Fast Ferroresonance

158 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 18, NO. 1, JANUARY 2003

Fast Ferroresonance Suppression of CouplingCapacitor Voltage Transformers

Milan Graovac, Reza Iravani , Senior Member, IEEE , Xiaolin Wang, and Ross D. McTaggart , Member, IEEE

Abstract—This paper describes a procedure for fast suppres-sion of the phenomenon of ferroresosnance in coupling capacitorvoltage transformers (CCVT) without major change in the CCVTdesign. It will be shown that it is possible to adjust parameters of the secondary overvoltage protection and the filter circuit so thatthe ferroresonance can be cleared in a very short time interval.The study cases reported in this paper show that ferroresonance iseffectively cleared within two cycles. An implementation of metaloxide varistors (MOV) as part of passive ferroresonance protec-tion is also addressed. The Electromagnetic Transients Program(EMTP) is used for modeling transients and fine-tuning the fer-roresonance suppressing circuit. The studies are conducted on theTrench TEHMP161A CCVT.

Index Terms—CCVT, CVT, electromagnetic transients, EMTP,ferroresonance.

I. INTRODUCTION

CCVT is a widely used apparatus for voltage measurement

at transmission and subtransmission voltage levels. The

output voltage of a CCVT is used for monitoring, protection

relays and control applications. Proper design and tuning of

CCVT components guarantee that its output is the required

replica of the input (system voltage) under steady-state con-

ditions. However, due to the CCVT energy storage elements

and magnetic saturation nonlinearity, its output waveform

deviates from the input waveform during transients [ 1], [2].

The phenomenon of ferroresonance is of particular concern

during CCVT transients, and can cause noticeable deviation of

CCVT response from the actual input waveform.

CCVTs are equipped with overvoltage protection schemes

and suppression circuitries to mitigate the phenomenon of fer-

roresonance. The required time to fully mitigate ferroresonance

can be up to several cycles (of 60 Hz). The phenomenon of

ferroresonance highly distorts the CCVT response during the

first three to five cycles, and results in relatively mild distortion

during the next four to six cycles. This characteristic behavior

of CCVT is well understood and imposes no major difficulty in

conventional applications.

Rapid proliferation of 1) digital protection relays and 2)

power electronic apparatus which require accurate voltage

monitoring for their controls, indicates ever increasing fidelity

Manuscript received September 10, 2001; revised February 27, 2002.M. Graovac, R. Iravani, and X. Wang are with the Center for Applied

Power Electronics (CAPE), Electrical and Computer Engineering Depart-ment, University of Toronto, Toronto, ON M5S 3G4, Canada (e-mail:[email protected]; [email protected]; [email protected]).

R. D. McTaggart is with the Instrument Transformer Division, Trench Group,Scarborough, ON M1P 3B5, Canada (e-mail: [email protected]).

Digital Object Identifier 10.1109/TPWRD.2002.803837

Fig. 1. CCVT circuit diagram for EMTP simulation.

of CCVT response during transient regimes. This paper demon-

strates that by means of fine-tuning of overvoltage protection

and suppression circuits, the phenomenon of ferroresonance

can be effectively mitigated within two cycles for the study

system. The objective is to achieve this improvement without

major change in the design and circuit topology of the CCVT.

II. STUDY SYSTEM

Fig. 1 shows schematic diagram of the Trench TEHMP161A

CCVT used for the studies. The CCVT parameters are given in

the Appendix.

The CCVT is composed of 1) capacitive voltage divider (

and ), 2) Step-down transformer (SDT), 3) compensating re-

actor (CR), 4) overvoltageprotection device and its seriesre-

sistor , and 5) ferroresonance suppressing circuit consisting

ofsaturablereactor( and ) inparallel withpermanentload

. In some designs, the burden side can also be equipped with

overvoltage protection device and its loading resistor

as part of the ferroresonance suppressing circuit. However, this

is not a necessary part for TEHMP161A CCVT. Switch in

series with is only introduced in the simulated circuit to im-

pose transients, otherwise it is not part of the CCVT system.

is the lumped resistance of the CCVT high voltage capacitive

branch.

Under normal conditions, the suppression circuit provides a

constant burden of . A salient feature of a CCVT equipped

with this kind of ferroresonance suppression circuitry is its flat

frequency response up to several hundred Hz. To the contrary,

a CCVT with tuned RLC filter based ferroresonance protection

circuit does not exhibit flat frequency response [ 3], [4].

0885-8977/02$17.00 © 2003 IEEE

Authorized licensed use limited to: Universidad de chile. Downloaded on November 19, 2008 at 12:26 from IEEE Xplore. Restrictions apply.

http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-

GRAOVAC et al.: FAST FERRORESONANCE SUPPRESSION OF COUPLING CAPACITOR VOLTAGE TRANSFORMERS 159

Fig. 2. Magnetization curve of .

Studies also show that ferroresonance suppression RL circuit

with saturable reactor has little effect on the CCVT transients.

The current through the reactor is negligible unless the ferrores-

onance is present so the only ferroresonance suppression circuit

component affecting the transient response is loading resistor

[1]. Therefore, the CCVT transient response to a primary

voltage collapse is less distorted when ferroresonance suppres-

sion RL circuit, with saturable reactor, is used. This results in

significantly smaller distance relay overreach [ 2].

has a knee point at about 150% of the rated voltage, Fig. 2,

and unless saturated has no effect on the CCVT response. Fer-

roresonance is accompanied with secondary side overvoltage

which saturates and triggers conduction of (if it exists in

the system). Consequently, the accumulated energy dissipates

in and .

It should be noted that ( ) represents either a spark-gap

with pre-specified break-down voltage or a semiconductor

switch (e.g., triac) which is triggered at a pre-specified voltage

level. TEHMP161A CCVT utilizes a spark-gap.

III. STUDY RESULTS

The CCVT ferroresonance response is investigated under the

following conditions. Initially switch is open and burden is a

series RL branch which consumes 1 VA at 115 and lag-

ging power factor of 0.85. Switch is closed ( m )

and after approximately seven cycles opened. Opening satu-

rates SDT and results in ferroresonance. DCG EMTP [5] isused

for modeling the CCVT ferroresonance test.

A. Case 1

This case investigates the effect of suppressing filter on the

CCVT ferroresonance phenomenon when and are not

included in the CCVT circuit. The ferroresonance suppressing

circuit consists of resistor in parallel with the sat-

urable reactor. Filter reactor has the magnetization curve shown

in Fig. 2 and the internal resistance of . is set

to fire at 250 V.

Fig. 3 shows the CCVT ferroresonance behavior. The first

diagram shows voltage during the test. The second dia-

gram shows RMS values of the same voltage calculated from

zero crossing to zero crossing. The third diagram is the enlarged

version of the second one (around the rated RMS value). It is

assumed that the ferroresonance is cleared if the RMS voltage

value deviates less than 5% from the rated value of 115 V. It

should be noted that half-cycle RMS results in higher deviation

as compared with full-cycle RMS.

Fig. 3. CCVT response to the ferroresonance test when and are notincluded in the system.

Fig. 4. Measured CCVT secondary voltage during the ferroresonance testwhen and are not included in the system. Time scale is 100 ms/div.

Fig. 3 indicates that ferroresonance is cleared after nearly

ten cycles. Overvoltage during the first cycle is very high. The

reason is that current through lags voltage by almost

90 . Therefore, reactor saturates only after the voltage has al-

ready reached the peak value. The CCVT ferroresonance re-

sponse of Fig. 3 closely agrees with the laboratory test result

shown on Fig. 4 and satisfies ANSI C93.1 standard [6]. Figs. 3

and 4 clearly illustrate that CCVT output voltage is highly dis-

torted during the first 4 cycles, and mildly distorted during the

next several cycles.

B. Case 2

This case investigates the phenomenon of ferroresonance

when gap and are also included in the CCVT

model. Breakdown voltage of is set to 250 V. Fig. 5 shows

that ferroresonance is cleared in 13 cycles.

Comparison of Figs. 3 and 5 indicates that presence of

and has a detrimental effect on the CCVT response. The

reason can be explained using Fig. 6.


http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-

http://-/?-


Fig. 5. CCVT response to the ferroresonance test when and areincluded in the system.

Fig. 6. Current components through and saturable reactor duringferroresonance.

Fig. 6 shows distribution of current between the saturable re-

actor branch and branch during ferroresonance. Secondary

voltage experiences a steep rise when the short-circuit cur-

rent is interrupted by switch . Saturable reactor saturates be-

fore voltage reaches to the voltage breakdown level of . Thus

reactor stores some energy during the first half-cycle of tran-

sient and returns a portion of this energy to the CCVT system

during the next half-cycle. If branch – is not included in

the system, this energy will result in the peak voltage of about

500 V in the second half-cycle as shown in Fig. 3. Presence

of – branch limits this peak voltage to about 400 V as

shown in Fig. 6. Reactor once again saturates during the

second half-cycle and also returns some of its stored energy to

the system in the third half-cycle. This in turn results in over-

Fig. 7. CCVT response to ferroresonance test for .

Fig. 8. CCVT response to ferroresonance test for .

voltage in the third half-cycle, but with a noticeably smaller

peak value as compared with the previous half-cycle. After the

second half-cycle, the SDT core is not saturated, but the tran-

sient will be experienced during the next few cycles. Duration

of this transient behavior primary depends on the overvoltage

during the second half-cycle.

Fig. 6 also indicates that the ferroresonance can be more ef-

fectively mitigated if we have the following.

• Phase shift between and the current through is

reduced.

• Dissipated energy in is increased to improve distribu-

tion of dissipated energy between and .

Both issues are addressed by increasing . Fig. 7 shows the

effect of different values of on the CCVT response to the

ferroresonance test when . It should be noted that

increasing beyond a certain value reduces the effectiveness

of since it can limit to a value below the breakdown

voltage of .

Fig. 8 shows that the CCVT response can be further improved

by fine-tuning .

Fig. 9 shows that the phenomenon of ferroresonance is effec-

tively damped out in one cycle when and

.



Fig. 9. CCVT response to ferroresonance test when and

.

Overvoltage protection device can be realized either by a

spark-gap or a triac. In any case must conduct only during

ferroresonance conditions and not as a result of systemovervolt-

ages which are reflected to the CCVT burden side. In this re-

spect, triac is more advantageous as its firing instant and firing

voltage can be more accurately controlled. Furthermore, triac

has capability for large transient overcurrents without reaching

to its thermal limit.

Fig. 10 shows the CCVT response to the hypothetical sce-

nario in which malfunctions andpre-maturely conducts. Pre-

mature conduction of initiates a transient process which re-

sults in brief saturation of SDT after a few cycles. The transientis promptly cleared by the saturable reactor . Fig. 10 indi-

cates that saturable reactor also provides an inherent self-pro-

tection against internal failures. Tuned RLC ferroresonance sup-

pression circuits do not have such a capability.

C. Case 3

An alternative to overvoltage protection device (either

spark-gap or triac) and is a Metal Oxide Varistor (MOV).

Advantages of MOV are 1) stable characteristic, 2) simple de-

sign and robust structure, and 3) high performance reliability.

However, the concern is the limited amount of energy that MOV

can dissipate.

To examine the effectiveness of the MOV in the CCVT sys-

tems, and its series resistance were replaced by a 130

MOV. The energy rating of MOV is 210 J within 2 ms.

is changed within the acceptable range to fine-tune the CCVT

system. Fig. 11 shows the CCVT response to the ferroresonance

Fig. 10. CCVT response to premature conduction of .

Fig. 11. CCVT response to ferroresonance testwhen and are replacedby MOV.

test when . Fig. 11 shows that ferroresonance is

cleared in four cycles. The total dissipated energy in MOV is 93

J which is significantly less than the rated value of 210 J.



Fig. 12. Sharing theburden between theMOVand thesaturable reactor duringthe ferroresonance test.

Fig. 13. CCVT response to the ferroresonance test for V,

, and .

Fig. 12 shows current components of MOV and saturable re-

actor during ferroresonance. MOV conducts when is

larger then 280 V. Therefore, it dissipates energy only during

part of the second half-cycle. Fig. 12 also illustrates that the sat-

urable reactor remains active and provides protection during the

rest of the transient period.

IV. DISCUSSION

For conventional applications where the CCVT ferroreso-

nance mitigation is acceptable within several cycles, e.g., 10

cycles, the suppression circuit of Case 1 (i.e., , and

) provides the desired solution. This approach is currently

adopted in the Trench TEHMP161A CCVT. If high-speed (e.g.,

two-cycle) ferroresonance clearing is required, the suppression

circuit can be augmented with the branch composed of

and ; see Fig. 1. represents either a spark-gap or a

triac. Theoretically, both triac and spark-gap provide the same

performance when fired at the same voltage level. However,

firing of the triac can be precisely controlled and practically the

required performance can be achieved with a higher degree of

precision. One solution to assure that spark-gap does not fire

prematurely is to utilize a gap with a higher breakdown voltage.

However, increasing the breakdown voltage will reduce ef-

fectiveness of the overall ferroresonance mitigation system.

Fig. 13 shows the CCVT response to the ferroresonance test

when the gap breakdown voltage is increased from 250 (Fig. 9)

to 350 V. Comparison of Figs. 9 and 13 indicates that increasing

breakdown voltage has increased the required time for clearing

ferroresonance from one to five cycles, respectively.

The studies also indicate that the combination of and

can be replaced by a MOV. Although MOV is not as effec-

tive as a triac/spark-gap in clearing ferroresonance, it can be

used in combination with the suppression filter for ferroreso-

nance mitigation within two cycles. An advantage of MOV over

triac/spark-gap is that it has a more stable – characteristic,

and it is easier to install, operate, and maintain.

V. CONCLUSIONS

The study results presented in this paper demonstrate that

the CCVT ferroresonance protection can be improved, without

a major change in the design, to mitigate the phenomenon of

ferroresonance within a prespecified time period. It is shown

that the well-tuned triac/spark-gap overvoltage protection can

clear the ferroresonance within two cycles. For cases when

pre-mature breakdown failure of the burden-side overvoltage

protection device is a concern, the MOV may be used. MOV

based ferroresonance mitigation may not be as effective as

triac/spark-gap protection, however it noticeably improves the

CCVT ferroresonance response. In all the studied cases, the

suppression circuit, i.e., – – , is the primary counter-

measure to the phenomenon of ferroresonance and either triac,

spark-gap or MOV augments its ferroresonance damping effect.

APPENDIX

Tables I-IV contain technical data for use throughout this

paper.

TABLE ITRENCH TEHMP161A CCVT TECHNICAL DATA



TABLE IICCVT CIRCUIT PARAMETERS

TABLE IIISDT MAGNETIZATION CURVE DATA

TABLE IV MAGNETIZATION CURVE DATA

REFERENCES

[1] M. A. Hudges, “Distance relay performance as affected by capacitorvoltage transformers,” Proc. Inst. Elect. Eng., vol. 121, no. 12, pp.1557–1566, December 1974.

[2] D. Hou and J. Roberts, “Capacitive voltage transformers—Transientoverreach concerns and solution for distance relaying,” in Proc. 22nd

Annu. Western Protective Relay Conf..[3] M. Kezunovic, Lj. Kojovic, V. Skendzic, C. W. Fromen, D. R. Sevcik,

and S. L. Nilsson, “Digital models of coupling capacitor voltage trans-formers for protective relay transient studies,” IEEE Trans. Power De-livery, vol. 7, pp. 1927–1935, Oct. 1992.

[4] H. J. Vermeulen, L. R. Dann, and J. van Roojen, “Equivalent circuitmodeling of a capacitivevoltage transformer for power system harmonicfrequencies,” IEEE Trans. Power Delivery, vol. 10, pp. 1743–1749, Oct.1995.

[5] DCG/EPRI, “Electromagnetic Transients Program (EMTP), Version3.0,” in Rule Book . Palo Alto, CA: Main Program, Nov. 1996, vol. 1.

[6] Power-line carrier coupling capacitors and coupling capacitor voltagetransformers (CCVT)—requirements, ANSI C93.1-1999, May 1999.

Milan Graovac received the B.Sc., M.Sc., and Ph.D. degrees from the Univer-sity of Belgrade, Yugoslavia, all in electrical engineering in 1980, 1987, and1992, respectively.

From 1981 to 1993, he was with Electrical Engineering Department, Univer-sity of Belgrade. He is now a Senior Research Associate at the University of Toronto, Toronto, ON, Canada. His research interests include power system re-liability, dynamics and control, FE modeling, and power electronics.

Reza Iravani (M’85–SM’00)received the B.Sc.degree in electrical engineeringin 1976 from Tehran Polytechnique University and started his career as a con-sulting engineer. He received the M.Sc. and Ph.D. degrees in electrical engi-neering from the University of Manitoba, Canada, in 1981 and 1985, respec-tively.

He is a Professor at the University of Toronto. His research interests includepower electronics and power system dynamics and control.

Xiaolin Wang received his B.Sc. and M.Sc. degrees from Xi’an Jiaotong Uni-versity, China, in electrical engineering in 1982 and 1988. From 1982 to 1994,he was with Electrical Engineering Department at Xi’an Jiaotong University.

He is a Senior Research Associate in the Department of Electrical and Com-puter Engineering, University of Toronto, Canada. His research interests areelectromagnetic transients and control/protection in power system.

Ross D. McTaggart (M’85) was born in Oshawa, ON, Canada, in 1952. Hestudied electrical engineering at the Universityof Toronto and receivedthe B.Sc.degree in 1976.

He becamea ProfessionalEngineerin 1979. He worked as an Engineer at sev-eral electrical manufacturers before joining Trench Electric in 1986 to developan SF6 current transformer product line up to 800 kV. When this was complete,he became the Engineering Manager and is now the R&D Manager at the In-strument Transformer Division.

Mr. McTaggart is an active member of the IEEE and IEC instrument trans-former standards committees and working groups.


Documents

Fast Ferroresonance