electrical and chemical diagnostics of transformer insulation abstrac1

7/31/2019 Electrical and Chemical Diagnostics of Transformer Insulation Abstrac1

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ABSTRACT

This paper describes the application of two relatively new diagnostic

techniques for the determination of insulation condition in aged transformers. The

techniques are (a) measurements of interfacial polarization spectra by a DC method

and (b) measurements of molecular weight and its distribution by gel permeation

chromatography. Several other electrical properties of the cellulose polymer were

also investigated. Samples were obtained from a retired power transformer and

they were analysed by the developed techniques. Six distribution transformers

were also tested with the interfacial polarization spectra measurement technique,

and the molecular weight of paper/pressboard samples from these transformers

were also measured by the gel permeation chromatography. The variation of the

results through different locations in a power transformer is discussed in this paper.

The possible correlation between different measured properties was investigated

and discussed in this paper.


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1. INTRODUCTION

The main function of a power system is to supply electrical energy to its

customers with an acceptable degree of reliability and quality. Among many other

things, the reliability of a power system depends on trouble free transformer

operation. Now, in the electricity utilities around the world, a significant number of

power transformers are operating beyond their design life. Most of these

transformers are operating without evidence of distress. The same situation is

evident in Australia. In PowaaerLink Queensland (PLQ), 25% of the power

transformers were more than 25 years old in 1991. So priority attention should be

directed to research into improved diagnostic techniques for determining the

condition of the insulation in aged transformers.

The insulation system in a power transformer consists of cellulosic

materials (paper, pressboard and transformerboard) and processed mineral oil. The

cellulosic materials and oil insulation used in transformer degrade with time. The

degradation depends on thermal, oxidative, hydrolytic, electrical and mechanical

conditions which the transformer experienced during its lifetime.

The condition of the paper and pressboard insulation has been monitored by

(a) bulk measurements (dissolved gas analysis (DGA) insulation resistance (IR),

tan and furans and (b) measurements on samples removed from the transformer

(degree of polymerization (DP) tensile strength). At the interface between the


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paper and oil in the transformer, interfacial polarization may occur, resulting in an

increase in the loss tangent and dielectric loss. A DC method was developed for

measuring the interfacial polarization spectrum for the determination of insulation

condition in aged transformers.

This paper makes contributions to the determination of the insulation

condition of transformers by bulk measurements and measurements on samples

removed from the transformer. It is based on a University of Queensland research

project conducted with cooperation from the PLQ and the GEC-Alsthom.

Most of the currently used techniques have some drawbacks. Dissolved gas

analysis requires a data bank based on experimental results from failed

transformers for predicting the fault type. When transformer oil is rep or

refurbished, the analysis of furans in the refurbished oil may not show any trace of

degradation, although the cellulose may have degraded significantly. DP

estimation is based on a single-point viscosity measurement. Molecular weight

studies by single-point viscosity measurements are of limited value when dealing

with a complex polymer blend, such as Kraft paper, particularly in cases where the

molecular weight distribution of the paper changes significantly as the degradation

proceeds. In these instances, a new technique, gel permeation chromatography

(GPC), is likely to be more useful than the viscosity method, because it provides

information about the change in molecular weight and molecular weight


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distribution. Investigation of the GPO technique has been included in this research

to assess its effectiveness in determining the condition of insulation.

Conventional electrical properties (dissipation factor and breakdown

strengths) of cellulosic materials are not significantly affected by ageing .so very

little recent research has been directed to electrical diagnostic techniques, in this

research project, thorough investigations were also undertaken of the conventional

electrical properties, along with interfacial polarization parameters of the cellulosic

insulation materials. The interfacial phenomena are strongly influenced by

insulation degradation products, such as polar functionalities, water etc. The

condition of the dielectric and its degradation due to ageing can be monitored by

studying the rate and process of polarization and can be studied using a DC field.

Furthermore, this is a non-destructive diagnostic test.

A retired power transformer (25 MVA, l1/132 kV) and several distribution

transformers were used for the experimental work. The results from these

transformers will be presented and an attempt will be made to correlate the

electrical and chemical test results. The variation of the results through the

different locations in a power transformer will be discussed with reference to their

thermal stress distribution. Accelerated ageing experiments were conducted to

predict the long term insulation behaviour and the results are presented in the

accompanying paper.


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2. EXPERIMENTAL TECHNIQUES

Experimental techniques used for the assessment of insulation condition in

aged transformers are described in the following section.

2.1 Conventional Electrical Tests

The dissipation factor and capacitance were measured at 50 Hz using a

Schering bridge. The partial discharge inception voltage was measured by a partial

discharge detector using a circuit with an input unit in series with the test sample.

Power frequency breakdown strength was measured by using the step by step

method. The standard wave shape of l. was used for determining the negative

lightning impulse breakdown strength. The electrical tests were carried out in

accordance with relevant ASTM standards.

2.2 Interfacial Polarization Spectra (IPS) Measurements

When a direct voltage is applied to a dielectric for a long period of time,

and it is then short circuited for a short period, after opening the short circuit, the

charge bounded by the polarization will turn into free charges i.e, a voltage will

build up between the electrodes on the dielectric. This phenomena is called the

return voltage. After applying the field for a time t, the polarization is expressed by

P(t) = P0 F(t), where P0 = E is the steady state value of the polarization, a is a

proportionality factor between the polarization and the field strength (E), called the


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polarizabiity, F(t) is the relaxation function of the polarization describing the

development of polarization in time and P is the bound charge density.

After opening the short circuit, the charge bound by the polarization will

turn into free charge, and a voltage will build up. So an increase in the polarization

due to bound charges will increase the voltage build up. Again, polarizabiity will

increase when polarization increases. So the maximum return voltage can be

correlated with the polarizability of the material.

With the development of polarization, the charge bounded on the electrodes

tends to grow. In the external circuit maintaining the field, this growth will cause

an absorption current given by Ja(t) = P(t) = d/dt P (t). With polarization

approaching a steady state value, the current decays in time to zero. As for

polarization, the absorption current is proportional to the field strength. So the

initial value can be written as Ja (0) = E, where is the proportionality factor

between absorption cur rent and field strength, and is called polarization

conductivity. It can be shown that the initial slope of the return voltage is

proportional to the polarization conductivity [10]. When the return voltage

approaches its maximum value quickly, the initial slope of the return voltage is

larger. Another parameter termed as central time constant, i.e. the time at which

the return voltage is maximum, is also dependent on the polarization conductivity.

Hence the fundamental characteristics of the dielectric can be measured by return

voltage measurements.


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Figure 1: A typical return voltage wave shape of a specimen from the retired

transformer

An experimental set up with an IBM PC and a programmable electrometer

(Keithley 617 model) was developed and implemented to measure the return

voltage of a two terminal dielectric system. The charging voltage was 100 volt DC

for the retired transformer insulation samples. The developed software was used to

control the electrometer. Adsorbed moisture and temperature of the oil-paper

insulation adversely affects the re turn voltage measurement. So the return voltage

measurement was always conducted at a known and low oil-paper moisture content

and at ambient environmental conditions (20 25 C).


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A typical return voltage wave shape of a specimen from the retired

transformer is shown in Fig. 1. The relevant parameters (maximum return voltage,

initial slope and central time constant) are identified in Fig. 1. Initial slope is the

slope of the return voltage graph (with linear approximation) for first few seconds.

As interfacial polarization is predominant at longer time constants, the spectrum of

the return voltage was investigated by changing the charging and discharging time

over a range of times greater than 1 second until the peak value of the maximum

return voltage was obtained The ratio of charging and discharging time was

two.then the spectra of maximum return voltage and initial slope were plotted

versus the central time constant (the time at which the return voltage is maximum).

The peak value of the maximum return voltage (from the return voltage spectrum)

and the corresponding initial slope (from the initial slope spectrum), along With

central time constant (from either of the spectrum), are the parameters used to

assess the insulation condition from the return voltage measurements.

2.3 GPC Analysis

Gel permeation chromatography provides a detailed molecular weight

distribution of the polymer. GPC is a chromatographic technique which uses

highly porous, non-ionic gel beads for the separation of polydispersed polymers in

solution. GPC separates polymer molecules on the basis of their hydrodynamic

volume. Detailed information of GPC technique is presented elsewhere [7].

Cellulose is not soluble in any common GPC solvents. Hence, for GPC


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measurements the cellulosic materials had to be derivatized to enhance their

solubility in these solvents. For this purpose, a cellulose tricarbanilate derivative

was prepared by the method of Evans et al. .

The molecular weight distribution of the cellulose tricarbanilate was

measured using a Waters Chromatograph equipped with a variable wavelength

tunable absorbance detector. Four ultrastyragel columns were used in series in the

Chromatograph, with tetrahydrofuran (THF) as the eluent. Detection was carried

out using the absorbance at 236 nm and the elution profile was acquired by

interfacing to an IBM computer. The elution profiles were converted to molecular

weight distributions using a calibration based upon narrow molecular weight

distribution polystyrene standards.

3. RESULTS AND DISCUSSIONS

Paper wrapped insulated conductor specimens 200 mm long and pressboard

samples of dimension 80*80 mm were collected from an aged power transformer.

Several distribution transformers were also tested.

3.1 Case Study 1: Kareeya Transformer

A 25 year old, 25 MVA, 132/11 kV transformer from Kareeya power

station, North Queensland, had been gassing for some years. This transformer was

dismantled and was used to investigate the quality of the insulation using electrical


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and chemical testing techniques, Since the aged transformer had been exposed to

air after dismantling, the samples had to be processed. The moisture content of

processed samples varied in the range 0.5 to 1.3%.

From our discussions with the experts from the GEC-Alsthom, it was

revealed that the top coils are subjected to higher thermal stress than the bottom

ones. To examine the differences that exist between the high stress and low stress

insulation samples, the samples were collected from top, middle and bottom coils

of low voltage and high voltage windings of the transformer. The schematic

diagram of a low voltage winding is shown in Fig. 2.


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Figure 2: Schematic diagram of one phase of LV winding of 25 MVA Kareeya

transformer

There were 90 coils/phase and 18 turns or layers of conductor/coil in the

low voltage winding. There were 10 conductors in parallel in the low voltage

winding. There were 60 coils/phase and 19 turns or layers of conductors/coil in the

high voltage winding. The HV and LV conductors were of rectangular cross

section 13.9 and 12 mm wide respectively and 2.6 mm thick with rounded corners.

The test specimens for insulated conductor samples were made up by placing two

samples side by side in a Perspex assembly, so that they overlapped each other for

a length of 100 mm. With two insulated conductors placed side by side to form the

specimen, the thickness of paper insulation between them was 1.0 mm and 0.8 mm

for the HV and LV specimens respectively. Pressboard (of 0.2 mm thickness)

samples were collected from the main bulk insulation between the high voltage and

low voltage windings.

Table 1: Results of conventional electrical tests on samples from LV A phase

winding of Kareeya transformer


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3.1.1 Conventional Electrical Test Results

To obtain an understanding of the effects of varying stresses along complete

windings, samples were taken from various locations of the LV A and HV B phase

windings and were tested. Two sizes of new (unaged) paper wrapped conductors

(New1 and New2) and new pressboard samples of similar composition and

thickness were obtained from the transformer manufacturer GEC-Alsthom.

Conventional electrical test results on paper wrapped insulated conductor

specimens from the LV A phase and HV B phase windings are presented in Tables

1 and 2 respectively.

In LV A phase, coils 1,2/44,45/89,90 are from top/middle/ bottom locations

respectively and layers 1,2/18 are from outer and inner locations. For each of the


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samples at least two samples were tested and average was taken. The partial

discharge inception stresses were usually very close to the power frequency

dielectric strength, and hence these results are not presented. In HV B phase, coils

1/29,30/59,60 are the top/middle /bottom locations and layers 1/12/19 are the

outer/medium/ inner locations.

The following comparison can be made between the results of the aged

insulation samples and those of the new insulation.

1. The average dissipation factor of LV A phase samples is 0.017 and that of

the New1 (similar to LVA phase) sample is 0.008. The average dissipation

factor of HV B phase samples is 0.015 and that of the New2 (similar to HV B

phase) sample is 0.009. The dissipation factor of aged samples is significantly

different from that of new insulation.

2. The average power frequency dielectric strength of the LVA phase samples

is 48.4 kVp/mm (with a SD= 3.16) and that of the New1 sample is 50.0

kVp/mm. The average power frequency dielectric strength of the HV B phase

samples is 41.6 kVp/mm (with a SD=3.5) and that of the New2 samples is 45.0

kVp/mm. The difference between the average value of the power frequency

breakdown strength of the LV A phase and the new samples is not significant,

whereas the variation of the HV B phase samples is 7.5% lower than the

corresponding new samples.


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3. The average lightning impulse breakdown strength of the LV A phase

samples is 77.0 kVp/mm (with a SD=7) and that of the New! samples is 81

kVp/mm. The average lightning impulse strength of the HV B phase is 68.5

kV/mm (with a SD=8.1) and that of the New2 samples is 84 kVp/mm. Again

the variation is not very significant for the LV A phase samples. The LI

strength of HV B phase sample is about 18% lower than the corresponding new

sample strength.

Table 2: Results of conventional electrical tests on samples from HV B phase of

Kareeya transformer.

3.1.2 Interfacial Polarization Spectra Results


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The same aged insulation specimens from LV A and HV B phase and

unaged insulation samples (New1and New2) were tested using interfacial

polarization spectra (IPS) measurements. The results from the IPS measurements

are presented in Tables 3 and 4.

In Table 3 all the samples from the aged transformer show large peak

maximum voltages, short central time constants and large initial slopes by

comparison with the values for new samples. There are significant variations

between the aged samples from different locations. For example, we see the

maximum return voltage of top coil 1-1 (1st coil from top, 1st outside layer)

reached its peak at 31 s, whereas 89-1 reached its peak value at 75 s, which is more

than twice the time constant of 1-1. The dissipation factor for sample 1-1 is fifty

percent larger than the 89-1 sample (Table 1). It is also observed that 45-1, 45-18,

90-2 and 90-18 samples have maximum return voltages at very small (low) central

time constants and large values of initial slope and dissipation factor. The variation

of insulation status between top, middle and bottom coils of LV A phase, observed

from the conventional tests is consistent with the data from IPS measurements. It

suggests that degradation due to ageing is characterised by higher dissipation factor

and consistent changes in IPS e.g. higher return voltage and initial slope, and low

central time constant.

Table 3: Results of IPS measurements on samples from LV A phase winding of

Kareeya transformer


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Table 4: Results of IPS measurements on samples from HV B phase winding of

Kareeya transformer


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In Table 4 for HV B phase, the peak maximum values of return voltage are

somewhat lower than for the LV A phase. The variation of the peak maximum

return voltage for the HV B phase is not as significant as LV A phase by

comparison with the corresponding new samples. For example, the mean of the

peak maximum return voltage of LV A phase is 3.2 volt and that for the

New1sample is 1.6 volt, whereas, the mean of the peak maximum return voltage of

HV B phase is 2.0 volt and that for the New2 sample is 1.8 volt. There are large

variations in central time constant. Again, all but one (29-12) of the samples from

the aged transformer show lower central time constant and larger initial slope by

comparison with the new samples. The maximum return voltage of 1-12 reached

its peak value at 43 s, where as 1-1 and 1-19 reached their peak values at 94 s. The


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dissipation factor for sample 1-12 is at least fifty percent larger than that for the 1-1

and 1-19 samples. This again illustrates consistency between dissipation factor and

IPS characteristics. Also, for HV B phase samples, it was found that the condition

of the insulation varies even between the layers. It is generally correct to say that

whenever samples have peak values of maximum return voltage with a fast (low)

central time constant, the associated values of the initial slope and dissipation

factor are large. This is illustrated by the examples of samples 60-1, 60-12, 60-19

and samples 29-1, 29-12, 29-19 (29-12 has already been mentioned as an

exception, but even this sample demonstrates the link between better dissipation

factor and better IPS characteristics). Although there are significant differences

of insulation characteristics between top, middle and bottom coils, it is not possible

to draw any conclusion about the trend of variation of insulation status between the

coil locations.


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3.1.3 GPC Test Results

Figure 3: GPC chromatogram of insulating paper samples obtained from new stock

and aged transformer

The GPC chromatograms of typical new and aged (from Kareeya

transformer insulation papers) cellulose are shown in Fig.3. The chromatogram of

new paper shows the presence of two components. The major component at lower

elution volume, high molecular weight, is due to cellulose, while the smaller, lower

molecular weight component is due to hemi-cellulose. The peak molecular weight

of the cellulose is 1.5 * 106 g/mol, while that of the hemi-cellulose is 5.8 * 104

g/mol.

The chromatogram of the cellulose paper taken from the aged transformer

shows that the molecular weight of the cellulose component has decreased

significantly, with the peak molecular weight falling to approximately 2 * 10

5


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g/mol. The molecular weight distribution of the cellulose has also broadened

considerably, and the peak due to the hemi-cellulose has become barely

discernible, suggesting that the hemi-cellulose component of the paper may have

been largely degraded.

In order to obtain an understanding of the nature of the reduction in the

molecular weight of the paper, separate studies were made on the effects of

accelerated ageing on paper and cotton linter (which only contains cellulose).the

results enabled the chromatograms of the new and aged insulation samples to be

simulated by combinations of components.

The simulated chromatograms of the new paper are shown in Fig. 4. The x

axis and y axis of Fig. 4 are in elution volume (ml) and in absorbance respectively

(similar to fig.3). This profile can be simulated reasonably well by a combination

of three components with three peaks, using the computer program peakfit and

an exponential Gaussian function to simulate the peak shape. Of the three

components used, two may be attributed to the cellulose component of the paper,

and the third may be attributed to the hemi-cellulose component. The molecular

weight at the peaks were calculated by employing the universal calibration

procedure to correct the polystyrene calibration curve. Similar simulations were

made for the transformer aged insulations, and the results of some selected samples

have been summarised in Tables 5 and 6.


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Table 5: Results from GPC analysis on samples from LV A phase of Kareeya

transformer

Figure 4: The simulation chromatogram of the new insulating paper


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In Tables 5 and 6, the molecular weights of the peaks P1 and P2 of the

insulating paper in the LV A phase and in the HV B phase of the transformer fall to

about one half to one third of the molecular weights at the corresponding peaks for

P1 and P2 of new insulating paper. A comparison between the LV A phase and the

HV B phase papers indicates that the largest change in molecular weight occurs in

the outermost layers (1-1,45-1,89-1) of the LV A phase conductors. The paper near

the top of the transformer, where the temperature is greatest, shows the greatest

decrease in molecular weight.

3.1.4 Results From Pressboard Samples

The conventional electrical test results of new pressboard and pressboard

collected from Kareeya transformer are presented in Table 7. Unequal electrodes

(ASTM D 149) were used for these tests.

The results show that the dissipation factor of aged transformer pressboard

is much higher than that for new pressboard samples. The power frequency

breakdown strength for aged transformer pressboard is reduced by 29.7% from that

of new unaged pressboard. The lightning impulse breakdown strength of aged

pressboard is reduced by 14.3 %.


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Table 6: Results from GPC analysis on samples from HV B phase of Kareeya

transformer

Table 7: Results obtained from aged and new press board: Unequal Electrodes

(ASTM D 149)


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The IPS results are shown in Table 8. The maximum return voltage of aged

pressboard from the Kareeya transformer reached its peak value at 21 s, whereas

the new pressboard sample reached its peak value after 360 s. The value of the

peak maximum return voltage and initial slope of aged pressboard are also much

larger than those of the new pressboard. Again, these findings are consistent with

those presented earlier for pa per insulation. So, from both the conventional

electrical tests and IPS measurements, it can be concluded that, the degradation of

pressboard in the Kareeya transformer was much more severe than for the paper

insulation.

The GPC technique was applied to new pressboard samples and samples

from the Kareeya transformer. The chromatograms were analysed in a similar way

to that described previously. The results are summarised in Table 9. The reduction

in the molecular weights of peak I and 2 for old pressboard relative to that of new

pressboard shows the deterioration in the condition of the insulation. However, in

contrast to what was observed for paper insulation, there was little change in the

molecular weight of peak 3 (P3) for pressboard.


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Table 8: Results of IPS measurements on new and aged pressboard samples

Table 9: GPC Results obtained from aged and new pressboard Sample

Figure 5: Peak molecular weight versus the central time constants for different

samples of Kareeya transformer


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3.1.6 Correlation Between Methods

Attention has already been drawn to the consistency in changes to electrical

(dissipation factor and IPS data) and chemical(peak molecular weight data)

properties caused by ageing induced degradation. The results are now re-examined

more closely to determine the level of consistency between the electrical and

chemical test methods. Coil 1-1from LV A phase(Table 3 ) shows that the peak

maximum return voltage is attained with a fast (short) central time constant, it has

a large initial slope and its peak molecular weights P1 and P2 (Table 5) are both

very low. So the same conclusion can be drawn from both the tests; that the

insulation has been severely degraded by ageing, and sample 1-1 is one of the most

degraded samples. Similar conclusions can be drawn for the samples 45-1 and 45-

18. Sample 44-18 shows peak maximum return voltage at a larger central time

constant, and as expected this is associated with a larger molecular weight. To

examine the extent of correlation between the electrical and chemical properties,

peak molecular weights P1 of the LV A phase and HV B phase specimens are

plotted against central time constants and initial slopes of the return voltages.

Although there were a few outlying points in both the graphs, Fig. 5 shows that the

decrease in the peak molecular weight corresponds to a decrease (fast) in the

central time constant and Fig. 6 shows that a decrease in the peak molecular weight

corresponds to an increase in the initial slopes.


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To test the statistical independence of the measured parameters, rank

correlation coefficients [4] were calculated for both the cases (with all data points

and with outlying omitted data points). These values are shown in Table 10.

Critical values of the rank correlation coefficients for two sided test with

significance level = 0.05 [4] are also shown in the Table 10. If the observed

value of the rank correlation coefficient is greater than the critical rank correlation

coefficient, then the statistical independence between the tested parameter is

rejected. With omitted outlying data points both graphs show good linear

correlations (with correlation coefficients greater than 0.9). At the same time their

rank correlation coefficients are also greater than the corresponding critical

correlation coefficients. When all the data points are considered, Fig. 6 show linear

correlation with correlation coefficients greater than 0.5. With all the data points,

the observed rank correlation coefficient is greater than the critical rank correlation

coefficient for the data points in Fig. 6. Although the test programme was

necessarily limited, a good trend has been emerged between the IPS parameters

and the chemical test results.

Table 10: Results of correlation coefficients from the correlation Figs. 5 and 6


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Figure 6: Peak molecular weight versus the initial slopes for different samples of

Kareeya transformer

3.2 case study 2:distribution transformers

3.2.1 Conventional E1ectrical Tests Results

Six distribution transformers were provided by electricity distribution

authorities. The dissipation factors of these transformers were measured by the

Schering bridge. For a single phase transformer, the shorted low voltage winding

was connected to the lower voltage arm of the Schering bridge, and shorted high

voltage winding was connected to the high voltage supply. For a three phase

transformer, three phases in the LV winding were short circuited and connected to


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the lower voltage arm of the Schering bridge and three phases in the HV winding

were short circuited and connected to the high voltage supply. Dissipation factors

were measured at two different voltages and the average was determined. This

arrangement measures the dissipation factor of the bulk insulation (insulation

between HV and LV windings) of the transformers. Results from the Schering

bridge measurements are shown in Table 11. Dissipation factors varied from 0.003

to 0.067 for single phase transformers and 0.006 to 0.081 for three phase

transformers. Transformers TI, T3 and T6 show high dissipation factors compared

to the other transformers.

3.2.2 Interfacial Polarization Test Results

All the six distribution transformers were tested for IPS measurements. The

charging voltage was 1000 volt DC and the procedure was similar to that followed

for the specimens made

Table 11: Results of Dissipation factors and capacitances of distribution

transformers


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Table 12: Results of IPS measurements of the single phase distribution

transformers

with two paper wrapped insulated conductors. The connection arrangements

of the transformers (two terminal dielectric) were the same as those used for

dissipation factor measurements. In this case, the bulk insulation between HV and

LV was tested.

From Tables 12 and 13, it is observed that the initial slopes and central time

constants vary significantly between the trans formers. The variation of the peak

maximum return voltage did not follow any specific trend. Some form of

normalisation may be necessary to compare different peak maximum values, when

the transformers are of different insulation resistances and capacitances. In general,

higher initial slopes are associated with shorter central time constants, and this is

consistent with previously presented results. Transformers with these

characteristics also tend to have large dissipation factors. For example, transformer

T1 and T3 show larger dissipation factors and higher initial slopes and lower

central time constants than the transformer T2. The oldest transformer of the three


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phase trans formers, T6 shows high dissipation factor, high initial slope and low

central time constant compared to the corresponding values for the transformers T4

and T5. Thus, a good correlation exists between initial slope, central time constant

and dissipation factor.

3.2.3 GPC Test Results

Several paper and pressboard samples were taken from the distribution

transformers (T1, T2, T3 and T5) for the GPC analysis. The results are shown in

Table 14. From the Tables 11 and

Table 13: Results of IPS measurements of the three phase distribution transformers

Table 14: GPC Results of cellulose samples from distribution transformers


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13, it is observed that the transformer T5 has a very low dissipation factor

with a low peak maximum return voltage, initial slope and large central time

constant. From Table 14, both paper and pressboard samples from T5 show high

peak molecular weight P1, close to that of new paper. From both the electrical and

chemical tests, it is evident that insulation of T5 is in very good condition. Both

paper and pressboard samples from T1 and T3 show a large reduction in peak

molecular weights compared to new ones. From Tables 11 and 12, it is observed


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that the transformer T1 and T3 have high dissipation factors with large initial

slopes and low central time constants compared to the transformer T2. A good

correlation is observed between the IPS parameters, dissipation factor and the GPC

results from the limited number of samples analysed from the distribution

transformers. The important point is that this finding is consistent with similar

findings for the aged power transformer, and as can be seen in the adjunct paper

with the results for artificially aged paper [12].

4 CONCLUSIONS

Conventional electrical tests and IPS measurements were applied to

insulated conductors and pressboard samples collected from a retired power

transformer. The molecular weights of the samples were also studied by GPC

analysis. Significant differences in the condition of the insulation have been ob

served throughout different locations within the Kareeya transformer. The

electrical test results (in particular dissipation factor and the IPS parameters) on the

Kareeya transformer insulation specimens were found to be consistent with the

GPC results. A good correlation has been observed between the electrical test

results and GPC analysis for detecting changes in the properties of the insulation

samples. The condition of aged pressboard from the Kareeya transformer has been

found to be significantly deteriorated compared to new pressboard. This was also

evident from both the electrical and chemical teat results.


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Several distribution transformers were also studied, Dissipation factors and

IPS measurements showed a good consistency in explaining the condition of

insulation in distribution transformers. GPC results from the distribution

transformers also correlated well with the dissipation factor and IPS parameters.

5 REFERENCES

AISN Software, Peakfit, vol. 2.0.

A. Bognar and et al., Diagnostic Test Method of Solid/Liquid Electrical

Insulations Using Polarization Spectrum in the Range of Long Time Constants,

7th International Symposium on High Voltage Engineering, pp. , Dresden,

Germany, 1991.

P. J. Burton and et al, Applications of Liquid Chromatography to the Analysis of

Electrical Insulating Materials, International Conference on Large High Voltage

Electric Sys tem, CIGRE Proceedings of the 1988 Session , Paper 15-08,

pp. , Paris, France, 28 August-3 September, 1988.


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K. V. Bury, Statistical Models in Applied Science, USA, John Wiley and Sons,

1975.

R. Evans, R. H. Wearne and A. F. A. Wallis, Molecular Weight Distribution of

Cellulose as Its Tricarbanilate by High Performance Size Exclusion

Chromatography., Journal of Applied Polymer Science, Vol.37, pp. , 1989.

D. J. T. Hill, P. T. Le, M. Darveniza, T. K. Saha and B. Williams, Gel Permeation

Chromatography of the Thermal Degradation of Cellulose Insulation Paper in

Aged Power Transformer, Materials Forum, Vol.16, pp. , 1992.

D. J. T. Hill, T. T. Le, M. Darveniza and T. K. Saha, Degradation of Cellulosic

Insulation Materials in a Power Transformer-

Part 1: Molecular Weight Study of Cellulose Insulation Pa per, Polymer

Degradation and Stability, Vol.48, no. 1, pp. 1995.

LEO, LEG 599-78, Interpretation of the Analysis of Gases in Transformers and

Other Oil-filled Electrical Equipment, iii Operation, 1978.

H. P. Moser and et al., Application of Cellulosic and Non cellulosic Materials in

Power Transformers, International Conference on Large High Voltage Electric


36/36

System, CIGRE Proceedings of the 31st Session (Paper 12-12), Vol.1,27 August 4

September, 1986.

B. Nemeth, Proposed Fundamental Characteristics Describ ing Dielectric

Processes in Dielectrics, Periodica Polytechnica, Electrical Engineering,, Vol.15,

no. 4, 1971.

T. V. Oomen and L. N. Arnold, Cellulose Insulation Materials Evaluated by

Degree of Polymerization Measurements, Proceedings of the 15th

Electrical/Electronics Insulation Conference, pp. , Chicago, IL, USA 19-22 Oct.

1981.

T. K. Saha, M. Darveniza, D. J. T. H.ill and T. T. Le, Electrical and Chemical

Diagnostics of Transformers Insulation-Part B: Accelerated Aged Insulation

Samples, Accompany ing Paper.

D. H. Shroff and A. W. Stannett, A Review of Papi Aging in Power

Transformers, lEE Proc. , Vol. 132, pt.c, no. 6, pp. , Nov. 1985.

J. Unsworth and F. Mitchell, Degradation Of Electrical Insulating Paper

Monitored With High Performance Liquid Chromatography, IEEE Transaction on

Electrical Insulation, Vol.25, no. 24, pp. , August 1990.

electrical and chemical diagnostics of transformer insulation abstrac1

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