analysis and location of partial discharges in power transformers by

7/27/2019 Analysis and Location of Partial Discharges in Power Transformers By

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ANALYSIS AND LOCATION OF PARTIAL DISCHARGES IN POWER TRANSFORMERS BYMEANS OF ELECTRICAL METHODSH Tatizawa , G F Buranilnstituto de Eletrotecnica e Energia da US P (IEEUSP)BRAZIL

Abst ract

Considering the number of failures in highvoltage ' power transformers caused byproblems in bushing and winding insulation, itis important to study means for diagnosingincipient failures, to avoid long unavailabilityperiods and its consequences.The aim of this work is to study detection andmeasurement procedures, concerning theevaluation of position of partial dischargeinside the transformer, in order to evaluatethe degree of danger for the equipment. Themethodology is based on a noninvasivemeasurement of partial discharges from thetransformer bushing. The position of thepartial discharge source is estimatedconsidering the transformer winding model,and the measurement and proper evaluationof the response to the partial dischargepulses. By means of a proper modeling,measurement and interpretation, thelocalization of the problem can be performed.For the measurements, the conventionaldetection technique proposed by IEC 270Standard was used

The authors wish to-acknowledgeFundaGIode Amparo a Pesquisa do Estado de SloPaulo, proc. 98/10799-4, for their supportto this study

Introduction

Considering the number of failures in highvoltage power transformers caused byproblems in bushing and winding insulation, it

is important to study means for diagnosingincipient failures, to avoid long unavailabilityperiods and its consequences.The aim of this work is to study detection andmeasurement procedures, concerning theevaluation of position of partial dischargeinside the transformer, in order to evaluatethe degree of danger for the equipment.The methodology is based on a noninvasivemeasurement of partial discharges from thetransformer bushing. The measured responseis a function of the transformer windingelectrical parameters, and the partialdischarge location. The most important isparameter m, which is the relationship of thecapacitive coupling between windings andbetween windings and neutral (or transformermetallic enclosure/core).

with C2 denoting winding equivalent seriescapacitance and C1 equivalent capacitancebetween winding and transformer core (andtank)

Determination of equivalent capacitances C1and C2 can be done by using knownmethods, for example by considering thegeometry of transformer (winding, core andtank) and dielectric properties of insulating oiland paper.Partial discharge occurrence in thetransformer winding comprises the incidenceof a very short duration electrical pulsepropagating from discharge location towardtransformer bushing. Considering theequivalent electrical circuit of the transformerwinding, for high frequencies, which iscomposed by the distributed elements

CIRED2001. 18-2 June 2001, Conference Publication No. 4820 EE 2001


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resistances, conductances, inductances andcapacitances the propagation of the pulse inthe winding will show two distinct moments.In the first, the effect of the capacitivecomponents are preponderant, and theinfluence of others components can beneglected (see figure 1

& L ---- I

t" t" T C 2 * T c 2 t C 2Figure 1 - Transformer winding equivalentcircuit

Voltage distribution along the winding, in caseof incidence of a pulse in the bushing, isexpressed by:

Z ( p ) . a p c o s h m ( l - x) - i nhm( l - )[ Z ( p ) . m p o s h m l - s i n h m l ]

(2)e, = E .

where e is the voltage at position x, I is thewinding length, x is per unit position, Z(p) isthe terminal impedance, E is the voltagepulse amplitude.With the incidence of a pulse betweenwinding ends, the voltage distribution will beexpressed by:

v ( x )= Ecosh(mx) O < x < n ( 3 )coSh(mn)E COSh[m( - x ) ]cosh[m(l- n ) ]v ( x ) = n l x l l (4)

The voltage at the bushing is obtained for x=O(bushing H) and x= l (bushingN)BushingH:

Bushing Nc

Capacitance at point n, which is obtainedfrom voltage and current distribution alongthe winding, is given by:C, = , j m [ t a n h ( m . n ) + tanh[m(l - n)] ]

(7)Apparent charge, as defined in IEC270standard, measured at bushing H and causedby the occurrence of a discharge at point n inthe winding is expressed by:

ExperimentalA specially adapted power transformer, withaccessible taps along the windings (figures 2and 3) was used in the tests. Pulses withperiod of about 70 nanoseconds, forsimulating partial discharge along thewinding, were injected in the winding taps. Inthe transformer bushings, those pulses weremeasured by means of a partial dischargedetector. For the measurements, theconventional detection technique proposedby IEC 270 Standard was used,complemented by digitalization equipment.

where n is the point of discharge, E the pulseamplitude, V(x) the voltage at x.


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h igh voltagewinding7

. .

Figure 2 - Power transformer with accessible taps,adapted for the study

Figure 4 a) and b) shows measurementresults, compared with the theoreticallyexpected values, represented by equation 9(fig. 4a - bushing H) and equation 10 (fig. 4b- bushing N) . In the figure, it is shown theexpected relationship between the PDmeasurement on the bushing (in ordinate)with the position of PD along the winding, inper unit (in abscissas). In test procedure,pulses of 1OOpC were injected in eachaccessible tap of the transformer winding.

Similar measurements of apparent chargewere performed on the winding taps, but withthe injection of the simulated partialdischarges pulses applied to the bushinginstead. The measurement results arepresented on figure 5, compared with thetheoretical curve.

PDdetectortransformer couplingcapacitorFigure 3 - Experimental setup

n - istancefiomH@u)measurement on buslung N (theoretical)

-e - measurement on bush ug N (negative charge)E measurement on buslungN (positive charge)-

Figure4- Apparent charg e measurement on bushing H(a) and bushing N(b)


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DP njected on bus& H

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8n - measurcmentpositonfromHU)- pparent charge (thcorehcd)

apparent charge @os)- pparent charge (negahve)

DP iniected on bushnu N

U

n measurement positionhomH @U)theoretical apparent charge

8 measured positive charge- - measured negabvc charge

4 b)Figure 5 - Apparent charge measurem ents along the windings taps, with dischargeinjection on the bushings: a)DP injection on bushin g H b)DP injection on bushing NConclusions

The obtained results show good agreementbetween the discharge position andtheoretical expectationsThe apparent charge measurementsperformed along the winding taps show goodagreement with expected values, indicatingthe winding electrical model adequacy.The method shows better results fordischarges located in regions near the centralpart of the winding.In this way, this methodology can be used forthe location of PDs inside the transformer.References

2 - IEC Publication 270, 1981, Partialdischarge measurements3 - CIGRE Working Group 05 - SC12(Transformers), 1983, An internationalsurvey on failures in large power transformersin service, Electra, n884 - Kallberg B., 1980, Location of partialdischarges in power transformers bycomputation and measurement ofcapacitively transmitted voltage pulses, !EET-PAS vol PAS-99, n.02,5 - Mendes J.C.; 1995, ReduqZio de falhasem grandes transformadores de alta tensao,Tese de doutorado, Epusp6 - Coulthard W 6, Transients in electriccircuits, Pitman & Sons, London, 1941

1 - James R. E., Trick F. E, Phung B . T., and 7 - Cornick, K et al., Distribution of very fastWhite P. A., 1986, Interpretation of Partial transient overvoltages in transformerDischarge Quantities as Measured at the windings. Paris, Cigre, 12-204, 6p, 1992Terminals of HV Power Transformers. l EEET-El Vol. 21, pp. 629-638. 8 - Karsai, K, Kerenyi, D, Kiss, L, Largepower transformers, Elsevier Science

Publishers, Amsterdam, 1986

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