048_venkatesh_aupec01paper(1)

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DETECTION OF FAULTS AND AGEING PHENOMENA IN TRANSFORMER BUSHINGS

BY FREQUENCY RESPONSE TECHNIQUE

R Venkatesh Dr. S R KannanChief Research Engineer Sr. General Manager

Corporate R & D Centre, Crompton Greaves Ltd. Mumbai, India.

Abstract

Large power transformers belong to the most expensive and strategically important components of

any power generation and transmission system. Assessment of insulation quality in large H.V.

power transformers at any point in time, also called Condition Monitoring is an area of work

currently being pursued by many laboratories and utilities. Several techniques are available for

monitoring of several parameters, which could indicate the condition on the insulation. This paper

examines the possibility of monitoring the condition of the bushings, a critical component of

power transformers, using frequency response techniques.

1. INTRODUCTION:

Large power transformers belong to the most

expensive and strategically important components of

any power generation and transmission system. A

serious failure of a large power transformer due to

insulation breakdown can generate substantial costs

for repair and financial losses due to power outage.

Therefore, utilities have clear incentive to assess theactual condition of their transformer, in particular the

condition of the HV insulation system, with the aim

to minimize the risk of failures and to avoid forced

outages of strategically important units.

Assessment of insulation quality in large H.V. power

equipment at any point in time, also called Condition

Monitoring is an area of work currently being

pursued by many laboratories and utilities. Several

techniques are available for monitoring of several

parameters, which could indicate the condition on the

insulation. From the literatures as well as field data it

has been established that bushings are one of the

major reasons for transformer failure [1], [2]. With

this background, it has been the theme of this

research work to establish an on-line conditioning

monitoring technique to monitor the status of a

bushing.

2. CONDITION MONITORING

TECHNIQUES:

Though a large number of techniques are available,none of then have really been applied to online

monitoring on a commercial scale, due to the

limitations associated with each one of them. This

has created a need for a new technique suitable for

online monitoring. In the recent past TFA / FRA has

shown promising characteristics to fill in the need

[3],[4],[5].

Though transfer function analysis / frequency

response is a relatively a new technique, it is fastgaining importance due to its simplicity, low cost and

ease of implementation. Also with this technique it

might be able to identify the type and location of

fault, which is not possible with most other methods,

involving terminal measurements.

In spite of all its advantages, frequency response

technique has still not become popular due to some

inherent limitations associated with practical

implementation, noise & interference being one of

them. The second limitation has been a lack of

availability of correlation between the signature and

the changes in parameter of the equipment. The

present work is towards eliminating / minimizing

these two limitations.

3. THE BASIC PRINCIPLE

It is well established that the transfer function or the

frequency response of the bushing would depend on

the basic parameters of the electrical equivalent

circuit and any change in the parameters would result

in a change in the TF or FRA. The basic parametersof the bushing insulation namely its capacitance and


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resistance undergo changes due to ageing as well as

other faults that might develop during their lifetime.

The new technique aims at detecting these changes

and correlating them with the basic parameters and

then finally to conventionally accepted electrical

quantities, which are normally used for assessing

quality of insulation.

4. BUSHING CIRCUIT MODELING:

To establish the feasibility of using TFA / FRA for

fault detection in bushings a bushing equivalent

circuit model is considered. Though in general

bushings are considered as non-inductive capacitor

and resistive element network, in the present study

the residual (self) inductance is also considered to

study the effect of this inductance on the sensitivity

of the technique.

The total losses in the bushing can be either

represented as an equivalent series or parallel

resistance. In the models considered both series as

well as parallel representations are considered along

with some possible combinations of both series and

parallel resistors (eight combinational circuits were

analyzed). This has been done in order to establish

the sensitivity of the technique to equivalent circuit

representation and understand the physical

phenomena.

For obtaining the numerical values of the parameters,

a typical EHV busing is considered with the

following values:

Capacitance = 250 pF

Inductance = 500 nH

Loss factor = 2.4 x 10-3

The equivalent series and parallel resistance are

computed from the loss factor value using the

expressions:

Tan p= IR/ Ic= 1/(CpRp)

Tan s=VR/Vc =CsRs

And the equivalence equations:

Cp= Cs/ (1+ tan2s) = C s/ (1+(CsRs)

2)

Cs= Cp(1 + tan2p) = Cp(1+ 1/((CpRp)

2)

Rp=Rs(1 + 1/ tan2s) = Rs(1 + 1/((CsRs)

2)

Rs= Rp/ (1 + 1/ (tan

2p))= Rp(1+(

CsRs)

2

)

The bushing is represented as a 10 series section

equivalent circuit. Two of typical equivalent circuit

representations are shown in figures 1 & 2.

5. FREQUENCY RESPONSE ANALYSIS:

The frequency response of the constructed bushing

model is studied by analytical method and computer

simulation.

5.1 Analytical method:

Analytical method is used as this lends itself to study

the variations in parameters quickly for a given

equivalent circuit representation. In analyticalmethod mathematical models are generated based on

the equivalent circuit parameters and these are used

to derive the output functions for any given input

signal. The output signal is derived across the last

section of the equivalent circuit and the input is

applied across the entire string of series sections. The

output is studied for various conditions like no fault,

3 % and 10% change in resistance and capacitance.

The analytical models are of the form

VI = I (Z1+ Z2 +..+Z10+ Zo ) and Vo= IZoTransfer function =Vo (s) / Vi (s)

= Zo / (Z1 + Z2 +Z3 +.Z10 + Zo )

Output

Rp10

Rs10

Rpo

Co

Lo

Rso

C10

L10mHz - MHz

Rp1

Rs1

L1

C1

Figs. 1 and 2. Eq. Circuit model of bushing


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Table II. Sensitive frequencies for some typical

models obtained by simulation.

Sensitive frequency for detecting changes

in

Model $

Rs Rp C

MOD 1 16 KHz. -- 16 KHz.

MOD 2 -- 0.1Hz 0.1 Hz

MOD 3 17 KHz. 0.1 Hz. 0.1Hz, 17 KHz.

$ : MOD 1 is simple Rs, C series, MOD 2 is simple

Rp, C parallel and MOD 3 is Rp, C parallel in series

with Rs

Some of the salient observations include

Both the analytical method and the computer

simulation gave similar results, with slightdeviations, that are attributable to the choice of

time step in the simulation.

There is a distinct difference in the phase and

magnitude change for various quanta of changes

in parameters, thus establishing the feasibility of

this technique for not only fault detection but

also identification. (e.g. 3 % and 10% change in

C1).

There is a distinct difference in the phase and

magnitude change for the same quantum change

in the same parameter at different locations, thus

establishing the feasibility of the technique forfault location. (e.g. 10 % change in R1 and R2)

It was found that there exists three distinct

sensitive frequencies, near DC (0.2 0.7 Hz),

medium frequency (1 kHz 300 kHz) and very

high frequency (>MHz.), where the changes in

parameter are easily identified by changes in

phase and magnitude.

For the selected set of parameters the most

sensitive frequency is found to be in 26 kHz,

where the maximum phase change occurs.

It has been observed that the equivalent circuit

representation does have an effect on this

sensitive frequency.

It has been observed that this frequency is the

frequency at which the capacitive impedance

becomes equal to the resistive impedance in the

network. For a simp le series representation this

frequency is in the region of 10Khz to 60Khz,

depending on the loss factor ( = 1/ CsRs) andfor a simple parallel representation thisfrequency is in mHz (0.1 0.7 Hz) again

depending on the loss factor ( = 1/CpRp ).

For combinational circuits, the sensitive

frequency includes both DC and medium

frequencies. Addition of the self inductance does

not affect this frequency much, but add another

sensitive frequency, which is in the high

frequency range (MHz)

It was also observed that different quantum of

changes in circuit parameters produce different

phase and magnitude changes thus enabling the

technique to be used for fault identification.

The technique also lends itself for fault location

as faults in different sections produce different

signatures.

7. FUTURE WORK:

The following are being done as an extension to thepresent work.

Experimental investigations on model with

discrete components, which closely represent an

actual bushing. Preliminary investigations

indicate a close correlation with the simulation

and feasibility of the technique to detect faults.

Analytical work to establish a correlation

between the change in various parameters and

aging and / or changes in loss factor, partial

discharge etc.

Work to establish a correlation between change

in phase and changes in circuit parameters. Accelerated ageing studies on 12kV model

bushings to establish the property correlation

with ageing and establish TFA / FRA method for

practical application.

8. CONCLUSION

From the study the following could be

established:

It is feasible to detect even small changes in the

bushing circuit parameters using FRA / TFA

approach. As ageing or operation under

abnormal system conditions would lead to a

change in electrical, mechanical, chemical or

physical property of the insulation, this would

manifest itself in the form of change in the

equivalent circuit parameters. By detecting the

changes in these parameters one could detect the

change in the bushing.

There exists a sensitive frequency (or a very

narrow band of frequencies, typically in the

range of 10 KHz to 60 KHz, where the fault

detection is more sensitive. This enablesconfiguring the measurement system tuned


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exactly to that frequency so that the noise can be

eliminated and detection sensitivity further

increased. This has been one of the most

important outputs as this makes the configuration

of the online monitor much more simplistic,

operating at a single frequency

The sensitive frequency depends on the loss

factor of the bushing and its capacitance. For the

bushing model considered this frequency was

found to be 26 kHz.

Various quanta of changes in circuit parameters

produce various responses, thus the technique

could be used not only to detect the fault but also

identify the type of fault.

Same amount of change in the parameter, but at

different locations produces different responses

and this could be used to locate the fault.

9. ACKNOWLEDGEMENT:

The authors wish to place on record their sincere

thanks to the management of Crompton Greaves Ltd.

for supporting the project and giving permission to

publish the work.

10. REFERENCES:

1. V.Smekalov Bushing insulation monitoring in

the course of operation a transaction in CIGRE

1996: 12-106.2. S.D.Kassihin, S.D. Lizunov, G.R. Lipstein,

A.K.Lokhanin, and T.I.Morozova Service

experience and reasons of bushing failures of

EHV transformers and shunt reactors a

transaction in CIGRE 1996:12-105.

3. E.P. Dick and C.C. Erven, Transformer

Diagnostic testing by Frequency response

Analysis, IEEE Transactions on Power

Apparatus and systems, Vol. PAS-97, No.6,

Nov/Dec. 1978.

4. J.Bak-Jensen, B.Bak-Jensen, and S.D.

Mikkelsen, Detection of Faults and ageingPhenomena In Transformers by Transfer

Functions, IEEE Transactions on Power

Devivery, Vol. 10, No. 1, January 1995.

5. P.T.M. Vaessen and N.V. Kema a new

frequency response analysis method for power

transformer, IEEE Transaction on Power

Delivery, Vol.7 No.1, January 1992.

6. A Feasibility Study for Fault detection in

Transformer Bushing, M.Tech. Thesis of

S.J.Kinge, I.I.T Kharagpur, 1997.

048_venkatesh_aupec01paper(1)

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