Electrical and chemical diagnostics of transformer insulation
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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.

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), tanö and furans and (b) measurements on samples removed from the transformer (degree of polymerization (DP) tensile strength). At the interface 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 and implimentation, 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.
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 = aE is the steady state value of the polarization, a 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.

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) = ßE, where ß 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 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 (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.
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 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


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

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 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.
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

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 (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 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.

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

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.
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.
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 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


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 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 a = 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


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.
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 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

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 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


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 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.

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 and presentation 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 and presentation 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.


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
3.1.3 GPC Test 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
3.2.3 GPC Test Results
4. CONCLUSION
5. REFERENCES
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REMYA JAYAN
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