REACTIVE COMPENSATION

EHV series capacitor equipment protectionand controlN T . Fahlen, M.Sc. (Eng.)

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

Abstract: Since the introduction of static VAR systems (SVS), interest in series compensation hasbeen somewhat reduced. However, in aiming at an economical solution of maximum power transfer on asystem, both methods have to be studied together objectively. Basically, to control the series impedance of apower system is, in fact, more effective than control of the shunt impedances. To meet transient stabilitydemands, high-speed controls of the series capacitor have been developed, which have not always givenexpected service experience, perhaps because of an unnecessary degree of sophistication. The paper describesvarious schemes designed with the motto 'The simpler the better'. Also, the subsynchronous resonance(SSR), at present giving a somewhat negative attitude to series compensation, is discussed in the paper on thebasis of computer studies and tests.

1 Introduction

EHV series capacitors have been used over a long period oftime as one of the most valuable means for improving powertransmission over long distances. The operating experience inSweden since 1954 has been excellent, resulting in the opinionthat unattended stations should have simple basic circuits withonly a few robust standard components and as simple aprotective system as possible.

The design of an EHV series capacitor installation startswith system planning which involves economic considerationsover the period of system development, as well as systemstudies including load flow, stability and voltage controlproblems.

To meet transient stability requirements, high-speedcontrols for the series capacitor have been developed, whichhave not always given expected service experience, perhapsbecause of an unnecessary degree of sophistication.

These requirements for transient stability, which depend onhigh-speed capacitor reinsertion after protective gap flashoverand fault clearing, may be met by using a dual-gap arrangementin which simple basic circuits with robust components areretained. A development of the dual-gap principle, with theaddition of a nonlinear bypass resistor, may improve stabilityand/or mitigate subsynchronous resonance (SSR) problems.

Another recent development is signal transmission fromEHV potential to earth performed by fibre optics. However,cascade coupled current transformers, without need for relaysand electronics to be inaccessibly located outdoors, are anattractive alternative, since the most sensitive parts areprotected against damage from the environment.

This paper first reviews the characteristics of series capacitorprotection methods, emphasising the importance of the bypassequipment design and the form of voltage limiting spark gap,the performance of which can have a significant influence onseries capacitor cost. The paper then describes the newconcepts which relate to signal transmission and the dual gapbypass methods pointing out their advantages.

Reference is then made to countermeasures for sub-synchronous resonance and the monitoring of turbogeneratorshaft oscillations to detect potential danger conditions online.

2 Protection

The protection scheme is required to operate reliably toprotect the capacitor banks, the constituant segments, andcapacitor units as well as the ancillary equipment under allservice conditions and modes of operation.

Paper 1625C (PI 1/P9), received 25th September 1981The author is with the Consulting Department, ASEA, S-721 83Vasteras, Sweden

In addition to the usual power frequency excess currentand voltage protection, means are necessary to limit withinacceptable levels both current and voltage transients and at thesame time permit a rapid reinsertion of the capacitors aftervoltage limiting equipment has operated as the result of systemfaults.

The limitation of transient voltage is perhaps the mostimportant aspect, and protective spark gaps are the firstconsideration.

2.1 Protective spark gapsFor reasons of economy, series capacitors are protected byspark gaps, which are set to operate at a voltage level abovewhich the cost of the capacitor dielectric required to with-stand the voltage increases rapidly. For modern capacitor unitsthis level is about 2.6 to 2.8 times the rated voltage (50Hzcrest value).

In general, the spark gap is the most vital protective item ofa series capacitor and must be fail safe. Because of theeconomics and need to limit switching transients the gap mustbe robust and able to protect large capacitor modules. Aschematic diagram of a conventional series capacitor scheme isshown in Fig. 1.

Fig. 1 Conventional series capacitor schemea capacitor segmentb unbalance transformerc discharge reactord damping impedancee protective spark gap/ bypass breaker

The most commonly used spark gaps are of two types, air-blast gaps and open-ventilated gaps. Air-blast gaps may give arapid extinction and deionisation of the spark but are rathercomplicated because of the air-blast valve control whichinvolves monitoring of a number of checking points. The open-ventilated gap is much simpler but requires the operation ofthe bypass breaker to extinguish the spark, and additionallyrequires a longer time for deionisation and cooling of theelectrodes than air-blast gaps.

Because of the limited size of air-blast gaps and controlcomplications, the open-ventilated type is preferred and hasbeen widely used. The withstand level between open-ventilatedgap electrodes is, however, rather imprecise, and to stabilise

394 0143-7046/81/060394 + 08 $01.50/0 IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981

the initiation of sparkover, it is necessary to apply a trigger,which results in an accuracy of about ± 5% of the nominalsparkover voltage. For a power gap consisting of two pairs ofelectrodes in series, the trigger circuit may be arrangedaccording to Fig. 2.

POWERGAP

TRIGGERCIRCUIT

Fig. 2 Spark gap scheme

The sparkover level is based on monitoring the totalcapacitor voltage. The grading circuit includes two pilot gapsin series, each factory adjusted to within 3% of half therequired sparkover level. These gaps are hermetically sealed inorder to prevent them from being influenced by atmosphericconditions. This allows high accuracy and reliability tobe maintained. The scheme is straightforward and initiates ininstantaneous voltage rise and sparkover in the power gap.

2.2 Bypass breakerThe bypass breaker is used for deliberate bypassing andinsertion of the series capacitor, and for automatic bypassingin case of faults and disturbances. Fig. 3 shows a typicalinstallation.

The bypass breaker must be capable of inserting the seriescapacitor without restrikes, and it must withstand a highcapacitor discharge current on closing. It is, however, notcalled on to interrupt short-circuit currents.

The required voltage rating across the breaker pole isdetermined mainly by the capacitor bank rating and isnormally considerably less than the line voltage. The bypassbreaker may, therefore, be a modified circuit breaker with theextra insulation to earth provided by mounting on thecapacitor platform.

The requirements for the operating gear are somewhat

Fig. 4 Damping impedance

different from those of a normal circuit breaker because of thehigh making current duty without there being a short-circuitcurrent breaking duty.

2.3 Damping equipmentThe effect of capacitor discharges after a gap flashover orclosing of the bypass breaker must be reduced and damped outquickly with respect to

(a) capacitor dielectric(b) capacitor fuses(c) breaker extinguishing chambers(d) type of spark gap.

If most of the capacitor energy is absorbed outside thecapacitor the remaining energy will not blow the fuses.Damping should be effective in a short time, say 1 to 2 ms.This is, of course, very important when capacitor reinsertionis required immediately on fault clearing.

The damping equipment consists, in principle, of a reactorin parallel with a resistor. Fig. 4 shows a typical installation,and a discharge characteristic is shown in Fig. 5.

Consideration must be given to the total transient currentpassing through the spark gap and damping impedance. Thiscurrent consists of the capacitor discharge current super-imposed on the transient fault current, and the latter can besignificant for capacitors located close to a substation.

100- 200 r

50-

<•* OH

-50-

Fig. 3 400kV series capacitor bank with modular oil-minimumbypass breakers of standard design, given a special operating cycle withthe aid of double operating mechanisms of the motor-operated spring-closing type

i-Capacitor voltage

I-Capacitor current

2.0

-100-

-100 J -200 U

Fig. 5 Typical damping oscillations

IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981 395

In Fig. 1 the damping equipment is shown in series with thespark gap and bypass breaker. The location of the dampingequipment in the line in series with the capacitor could how-ever be a justified alternative, since this gives the minimumreactor cost. This method also reduces the secondary arccurrent arising from trapped charges.

Normally, a spark gap is connected in series with theresistor, which sparks over on capacitor discharge, therebyconnecting the resistor in circuit. In steady-state operation, theline current flows through the reactor only, and this results inreduced losses.

2.4 Con trol o f trapped chargesWhen a series-compensated line is disconnected by the linebreakers without previous or simultaneous operation of theprotective spark gap or the bypass breaker or any other voltagelimiting device, the capacitor will be left with a trapped DCcharge. .

If the line is disconnected permanently, then the charge willdisappear through the built-in resistors in the capacitor units.These resistors are, basically, intended for security duringmaintenance, and allow the capacitor to be discharged fromrated voltage to 50 V in 5 min.

For a series capacitor, however, the discharge may have tostart from a voltage corresponding to the maximuminstantaineous value before, or during, operation of the over-voltage limiting device. If the line under these circumstances,is disconnected owing to a line fault and high-speed linereclosure is used, it is very likely that when the line isreinserted together with its series capacitor, the reclosingtransient will cause the overvoltage limiter equipment tooperate. This will result in a temporary loss of compensation.

If the line reclosure is made at an unfavourable phase angle,then the combined effect of the reclosing transient and the DC

voltage on the capacitor will result in an increased switchingsurge voltage across the series capacitors. This would increasethe risk of loss of compensation. Series capacitors in adjacentunfaulted line sections could also become bypassed in thesecircumstances.

In order to prevent such disturbances, the trapped DCvoltage has to be sufficiently reduced during the 'dead' intervalbefore the high-speed line reclosure. This can be achieved byeither

(i) a close-open operation of the bypass breaker during theline 'dead' interval

(ii) a saturable reactor connected in parallel with thecapacitor to form a discharge path. Fig. 6 shows a typicaldischarge reactor.

To close an ordinary bypass breaker takes about 0.1s;additionally, a few milliseconds are required to complete thedischarge through the damping impedance. A drawback in thisscheme is the need for relay controls to operate the breaker,requiring more frequent maintenance compared with thesaturable reactor scheme.

The saturable reactor becomes saturated rapidly whenexposed to a DC voltage, and thereby draws a current of therequired magnitude to meet the short time available fordischarge, as shown in Fig. 7. The energy stored in thecapacitor is absorbed by the reactor winding and/or anadditional resistor. A secondary winding for a low-voltageoutput to signal relay protections can be added. During normalservice, the reactor current must be negligible so that losses aresmall.

Fig. 6 Single-phase discharge reactor under test

396

Fig. 7 Voltage U and current I on discharging the capacitor

The use of discharge reactors, such as the saturable reactor,in a suitable complementary capacitor equipment designprovides a simple and natural discharge method with minimummaintenance of circuit breakers. It also introduces thepossibility of excluding built-in resistors, thereby simplifyingmanufacture, and reducing losses and temperature. This inturn leads to reduced failure risk and increased life time.

2.5 Excess curren t pro tection

2.5.1 Overcurrent protection: This protects the capacitoragainst high currents of short duration not covered by otherprotection methods. The overload protection (followingSection) is normally equipped with an instantaneous trip relayfor this purpose.

The spark gap has also to be protected against overcurrentscausing excessive heat. Harmonic currents need not beconsidered in EHV compensated lines.

2.5.2 Overload protection: Normally, a thermal relay with aninverse time characteristic and adjustable thermal timeconstant is connected to a line CT with the primary currentthe same as the capacitor current. When operating, the

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

protection will close the bypass breaker and automaticallyreinsert the capacitor when the current has dropped to itsnormal value. The setting of the overload relay is chosen tocope with the recommended permissible overloads, e.g. 1.35times rated current for 2h [1]. Instantaneous trip facilitiesmay also be included to operate on excessive peak overloads.

In some schemes, overload protection is omitted, in whichcase the overload is controlled by the operators. This is toprevent loss of compensation during emergency conditions.

2.6 Overvoltage protectionAs considered in Section 2 transient overvoltages must belimited to 2.6 to 2.8 p.u. of rated capacitor voltage in order toachieve the lowest capacitor costs. Although this is preferablydone with a robust air-gap self-triggered on a preset voltage levelwith high accuracy [1], additional methods are used, such asthe opening of a series breaker, or line breaker, or the closingof a parallel breaker.

The appropriate method is initiated automatically,depending on the state of the service conditions in relation tocontrol schemes. The time for dionisation and obtaining thenecessary voltage withstand after extinction of the sparkdepends mainly on the value and the duration of the short-circuit current. The main task of the spark gap is for protectivereasons. Therefore complication in the design should beavoided and special attention paid to the self-triggered scheme.

An overvoltage relay with a relatively short time delaybefore initiating breaker action may be used to protect againstsustained overvoltages below the spark gap level. Such over-voltages could result from low-frequency currents.

2.7 Subharmonic protectionThe tendency towards subharmonics in a network increaseswhen series capacitors are used and the load is low. The sub-harmonics may be hazardous, especially for transformers.However, there have never been reports of such disturbanceson EHV transmission schemes.

The subharmonic protection comprises a low-pass filter forlow-frequency currents below about 30—40 Hz and a sensitiverelay. The relay is normally set to detect currents of about10% of rated current and given a time delay before trippingis signalled to allow for any natural damping to take place andfor a safe current detection through the filter. In case ofpersistent subharmonics, the series capacitor is bypassed andautomatically reinserted after a few seconds. For many seriescapacitor applications however, subharmonic protection is notincluded in the protection scheme.

2.8 Fiashover protectionInsulation failures resulting in flashovers between the capacitorand platform, or between the control gear and platform, aremet by means of a current transformer in equipotentialconnection between the line and the platform. In case of plat-form faults, a current flows through this current transformer,and the protection signals the bypass breaker to closepermanently.

It should be observed, however, that a fiashover mightbypass the damping impedance as well as part of the capacitor.Therefore the fiashover protection should signal instantaneoustripping. To avoid flashovers, the voltage distribution withinthe capacitor should be considered with care and corre-sponding BIL levels chosen [1].

2.9 Imbalance protectionThis protection detects asymmetry in the capacitor bank,owing to blown capacitor fuses or short circuits caused byfiashover across bushings or between capacitor units and racks.

The protection scheme may differ, depending on the size

and physical layout of the bank as well as the method offusing, i.e. internal element fuses or external unit fuses.Recommendations are given in IEC 143 [1].

A sensitive protection is obtained by arranging each phaseof the capacitor bank into a balanced bridge. Inadvertentimbalance due to manufacturing tolerances is avoided bymarking the units with a selected number of plus and minus,depending on the discrepancy from rated capacitance. In eachrack, if possible, the number of plus should be equal to thenumber of minus. As a consequence, the balance will be goodand the voltage distribution even. Asymmetry is detected by aCT in the zero branch and designed with regard to specialduties.

The main objective of the imbalance protection is toprotect sound elements, in the case of internal fuses, or soundunits, in the case of external fuses, from over-voltages in theorder of 10—15%. The fuses should blow before the imbalanceprotection operates. This might be difficult to achieve withexternal fuses because, to obtain fuse operation, the entireunit has to be short circuited. This means that all other unitsin parallel, about 20, will also be short circuited, and thisresults in a very high temporary asymmetry compared with theset level of the protection.

The various imbalance protection schemes depend on fusemethod and could be as follows.

2.9.1 Internal fuses [2]: A low-set signal will cause trippingabout 0,4 s after sufficient element fuses have blown for theremaining voltage across parallel sound units to increase10—15% above rated voltage. Commonly, about five to sevenfuses in a parallel group of elements may have blown beforethis trip level is reached. Additionally, a high-set signal with atime delay less than 0.4 s will cause tripping should anexternal fiashover or short circuit occur. The reason for thisis that internal fuses will not detect such faults..

2.9.2 External fuses: A low-set signal will initiate an alarm ifone or two fuses have operated. A high-set signal will causetripping if three or more fuses have operated.

It should be observed that the trip function and time delaymust be co-ordinated with the fuse characteristic down toabout 50% of rated current.

It should be understood that a capacitor bank tripped byimbalance protection may include unsound units withunblown external fuses. Therefore it is recommended thatcapacitor units should be checked every year by means of atong instrument, which permits rapid checking of capacitancewithout loosening any connections.

One advantage with external fuses is that a fault detectorrelay based on counting blown fuses could be easily arrangedto replace the conventional imbalance protection which isfairly imprecise.

2.10 Subsychronous reasonance (SSR) protectionDepending on calculated or assumed risks of interactionbetween a series-compensated network and turbogenerators,it may be found desirable to introduce a protection able todetect critical subfrequencies of a steady-state nature and of amagnitude of about 3% of rated current [3]. Shouldundamped oscillations persist, part of the compensation mustbe bypassed in order to bring the system out of resonance.

Other protective methods are:(a) The use of a dual-protective spark gap scheme with one

gap set for a very low fiashover voltage to give a rapid insertionof a nonlinear bypass resistor. The other gap is set at thenormal fiashover level.

(b) The monitoring of shaft speed and torque oscillationson large turbogenerators for deriving input signals to

IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 19&1 397

supplementary excitation damper control (SEDC) and/orthyristor-controlled shunt reactors designed to inhibitoscillations. Similar signals may be used as inputs to seriescapacitor thyristor bypass damping equipment or for thetelemetric switching of series capacitor modules to stopresonance.The onset of conditions liable to lead to emergencies aredetected by the shaft torque monitor operating in relation tomaximum permitted shaft stresses and cycle fatigue capabilityinformation and causes tripping of generators should this benecessary [4].

3 Signal handling

In series capacitors for power stations with line voltages up toaround 130kV, each relay protection scheme is supplied fromindividual instrument transformers fully insulated to meet theline voltage. For higher system voltages, this method issomewhat expensive. In these cases, fault detecting devices canbe located on the capacitor platform, with the signalstransmitted to ground by means of light signals through fibreoptics. However, to give the relay protection of seriescapacitors the same simplicity and standard quality as forordinary substations, a number of signals may be transmittedbetween EHV potential and ground via a modified currenttransformer. By this method, which is economically justifiedup to 500kV, no relays, electronics, Optronics or any othersophisticated control equipment has to be inaccessibly locatedoutdoors. Consequently, the most sensitive equipment isprotected against the environment.

The signals are taken out through primary transformersconnected in cascade with the fully insulated currenttransformers. The primary transformers are located on theEHV platform, and thus given a reduced insulation. Signalseparation takes place according.to the time signal level chartshown for multicut protection in Fig. 8. The principle usedis that the signal for the shortest operating time has the highestsignal level, whereas signals with a longer operating time havea lower signal level. A typical schematic diagram is shown inFig. 9.

4 Capacitor reinsertion using dual-gap schemes

With regard to statistical current magnitudes, control systems,environmental conditions and, especially, the state of gapelectrodes, reinserting the series capacitor very rapidly afterthe extinguishing of the gap current is very much a matter ofprobability.

Sophisticated items of protection and control gear havebeen developed, located inaccessibly at high potential, thusgiving the series capacitor a reputation for being a complicatedpiece of equipment. Therefore due attention should be paid tothe mode of control and switching of a series capacitor [5,6] .

4.1 GeneralBasically, the new concept consists of an extra branchconnected in parallel with a conventional series capacitorprotected by traditional heavy-duty spark gap equipment, see

GROUND

CONVENTIONAL

SIMPLE

RELIABLE

Fig. 8 'Multicut' circuit

LINE

C — i UNBALANCE

SPARK GAP

FLASHOVER

SIGNAL LINK

SIGNAL LEVEL

A

SIGNAL TIME

Fig. 9 CT protection scheme for series capacitors

1) Two phase only (optional)2) If discharge reactor omitted3) Damping impedance

Alternative location in serieswith capacitor

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

Fig. 10. The alternative, chosen, b, c, or d in Fig. 10 dependson the design and operation of the power network as well ason cost considerations. A feature common to all thesealternatives is that the gap (Gl) in the extra branch has a lowersetting than the ordinary gap (G2). The latter should normallybe set to correspond to a reasonably low capacitor cost, i.e.2.6-2.8 p.u. rated voltage. Consequently, after gap Gt hassparked over and the breaker Bj subsequently opened, theseries capacitor will be reinserted against a clean power gap(G2) not subjected to any spark, and therefore having a welldefined withstand level.

4.2 Cost evaluationIf trigger tolerances, withstand levels ofthe power gap andvarious reinsertion schemes based on a reasonable equalprobability of successful reinsertion are considered, theproblem of cost evaluation of the capacitor may be illustratedas shown in Table 1. The Table shows that, with regard to thedifferences in capacitor costs for the three reinsertion schemesdesigned to give a reasonably good safety margin, the dual-gapscheme is likely to give the cheapest solution.

SERIES CAPACITOR ]

ORDINARY GAP >J

CONVENTIONALSCHEME

ORDINARY BREAKERJ

DUAL-GAP SCHEME

DUAL-GAP/BY-PASS RESISTOR SCHEME

DITTO. TWO STEPS

DITTO, STRIPPED

Fig. 10 Diagram showing various dual-gap scheme alternatives forcapacitor high-speed reinsertion

Breaker BI in schemes c and d has pole Y mechanically delayedrelative to pole X.

P.U. CAPACITOR VOLTAGE

DUAL GAPS SINGLE GAP

GAP 2

GAP1

Fig. 11scheme

NOMINAL SETTINGSIMPROVEMENT

2.45 (0.8x3.05)RECOVERY WITHSTAND

Comparison of capacitor voltages according to bypass gap

4.3 Gain and impro vemen tThe example in Fig. 11 shows another interesting comparisonbetween a single-gap scheme with a recovery withstand equalto 80% of the nominal value and a dual-gap scheme withreduced gap settings. The improvement is the differencebetween the minimum sparkover of gap 2 and the recoverywithstand of the single gap.

Fig. 12 shows a comparison between a dual-gap schemewith a fixed delayed reinsertion of 6 cycles after faultinception and an air-blast gap designed to reinsert 3 cyclesafter a 3 to 5 cycles' fault clearing.

Rapid capacitor reinsertion is possible with the dual-gapscheme if fault clearing is normal or even delayed.LINE FAULT CLEARING

CYCLES5

3-

FAULT INCEPTION

' CAPACITOR REINSERTION

1 3 6 7 8 CYLLtS

Fig. 12 Improved capacitor reinsertion

Both the Turkish [7] and the Mexican 400kV transmissionsystems include series capacitors with dual gaps and modifiedcurrent transformer signal links. Fig. 13 shows a typicallayout.

5 Dual-gap/by-pass resistor scheme

The operating performance of a series capacitor installationwill be considerably improved by means of the principleillustrated in Fig. 10. The circuit alternatives which include anonlinear series impedance have the following advantages inconnection with line faults:

(a) improved compensation efficiency(b) increased damping of subsynchronous current(c) reduced voltage peak transient(d) reduced stresses on capacitor breakers.

Such a scheme can be built up relatively simply by adding to aconventional series capacitor an extra branch consisting of aspark gap Gt , a nonlinear resistor and a series breaker Bx.

Typical settings are: resistor gap Gj ~ 2.2 p.u., andordinary gap G2 ~ 2.7 p.u.

These gap settings are chosen with regard to minimumcapacitor cost, gap tolerances and the tolerance of the non-linear resistor so as to obtain selective gap operation.

The resistor is dimensioned with respect to:(i) fault levels at line terminals

(ii) line parameters(iii) series capacitor reactances(iv) fault clearing times(v) damping requirements

Table 1: Relative costs of capacitor schemes according to bypass method

Dual-gap schemeslow-speedbypass breaker

Single-gap schemeultra-fastbypass breaker

Self-clearingspark gap withrepetitive arc

Trigger tolerance (+/—%)Withstand level (%)*Minimum sparkover (p.u.)Nominal setting (p.u.)Uum (P.u.)L / S ( IEC) (p.u.)Relative capacitor cost

Before reinsertion

31002.832.93.01.2 X 3.0 = 3.61

585

2.832.83/0.85 = 3.331.05X3.33 = 3.51.2 X 3.5 = 4.2(4.2/3.6)2 = 1.36

585

2.833.333.51.4X3.5 = 4.9(4.9/3.6P = 1.85

IEEPROC, Vol. 128, Pt. C, No. 6, NOVEMBER 1981 399

Furthermore, if it is assumed that a fault occurs in an externalline section, causing the resistor gap to spark over, the upperlimit of the voltage/current characteristic of the resistor shouldbe such that the capacitor voltage should not exceed 2.8 p.u.minus the gap tolerance at any time during the fault period inorder to avoid a complete loss of the capacitor.

The bypass resistor scheme gives a considerable reductionof the voltage transient caused by fault tripping and gap spark-overs, thereby reducing the capacitor cost. This aspect alonemay, in some cases, justify the use of the bypass resistor.

5. 7 Fast reinsertionAfter fault clearing, the capacitor will retain its normal voltage.At the same time, the resistor current decreases considerablybecause of the nonlinear characteristic, resulting in automaticreinsertion of the capacitor immediately on fault clearingwithout any switching action being necessary. The use of fastreinsertion of capacitors by means of the bypass resistorscheme increases the effectiveness of the compensation undertransient conditions.

The gap is extinguished by means of the opening of thebreaker B1} which is not critical with respect to time. This,together with the small current with a prolonged current zero,makes the stresses on this breaker fairly small.

type of spark gap by a resistor having the same type ofcharacteristic as a gap-less surge arrester. This should give anexcellent high-speed reinsertion, but the cost might be alimiting factor. Bypass breakers, isolating switches, imbalance,overload and flashover protections continue to be used.Imbalance protection for the resistor is also proposed and, incase of high short-circuit current, a protective spark gapforcibly triggered, based on monitoring the current, may berequired, with sophisticated controls.

5.4 Effect on subsynchronous resonance (SSR)The influence of the bypass resistor scheme on the SSRphenomenon has been investigated for the. 400 kV linebetween Forsmark power station and Vargfors in Sweden,which has 32% series capacitor compensation and a sub-synchronous natural frequency of 24.8 Hz [8].

Calculations have been made for a 3-phase short circuit atVargfors and with a duration of 0.08 s, both with and withoutthe resistor. Each of the shaft sections displayed loweroscillatory torques with the resistor in use than with theresistor disconnected.

Moreover, the torques oscillated with mode shapesdetermined by the turbine generator itself, i.e. withoutinteraction between the turbine generator shaft and theelectrical system.

L-B A-A

i

8 m

I

c

\

c

CT1

-12.5 m

UR|

l,J]R

RC

LBi'

r

—»G2

V

fG1

s

y

CT2

B2

B1

-fllol

J'

1=1100 A

BIL 1550to ground

Plan view Circuit diagram

Fig. 13 400kV series capacitor, dual-gap scheme; layout for onephase

5.2 Type of resistorThe silicon-carbide resistor slope characteristic allows the gapG2 to trigger on high short-circuit current in a natural modewithout complication, thereby protecting the resistor fromoverheat. Should the breaker Bx be opened to prevent theresistor from thermal overheat, the capacitor can still be inservice.

A physical arrangement of the resistor is shown in Fig. 14.This is for the El Chocon transmission system in Argentina.The resistor discs are assembled in nitrogen-filled procelains ofsurge-arrester type with safety valves and utilising standardcomponents. For this application, the resistor can absorbabout 100 MJ of heat energy, which covers two short circuitson the capacitor busbar, each cleared after five cycles, plus areasonable safety margin.

5.3 Zinc-oxide resistor schemeThe Bonneville Power Authority has used this scheme [5]where it has been found desirable to replace one particular Fig. 14 El Chocon bypass resistor

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

The investigation reveals that the resistor reduces theamplitude of the torque oscillations just after a disturbanceand in this case would solve the transient SSR problems. If thesystem is not self-excited, the resistor will also prevent theoscillations from reappearing should the resistor be switched infor some period of time, e.g. about 10 s.

6 Conclusions

The conventional arrangement of protection methods forseries capacitors has been described. It has been seen thatcapacitor units with internal fuses have advantages overcapacitor units with external fuses. Of the two principalmethods of voltage protection and capacitor reinsertion, theuse of open-ventilated spark gaps are preferred primarilybecause of simplicity and robustness.

Modern methods include the dual-gap schemes and theresistor-bypass scheme, and it may be concluded that:

(a) the dual-gap scheme has a higher voltage withstand levelprior to reinsertion than an ultrafast bypass breaker or a singleself-clearing gap scheme and can therefore be assigned a lowergap setting. Consequently the test voltage of the capacitorunits and the costs would be materially reduced

(b) with respect to the reduced withstand levels of thepower gaps prior to reinsertion, a single-gap scheme cannotgive the same security as that obtained with a dual-gap scheme.The capacitor can remain in service even if the breaker in thebranch of the extra gap should fail

(c) the bypass resistor scheme as designed for the ElChocon transmission system has improved the stability incomparison with a conventional series capacitor, partlybecause of the automatic reinsertion immediately on faultclearing and partly because of its braking effect on thegenerators. Further, the overall series capacitor cost wasreduced owing to the lower spark-gap setting made possible

(d) it has been found possible to design the characteristic of

the bypass resistor so as to damp transient and steady-stateSSR disturbances on systems with series capacitors in Sweden

(e) the bypass resistor scheme is also well suited forlocations with high short-circuit levels because of the stand-by gap (G2), which will spark over, together with the resistorgap (Gj), for adjacent line faults, thereby protecting theresistor from very high thermal stresses. This feature isobtained by means of a high-ohmic resistor value, which willalso support the high-speed reinsertion.

7 Acknowledgments

The author wishes to express his gratitude to TEK, Ankara andthe consultant Prof. Diceto,'as well as CFE, Mexico City, fortheir valuable support in the application of the dual-gapscheme with CT signal links.

8 References

1 International Electrochemical Commission (IEC) publication 1432 JANCKE, G., FAHLEN N., and NERF, O.: 'Series capacitors in

power systems'. IEEE symposium, 1973, pp. 443-4493 IEEE symposium on countermeasures for subsynchronous resonance,

19814 DUNLOP, R.D., HOROWITZ, S.H., JOYCE, J.S., and

LAMBRECHT, D.: Torsional oscillations and fatigue of steam-turbine generator shafts caused by system disturbances andswitching events, CIGRE, Aug. 1980, no. 11-06

5 BATHO, J.L., HARDY, J.E., and TOLMUNEN, N.: 'Series capacitorinstallation in the BC Hydro 500 kV System'. IEEE symposium,1977, pp. 240-245

6 COURTO, A.L., and STARR, E.C.: 'Experience with 500kV seriescapacitor installations and new protection schemes on the BPAsystem'. CIGRE, Aug. 1980, no. 31-09

7 ILICETO, F., and TURKOGLU, G: 'Planning features of theextension of the 420kV Turkish system', CIGRE, Aug. 1980, no.31-12

8 AHLGREN, L., FAHLEN, N., and JOHANSSON, K.E.: 'EHV seriescapacitors with dual gaps and non-linear resistors provide technicaland economic advantages'. IEEE 1979 Power Engineering SocietyWinter Meeting, New York p. A79 049-8

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