229 Palmer-Buckle Butler Sarma

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CHARACTERISTICS OF TRANSFORMER PARAMETERS DURING INTERNALWINDING FAULTS BASED ON EXPERIMENTAL MEASUREMENTS

Peter Palmer-Buckle Karen L. Butler N. D. R. SarmaStudent Member, IEEE Member, IEEE Member, IEEEPower System Automation Laborato~Department of Electrical Engineering

Texas A&M Universi~College Station, TX 77840-3128

Abstract: This paper describes in detail, field experimentsperformed on a single-phase, distribution transformer tostudy the behavior of transformer terminal parameters duringinternal winding faults. A custom-built transformer providedwith external taps was used for these tests. The taps wereused to stage various internal winding faults in thetransformer. Terminal values of voltages and currents weremonitored and the results presented. A comparison of theseresults with simulation results is also presented.Keywords: dist ribution transformers, windings, internal windingfaults, fault detection, fault diagnosis, incipient faults.

I. INTRODUCTIONInternal winding faults comprise about seventy to eightypercent of modern transformer breakdown [1] and this islikely to increase since loading transformers to their optimumcapacity is becoming normal practice. These winding faultsare a result of the degradation of the transformer winding dueto aging, high voltages, etc., which tend to cause a

breakdown in the dielectric strength of the insulation. Thisbreakdown either causes adjacent windings to short (turn-to-tum) or a winding to be shorted to ground (turn-to-earth).Several techniques have been developed for the detection anddiagnosis of these faults with a large proportion using thegases dissolved in the transformer oil (dissolved gas analysis)[2, 3] or determination of the degree of polymerization [4] ofpaper insulation. Other parameters used are temperature,thermal and electrical stress of insulation, insulation agingand overloading incidents.

This research investigates the viability of utilizingelectrical parameters (mainly terminal voltages and currents)of a transformer in an on-line transformer incipient faultdetection method. The magnetizing current in a normaltransformer is about ten percent of the full load current.During internal winding faults, depending on the location ofthe fault, the magnetizing current increases rapidly, Thedistribution of transformer current subsequent to an internalelectrical fault therefore differs entirely fi-om the distributionof normal load or no-load currents and is governed mainly bythe internal reactance of the windings [5].This paper presents results of experiments performed on a

custom-built, 25kVA, 60Hz, 7200V1240VI120V, single-phase transformer connected to a 25kW load. To preventdamage to the transformer during faults due to highcirculating currents, the experiments were performed at a lowvoltage level. The results presented in this paper are thereforeat the reduced voltage level. Future work would beperformed at the rated values. The transformer was providedwith taps on both the primary and secondary windings whichwere used to stage the following cases of faults:1. Turn-to-earth and turn-to-turn faults on the primary

windings.2. Turn-to-earth and turn-to-turn faults on the secondasywindings.A variac capable of supplying up to 140V was used to supplypower to the primary side of the transformer and digitalmeters were used to monitor the terminal voltages andcurrents.The recorded values of voltages and currents were used tovalidate computer models compatible with the AlternativeTransients Program (ATP) [6] to simulate internal windingfaults of single-phase, distribution transformers. The modelsused were based on those developed for three-phase, powertransformers by Bastard et al. [7].

II. COMPUTER SIMULATION MODELSIn ATP, a single-phase, two-winding transformer shown as aT-circuit in fig. 1 is represented by 2X2 matrices of [R] and[L].

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R]E1L]=[$l1 (1)where the Ri represents the winding resistance, Li, thewinding inductance and A4~,the mutual inductance betweenwindings i andj. Subscript 1 refers to the primary windingand 2 the secondary winding.

11 R1 L, L2 R, 12

+ +

1, 12

- L4 ~na,i= +

v, b, b T NJ V2

nmi= J Fig. 3. Single-phase transformerwith a turn-to-turn fault on primary.

v

. IL. [R] =V*Fig. 1.T-Circuit Representation of a Single-Phase, Two-WkdingTrrmsfonner.

BCTRAN, a supporting routine of ATP is used to derive alinear [R] and [L] representation for the single-phase, two-winding transformer using open-circuit and short-circuit testdata at rated frequency.To model internal faults, the [R] and [L] matrices arerevised where some of their elements are from the BCTRANoutput and the other elements computed using mathematicalequations modeling the faulted transformer as proposed byBastard et al [7].A turn-to-earth fault on the primary (originally having N1turns with Nz turns on the secondary) divides the windinginto two sub-coils as shown in figure (2).In this case the faulted transformer is represented by 3X3matrices of [R] and [L] shown in (2).

Fig. 2. SingIe-phase transformer with a turn-to-earth fault on primary.

R]=lNIL]=E::~l2)Figure (3) shows the case of a turn-to-tun fault on theprimary. This divides the winding into three sub-coils and asshown in (3a) and (3 b), it is represented by 4X4 matrices of[R] and [L].

[L] =

R(?O o 0OR bi)oOORCOo 0 OR2 I

La ~.b Mac M.2Mb. Lb Mb. Mb2Mca Mcb L. Mc2LM2. M2b M2c L2

(3a)

(3b)

In both cases, the values in italics for the resistance matricesare computed using proportionality while those of theinductance matrices are computed using the rules ofproportionality, leakage and consistency as outlined in [7].R2 and L2 are given by BCTRAN.

III. FIELD TESTSA. EXPERIMEN TAL S ETUP

The experimental setup (shown in figure 4) comprisesmainly the transformer, primary and secondary panel boardsand a variable resistive load bank.

Fig. 4. Experimental setup for the field tests

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The transformer, shown in fig. 5, is custom-built based on thefollowing specifications:

Fig. 5. Single-phase, two-winding transformer with tapsq Power rating 25kVA. Rated primary voltage 7200Vq Rated seconda~ voltage 1201240V. Rated primary current 3.472A. Rated secondary current 208.4I1O4.4A. Primary turns 780. Secondary turns 13/26The transformer is provided with taps on both the primaryand the secondary sides. There are 12 taps on the primary and10 taps total on the secondary. These taps are external to the

transformer as shown in figure (5). For ease of simulation ofinternal winding faults, these taps are connected to panelboards through cables. The cables used on the primary sideare 15kV class capable of withstanding not more than10Arms of current and the secondary side cables are of the600V class. They can withstand up to 400Arms.

Fig. 6. Primary and secondmy panei boards showing connectors.

Both panel boards (fig. 6) are made of G-10 (epoxy), 0.5thick. Copper strips bolted to the panel board at 1.0separation are used as bus bars on the primary to facilitateease in connections while on the secondary side, mechanicallugs are used due to the thickness of tihecables. Instrumenttransformers on both the primary and the secondary sidesreduce terminal voltages and currents to measurable levels.B. R EDUCED VOLT AGE T ES T P ROCEDUR E

The simulation revealed very high circulating currents inthe shorted windings. Currently we are investigating ways ofreducing these currents to levels that would not damage thewindings of the transformer. The results presented in thispaper therefore represent values obtained when the suppliedvoltage was far less than the rated on the primary.The primary was supplied using a single-phase variaccapable of supplying up to 140V, 22A. The variac wasconnected to the primary panel board (fig. 7) and a resistiveload of 2.3040hms connected to the secondary side. Digitalmeters were used to monitor primary and secondary voltagesand currents as well as circulating currents during faults. Tostage an internal fault, a tap is either connected to the ground(turn-to-earth fault) or to another tap (turn-to-turn fault).The voltage on the variac is varied sllowlytill a voltage of

about 10OVis reached. The primary voltage was limited to100 V due to high circulating currents flowing through theshorted windings. This could damage the transformer. Themaximum allowable current that could flow through thewindings was calculated based on (4), supplied by thetransformer manufacturer. The terminal voltages and currentsas well as the circulating current were recorded.

n =

Load

IL--+JJ,

Fig. 7. Setup for reduced voltage tests.The A ,s are ammeters, Vs are voltmeters, AC is a 120V,60Hz mains, Vu is single-phase, 60Hz, 120V input, 140Vmaximum output, variac, HI and H2 represent terminals ofthe primary winding and XI and X.2are the terminals of thesecondary winding.

AT= K .12. T~in,. (Q/#) (4)

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where AT isthe temperature rise in degrees Celsius,K = 0.343 is a constant,I is current in amps,T is time in minutes andQ/# isthe ohms per pound of conductor.

The maximum AT for the transformer was limited to 115C.Knowing the ohm per pound of the conductor used (which iscopper for the primary and aluminum for the secondary inthis case) and the time, the maximum current was calculated.

IV. RESULTSA. FIELD TESTS

Tables 1 and 2 give recorded values of voltages andcurrents for turn-to-turn faults on the primary and secondaryrespectively. Both tables show an increase in primary currentas the number of turns shorted increase. The primary voltagewas kept almost constant at 100V. The secondary voltage inTable 1 remained fairly constant as expected. Since the loadwas purely resistive and maintained at 2.304 ohms, thecurrent flowing through the load was in direct proportion tothe secondary voltage. In Table 2, the secondary voltagedecreased as the number of shorted turns increased. This isbecause as more turns are shorted, few effectively remain incircuit.Tables 3 and 4 give results of turn-to-earth faults. Like inTables 1 and 2, the primary current increases with increasingshorted turns in both cases. The secondary voltages in thiscase decrease with increasing shorted turns for turn-to-earthfault on both windings.All voltage and current values are rms.

Table 1, Terminal values of turn-to-turn faults on primary.Fault on primary

Short I Number I Primary I Secondwy I Primary SecondaryTurns [ Voltage I Voltage I Current CurrentNormal I 01 100.OI 3.3091 0.03 I 1.49E9-lof 21P4-5f 28P5-6f 25P1-2f 41P4-6f 5(P6-7f 57P5-7f 8$P4-7f 112P4-8f 22L 99.8100.2100.C100.C100.3100.3100.1100.4100.2 3.326 0.38 1.463.316 0.51 1.463.320 0.53 1.533.234 0.78 1.493.334 1.02 1.543.337 1.03 1.533.336 1.61 1.533.348 2.29 1.533.467 5.34 1.61

Table 2. Terminal values of turn-to-turn tkadtson secondary.

EhortNormalS16-17fs19-2ofs2-4fS8-10fslo-13fE16-20fs13-17fS16-19fS8-13fS2-8fs4-lofs13-2ofs2-lofs4-13fs2-13f 2 99.8 3.165 0.14 1.372 99.8 3.166 0.12 1.283 100.2 2.987 0.41 1.323 100.2 3.021 0.39 1.374 100.0 2.682 0.68 1.214 100.1 2.390 1.11 1.045 99.9 2.333 0.92 0.975 100.4 2.665 0.82 1.346 100.0 2.076 1.20 0.917 100.0 1.933 1.30 0.867 100.1 1.931 1.32 0.918 100.3 1.622 1.54 0.659 100.0 1.450 1.55 0.5810 100.4 1.190 1.75 0.5912 100.4 0.970 1.88 0.36Table 3. Terminal values of turn-to-earth faults on primary.m=hort Number Primary Secondruy PfimrrryTable 4. Terminal values of turn-to-earth faults on secondary.kRiiF=$=Fhort Number Primary Secondmy Primary

B. COMPARIS ON BETWEEN FIELD AN D S IMULATIONRESULTS

Figures 8, 9 and 10 show a comparison between field andsimulation results of secondary voltage, primary current andsecondary current for a turn-to-turn fault on the primaryrespectively.

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taps shortedIEiField s Simulation \

Fig. 8. Comparison between field and simulation results: secondmyvoltage, turn-to-turn fault on primary.

5.0g 4.0: 3.0x2 2.0~? 1.0

0.0

taps shorted

H Field H SimulationFig. 9. Comparison between field and simulation results: primaryeurren~ turn-to-turn fault on primary.

2.001.801.601.401.201.000.800.600.400.200.00

taps shortedu Fie ld . S im ula t ion

V. DISCUSSIONS AND CONCLUSIONResults of field experiments carried out on a custom-built

transformer fitted with taps to simulate internal windingfaults are presented. Comparisons between field andsimulation results are also given. The terminal voltages andcurrents behaved as the simulation. The slight discrepanciescould be due to the following factors:q During the field experiments, it was not easy to get theexact primary voltage as used in the simulation.q Leakage factors were assumed to be negligible in somecases while in others, they were assumed to be small.This was for ease in calculation of the inductance

parameters using the method proposed in [7].. The cables used in the field for (connection to the tappositions on the panel board were modeled as pureresistors.The primary current as stated earlier increased withincreasing number of shorted turns. In the case of faults onthe primary winding, as the shorted turns increase, the

effective number of turns across the primary decrease.However, the primary voltage is maintained constant at100V, This causes the magnetizing current and hence theprimary current to increase rapidly [5]. For faults on thesecondary side, the increase in primary current is not rapid.The increased primary current is due to the high circulatingcurrent in the shorted windings. This current flows inopposition to the normal flow of current in the winding. Theeffective flux which is dependent on the primary voltage isreduced. More current must therefore be drawn from theprimary to bring the flux to the value proportional to theprimary voltage.As expected, the secondary voltages were not significantlyaffected when the faults were on the primary winding.However, for faults on the secondary winding, the voltagedecreased with increasing shorted turns, This is due to thedecrease in effective number of turns across which the load isconnected, The secondary current as stated earlier is in directproportion to the secondary voltage in adlcases and hence hasthe characteristics of the secondary voltage.These experiments were performed to validate and ifnecessary fine-tune the models used in the simulations. Fromthe results of the comparison plots, it can be concluded thatthe models agree well with the actual transformer underinternal winding fault conditions. Future work wouldinvestigate the dependence of the voltages and currents onthe fault position.

VI. ACKNOWLEDGMENTThe authors greatly acknowledge the National ScienceFoundation through grant ECS-9522208 for their support ofthis work.

Fig. 10Comparison of field andsimulation results: secondary current, turn-to-tum fault on primwy

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VI. REFERENCES[1] StigrmLA . C. Franklin, The .J&PTrarr

229 Palmer-Buckle Butler Sarma

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