Bushing Test Errors

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1350 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY2006 Analysis of Some Measurement Issues in Bushing Power Factor Tests in the Field S. Zhang, Member, IEEE Abstract—The power factor measurement is widely used in the evaluation of bushing insulation conditions in the field. Some mea- surement issues occur in the field tests which give abnormal read- ings of bushing and/or power factors. This paper provides an engineering analysis of the possible measurement issues in the bushing field tests such as testing a bushing on a transformer or in its shipping crate, or a bushing with wet/contaminated voltage/test tap or wet/dirty porcelain surface. Index Terms—Bushing, , , crate, porcelain, power factor, test tap, transformer, voltage tap. I. INTRODUCTION T HE measurement of power factor is widely used in the evaluation of bushing insulation conditions in the field. The insulation of a bushing can be illustrated with a capacitor and a resistor in parallel (Fig. 1). With the voltage applied to the bushing, both capacitive current and resistive current flow through the bushing insulation. The bushing power factor is the cosine of the angle , by which the total current leads the applied voltage . The magni- tude of bushing power factor is the ratio of the magnitude of the resistive current to that of the total current (1) The of a bushing is very small and is very close to 90 . For example, an oil-impregnated paper-insulated bushing typically has , which gives . The measurement of is very sensitive to the small changes of the bushing insulation as well as the testing environ- ment. The leakage currents could flow into the test circuit and cause measurement issues. The “guard circuit” of some general portable measurement equipment is used to have unwanted currents bypass the measurement circuit. The measurement errors could occur if testers do not follow certain instructions when testing bushings on transformers. Some conditions such as a wet/contaminated bushing voltage tap or test tap, or wet/dirty porcelain surface could cause mea- surement issues. Testing a bushing in its shipping crate could also give abnormal readings. This paper analyzes the measurement issues that occurred in these cases. For simplicity, we use capacitors to represent and of bushing insulation in our discussions below. Manuscript received June 20, 2005; revised December 1, 2005. Paper no. TPWRD-00347-2005. The author is with PCORE Electric Company, Inc., LeRoy, NY 14482 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/TPWRD.2006.874616 Fig. 1. Illustration of bushing insulation. (a) Equivalent circuit. (b) Phasor diagram. Fig. 2. Testing of bushing H1 on the transformer using the UST method (two phases of a three-phase transformer or a single-phase transformer are shown). II. TESTING BUSHINGS ON TRANSFORMERS It is well known that we must follow the following instruc- tions when testing bushing insulation on a transformer using the ungrounded specimen test (UST) method (Fig. 2) [1]. 0885-8977/$20.00 © 2006 IEEE

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Bushing Test Errors

Transcript of Bushing Test Errors

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1350 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY 2006

Analysis of Some Measurement Issues inBushing Power Factor Tests in the Field

S. Zhang, Member, IEEE

Abstract—The power factor measurement is widely used in theevaluation of bushing insulation conditions in the field. Some mea-surement issues occur in the field tests which give abnormal read-ings of bushing 1 and/or 2 power factors. This paper providesan engineering analysis of the possible measurement issues in thebushing field tests such as testing a bushing on a transformer or inits shipping crate, or a bushing with wet/contaminated voltage/testtap or wet/dirty porcelain surface.

Index Terms—Bushing, 1, 2, crate, porcelain, power factor,test tap, transformer, voltage tap.

I. INTRODUCTION

THE measurement of power factor is widely used in theevaluation of bushing insulation conditions in the field.

The insulation of a bushing can be illustrated with a capacitorand a resistor in parallel (Fig. 1). With the voltage applied tothe bushing, both capacitive current and resistive currentflow through the bushing insulation.

The bushing power factor is the cosine of the angle , bywhich the total current leads the applied voltage . The magni-tude of bushing power factor is the ratio of the magnitudeof the resistive current to that of the total current

(1)

The of a bushing is very small and is very close to90 . For example, an oil-impregnated paper-insulated bushingtypically has , which gives .

The measurement of is very sensitive to the smallchanges of the bushing insulation as well as the testing environ-ment. The leakage currents could flow into the test circuit andcause measurement issues. The “guard circuit” of some generalportable measurement equipment is used to have unwantedcurrents bypass the measurement circuit.

The measurement errors could occur if testers do not followcertain instructions when testing bushings on transformers.Some conditions such as a wet/contaminated bushing voltagetap or test tap, or wet/dirty porcelain surface could cause mea-surement issues. Testing a bushing in its shipping crate couldalso give abnormal readings.

This paper analyzes the measurement issues that occurred inthese cases. For simplicity, we use capacitors to representand of bushing insulation in our discussions below.

Manuscript received June 20, 2005; revised December 1, 2005. Paper no.TPWRD-00347-2005.

The author is with PCORE Electric Company, Inc., LeRoy, NY 14482 USA(e-mail: [email protected]).

Digital Object Identifier 10.1109/TPWRD.2006.874616

Fig. 1. Illustration of bushing insulation. (a) Equivalent circuit. (b) Phasordiagram.

Fig. 2. Testing C of bushing H1 on the transformer using the UST method(two phases of a three-phase transformer or a single-phase transformer areshown).

II. TESTING BUSHINGS ON TRANSFORMERS

It is well known that we must follow the following instruc-tions when testing bushing insulation on a transformer usingthe ungrounded specimen test (UST) method (Fig. 2) [1].

0885-8977/$20.00 © 2006 IEEE

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• Terminals of transformer windings at the same voltage asthe bushing under test should be connected together to theequipment voltage source (e.g., H1 and H2 in Fig. 2).

• Terminals of windings at other voltages should begrounded (e.g., X1 and X2 in Fig. 2).

• All of the covers of bushing voltage tap or test tap shouldbe closed except for the bushing under test.

We see from time to time that testers do not follow these in-structions and the test results turn out to be wrong. The logicbehind these recommendations is not well explained in any pub-lished document. With the inductance of the transformer wind-ings and the stray capacitance between the transformer windingsand the bushing foils, leakage currents flow into the test circuitand could cause measurement errors during bushing tests.

The following sections present the engineering analysis ofhow these steps minimize the effect of the inductance of trans-former windings when followed correctly.

A. Stray Capacitances Inside Transformer

As most transformers are core-form transformers, we usecore-form transformers for discussion in this paper. Usually,the high-voltage (HV) coil and the low-voltage (LV) coil inone phase of a core-form transformer are wound concentricallyaround the grounded core inside the transformer (Fig. 2). Sincethe windings are physically very close to each other and verytall, the capacitance between the HV and LV coils,1 , andthe capacitance between the LV coil and grounded corecould be very large compared to bushing capacitances.Depending on the actual dimensions, the capacitance betweenthe HV coil and the grounded tank wall could be very largetoo.

The capacitance-graded bushings have conductive foils in thebushing core extended below the grounded flange. Therefore, astray capacitance exists between the bushing foils and trans-former windings. When the bushing is tested on a transformer,the applied voltage will be also impressed on the windings.Then, there will be a leakage current flowing from the trans-former winding, through the stray capacitance , and into thebushing core (Fig. 2). Since this leakage current flows directlyinto the bushing core ( insulation) rather than the guard cir-cuit, the guard circuit cannot eliminate it from the test circuit.Therefore, it could cause measurement errors.

B. Terminals of Transformer Windings at the Same Voltage asthe Bushing Under Test Should be Connected Together to theApplied Voltage During Bushing Tests

As discussed before, when we apply a voltage to the bushingfor test, the stray capacitance carries a leakage current from thewinding into the bushing core and, hence, into the test circuit.If the terminals of windings at the same voltage as the bushingunder test are not connected together during bushing tests, theleakage current could cause measurement errors.

We will discuss the single-phase transformer first and thenmultiphase transformers next.

1It is a U.S. convention to use the letter “X” rather than “L” to representtransformer LV windings and terminals.

Fig. 3. Testing bushing H1 without connecting winding terminals togethercauses measurement error (shown in two terminals of one winding in asingle-phase transformer). (a) Leakage current flows into the measurement.(b) Equivalent circuit (C of bushing H1 consists of C and C in series).(c) Phasor diagram and power factor change.

1) On a Single-Phase Transformer: Fig. 3 shows the case oftesting bushing H1 on a single-phase transformer without con-necting terminals of the windings at the same voltage as thebushing under test. For simplicity, we only show the two termi-nals of one winding and put the effect of the other windings (aswell as the bushings connected to those windings) into the ca-pacitors and . In the equivalent circuit, the bushing

is represented with two capacitors and in series, andthe winding inductance is represented with two inductorsand in series. The points separating and winding induc-tance are the two terminals of the lumped stray capacitance .

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The transformer windings have resistance. However, for sim-plicity, we use inductors to represent the windings, and in thephasor diagram, the current through a winding lags the appliedvoltage across it by nearly 90 .

The currents through and will flow into the ground(which is the same connection as the guard circuit in the USTtest) and cannot influence the measurement. Since the reactanceof the winding inductance is much smaller than that of thecapacitances between windings and bushings ,we can use a simplified circuit in Fig. 3(b) for analysis, where

is the total of , and .Since is much smaller than , the current is much

smaller than . Therefore, and have almost thesame phase and magnitude. Since leads by nearly90 and leads by nearly 90 leads bynearly 180 .

Because the reactance of is much smaller than that ofis much smaller than . Since the combination

of and is the applied voltage and havealmost the same phase and magnitude. Hence, both and

lead by nearly 90 . The difference between them is theleakage current .

The actual current through the bushing insulation is (Fig. 1).Without leakage current , the measured current .However, flows into the test circuit and makes the measuredcurrent different from the actual current , which causesthe measurement error.2 The phasor diagram shows that themeasured is smaller than the actual ; hence, the measuredpower factor is higher than the actual value of the bushinginsulation.

It should be noted that the leakage current changes themeasured values of both resistive and capacitive currents. Eventhough is mainly capacitive, it affects the measurement of re-sistive current (Fig. 1) more than it does to the capacitive cur-rent (Fig. 1) because . Therefore, the testers wouldnotice the abnormal resistive current (or power loss) during thebushing power factor test. Depending on the value of the straycapacitance and the leakage path, the effect of the leakage cur-rent might also be so insignificant that the testers do not noticeits effect.

This leakage current cannot be eliminated using the method,such as the guard circuit, as was discussed before. However, wecan connect two terminals of the winding together to the appliedvoltage to minimize its effect.

Fig. 4 shows that when we connect the two terminals of thewinding together, we actually make the currents through thecapacitors , and flow into the guardcircuit (ground) and these currents cannot influence the mea-surement. Therefore, we can simplify the equivalent circuit inFig. 4(b), where the effective inductance is actually the re-sultant inductance of and in parallel.

2Strictly speaking, current i changes after the introduction of the leakagecurrent i , and is no longer equal to current i. However, we assume that i doesnot change here to simplify the diagram, and this assumption does not affect theanalysis results.

Fig. 4. Connecting terminals together minimizes the effect of the leakagecurrent and makes the measured power factor closer to the actual value ofthe bushing insulation. (a) Connecting terminals to minimize the effect of theleakage current. (b) Equivalent circuit (C of bushing H1 consists of C andC in series). (c) Power factor is closer to the actual value.

is smaller than , and the reactance of is muchsmaller than that of . Therefore, the branch of - in the

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simplified circuit is capacitive and its reactance is almost thesame as that of . Hence, the leakage current flows onlythrough the capacitance and it leads the voltage as well asthe total applied voltage by nearly 90 .

For comparison, we put the phasor diagrams of both beforeand after connecting the winding terminals together in Fig. 4(c).It is clear that by connecting the two terminals of the windingtogether to the applied voltage, the measured is much closerto the actual than not connecting the terminals; hence, themeasured power factor is much closer to the actual value of thebushing.

Therefore, connecting the terminals of the winding togetherto the applied voltage is to make the leakage current more capac-itive. As the leakage current is much smaller than the capacitivecurrent through the bushing insulation, the effect of this leakagecurrent is minimized.

2) On a Multiphase Transformer: When we test a bushingon a three-phase transformer with -windings connected to thebushing under test, each winding has two terminals connectedto two phases as well as two bushings. Each phase of the

-windings is similar to a single-phase transformer. Therefore,the single-phase transformer case applies to this case.

When testing a bushing on a three-phase transformer withY-grounded windings connected to the bushing under test, theneutral point should be ungrounded during the bushing tests.The reason is that by connecting the three phases together tothe applied voltage during the bushing test, the applied voltageis zero-sequence voltage since all three phases have voltages ofthe same polarity. With the neutral point still grounded, in casethe secondary winding has a good zero-sequence path (such asthe -winding), large zero-sequence current will flow throughthe circuit. The limiting factor of this zero sequence will bethe short-circuit impedance of the measuring equipment and theground resistance. Therefore, this current will be considerablyhigher than the maximum current that the measuring device canprovide. As we disconnect the neural point from the ground andconnect it to the applied voltage, such as the phase terminals ofthe windings, the situation is similar to the case of a single-phasetransformer.

When testing bushings on a three-phase transformer with un-grounded Y windings, the analysis is similar to the conditionon a single-phase transformer. By connecting the three-phaseterminals of the windings together, the effect is similar to thatwhich we discussed for the single-phase transformer.

C. Terminals of Transformer Windings at Different Voltagesfrom the Bushing Under Test Should be GroundedDuring the Tests

When testing a bushing on a transformer, the terminal oftransformer windings at different voltages from the bushingunder test should be grounded. Otherwise, when we apply thevoltage to bushings as well as windings, these ungroundedterminals will have floating potentials. As shown in Fig. 5,without grounding X1–X2 terminals, there is some voltage onX1–X2 during the tests. It certainly is unsafe if touched bytesters.

Fig. 5. Not grounding X1-X2 when testing bushing H1 will apply voltage onX1 and X2 and is an unsafe condition to testers.

Fig. 6. Leakage current through the wet tap increases the C power factor.

D. Covers of Voltage Tap or Test Tap Should be Closed forBushings Not Under Test

The covers of voltage tap or test tap of bushings not undertest should be closed during the tests. Energizing the bushingunder test will also energize some other bushings not under test;hence, leaving the cover open could leave very high voltage onthe bushing tap (and ) and damage the bushing insulation.It is also unsafe to testers.

III. EFFECT OF WET/CONTAMINATED TAP

If the voltage tap or test tap of a bushing is wet and/or con-taminated, a leakage current will flow from the tap pin, throughthe wet/contaminated tap, and to the grounded bushing flangeduring the bushing test. This would affect the test results.

The leakage current is resistive, and the leakage path is in par-allel with the bushing . Fig. 6 shows the effect of the wet/con-taminated tap on bushing and tests. In the UST test forbushing , the leakage current actually flows into the guard cir-cuit (ground). Therefore, the measured value of power factoris not affected by the tap condition.

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Fig. 7. Salty water is sprayed onto the tap to simulate the wet and contaminatedtap condition. (a) Wet bushing tap. (b) Bushing in the test.

TABLE IEFFECT OF WET AND CONTAMINATED TAP ON BUSHING TEST

In the grounded specimen test (GST) for bushing , theresistive leakage current flows into the measurement circuit.Therefore, the measured value of the power factor is muchhigher than the actual value.

We soaked the tap insulator in boiling water, put salt on thetap insulator, and sprayed the salty water onto the tap insulatorto simulate the condition of wet tap (Fig. 7). The test resultsafterwards on several bushings are shown in Table I.

Due to the tap condition, the tester may not be able to get anyreadings during the test because the protection of the testequipment could be triggered by the high leakage current.

It should be pointed out that the condition of the wet/contam-inated tap may occur together with a wet/dirty porcelain sur-face which will be addressed in the next section. Usually, thewet/contaminated tap is caused by some moisture on the outsidesurface of the tap. This condition may be corrected by cleaningand drying the tap before any tests in order to have the normalreading of the power factor. When using the tap adaptor inthe bushing test, it is very important to keep the tap adaptor frombeing contaminated.

Fig. 8. Effect of wet/dirty porcelain on the bushing UST test for C .(a) Equivalent circuit. (b) Phasor diagram.

IV. EFFECT OF WET/DIRTY PORCELAIN SURFACE

If the bushing porcelain surface is dirty and wet with contam-ination and moisture, leakage currents will flow on the porcelainsurface. If a bushing is tested in this condition, measurement is-sues could occur.

Fig. 8 shows the effect of the wet/dirty porcelain surface onthe bushing test using the UST method. The contaminationis usually evenly distributed on the porcelain surface, which hasan effect of two equivalent resistors in parallel with bushingand , respectively; and, hence, the two corresponding leakagecurrents and . The voltage drop across is so small that wecan ignore which is the actual current crossing during the

UST test. The leakage current flows into the test circuitand increases the measured power factor while flows awayfrom the test circuit and decreases the measured power factor.Therefore, when , the measured is smaller than theactual which gives a higher value of bushing power factor

; when , the measured is bigger than the actual, which gives a lower value of bushing power factor .

In the case that and the difference between them isgreater than the actual resistive current crossing bushing , themeasured , which gives both negative power factor andnegative resistive current (power loss).

When we test bushing with a wet/dirty porcelain surface,the analysis is shown in Fig. 9 and is similar to that of bushing

. The two equivalent leakage currents and , as well asthe resistive current crossing bushing , will together alter themeasured bushing power factor. Therefore, we may expectthe measured power factor to be higher than the actual value,lower than the actual value, or even negative.

To see the effect of the wet/dirty porcelain surface, wesprayed salty water on bushing porcelains and performed thebushing tests. The test results are shown in Table II.

The test results show that the spray condition is not repeat-able and the results are random from test to test. However, thewet/dirty porcelain surface does have a significant effect on the

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Fig. 9. Effect of wet/dirty porcelain on the bushing GST test for C .(a) Equivalent circuit. (b) Phasor diagram.

TABLE IIEFFECT OF WET/DIRTY PORCELAIN SURFACE ON BUSHING TESTS

test results and could cause the measurement issues on bothand power factors. Therefore, it strongly suggests that weshould clean and dry the porcelain surfaces before the bushingtests.

A similar situation was discussed regarding testing bushingswith resistance-graded (RG) porcelain [2].

As we pointed out before, the condition of a wet/dirty porce-lain surface may occur together with a wet/contaminated tap.Therefore, the tap should also be cleaned and dried.

V. TESTING BUSHING IN CRATE

Testers may incorrectly test a bushing in its shipping woodcrate. The resistance of the wood is not as high as that ofbushing porcelain. Therefore, there could be some leakagecurrent flowing through the wood during the bushing test. How-ever, the effects of wood on bushing and are different.

Fig. 10. Effect of crate on the bushing UST test for C .

Fig. 11. Effect of crate on the bushing GST test for C .

If we test bushing in the crate using the UST method, theequivalent circuit and diagram are shown in Fig. 10. The cratewood has contacts with the bushing porcelains and flange, whichmake a path for leakage current from part of the bushingthrough the crate wood and then to the grounded flange. As thevoltage drop across is very small, the leakage current isformed from the voltage across the leakage resistor and is inphase with . This resistive leakage current will turn the cur-rent away from and, hence, the voltage from . Sincethe test equipment only measures the angle between the total ap-plied voltage and the current flowing into the test circuit , themeasured angle is greater than the actual , which reduces themeasured value of bushing power factor . Further anal-ysis shows that the current is also reduced which makes themeasured value of capacitance lower than the actual value.

However, during the GST test for bushing , the leakagecurrent flows into the measurement circuit (Fig. 11). Therefore,the measured current has a phase shift from the actual cur-rent and its magnitude is greater than . Therefore, both themeasured power factor and capacitance are higher thanthe actual values of the bushing insulation.

We tested a 115-kV bushing (POC550G0800S) in its crate(Fig. 12), and the test results are shown in Table III. The bushingwas stocked outside in the crate, and experienced a scatteredshower and sunshine afterwards but before the tests. The crateappeared dry.

A bushing is usually shipped with plastic covers. The testshows that if the testers test the bushing in the crate with theplastic bags still on the bushing, the test results will be far dif-ferent from the actual values of the bushing.

A similar result was reported when a capacitor was hung witha crane sling which gave a negative power factor in the UST test[3].

It should be pointed out that the effect of the crate dependslargely on the condition of the wood. The testers may not seesignificant differences if they test the bushings in the crate. In

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Fig. 12. Experimental test of the bushing in crate.

TABLE IIIEXPERIMENTAL TESTS OF THE BUSHING IN CRATE

some cases, even if the bushing has no contact with the woodcrate, the stray capacitance could be large enough to cause mea-surement errors.3 Therefore, the bushing should not be testedlying on the crate.

VI. CONCLUSION

We draw the following conclusions from the discussionsabove.

1) When testing a bushing on a transformer without con-necting terminals of transformer windings at the samevoltage as the bushing under test to the applied testvoltage, the leakage current between the transformerwinding and bushing insulation could cause measure-ment errors. The testers could see the abnormal powerloss in this situation. If the terminals of transformer

3Personal communication with Mary Foster of Doble Engineering Company.

windings at the different voltages from the bushing undertest are not grounded, there will be some potential onsome bushings as well as transformer terminals, which isunsafe to the testers.

2) When testing a bushing on a transformer, all of the coversof bushing voltage/test taps should be closed except forthe bushing under test. Otherwise, these bushings couldbe damaged and it is also unsafe to the testers.

3) The wet and/or contaminated tap makes the bushingpower factor higher than the actual value but has no effecton the test results of the bushing power factor. Thetap should be cleaned and dried in this situation; the tapadaptor, if used in the tests, should have no contamination.In most cases, the issue of wet/contaminated tap occurstogether with the wet/dirty porcelain surface.

4) The wet and/or dirty porcelain surface can cause randommeasurement issues on both and power factorsduring bushing tests. Sometimes the test values are evennegative. We should clean and dry the porcelain surfacebefore bushing tests.

5) Testing bushing in crate could give a lower powerfactor but a higher power factor than the actual valuesof the bushing insulation. A bushing should not be testedin its shipping crate.

ACKNOWLEDGMENT

The author would like to thank the valuable discussions withM. Foster of Doble Engineering Company during the revisionof this paper.

REFERENCES

[1] Type M2H Instruction Manual—Electrical Insulation Testing, Water-town, MA, 1988. Doble Eng. Co.

[2] D. Zeng, “An improved method of measuring C power factor of re-sistance-graded bushings,” IEEE Trans. Power Del., vol. 14, no. 2, pp.437–442, Apr. 1999.

[3] L. Pong, “Review negative power factor test results and casestudies—Analysis and interpretation,” presented at the Int. Conf.Doble Clients, Boston, MA, Mar. 2002.

Shibao Zhang (S’99–M’02) received the B.S. degree in electrical engineeringand the M.S. degree in high voltage engineering from Tsinghua University,Beijing, China, in 1994 and 1997, respectively, and the Ph.D. degree in elec-trical engineering from Kansas State University, Manhattan, in 2001.

Currently, he is a Senior Engineer with PCORE Electric Company, Inc.,LeRoy, NY. His responsibilities include bushing design, development, test,and repair. He was an Electrical Engineer with Beijing Xuji Electric Company,Beijing, China, for one year.