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Power Transfomer Core Fault Diagnosis UsingFrequency Response Analysis.

Jorge Pleite, Carlos Gonzlez, Juan Vzquez, Antonio Lzaro.

Electronics Technology Department.Universidad Carlos III de Madrid.

Avda. de la Universidad 30, 28911-Leganes, Madrid, SPAINe-mail: pleite@ing.uc3m.es http://www.uc3m.es/gsep

Abstract . Power Transformers are main devices in the

performance of Energy Supply Systems. Therefore, there is agreat interest in its health state. Frequency ResponseAnalysis (FRA) is nowadays an appreciated preventive

technique used for transformer maintenance in order todetect winding displacements. But, in spite of the advantages

of FRA, a systematic diagnosis procedure has not been stilldeveloped due to the physical meaning of the FrequencyResponse is not really obtained. Our research group hasdeveloped a three phase magnetic core model that provides

the relations among the raw Frequency Response and the

different parts inside the three phase transformer. Differenteffects of the magnetic core are recognized through differentparameters of this model. A diagnosis procedure is available

taking into account these parameters because the internalparts of the transformer are distinguished.

I. INTRODUCTION

Efficiency is a main target for Energy Power Supply

Companies, since it must operate in a competitive market.Power Transformers are mainly involved in the energytransmission and distribution. For this reason, thereliability, quality and economic cost of the companydepends strongly of the transformer health and, therefore,an intensive maintenance of the electric machine isrequired. In spite of corrective and predictive maintenanceis carried out, preventive maintenance of the powertransformer is getting ahead and it is must be taken intoaccount in order to obtain the highest reliability [1].

In the last years, preventive maintenance techniquessuch as Dissolve Gas Analysis (DGA), tg and capacitancemeasurement or Frequency Response Analysis (FRA) have

appeared. The FRA technique measures the frequencyresponse of the transformer which allows to know theinternal state of the machine. It has been proved that FRAis specially useful detecting winding displacements anddeformations,[2]. But, as the entire transformer state isknown by means of FRA measurement, not only thewinding but also the magnetic core behaviour can beanalyzed.

However, although Frequency Response Analysis hasbeen developed in the last years and it is widely used allover the world, a systematic diagnosis procedure has not

been achieved due to it is no so easy to have a physicalinterpretation. A physical meaning of the core using theFrequency Response is presented in this work. In sectionII, a core model for the different phases of a three phasetransformer is proposed. The relations among model

parameters and the different internal parts of thetransformer are presented. In section III, FrequencyResponses of an actual transformer are shown and the

parameters of the model are calculated. By means of theknowledge of the different parameters that has a relationwith the physical parts of the transformer, a systematicdiagnostic procedure can be achieved. Finally, section IVgives the achieved conclusions.

II. DIAGNOSIS PROCEDURE

It is not easy to obtain a physical interpretation lookingat the raw Frequency Response of a single phase in a

power transformer. Some experts are able to interpret thisresponse but from a subjective and non systematic point ofview.

In this section, a model of the magnetic core thatallows to interpret and localize the internal parts of thetransformer is presented. Some algorithms have been alsodeveloped in order to calculate the values of the

parameters of the model. As soon as these parameterssupport the physical meaning, the diagnosis can be carriedout observing the evolution of its values.

A. The model.

Our research group has developed a frequencydependent model of the entire transformer (including not

only the core but also the winding effects) in order toobtain the physical interpretation of the FrequencyResponse, [3]. A specific model for the magnetic is core

presented in this paper based on the general previousmodel, taking into account the physical interpretation ofthe magnetic field distribution inside the core.

A three phase transformer is represented in Figure 1.Each phase is placed in a different column, and may havedifferent winding sides (primary, secondary, etc). In Figure2. the proposed magnetic core model of the transformer is

presented. The magnetic path is the same for lateral phases

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while it is different for the central phase, due to thesymmetric structure of the transformer. Equation (1)represents this effect where the value ofkdepends on howsymmetric the transformer is.

1k0erewh

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equivalent circuit is obtained from the phase R point ofview but can be analogue obtained for T phase.

Considering that NR=NS=NT and (1), the relation between the inductance of lateral and central phase isgiven by

kL

L

S

R = (3)

Taking into account (3), the equivalent impedance isgiven by4), where LLAT is the measured inductance of theFrequency Response of a lateral phase. By this way, anexpression for the value of the inductance due to the lateralleg of the transformer (R o T phase) is obtained.

12

1)(II =+

k

kLLLLL RTSRLAT (4)

The procedure is similar for the C parametercalculation looking at Range 4. In this case, highfrequencies of the magnetic core effects are considered.The equivalent circuit is shown in Figure 6.

Tacking into account that the windings of the threephases are built in the same way (which is fairly the way itis), their capacitances between winding and core are thesame5).

TSR CCC == (5)

Looking at Figure 6. , the value of the capacitance for alateral phase is given by6).

3

2)withcontectedseries(II RTSRLAT CCCCC =

(6)

LR

LS

LT

LLAT

Figure 5. . Magnetic core equivalent circuit at Range 1.

CR

CS

CT

CLAT

Figure 6. . Magnetic core equivalent circuit at Range 4.

2) Central Phase parameters calculation.The calculation procedure for the central phase is equal

to the lateral phase. It is necessary to interchange S for R

in Figure 3. , Figure 5. , and Figure 6. Equation (7) is the

analogous to4) where LCEN is the measured inductance ofthe Frequency Response of the central phase.

kLLLLL RTRSCEN

21

2)(II =+ (7)

3) Obtaining the parameters of the core modelLR and k can be calculated from4) and (7), through

expressions (8) and (9).

12

=CEN

LAT

L

Lk (8)

22 CENLATR

LLL = (9)

CR is obtained directly from CLAT through expression(6).

III. EXPERIMENTAL RESULTS.

The Frequency Response of the ferromagnetic core ofan actual three phase transformer is shown in Figure 7. ,which corresponds with the behaviour of R and S phases.For confidential requirements, actual numerical results arenot shown. The graph scales are not represented and thenumerical values have been rescaled through a factor. Thisdoesnt mean any missing information for this paper sinceall the processes can be explained through the example

with these corrections.

In this Frequency Response the core effects are mixedwith the winding effects (e.g, Leakage inductance). So, itis difficult to obtain the LLAT, LCEN and CLAT due to thecore effects directly from this measurement shown inFigure 7. In order to distinguish the actual value of theimpedance due only to the core effects, a complexmodeling procedure was previously developed, [3]. Thismodel is able to discriminate the different effects coreand windings- mixed in the actual measurement. Thevalues obtained from the Frequency Response by means ofthis previous model are presented in TABLE I. The valuescalculated by means of (4) and (7) are presented in

TABLE II.

Observing the results, some conclusions can beobtained:

LR, LS, LT parameters have been calculated. Bythis way, the inductances that represent the threelegs of the three phase transformer have beendistinguished. These parameters will be useful toobtain a diagnosis. If any of the parameters haschanged in different states of the transformer itmeans that some parts inside the magnetic core haschange. By means of the physical interpretation ofthe model is possible to know what is the part thathas change and how much it has change, e.g.: ifonly LR has change it can be conclude that thefault is placed in the R phase leg.

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Figure 7. Frequency Response in terms of a ferromagnetic core in athree phase Power Transformer of phase R an S respectively.

TABLE I. MEASURED VALUES.

Values obtained from Frequency Response

LLAT LCEN CLAT CCEN

1 1.378 1.433e-9 1.443e-9

TABLE II. CALCULATED VALUES.

Parameters Values of the model

LR LT LS CR CT CS k

1.311 1.311 2.906 2.15e-9 2.15e-9 2.17e-9 0.451

The value of LS is more than the double of LRvalue. This fact has a physical interpretation because the reluctance of the central phase isminor than the reluctance of the lateral phase.Looking at Range 1 in Figure 7. , it is possible tocheck that the impedance of S phase is higher thanR phase.

The value of the capacitance formed between eachwinding and the ferromagnetic core is calculated.Therefore it is possible to locate a failure if the

parameter changes among different states of thetransformer.

The three values of the self phase capacitances CR,CS and CT are fairly similar. This fact agrees withthe physical interpretation because the threewindings structure are quite similar. Looking theRange 4 in Figure 7. it is possible to check thatthe response of lateral and central phases are thesame.

IV. CONCLUSSIONS.

A core effects model based on the frequency responseof transformers is presented. It has the facility of offering a

physical meaning of the different effects in thetransformer. Therefore, an internal knowledge of thetransformer can be achieved by means of an externalmeasurement (as is the Frequency Response) through the

proposed model. The application of this model is veryuseful in order to carry out a diagnosis of the core health in

power transformers. Some experimental results have beenpresented in order to illustrate these concepts.

ACKNOWLEDGMENT

This work is been supported by the SpanishGovernment (Ministerio de Educacin y Ciencia -DPI2005-09039-C02-02) and Union Fenosa S.A. (Spanishelectric energy supplier).

REFERENCES

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