IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 8 August 1981

IMPROVEMENT OF TRANSMISSION CAPACITY BY THYRISTOR CONTROLLED REACTIVE POWER

A 01wegard K Wal ve G Waglund

Swedish State Power BoardSweden

Abstract

This paper shows that thyristor controlled reac-tive power can be used as an efficient tool to improvedamping of large power systems. If thyristor switchedcapacitors are used there will also be positive con-tribution to transient stability. Examples of powersystems where damping is the critical factor have beeninvestigated. The use of supplementary control ofgenerators and control of reactive power has beenstudied. Basic configurations of thyristor switchedcapacitors and thyristor controlled reactors arepresented.

INTRODUCTION

Damping of power oscillations is a basic factorwhen determining the transfer capacity of a powersystem. Primarily, the system must have positivedamping under normal operation. Furthermore, thedamping must also be sufficient to bring the powersystem back to stable operation after network faults.

In many cases damping is not critical. Thermallimits or transient stability might be dimensioning.This paper, however, will concentrate on damping andhow it can be improved. Examples of networks will beshown where damping is critical.

The interconnected Nordel system consi'sts of thenational systems of Sweden, Finland, Norway and Denmarkwith an installed capacity of 60,000 MVA. The commonNordel planning criteria demands that the networkshould withstand certain types of faults (single poleline faults, busbar faults, generator tripping etc.)without loss of stability or load shedding. That meansthat both damping and transient stability must be suf-ficient.

In some stages of the expanding Nordel power systemdamping has been a critical factor. In the 60's thetie-lines were often comparatively weak and the dampingwas bad. Even a small disturbance could create undampedoscillations. Now the ties have become much stronger.Still damping appears to be critical in the near futu-re as large units are installed in many regions withoutcorresponding expansion of the network. After se-vere network faults the damping between distant re-gions of the system will be insufficient if no special

81 WM 092-6 A paper repommended and approved by theIEEE Power System Engineering Comittee of the IEEE

Power Engineering Society for presentation at the

IEEE PES Winter Meeting, Atlanta, Georgia, February

1-6, 1981. Manuscript submitted September 2, 1980;made available for prihting November 14, 1980.

H Frank S Torseng

A S E A ABSweden

measures are taken. This was experienced in March 80when large oscillations appeared in the Nordel powersystem on a special occasion when some important trunklines were out of service and the transfers in someparts of the system were high.

tmthermal limit

transient stabilitylimit

_r _ m-_ _ v_ !s -idamping limit

Fig. 1. Transfer capacity limit.

When damping is the critical factor the powertransfer limit is determined as illustrated in Fig. 1.The question then arises:Which measures can be taken toimprove the damping of the system? The concept of thispaper is as follows:

- Damping can often be improved by control methods,without erecting new lines,

- Such methods could be supplementary control sig-nals in the excitation system of generators. Such tech-nique is already used in the Nordel system,

- The thyristor technique enables large capacitorbanks and reactors to be controlled. They can be usedas an efficient tool to improve damping of the powersystem. Even some positive contribution to transientstability can be achieved.

BASIC CONCEPTS OF THYRISTOR CONTROLLEDREACTIVE POWER DEVICES

The basic concepts of thyristor controlled ractivepower devices are:

- thyristor-switched shunt capacitors (TSC)- thyristor controlled shunt reactors (TCR)- thyristor-switched series capacitors

1981 IEEE

3930

3931Thyristor-switched shunt capacitors

The basic idea of thyristor-switched shunt capaci-tors is to split up a capacitor bank into sufficientlysmall capacitor steps and switch these steps on and offindividually, using anti-paral'lel-connected thyri storsas switching elements. Fig. 2 shows the basic scheme.

The TSC are characterized by:

- Stepwise control

- Average one half-cycle (max one-cycle) delay forexecuting a conmand from the regulator; as seenfor a single phase.

- Practically no transients

- No generation of harmonics

Fig. 3. Thyristor-controlled shunt reactor type.

The TCR are characterized by:

- Continuous control

- Maximum one half-cycle delaycommand from the regulator; asphase.

for executing aseen for a single

Arrangements

For many applications, a thyristor controlledshunt reactive power device built up with a few largesteps of thyristor-switched capacitors and one or twothyristor-controlled reactors seems attractive, Fig. 4.It combines favourable properties of the two thyristorschemes discussed above. It provides continuously vari-able reactive output from full lagging to full leadingcurrent. It has a high response and reduced harmonicgeneration.

Fig. 2. Thyristor-switched shunt capacitor type.

Thyristor-controlled shunt reactors

The basic idea of thyristor-controlled shuntreactor is to control the fundmental-frequency currentcomponent through the reactor by delaying the closingof the thyristor switch with respect to the naturalzero passages of the current. Fig. 3 shows the basicscheme. Harmonic currents are generated from the phase-angle controlled reactor. The magnitude of the harmo-nics generated can be reduced by two methods. In onemethod the reactor is split into small reactor stepswhile' only one reactor step is phase-angle controlled.The other reactor steps are either on or off. In thisway the amplitude of every harmonic is reduced.

The other method is the so-called twelve-pulsearrangement where two identical connected thyristor-controlled reactors are used, one operated from a wyeconnected secondary winding, the other from a deltaconnected winding of a step-up transformer.

[AVRi11

Fig. 4. Combined TSC/TCR type.

L4j

3932

Thyristor-switched series capacitors

The basic idea of a thyristor-switched series ca-pacitor is to switch on or off the capacitors by usinganti-parallel connected thyristors as switching ele-ments in parallel with the capacitors. Normally theseries capacitors are split up into two or more stepsas is the case for a shunt capacitor. Fig. 5 showsthe basic scheme.

The switching on of a series capacitor is accomp-lished by suppression of the firing pulses to the anti-parallell thyristors so that the thyristors will blockas soon as the current becomes zero. In principle, thecapacitor will now be charqed to the double peak vol-tage. The switching off instant of the capacitor is se-lected at the time when the capacitor voltage is zero.

Fig. 5. Thyristor-switched series capacitor type.

Step 3. The capacitor/reactor should be controlledin such a manner that it always introduces dampingpower on the oscillating machines. This requires anappropriate control signal from the oscillation thatis to be damped out. In most cases, line power trans-fers or bus voltages may be used.

Improvement of damping may be expressed by MW/Mvarwhich should be interpreted as the extra power transferthat could be accepted until the limit of damping isreached. Of course, the figures, MW/Mvar, may varydepending on whether a cornplete network or disturbednetwork with lines etc. out of service is considered.

Control method of TSC/TCR

How should the TSC/TCR be controlled to achievethe result as explained above? At first an appropri-ate control signal must be used that describes the be-haviour of the system. The control method must be ableto analyse the signal and order switchings at the rightmoments of oscillation. A method is described below.

The principal idea is illustrated by a few fig-ures. Consider a very simple two machine network withinterconnection, Fig. 6. A controllable capacitor bank,TSC, is installed at bus B. It is presumed to be con-trolled in 3 steps. Control signal is transfer powerpL

GENA AGj

The thyristor controlled reactive power device hasnot ruled out the synchronous condenser, but it is al-ready a severe competitor, in terms of benefits incost, performance and reliability.

IMPROVEMENT OF DAMPING BY THYRISTOR CONTROLLEDCAPACITORS/REACTORS

Small damping power

In a large interconnected power system, e.g.50,000 MVA installed, osci11ations after disturbancesmay be in the order 1,000 MW peak to peak. The dampingpower contributions from generator damper windings,governors and voltage control are much smaller. Intotal, sufficient damping power may be in the order of50-100 MW peak to peak. Thus, compared with total capa-city of the network only small amounts of dampingpower must be created.

How damping is achieved by TSC/TCR

Coupling of reactive elements will influence thevoltage and thus the generators of the system. In prin-ciple damping contributions are achieved by TSC/TCR asfollows.

Step 1. Switching of a capacitor/reactor willinstantaneously affect the voltage of the system.

Step 2. The electrical outputs of the generatorsare then immediately changed in two ways, either di-rectly as the voltage of transfer network is changed(compare expression P = E E2 sin P/X in a two ge-nerator network), or seconaarily because the loads areaffected by the voltage change. In the latter case thegenerator electrical output will be most affected formachines which are close to the varying loads.

D GEN EPpL

ROTOR ANGLETA-VE

IF

Fig. 6. Principles of control method.

Mode 1. Normally, when only small oscillationsoccur the capacitor bank is only switched according tothe voltage VB i.e. it serves as a slow reactiveback-up.

Mode 2. When a power swing starts i.e. the deriva-tive dP/dt is positive and beyond a certain value D2the capacitor is connec"ted by a step every 0.04 seconds.

3933Mode 3. During the following oscillations one or

several capacitor steps are disconnected after P maxi-mum is reached and reconnected after P minimum. To makesure that the extreme value has been passed, a condi-tion, IdP/dtl > D3 may be used. The number of stepsthat will be reconnected will depend on the magnitudeof the oscillation i.e. the latest peak to peak value.By this coupling sequence damping is improved.

Mode 4. In order to avoid unjustified operation ofthe TSC at disturbances of the input signal, specialfiltering has to be made. For example, too large deri-vatives, IdP/dtl > D1, may be neglected.

By the described method damping can be improved(mode 2 and 3). Transient stability will also beslightly improved if capacitors are used.

The control of a reactor, TCR, will be similarin so far as connection of a reactor is equivalent todisconnection of a capacitor.

Controlled series capacitors

The basic idea of the control method is in manyways the same as that of shunt capacitors. In order toincrease the transient stability, the full bank shouldbe connected during the first power swing of P.During the oscillations switchings should be made alterextreme values of the line power. That means, that thecontrol method may be described as a continuousbangbang method.

NETWORK EXAMPLES

Two types of networks are presented where thyris-tor controlled capacitors/reactors may be used to im-prove transmission capacity. The networks are chosenfrom the development of the Swedish and the Nordel net-work, although the studies have been rather conceptu-al and not limited by actual plans.

The case 1 network is a typical interconnectedpower system with weak ties compared with the powerinstalled. The case 2 network is a meshed and a longdistance transfer network.

The calculations have been performed by a tran-sient stability computer program. The control methodwhich in practice is handled by a online mini computerhas been represented by a certain subroutine in theprogram.

1. Interconnected power system with weak tie-lines

The Nordel system in late 1960's had comparativelyweak ties. The central Swedish system was on the otherhand relatively well meshed. The qualities of the totalsystem are illustrated by a simplified reactance dia-gram of the network, Fig. 7. In this system the transi-ent stability and damping of the tie-lines in southernNorway-Sweden and Zealand-Sweden were limited.

The tie-line to southern Norway is a 400 kV line.In the studied case the damping is very weak. Steadystate limit of the undisturbed network is about 300-400 MW. If a load of 140 MW is disconnected in Norwayundamped oscillations start as shown in Fig. 8 atpower transfers of about 300-400 MW. Oscillation fre-quency is about 0.35 Hz.

If a 300 Mvar TSC in 10 steps is installed inNorway close to the tie-line, the damping can be im-proved considerably as shown in fig. 9. About 300 MWextra power can be transferred i.e. the improvement isabout 1 MW/Mvar. Transient stability limit with respect

Fig. 7. Simplified reactance diagram of the case 1network (earlier stage of the Nordel network).

a.

b.

c.

Fig. 8. Oscillations after 140 MW load tripping inNorwaya) tieline power b) voltage at tieline endc) rotor angle of a generator in Norway rela-tive to northern Sweden.

to severe transient faults is also improved, but toless extent or about 0.3 MW/Mvar.

To illustrate the importance of finding the opti-mal location of the TSC sqme special calculations havebeen made on a simplified network. A capacitor step of20 Mvar has been switched at various locations alongthe tie-line or in the meshed networks. The increase ofthe electrical output from the generator groups isshown in Fig. 10. If the inertia of the groups is con-sidered, the relative acceleration of the Norwegianagainst the Swedish machine groups may be calculatedas in Fig. 11. It shows, for example, that TSC/TCR in-stalled at D are about 2-3 times as efficient as inst-alled at F.

3934

Switching of a capacitor step will influence thegenerator outputs in mainly two ways as was toldearlier, either directly by affecting the transferpower or indirectly by changing the loads. Calculationson the simplified network show that almost 50 % of theeffect on the oscillating generators may be derivedfrom the voltage dependence of loads.

0'

a.

MW

600

400

kV

b *00b.

3060

*o

c.

20

Mor300

d..I

5 10 15 S

S 0 15 S

I 10 15 S

I

Relative accelerationNorway-Sweden

High transferSweden to Norway

2 /HIgh transfer/ Norway

AB C D E F G H

Fig. 11 Relative instantaneous acceleration ofNorwegian generators versus Swedish, ifa 20 Mvar capacitor is connected to busA, B etc.

5 1O 1S S

Fig. 9. Oscillations after 140 MW load tripping in

Norway with TSC (300 Mvar) in Norway close totielinea) tieline power b) voltage at tieline endc) rotor angle of a generator in Norway rela-tive to northern Sweden d) reactive power

Norway SwedenTieline G

C D E F

275kV 400kV

MW

Total

Norway

RJ#/1 Sweden

~~~~~0

,+'

AAB C D E F G' H

Fig. 10. Instantaneous change of generator output when

a 20 Mvar capacitor is connected to bus A, Betc.

2. Network with distant transfers

A future Swedish network was studied that is pre-sumed to be disconnected from the rest of the neigh-bouring countries. As this is normally not the case,the assumption is somewhat academic. Furthermore,some extra power generation is presumed in the norththat is not actually planned.

In Fig. 12 a network scheme and a simplified reac-tance diagram are shown. The transfer capability fromnorth (A) to the central part of the system (E+F) havebeen studied. In case of a network fault in the Dregion the main oscillations will take place between Aand D+E. Studies show that in case of minor transientfaults, such as single pole line faults, damping willbe the critical factor if it is not improved by anymeans. Fig. 13 shows the behaviour of the network.

A large amount of power system stabilizers (PSS)will be very useful to improve the damping. They shouldpreferably be installed in A.

With TSC damping will be improved considerably cor-responding to higher power transfers of 3-4 MW/Mvar atthe best. But if supplementary control of the voltageregulators is presumed as the basic mean to improvedamping, the TSC is estimated to improve dampingfurthermore by some 1-2 MW/Mvar. The increase oftransient stability is estimated to be about 0.2-0.4 MW/Mvar.

The location of the TSC should be made somewherebetween A and D+E. Preferably, they should be locatedcloser to A. In order to be beneficial to transientstability, location in a weak section of the network isrecommended. Location at B or C would be advantageous.

In Fig. 14 calculations are shown in case 'a com-bination of TSC and TCR is used and located in regionC. The bus voltage is used as a control signal becauseit shows the oscillation between the main parts of thesystem. The power PL of a single line contains,oscil-lations of local machines and is unsuitable.

00

J-

.. r

3935

a. A

< 1 t~~~~~~~~~~~~23 4 s 6 7 8 S

U kV

b. o-.

H ~~~~~~~~~PiMW

C.~~~~~~~~~~~-

Fig. 12. Simplified reactance diagram of the case 2 0 Mvarnetwork (future Sweden without ties).

d.

A1N | D I U 1S~~~~135 6 8 S100 ~~~~~~~~D

Ea. 0

Fig. 14. Oscillations after line fault in region D/>t\2zs \ 1/ t with TSC + TCR in region C (300 Mvar +

2 3 49 6 >S300 Mvar)a) rotor angle b) voltage in region A(dotted) and C c) line transfersd) reactive power

U kV

/ < / \Controllable series capacitors have also beenb. studied. The capacitors on all major lines from D to

F+E have been assumed to have 1/3 of the banks control-lable. The results show a considerable effect on damp-ing. Still it has been difficult to find a good control

Pd SMW r\ ' method. If the capacitors are controlled by the localline power the various lines and capacitors will dis-turb each other. A common control signal would avoid

C,- < ,~\ 1 that problem and the improvement of damping would be asgood as that from shunt capacitors expressed in

A-BA MW/Mvar. However, the transmission of such a controlsignal to the banks is an unrealistic solution. More

c. practical would be to control a few major lines, 3 outof 7, which would have large controllable stand-by

D--E capacitors.|Fittous powercaJculated from The transient stability can be improved much more

I\ / /Iv\toltagevarlatlon. efficiently by series capacitors than by shunt ca-

!' \so/ \ 0pacitors provided they are in stand-by operation. Theswitchable series capacitors need only to be dimensi-

1__- oned for full load during periods of a few minutes.0 5 10 -SChoice of location, capacity and control of TSC/TCR

Fig. 13. Oscillations after line fault in region D The location of TSC/TCR in a network is normallywithout PSS or TSC/TCR determined by dynamic stability calculations, but somea) rotor angles b) voltage in region C general conclusions can be made from the studies refer-c) line transfers. red to in this paper.

3936

In networks with weak tie-lines the optimal loca-tion is close to the tie-line preferably within thesmallest of the two interconnected systems.

In a meshed network with a group of poorly dampedmachines connected to long transmission lines the bestlocation of TSC/TCR is in the transmission section butclose to the group of oscillating machines. It is ad-visable to divide the TSC/TCR-units into two groupsconnected to separate busbars. If there is a load areaclose to the location of TSC/TCR the control will beeven more efficient.

The best effect on transient stability is when TSC(not TCR) is installed in a part of the transmissionsystem where the busvoltage is sensitive to networkdi sturbances.

Typical capacity of TSC/TCR is 100-400 Mvar. WhenTSC is used it should be divided into 3-5 steps.

Control signals

The control signal to TSC/TCR should be derivedfrom the main grid. In case of tie-line problems theactive power transfer of the tie-line is often suitab-le. In a meshed system it is more complicated. The busvoltage will follow the main oscillation of the system.The sum of the active power on some trunk lines mayalso be used. In case the voltage is used extra caremust be taken to avoid feed back from the capacitorswitching or reactor control.

Other factors

In general subsynchronous resonance, SSR, betweenlarge turbine-generators and the network may occur un-der some special conditions. Our opinion is that thecontrol method of TSC and TCR will not cause any riskof SSR. There is only some risk when thyristor switchedseries-capacitors are used. Switching of those willchange resonance frequencies of the power system whichmight cause resonance with turbine-generators.

COMPARISON BETWEEN DIFFERENT MEANS TO IMPROVE STABILITY

Extension of network by erecting new lines isoften the natural way to increase the transfer ca-pacity. But economics and environmental considerationsmake it very important to find other means. In casestability, expecially damping, is the limiting factorsome methods can be found as shown in table 1.

Method Improvement ofTransient Dampingstability

Power system stabilizer,PSS 0 or (-)+++

Synchr. comp (normalcontrol) + 0 or +

Synchr. comp (specialcontrol + ++

TSC + ++++

TCR 0 +++

Series capacitor (static) ++ +

Series capacitor(thyr. switched) ++ +++

Table 1

Power system stabilizer (PSS) produces a sup-plementary signal to the voltage regulators, and damp-ing power is created in the generator output. Thisdevice is comparatively cheap and should be used as abasic method to improve damping of oscillations betweenmachines or groups of machines. Oscillation frequenciesmay be from 1 Hz to 0.2 Hz although the lowest fre-quencies are more difficult to control by PSS.

Synchronous compensators are mainly used to con-trol the vo tage level both at normal operation of thenetwork and at disturbances. However, special controlof such compensator by PSS has been studied. In thiscase PSS was controlled by a tie-line power. Because ofthe large internal time constants the rapid response ofthe compensator is limited. Such special control of asynchronous compensator could be advisable if the com-pensator is considered economical from other reasons.If its primary use is to improve damping, static com-pensators should be considered.

Thyri stor controlled capaci tors/reactors (TSC/TCR)have been discussed very much above. They should beconsidered if damping is the major problem. Capacitorswill also improve transient stability to some extent.

Thyristor controlled series capacitors should beconsidered if both damping and transient stability areto be improved. Furthermore, their use is most naturalif static banks of series capacitors are already in-stalled or planned.

CONCLUSIONS

This paper has treated power systems, where damp-ing is the critical factor for transfer capacity.Examples have been taken from the Nordel and theSwedish power systems.

Applications of controlled reactive power in largepower system to improve damping and transient stabi-lity have been studied. Comparison with othermethods has been discussed.

The conclusions could be made as follows:

- If expansion of a network by new lines is excludedthe only measures to improve damping seem to be supple-mentary control of generators combined with thyristorcontrol of reactive power.

- The basic means to improve damping of a power sys-tem is the use of supplemantary signals to the genera-tor voltage control, also called power system stabiliz-ers, PSS. But its contribution is limited with respectto frequency range.

- Thyristor controlled capacitors and reactors,TSC/TCR, can be a very good method to improve dampingboth in power systems with weak tie-lines and in longdistance transfer networks. Studies indicate an im-provement of 1-2 MW/Mvar, interpreted as extra MWtransfer that could be permitted due to damping perinstalled Mvar of TSC/TCR.

- Thyristor switched capacitors, TSC, will also givepositive contribution to transient stability. Improve-ment could be 0.2-0.3 MW/Mvar at severe transientfaul ts.

- Thyristor switched series capacitors can improveboth damping and transient stability. However, specialattention has to be paid to the control strategy tomake sure that the control due to damping will beefficient.

REFERENCES

1 A. Olwegard, L. Ahlgren, H. Frank, "Thyristor-controlled shunt capacitors for improving systemstability", CIGRE Paper No. 32-20, 1976.

2 H. Frank, T. Pettersson, "Thyristor-switched shuntcapacitors and their modeling for transmissionapplications", Paper A 78 105-9, presented at IEEEWinter Power Meeting, 1977-.

3 K. Engberg, H. Frank, S. Torseng, "Reactors andcapacitors controlled by thyristors for optimumpower system control", presented at EPRI Seminaron Transmission Static Var Systems, Oct. 24-25,1978.

4 K. Walve, "Reactive power and voltage regulationin the Swedish high voltage system", CIGRE PaperNo. 31-08, 1980.

5 L. Gyugyi, R.A. Otto, T.H. Putman, "Principles andapplications of static thyristor-controlled shuntcompensators", Paper F 78 096-0, presented at IEEEWinter Power MetiT977

6 L. Gyugyi, E.R. Taylor, Jr, "Characteristics ofstatic thyristor-controlled shunt compensators forpower transmission system applications", PaperF 80 206-0, presented at the IEEE PES WinterMeeting, 1980.

Discussion

R. Gutman and B. M. Pasternack, (American Electric Power ServiceCorporation, Columbus, OH): The authors have described an in-teresting and timely application of thyristor-controlled reactive powerdevices for the improvement of transmission system capability. Thepremise on which this application is founded is the well known relation-ship between transmission system transfer capability and planningcriteria, particularly the margin of stability, selected for a given powersystem. This key relationship is presented in Figure 1 of the paper, but itis left vaguely defined in terms of the following three factors: 1) thermallimit, 2) transient stability limit, and 3) damping limit. It would behelpful if the authors expanded on their notion of damping and tran-sient stability limits and the difference between the two limits from theviewpoint of transmission capability. Perhaps, the authors could alsocomment on the differences between their enumerated criteria and thecriteria set forth earlier by R. D. Dunlop, R. Gutman, and P. P.MarchenkoI'I

R. D. Dunlop, et al, has shown analytically that the load-carryingability of an EHV transmission line, or "loadability," is limited by oneof two basic performance criteria -- line voltage drop or stability margin-- depending on line length. For moderate-length lines of up to 200miles, see Figure A, the voltage drop limitation is normally the controll-ing factor on line loadability, while longer lines are stability-limited.The cross-over point can vary up or down relative to the level of accep-table voltabe drop and/or stability margin, both of which reflect the in-dividual judgment of a given system planning organization with regardto planning criteria and desired level of operating realibility. The ther-mal limitation is significant (from the loadability viewpoint) only forvery short lines strung with non-bundled phase conductors, typicallyoperating at below EHV levels.

Since the method of improving transmission loadability by means ofincreased damping is based on the assumption that such damping mightnot be adequate to restore stable system operation following a distur-bance, the application of this method will inherently be aimed at onlythose lines whose loadability is limited by the stability constraint.' Thisrequirement narrows down the application of the described method toareas characterized by long lines with no intermediate switching stations-- a condition not often encountered in a well interconnected systemserving many distrubuted load centers.To insure that the improvement in transmission loadability does not

come at the expense of increased risk of system instability, this type ofdevice must operate in perfect harmony with the system it is designed to

3937

stabilize, or the opposite effect of an aggravated disturbance will result.Such a device must also be highly reliable, as the reduced margin ofstability permitted by its proper operation will most likely be inadequateshould a system disturbance occur at the time of a sudden and unan-ticipated outage of the device. These considerations require not only a"fail safe" operation of the device but, in bulk transmission system ap-plications, also necessitate the establishment of a rapidly-actuatedspecial operating procedure to restore an acceptable margin of stability.

It is important to keep in mind that the proposed device does not addany new transfer capacity to the existing power system, but rather,through special control means, makes it possible to reduce the stabilityoperating margin, thereby, allowing higher loading levels on thestability-constrained transmission lines. Clearly, the application of suchdevices on the power system is not in any way a substitution for thecarefully planned network expansion; instead, it should be viewed as auseful tool capable of providing greater flexibilities in planning and tim-ing new transmission facility additions.

REFERENCE

[1] R. D. Dunlop, R. Gutman, and P. P. Marchenko, "AnalyticalDevelopment of Loadability Characteristics for EHV and UHVTransmission Lines." IEEE Transactions on Power Apparatus andSystems, Vol. PAS-98, No. 2, pp. 606-617, March/April 1979.

Manuscript received February 25, 1981.

C. W. Taylor (Bonneville Power Administration, Portland, OR): Theauthors have presented interesting concepts for stability enhancementby controlled reactive power.

Several mechanisms by which shunt compensation switching can im-prove stability are described. Another mechanism is by reduction of ef-fective transfer reactance [1]. This method is most beneficial at elec-trical centers and can improve both synchronizing and damping tor-ques. Have the authors considered this mechanism? Could the authorsindicate the electrical center locations for the networks of Figures 10and 12?The authors propose control based on the derivative of tie line power.

A tie line current rather than tie line power signal would be less sensitiveto changes in predisturbance tie line power level or operating point [2,31.The networks shown in Figures 7 and 10 are of interest to us because

the oscillation frequency (about 1/3 Hz) is nearly the same as the PacificAC Intertie [3]. Have spontaneous oscillations without large initiatingdisturbances occurred?At the oral paper presentation, two very interesting real system re-

cordings of tie line power following loss of generation incidents wereshown. One was stable but with large, lightly damped oscillations. Theother was unstable after several undamped (but not negatively damped)oscillations. Could the authors include these recordings in their closure?For the unstable case, were not synchronizing as well as damping forcesinsufficient?

Several questions come to mind regarding switching of rather largeblocks of shunt compensation for transient stability enhancement ordamping of large swings? First, could the author discuss the economicsof thyristor switching versus use of high speed switchgear? Second, verylittle reserve shunt compensation capacity in the "boost" directionwould typically be available during heavy intertie loading conditionswhen stability levels are lowest. Shunt capacitors would be on the reac-tors off. Do the authors consider installation of "reserve" shunt com-pensation (with short time overload capability) to be cost effective insome instances? If SSR is not a problem, are fixed series capacitors amore economic means to improve instability for both large and smalldisturbances?

Finally, could the authors describe existing or planned applications ofstatic var compensation on the Nordel system? Are the applications forsmall disturbance or large disturbance stability improvement? For largedisturbance control, are synchronizing or damping torque improvementor primary interest?

REFERENCES

[11 E. W. Kimbark, "Improvement of System Stability by Switched

3938

Series Capacitors" IEEE Transactions on Power Apparatus andSystems, Vol. PAS-85, pp. 180-188, February 1966.

[2] C. Concordia, "Performance of Interconnected Systems FollowingDisturbances", IEEE Spectrum, pp. 68-80, June 1965.

[3] R. L. Cresap, D. N. Scott, W. A. Mittelstadt, and C. W. Taylor,"Damping of Pacific AC Intertie Oscillations via Modification ofthe Parallel Pacific HVDC Intertie", CIGRE 14-05, 1978.

Manuscript received March 2, 1981.

A. Olwegard and K. Waive: We are grateful to the discussors Messrs.Gutman, Pasternack and Taylor for their interest and comments on thispaper which provide an opportunity to expand on some significantpoints.The main objective of this paper is to show that in large intercon-

nected networks some means may be needed particularly to improvesystem damping in order to increase the maximum power transfers ontrunk line sections and system interties. Figure 1 is only used as an in-dication of that fact with reference to the thermal limit of the transmis-sion section.

Messrs. Gutman and Pasternack are referring to the two basic perfor-mance criteria - line voltage drop and steady-state stability margin.These criteria are derived from a simplified two-machine model andthey can be this assumption be used as a preliminary estimate of atransmissions project. In large interconnected system they must be usedby caution regarding the loadability of both trunk lines and tie-lines.System stability should in such cases be based on a multi-machinestudy. The Nordel system is one example of that. In 1963 the steady-state stability limit of the 100 km, 400 kV tie-line between Norway andSweden was about 900 MW. In 1968 though increased short circuitpower at both ends of this limit had decreased to about 300 MW. In1971/72 the steady-state stability limit varied during the year from 300MW at summer light load to 0 MW at winter peak load (at times it wasthen disconnected).At an early stage of development the loadability of the trunk lines

from north to central Sweden (line lengths between 450 and 650 km)was limited by stability to about 0,7 SIL. Some of the trunk lines arenow limited to about 2,0 SIL despite much harder stability criteria whilesome of the lines are thermally limited with respect to loss of a parallelcircuit. In future stages the loadability of the trunk lines will depend ongeneration expansion and on the development of unidirectional powertransfers on other transmission sections in the Nordic system.

Regarding the thermal limit it is worth noticing that in networkswhere series capacitors and HV sea cables are used the thermalloadability of such network sections tends more to be a planning andoperating problem with respect to circuit configuration and systemdimensioning criteria rather than design criteria for a single line.

Regarding the line-voltage drop criterion the method applied in oursystem is to optimize various reactive power resources in order toachieve a suitable voltage profile and transfer capability.

In systems with stability constraints the proposed device is useful inthree respects. Firstly it will be used to increase the damping limit up tothe transient stability limit, secondly to improve transient stability andthirdly for improved voltage control particularly in postfault cases. Incombination with other means to improve stability such as seriescapacitors and supplementary excitation control devices the use ofTSC/TCR can economically contribute to an increased transfercapability and then reducing the number of EHV lines.We agree with Mr. Taylor that the reduction of the transfer reactance

is most beneficial at electrical centres. This method has been used to alarge extent in the Swedish trunk line system. All long 400 kV lines formnorth to central Sweden are series-compensated from 40% to 60%o (inlocations from C to E and F in figure 12).Another promising method that has been studied is to use extra

thyristor switched series capacitors on some of the long lines alreadyequipped with fixed series capacitors. By forcing the series compensa-tion between a lower and upper limit the transient, oscillatory andsteady state stability modes could be controlled.The electrical centre for the network in figure 10 is located close to

point D. see also figure 7 which shows the equivalent reactances andmachine ratings of the simplified network.By series compensation of the tie-line E-F in figure 10 preferably the

synchronizing torque would be improved corresponding to an increasedtransient stability limit by about 150 MW. If instead a thyristor-switched shunt-capacitor of about the same size is used the transientstability limit would be improved by about 100 MW but the oscillatory

stability limit by about 300 MW. The latter is to be preferred in thiscase.For the Swedish network in figure 12 the electrical centre is between

C-D andE-F.Spontaneous oscillations without large disturbances appeared in the

late 60's when the interconnected system in Norway and Sweden wasunder development and new generation areas were connected to thesystem. The oscillation frequency was then about 0,5 Hz (1). In 1980large oscillations occurred in the interconnected Nordic system. A cou-ple of important transmission lines were then out of service, but nolarge disturbance initiated the oscillations. Frequency of oscillation wasabout 1/3 Hz.The control method to improve system damping requires several

switchings within a short period of time. Thus conventional switch-gearwould be unsuitable.We do not consider shunt compensation with short time overload

capability. In case lines or units are tripped the device may be used asalternative to spinning reserve i.e. it will be used continuously for sometime to restore the voltage.The real system recordings referred to by Mr. Taylor are reproducing

400 kV voltage and tie-line power oscillations in Borgvik which is thesubstation on the Swedish side of the tie-line between West Sweden andSouth Norway.

In the first recording, see figure 1, the prefault loading of the tie-linewas 60 MW. At this disturbance a network fault resulted in loss of 1200MW generation in north Sweden. The stability of the tie-line betweenSweden and Norway was however maintained. Thanks to that systemfrequency only dropped to 49.6 Hz and normal operation could be easi-ly restored.

PMW ACTIVE POWER

4C

20)O A I 40MW60M

60

I 0 ZO 30 40 05

UKV VOLTAGE

420410400

FKue 1. Recording of voltage and tie- power in Borgikattrsplng of 1200MW generation innorth Sweden

In the second example, see figure 2, the prefault tie-line loading was430 MW. In this case the loss of 1500 MW generation in north Swedenlead to increasing rotor angle swings between the groups of machines inNorway and Sweden. After three swings, just before the tie-line trippedout the Norwegian system was close to loss of synchronism with theSwedish system. The consequence of the tie-line tripping was that thefrequency swiftly dropped in the Swedish system. The consequence ofthe tie-line tripping was that the frequency swiftly dropped in theSwedish system causing some further 500 MW thermal generation totrip and about 2000 MW of load to be shed. Due to insufficient damp-ing and synchronizing forces in the tie-line this case thus developed to aserious disturbance. Our paper shows means to improve the tie-linestability.

P MW

1000

800

600

ACTIVE POWER

420MWL-U K4 VOLTAGE 0ul a

400

080.

350l

300

Fgure 2. Recording of voltage and tie-ine power in Borgvlkat tripping of 1500MW generation in north Sweden

)01

i6 w 30 40 50 5

In figure 3 an example of a recording on the same tie-line shows howspontaneous oscillations are growing due to negative damping even at a

low tie-line power level. This particular case was stabilized after about 2minutes by reducing the generation in the Norwegian system by about400 MW.

Ukv VOLTAGE420-

410.

400

PMW ACTIVE POWER0oo4

200

0 \ fAAff A20MWptp

-400-

-600-

Fgue 3. Recordng of voltage and tie-kn power in Borgvk

at negativety damped spontaneous oscilations

3939

Series compensation must be regarded as a basic mean to improvetransient stability and damping. If the latter is the critical limitationTSC/TCR may however be the most economical method. Of course,supplementary signals to the generators should be the first method to beconsidered to improve damping.One application of static var compensation is decided. A 4 x 50

Mvar capacitor bank will be used in the Stockholm load area and it willbe connected to 220 kV in a 400/220 kV transformer station. The mainpurpose is to use it at disturbances to improve voltage and to improvedamping of some large nuclear units close to the area. Further installa-tions are discussed to be made in northern Sweden for large disturbancecontrol and where damping torque improvement are of greatest in-terest.

Thyristor controlled reactors that are installed in the Oslo area forlocal reasons are planned to be also controlled according to the methoddescirbed in the paper.

REFERENCE

[1] A. Olwegard, "Improvement of System Stability in InterconnectedPower Systems". CIGRE 32-17, 1970

Manuscript received May 4, 1981.