Electrical and chemical diagnostics of transformer insulation

8/7/2019 Electrical and chemical diagnostics of transformer insulation

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Electrical and chemical diagnostics of transformer insulation

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. In Power

Link 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

Dept. of EEE MESCE, Kuttippuram

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between the 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.

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.


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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 Electric Tests

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

Schering bridge. 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.

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, is a

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

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

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


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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. 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.

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

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


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adversely affects the return 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).

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

transformer

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


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

measurements the cellulosic materials had to be derivatized to enhance their

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

was prepared.

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. The elution profile

was acquired by interfacing to an IBM computer.


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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, was used to investigate the quality of the insulation using electrical 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%.

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.

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

low voltage windings. 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


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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 winding is shown

in Fig.2.

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

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.

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

phase winding of Kareeya transformer


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

Kareeya transformer.

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

locations respectively and layers 12/18 are from outer and inner locations. In HV

B phase, coils 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 sample is 0.008. The average dissipation factor of HV B phase


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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.

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 New1 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.


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3.1.2 Interfacial Polarization Spectra Results

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.

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

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


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(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.

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. 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 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


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samples 60-1, 60-12, 60-19 and samples 29-1, 29-12, 29-19. 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.

3.1.3 GPC Test Results

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 * 105

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.


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

stock and aged transformer

Figure 4: The simulation chromatogram of the new insulating paper


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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. This profile can be simulated reasonably well by a combination of

three components with three peaks, using the computer program. 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.

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.


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

transformer

3.1.4 Results From Pressboard Samples

The conventional electrical test IPS measurements and GPC analysis is

performed on new and aged transformers sample. The results are presented in

tables 7, 8 and 9.

The results show that the dissipation factor of aged transformer pressboard

is much higher than that for new pressboard sample. It is also observed that the

electric breakdown strength of aged sample is considerably reduced from new

sample.


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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)

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

aged pressboard reached it peak value at 21 s, whereas for new sample it is 360 s.

The value of the peak maximum return voltage and initial slope of aged


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pressboard is much larger than those of the new pressboard. So, from both the

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

degradation of aged pressboard at the Kareeya transformer was much more

severe than for the paper insulation.

The GPC results are shown in Table 9. The reduction in the molecular

weights at the peaks P1 and P2 for old pressboard relative to new pressboard

shows the deterioration in the condition of insulation.

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

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


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

samples of Kareeya transformer

3.1.5 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


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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.

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


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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 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 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. TI, T3 and

T6 show high dissipation factors compared to the other transformers.


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Table 11: Results of Dissipation factors and capacitances of distribution

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 with two paper wrapped insulated conductors.

In this case, the bulk insulation between HV and LV was tested.

From Table 12 and 13, it is observed that the initial sloped and central

time constants vary significantly between the transformers. In general, higher

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

consistent with previously presented results. Transformers with these


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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 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.

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

transformers

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

transformers



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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 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 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.

Table 14: GPC Results of cellulose samples from distribution transformers


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4. CONCLUSION

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 insulntion

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


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been found to be significantly deteriorated compared to new pressboard. This

was also evident from both the electrical and chemical teat results.

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

IEEE transactions on power delivery October 1997.

Degradation of electrical insulating paper monitored with high

performance liquid chromatography IEEE transaction on electrical

insulation august 1990.

Thermal ageing of cellulose paper insulation IEEE transaction on

electrical insulation February 1997.


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ACKNOWLEDGEMENT

I express my sincere gratitude to Dr. P.M.S Nambissan, Prof. &

Head, Department of Electrical and Electronics Engineering, MES College of

Engineering, Kuttippuram, for his cooperation and encouragement.

I would also like to thank my seminar guide Mrs. Sheeba Paulose.

(Lecturer, Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge,

Department of EEE) for their invaluable advice and wholehearted cooperation

without which this seminar would not have seen the light of day.

Gracious gratitude to all the faculty of the department of EEE &

friends for their valuable advice and encouragement.


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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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CONTENTS

1. INTRODUCTION

2. EXPERIMENTAL TECHNIQUES

2.1 Conventional Electric Tests

2.2 Interfacial Polarization Spectra (IPS) Measurements

2.3 GPC Analysis

3. RESULTS AND DISCUSSIONS

3.1 Case Study 1: Kareeya Transformer

3.1.1 Conventional Electrical Test Results

3.1.2 Interfacial Polarization Spectra Results


3.1.4 Results From Pressboard Samples

3.1.5 Correlation Between Methods

3.2 case study 2:distribution transformers

3.2.1 Conventional E1ectrical Tests Results

3.2.2 Interfacial Polarization Test Results


4. CONCLUSION

5. REFERENCES

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Electrical and chemical diagnostics of transformer insulation

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