Capacitive Divider Transformer

7/30/2019 Capacitive Divider Transformer

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1202 IEEE Transactions on Power Delivery, Vol. 12 , No . 3, July 1997Capacitive Divider Substation

L. Bolduc, MemberEEE B. Bouchard, Non-member G. Beaulieu, Non-memberInstitut de recherche Cegelec BG Checo Hydro-Qdbec&Hydro-Qu6bec (IREQ) R6gion MatapCdia1800 boul. Lionel-Boulet Anjou, QuCbec 355 boul. St-Gennain ouest,Varennes, Quebec Canada HIM 3R4 Rimouski, QuebecCanada J3X 1S1 Canada G5L3N2

7151, ue Jean-Talon est,

Abstract - The development of a passive damper-filter hasmade it possible to achieve a stable capacitive-couplingsubstation (SCC). This paper presents the underlying theory andanalyzes the impact of the SCC on a power network system. Therelated nuisance phenomena (overvoltage, ferroresonance) aredescribed together with the solutions applied. Test results arepresented as proof that the nuisance phenomena have beensatisfactorily eliminated: overvoltage is adequately limited at theworst node by protecting the equipment and ferroresonancenever lasts more than two cycles. Lastly, the paper describesHydro-Qukbec's first such capacitive-coupling substationinstalled at RivMre Ste-Anne (Qukbec).Keywords: Capacitive Substation, Capacitive Divider,Rural Electrification.

1 INTRODUCTIONThe capacitor divider has been known for quite awhile but using this technology to transform highvoltage (HV) to medium voltage (MV) for deliveringpower is more recent. Phenomena such asferroresonance made it difficult to do in the past.However, the development of a passive damper-filter,

patented by Hydro-Qukbec, has made it possible todesign a capacitive-coupling substation (SCC) with acapacity of up to 1.5-2 MW. The theory behind thedesign is presented in section 2 followed by details ofthe protection against overvoltage and ferroresonance,the two major problems faced by the SCC. Full-scalelaboratory tests have been conducted on a single-phaseSCC prototype with a view to validating the feasibility

of a complete three-phase system. The paper concludeswith a description of a 1.5MW SCC successfullyincorporated into the Hydro-Qu6bec power system.2 T Y

2.1 ModelThe principles of capacitive-coupling transformers

are not new, as they have long been used both bymanufacturers of the capacitor voltage transformersrequired by electrical utilities and by some electricutilities for feeding small loads [ l , 21, so only a briefreview is presented here. Figure 1shows the circuitmodel (a) and the Th6venin equivalent of the voltagedivider (C1 and C2) seen from the load side (b). TheThevenin equivalent voltage of the divider ( v lh) is

and its impedance is that of capacitors C1 and C, inparallel.

96SM 3806 PWRD A paper recommended and approved bythe IEEETransmission and Distribution Committee of the IEEE Power Fig. 1 Model of capacitive-coupling transformer (a)Engineering Society for presentation at the 1996 IEEUPES Summer and its equivalent Thevenin circuit seen fromthe load side (b)eeting, July 28 - August 1 , 1996,Denver, Colorado Manuscriptsubmitted December 22, 1995; made available for printing May2, 1996.

An inductor L is added to the capacitive voltagedivider to cancel the impedance due to c th =Cl + C2 at60 Hz. This is done by adjusting C1, C , and L so theysatisfy (2):

0885-8977/97/$10.00 0 996 IEEE


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1203Therefore, the voltage node V , needs to be protectedagainst short circuits at the output.A good way to do this is to add a surge arrester inparallel with the inductor to detune the system and limitVl; and V , during the time required to operate a breakeror a disconnecting switch. The arrester has to becalibrated for this protective function.

where w =2 ~ 6 0 z.Since the impedance is zero at 60Hz, the outputvoltage V , remains equal to and in phase with Vth,which is proportional to and in phase with Vi,, whetherthe load be resistive, capacitive or inductive.V,, the voltage at the terminals of C2, is also thevoltage between elements CthandL .2.2 Protection

The voltage node V, is subjected to high voltagebecause of the resonant circuit and the current drawnfrom the load. This can be seen in the phasor diagramof Fig. 2: when no load is connected (Fig. 2a), thecurrent through L is zero and the conventional rules forcapacitors apply, i.e. the current ic l supplied at Vi, is90" leading the voltage and equal to i the current incapacitor C2. In general,

(3)

where V c l is the voltage at the C1 terminals and io isthe load current. We also have v,=VL +Po where V Lis the voltage at the terminals of L.

Fig. 2 Phasor representation of currents and voltage ofFig.1 without load (a) and with load (b).When a real load (R ) is connected (Fig. 2b), the

current through R is in phase with the exit voltage V ,and, since io flows through L , it produces a voltage V Lleading io and V , by 90". Vl; equals VCth but is out ofphase by 180", so Vth=V,. Therefore, the voltage Vc1will be as shown in Fig. 2b, from which it can be seenthat, if a short circuit appears through the load at V,, V ,will become equal to Vl;,which can take infinite values.

2.3 FerroresonanceIf the capacitive-coupling transformer is used as asubstation for feeding loads or for rural electrification,it normally has distribution transformers installed onthe outgoing feeders. However, these can saturate andcreate ferroresonance with the capacitor divider. Manymethods exist for calculating ferroresonance oscilla-tions [3 , 41 but one useful way of evaluating the risk isto calculate the natural frequencies with saturated andnonsaturated transformer cores. Ferroresonance phe-

nomena can occur for frequencies between1 1 ( 5 )f2 d G m 5 2 n J W

Let us have an analytical view of the problem atfundamental frequency. Figure 3b [3] shows theelement impedances of Fig. 3a. It can be seen that thereare three different solutions for which A and C arestable. Solution A is the linear solution. For somereason, such as an output short circuit or upon systemenergization, solution B or C may be reached instead,and ferroresonance is produced.

Fig. 3 Electric circuit causing ferroresonance (a), ana-lytical view of ferroresonance phenomena (b)One solution consists in first designing the circuit insuch a way as to avoid the possibility of occurrence ofthe fundamental (point B or C ) and harmonic ferro-resonance. Choosing L as linear and satisfying (2)eliminates the possibility of points B and C existing. Asshown in Fig. 4, the impedance of L is then parallel to


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1204that of Cf h. If we add the transformer impedance to theimpedance of L, the magnetization curve will be higherand points B and C will no longer exist at fundamentalfrequency. However, subharmonic ferroresonanceremains possible, since cc)L decreases and l /(wCth)increases when cc)is reduced.

I

2.4 Impact on transmission linesWith Fig. la, using ( 2 ) ,we can calculate the currentdrawn from the line (6). The reactive part of thiscurrent is capacitive. The line is inductive so that if theSCC is placed at the end, it produces a resonant circuitwith the possibility of an overvoltage at the connectionpoint, as shown in Fig. 6. The longer the line, the

higher the impedance of Lt generating a higher voltagefor the same capacitive current drawn. Since there isonly one branch, the same current is goes through allelements but is leading the voltage of C by 90" andlagging the voltage across Le by 90". With the feedingsubstation regulated at V s and V ,=Vi, +V L ~ , longerline raises the voltage at the capacitor divider Vin, a factwhich has to be accounted for in the choice of C1 andC2. Figure6b also shows that the current drawn fromthe line is capacitive, since is is leading with respect to

Fig. 4 Effect of choosing L linear with the sameimpedance of Cfhat fundamental frequency

The second step is to add a damper-filter at V,,parallel to M , to deal with subharmonic ferroresonanceas well as harmonic transients. The filter has to act asan open circuit at 60 Hz and as a short circuit withlosses at all other frequencies. The impedance filter asa function of frequency in plotted in Fig. 5. Thisspectrum of impedance requires a resonant circuit.Also, filter losses have to be as low as possible at6 0 H z to reduce permanent losses but as high aspossible at other frequencies to damp harmonics andsubharmonics.So this filter will be made of a capacitorin parallel with an iron-core inductor tuned at 60 Hz,

vs.(6)Cl+c2

V L I

both in series with a resistor. (a> (b)12 I I I I I Fig. 6 Circuit of the transmission line feeding thecapacitor divider (a) and its phasor represen-tation (b)

$ 6i 4" 2

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However, with inductive substations (Fig. 7) , theoutput voltage is lower than V, if the line is notcompensated or regulated as shown in Fig. 7b. If placedsomewhere between two inductive substations(Fig. 7a), the SCC not only serves to feed loads but, inaddition, acts as a compensator, reducing the reactivecurrent in L1 and the losses in the corresponding

a

1000010 100 1000Frequency (Hz)


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-20o l - - - +

Fig. 7 Circuit of the transmission line feeding thecapacitor divider between two substations (a),the phasor diagram of the line without thecapacitor divider (b) and with the capacitordivider (c)3 FIELD TESTS

3.1 DevelopmentAfter many simulations to validate the theory, IREQin collaboration with a transformer manufacturer hasnow industrialized a 6OO-kVAR, 14.4 kV damper-filter.A full-scale single-phase 550-kVA SCC prototype,including the whole protection circuit, was built byCegelec BG Checo and tested under many differentoperating conditions at IREQs high-power laboratory.The prototype was built for an input voltage of

161 kV/& and an output voltage of 25kV/& . TheC1 capacitor bank is rated 0.98 pF (2 x 8 x 230kVAR,12.47kV) and the C2 bank, 5.34pF (2 x 4 x400 kVAR, 9.96 kV). The reactive power provided tothe line is estimated to be 2.7 MVAR in each phase.The air core reactance L is 1.1H.3.2 Energizing behavior

The most critical operating condition is energization,when only saturable loads are present at the output. Thetest results shown in Fig. 8 reflect this condition. Theload here is made up of ten 100-kVA unloadeddistribution transformers. Curve (a) gives the transmis-sion line voltage to ground V,, whose rms value is161 kV/& ;no effect is observed. Curve (b) illustratesthe input current of the system Isdc. The naturaloscillation frequency depends on the capacitance of C1in series with C2 and the inductance of the transmissionline. This oscillation is damped by the transmission lineand ground grid resistances. Voltage harmonicstraveling on the transmission line are also filtered bythe capacitor banks. The rms steady state input current

in capacitor C1 is 29.3 Arms. Curve (c) gives thedamper circuit current 4. This circuit reduces allharmonics and ferroresonance except those at fun-damental. Curve (d), which gives the load current Zch, isinteresting because it illustrates the tendency of part ofthe ten transformers to saturate in one polarity and theother part in the opposite polarity. In fact thetransformer inrush current tends to circulate throughthe damper filter and other transformers since it isblocked by the capacitors. Curve (e) presents the outputvoltage Vch. The ferroresonance phenomenon initiatedin closing is quickly damped within three cycles andonly very small distortions near the zero crossingspersist, owing to the magnetizing currents.

vs 160 V o l t r x l d ) (a) Tension de source 1800

-80-160Isdc 9060300

-3 0-6Ok I'

d) Courant de sortie du SCC-360

-400 Tension de sortle du SCC-3I I

-20I 4# I I I I I J

0 0.04 0.08 0.12 0.16 0.20 0.24 0.2840

Fig. 8 SCC behavior when energized with a saturableload: source voltage (a), input current (b),damper-filter current (c), output current (d) andoutput voltage (e).3.3 Short-circuit behaviorFigure 9 shows test results obtained under short-circuitconditions at the transformer output. Curve (a) illus-trates the source voltage V,, which is not affected by


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1206At the time the short-circuit is cleared, the output

voltage Vch, (curve (f)), overshoots to 2 p.u. for onehalf-cycle and returns to normal in less than 3 cycles.The damper-filter is essential to this quick return tonormal.3.4 Response to 1 ut-onlo"" I I I I I(c ) Courant du circuit amortisseurIf 40iAmphe I I I I

Fig. 9 SCC behavior for a short circuit at the output:source voltage (a), input current (b), damper-filter current (c), surge arrester current (d),output current (e) and output voltage (0.the short circuit on the distribution side while curve (b )shows the sh ape of the input current, Zsdc.Someharmonics, due particularly to surge-arrester operationand excitation of natural frequency, are added to thenormal current around 30 A,,,. Curve (c) is the currentI f flowing into the filter. Large current values at thebeginning and end of the short-circuit are partly due tothe damper's own discharging and charging currents,since the terminal voltage falls to zero, rises to a fairlyhigh overvoltage and falls again to zero. Notice that, inthe test procedure, the SCC was disconnected only twocycles after the short circuit had been cleared. Thefunctions of the damper circuit are to damp harmonicsand ferroresonance. Its current stays around zero,except during these and transient conditions. Curve (d)gives the current flowing through the speciallydesigned 21- kV surge arrester. The current peaks at120A. The arrester functions are to detune the L-Cthresonant circuit, limit the output current, and limit thevoltages at the L and C2 terminals. As illustrated incurve (e), the total short-circuit current Zch is aroundSOAm,, which is about twice the nominal loadingcurrent.

It is also interesting to see how the system respondsto loads being shut on and off. Figure 10 shows testresults obtained when a 500-kW load is connected anddisconnected at the transformer output. Curve (a)illustrates the output current. In steady-state conditions,35.2Am, go to the load. At opening, two peaks ofmagnetizing current are observed through thetransformers but the current is very quickly damped.Curve (b) gives the output voltage. At the time the loadis connected, the output voltage is seen to decrease forone cycle and then return to its normal value (slightlyreduced by 1% compared to the no-load condition).When the load is disconnected, the output voltageovershoots to less than 2 p.u. for one cycle and returnsto normal in the two ensuing cycles due to the action ofthe damper-filter. None of these transients created aproblem for the three kinds of personal computerconnected in parallel before the load breaker.

60

30

0

-30

. . . , . _ . _ . . . . . . . . _ . . . . .................I.............-690 L,(b) Tension de some du S C C J40 volt (X 103) ,A20

-20

Fig. 10 SCC behavior for load shut-on/off outputcurrent (a) and output voltage (b).


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4 APPLICATIONS4.1 Three-phase capacitive-couplingsubstation

The tests were followed by the development of athree-phase SCC, for a new 1.5-MW substation toreplace a station where the equipment had becomeobsolete (Fig. 11). Its purpose was to supply touristfacilities and other small loads in Hydro-QuCbec'sMataphdia region including "Gite du Mont-Albert"lodge in Gasp6 Park. The SCC is connected to a161-kV line operated at 171kV and the distributionvoltage is 25 kV. The substation is designed for easyadaptation to line voltage changes from 161 kV to230kV. This involves adding capacitors in the C1branch reducing it to a 0.65 pF (2 x 12x 230kVAR,12.47 kV), and using 1.16 H tap on L. The reactivepower provided to each phase of the line will become3.85 MVAR. A circuit switcher is used at the entrance.The cost of this substation (in CAN dollars) includingall the work and a remote control is around $2M($1.2 M for equipment including studies, $0.5M forconstruction, $0.2 M for telecommunication equipment,installation and training, $0.1 M for spare parts), whichis about one quarter of the cost of the smallestconventional substation built at Hydro-QuCbec. Thenew substation has been in continuous service sinceAugust 1994. The old installations were dismantled inOctober 1995 after more than 12 months' certificationtests.

Fig. 11 Hydro-QuCbec's Rivi6re Ste-Anne substationAmong other noticeable environmental character-istics, the substation contains a low quantity of oil (norecuperation pit). The noise level is lower than 72 dBand is produced by the damper-filter alone.

12074.2 Problems encountered

The system was originally designed to transform161 kV to 25 kV but, in view of the fact that thelines were normally operated at 168kV and wouldbe forced up to around 171 kV when the SCC wasconnected, all distribution transformer taps were setat the 105% position. The installation seemed to becorrect at the time of the acceptance tests in April1994 but it was not to last. The substation isconnected to a large line loop and could thereforefind itself at the end of the line if the loop is opened,increasing the mean voltage and voltage excursionsover the maximum threshold. This is what happenedin summer 1994, obliging the utility to modify someof the capacitors in the C1 branches, thus reducingthe output voltage by a total of 6%. Tests in open-loop mode confirmed this modification was correct.If the overvoltages produced in the open loop hadnot imposed so much stress on the substationequipment, the load-side voltage could have beencorrected by installing voltage regulators at thebeginning of the distribution feeder.An incident on the HV system revealed that thedisconnector part of the circuit switcher was notnecessarily opening during the switching operationso that there was no visible break. The control wastherefore modified to force the disconnector to openevery time following the opening of the switcher.Because of the very low short-circuit current and thethermal capacity of the detuning surge arrester, thefault is cleared in less than 1 s; otherwise, thesubstation protection systems start to operate. Arecloser fulfills this function at the substation out-put. Fuses are useless on the feeder because they donot operate fast enough. The same may be said ofother fuses that are either too large or too slow. Careis therefore needed to coordinate the protection andthe fault detection methods. The new SCC tech-nology lends itself to the use of high-speedprotection measures.

4.3 AdaptabilityThe system developed and installed at Rivigre Ste-Anne substation, the SCC-3, is the third type in the

development of capacitive-coupling systems [ 5 ] . Twosmaller systems using different capacitive-couplingtechnologies have been industrialized and installed inthe past. The SCCS can provide power for auxiliaryservices in a conventional substation and also repre-


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1208sents a good solution for feeding small loads and forrural electrification. The only limit to the system is theelement capabilities.

5 CONCLUSIONThe development of a damper-filter has made it pos-sible to achieve a stable capacitive-coupling substationbased on the simple principles of a capacitive divider.The SCC-3, as this system is known, does not require acostly power-transformer or circuit-breaker. It readilyadapts to lines operated at voltages of 100 to 245 kV.Its modest cost compared to a conventional substationmakes it an ideal choice for applications in areasalready strongly electrified and opens new avenues for

rural electrification projects for loads I 2 MW. There isno theoretical limitation except impact on the line,dimension of some equipment like inductors andarrestors and finally comparative price with conven-tional substation, Although it is essentially a feeder, theSCC is a shunt capacitor capable of withstanding thevoltage and reducing system losses by canceling someof the reactive current. There is a direct relationbetween the power output and the vars compensationeffect on the transmission line. 2 MW of power outputwill lead to about 10MVAR of capacitivecompensation which will bring beneficial effect to mostof transmission line.It also offers environmental advantages, is easilytransportable and can be adapted without difficulty tochanges in line voltages.The new substation at Rivihre Ste-Anne in Hydro-QuCbec's Matapedia region, the first application of thisdesign, has been in operation since August 1994 andthe utility's positive experience has given managementsufficient confidence to remove the old substation fromservice last October.

6. ACKNOWLEDGMENTSThe authors extend their sincere thanks to DanielTousignant, Irhne Lemon and Lesley Regnier for theircontribution to the preparation of this paper.

'7. REFERENCES113 H.G. Sarmiento, R. de la RosJ. Vilar, "Solving Electric Energy Supply to RuralAreas: The Capacitive Voltage Divider," IEEETransactions on Power Delivery, Vol. 5 , No. 1,

January 1990.[2] R.L. Wilde and J. Carr, "Customer service directfrom transmission lines," BEE Transactions onPower Apparatus and Systems, Vol. PAS-99,No. 1Jan./Feb. 1980,[3] J.R. Marti and A.C. Soudack, "Ferroresonance inpower systems: Fundamental solutions," IEEProceedings-C, Vol. 138,No 4., July 1991.[4] S.K. Chakravarthy and C.V. Nayar, "Ferroresonantoscillations in capacitor voltage transformers," IEEProc. Circuit Devices Syst., Vol. 142, No. 1,February 1995.[5] BG Checo Inc., "Capacitive Coupling Systems for

Rural Electrification: SCC-1, SCC-2 & SCC-3,"Anjou (Qukbec), June 1994.BIOGRAPHIES

Uonard Bolduc (M96) received his B.Sc.A in Engineering Physics in1970 and his Ph.D. in Physics in 1973 from Universite Laval in QuebecCity. After postdoctoral studies at the U niversite du Qu ebec in M ontr6a1, hejoined the high-power department of TREQ (Institut de recherche dHydro-Qukbec) in 1975. He has been involved in different theoretical and physicaldevelopments such as subharmonic ferroresonance damping, frequency-response-of-stray-losses diagn osis method for transfo rmer winding displace-ment; self-regulating variable inductors; capacitive-coupling systems forfeeding medium-power demand (C 2 MW). His main field of interest andresearch, however, has always been the effects of direct currents ontransformers and the power system in general. He is a mem ber of the O rdredes IngCnieurs du Que bec.BenoTt Bouchard is a graduate of Universitd Laval in Q uebec City, with aB.Sc. in Electrical Engineering in 1969 and M.Sc. in 1973. He has beenworking for CEGE LEC for the last 17 years. He is currently Director ofEngineering at having gained experience in variousspheres of this c design, fault disturbance recording,international pr international marketing , deprojects, e tc. Th cts in which he has played arange from capacitive-co upling systems (SCC) and control units (UAC) forHydro-Quebec to the supercon ducting magne tic energy (SMES)system with RE Q and rural telephony with Quebec-TW He is amember of the Ordre des Ingknieurs du Qukbec.Gadtan Beaulieu obtained a B.Sc. in Electrical Engineering in 1983 fromUniversitk Laval in Queb ec City. He joined Hydro-Quebec two years later asengineer in the transmission and distribution system planning departmentHis particular areas of activity include flicker, voltage sags, harmonics andother problems experienced by clients in the industrial sector as well as

power flow at the regional level and fault current analysis. He is also thecoordinator of private producers of hydro and wind energy at a regionallevel. He is a member of the Ordre des Ingenieurs du Que bec.


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1209DISCUSSION wherePi, s the power added to or supplied by the line, P is

the VA R power supplied to the line by the shunt capacitivebank and Po s the power provided to the load. Once thesystem is installed P is very little affected by PO ince theline voltage is practically unaffected by the load. P isnormally larger than Po.When designing a new substationWe ensure that the voltage vc at the terminals ofc2 (equal toVo at no-load) increases by a factor betweenh o 2 at full

D.A.McCALLUM (Hydro-Quebec, Varennes, Quebec, Canada).The authors are to be congratulated on an elegant solution toobtaining low amo unts of power in an economical w ay from an H Voverhead transmission line.Would the authors please advise what amount of shunt capacitiveMvar was added to the transmission line in the applicationdescribed in the paper, and i ts affect on the system voltage profile.Would they also please describe how this required shunt Mvar isaffected by the station ac line voltage available, the distributionvoltage chosen and the load required from the capacitive tap. guaranteed load. This implies that the voltage at the

terminals of L will not become larger than kV, , withManuscript received August 16, 1996. In this way P becomes dependent on the

maximum guaranteed output power P Algebra givesP,= - -I , l

Capacitive Divider Transformer

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