harmonics still further, withoutusing filters, is to adopt 12-pulse control, see Fig. 15. Thismeans that the 5th and the 7th harmonics are eliminated too.

6 Installations

The first static compensator in the world, using both TSC andTCR, was the ESCOM compensator connected to the ESCOM132 kV system to balance the voltage, see Section 4. Thiscompensator has single-phase control. All three phases areidentical and the regulation range is from 20 MVAR capaci-tive to 10 MVAR inductive (three-phase power). The SVScomprises two thyristor-controlled reactors, each rated5 MVAR, and four thyristor-switched capacitors, each rated5 MVAR. Figs. 16-19 show different views of the ESCOM

371

Fig. 17 ESCOM compensator

Centre: 5 MVAR TSC; right: three-phase stacked damping reactor

Fig. 18 Three single-phase TCR 5/3 MVAR each

The reactors are of air core type[ESCOM]

Fig. 20 Three-phase stacked water-cooled thyristor valve for AEP{Beaver Creek, Kentucky)

This valve controls one 62.5 MVAR TCR (see Fig. 21)

Fig. 19 Cubicle housing two three-phase, air-cooled thyristor valves

[ESCOM]

Fig. 21 62.5 MVAR reactor controlled by thyristor valve in Fig. 20

Two stacked reactors are connected in each delta branch. The valve iselectrically connected between the two reactors.

compensator. The thyristor valves are, in this case, air cooled,owing to the small size of the compensator.

Fig. 20 shows a water-cooled thyristor valve in theAmerican electric power (AEP) compensator, situated inBeaver Creek, Kentucky. This valve controls one of the twothyristor-controlled reactors, each rated 62.5 MVAR, seeFig. 21. The compensator also comprises two TSC, each rated62.5 MVAR. The total thyristor-controlled range is thus250 MVA. The single-line diagram is as shown in Fig. 8. Boththe reactors and the capacitors are connected in delta.

A thyristor with a silicon wafer area of 45 cm2 is shown inFig. 22. Using this thyristor in a valve makes it possible to

372 IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981

Fig. 22 Thyristor with silicon wafer area of 45 cm2

handle currents exceeding 3000 A, without parallel connectionof thyristors being necessary. A complete thyristor is designedas a stack of discs of suitable material, next to the siliconwafer in order to reduce the thermal expansion.

7 Conclusion

Thyristor-controlled static compensators provide the systemengineer with a new tool to meet reactive power generationand absorption demands. It can be used for different appli-cations such as, voltage control, voltage balancing and stabilityimprovement. The compensator can be used with any systemvoltage, and is normally connected via a transformer. It willbe frequently employed in industrial, distribution and trans-mission systems.

A 60 MVAR compensator, comprising thyristor-switchedcapacitors, has been in successful operation in Sweden since1972 for compensation of arc furnaces. In transmissionsystems, there are, so far, only a few static compensators inoperation; but several are under manufacture. The combinedTSC/TCR compensator offers several technical advantages,such as flexible design and low losses. Service experience fromsuch compensators in transmission systems is reported verygood.

A static compensator can comprise both thyristor-controlledand breaker-operated reactive-power elements. The control ofthese breakers is often incorporated in the SVS regulator.

8 References

1 ENGBERG, K., FRANK, H., and TORSENG, S.: 'Reactors andcapacitors controlled by thyristors for optimum power-system varcontrol'. Paper presented at EPRI seminar on transmission static varsystems, Oct. 1978

2 ENGBERG, K. and IVNER, S.: 'Static var systems for voltagecontrol during steady-state and transient conditions'. Paperpresented at EPRI/Hydo-Quebec seminar on controlled reactivecompensation, Sept. 1979

3 OLWEGARD, A., AHLGREN, L., and FRANK, H.: Thyristor-controlled shunt capacitors for improving system stability'. CIGRE,Paper 32-20, 1976

4 TORSENG, S.: Thyristor-switched capacitors'. Paper presented atIEE seminar on control of reactive compensation for AC powersystems, London, Sept., 1980

5 ENGBERG, K., HERMANSSON, L., and TORSENG, S.: 'Experienceand position report on thyristor-controlled capacitors and reactorsin transmission systems'. Paper presented at IEEE conference on'Overvoltages and compensation on integrated AC-DC systems',Winnipeg, Canada, July 1980

6 Engineering Recommendation P16. Electricity Council, London,June 1975

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