Distribution Transformer Bushing Casing Insulation Degradation Diagnosis

transformer bushing1

In this paper, the author introduces a 500kV main transformer bushing with abnormal dielectric loss, and theoretically analyzes the abnormal dielectric loss in the process of disassembly and testing. The results of the analysis show that the main cause of the abnormal dielectric loss is aging, which provides some reference for the fault diagnosis and operation and maintenance of the same type of equipment.

Distribution transformer is an important transmission and transformation equipment in power system, and its reliability affects the safe and stable operation of power grid. As an important part of distribution transformer, casing insulation failure will occur easily if aging, water and moisture enter into it. A large number of distribution transformers have been in service for several decades, and it is important to strengthen the operation and maintenance of these distribution transformers, so that problems can be detected in time to eliminate the hidden danger of casing insulation failure.

In this paper, I disassembled and tested a 500kV distribution transformer casing with excessive dielectric loss. Based on the test results, I theoretically analyzed the possible causes of the excessive dielectric loss of the casing and verified the correctness of the analysis by combining the disassembly test.

Table of Contents

Defects and field test situation analysis

In May 2019, a 500kV substation No.1 main transformer was insulated during routine maintenance, and it was found that the dielectric loss of the main screen of the medium voltage side casing (Am: 1.183%, Bm: 0.696%, Cm: 0.883%) (normalized to 20℃) exceeded or was close to the noted value (0.8% on the medium voltage side), which was higher than the test result in 2013 (Am: 0.40%, Bm: 0.43%, Cm: 0.40%) (normalized to 20℃). ) (normalized to 20°C), which is a significant increase.

The information of main transformer No. 1 and retired casing is shown in Table 1. In order to find out the cause of the defect, the diagnostic tests of 10 kV frequency dielectric loss and frequency-domain dielectric response (FDS) of the casing were carried out on the No. 1 main transformer bushing of the substation, and the results are shown in Figure 1.

Parameter

#1 main transformer

High pressure bushing

Medium pressure casing

Rated voltage

510/230±9×1.33%/36

500kV

245kV

Rated current

3849A(Low Pressure)

1600A

3150A

Model

 SUB-MRR

COT1500-1600

COT950-3150

Date of commissioning

1999-12-8

1999-5-25

1999-5-25

distribution transformer2

Figure 1: Frequency-domain dielectric response test results of a 500kV main transformer No. 1 medium-voltage bushing

From Figure 1, it can be found that the frequency domain dielectric response (FDS) curve is obviously high and convex in the low and medium frequency bands, indicating that there is some deterioration of the main insulation of the casing.

The main screen dielectric loss of the MV casing was retested in August 2019, and the test results did not change significantly from the test results in May 2019.

The equipment operation and maintenance management unit scrapped the entire main transformer in view of the demand for additional capacity in the area and the age of the main transformer in service.

At the same time, in order to further analyze the reasons for the abnormal dielectric loss of the casing, and to evaluate the insulation status of the old casing that had been in operation for more than 20 years, and to provide O&M reference for the casing with similar defects, the retired casing of main transformer No. 1 was returned to the plant for diagnostic tests and disassembly analysis.

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Return test and disassembly plan

Insulation resistance

After returning to the plant, the insulation resistance of the MV casing Am was measured first, and the test results showed that the insulation resistance of the MV casing of the distribution transformer was not abnormal.

FDS and water content test

The FDS test results of the casing are shown in Figure 2.

Based on the FDS test results, the following conclusions can be drawn.

(1) In terms of voltage level, there is no significant abnormality in the FDS curve (black line in the figure) of high-voltage casing C (the casing’s routine test data are normal), while the curve of medium-voltage casing Am (the main screen’s dielectric loss reached 1.183% in the routine test) is significantly upward in the low and medium frequency sections (blue line in the figure) and shows an obvious upward convex shape, which is consistent with the casing insulation deterioration characteristics. The moisture content of the casing insulation was roughly analyzed using the moisture assessment software included with the test instrument.
The results of the casing moisture assessment also showed that the insulation of the medium-voltage casing Am had deteriorated more severely, while the insulation of the high-voltage casing C was in good condition.

main transformer 2

In addition, for the same casing, the moisture assessment result is lower at high temperature, because the distribution of moisture between oil and paper insulation system is temperature dependent, and moisture migrates from paper to oil at high temperature, so the moisture content in paper is relatively low at high temperature.

(2) In terms of temperature, the correlation between the FDS curve of high-voltage casing and temperature is not significant, while the FDS curve of medium-voltage casing shifts significantly to the right as the temperature rises, which may be due to the increase of insulation medium conductivity loss caused by the increase of temperature.

Find more about Ultimate Dirstribution Transformer for Guide

Insulation oil test

Oil samples were taken from the casing and tested for oil chromatography, and the results are shown in Table 3. The oil chromatograms of medium pressure casing Am had similar CO and CO2 contents, indicating the possibility of deterioration of the solid insulation of the casing. At the same time, oil samples were taken and tested for micro water, withstand voltage, volume resistivity and other indicators.

Main Transformer #1 Decommissioned Casing FDS Test Results
Main Transformer #1 Decommissioned Casing FDS Test Results

The results of the transformer bushing moisture assessment also show that the medium voltage casing Am has a more severe insulation deterioration, while the high voltage casing C is in good condition.

This is because the distribution of moisture in the oil-paper insulation system is temperature dependent, with moisture migrating from the paper to the oil at high temperatures, so the moisture content in the paper is relatively low at high temperatures.

(2) In terms of temperature, the correlation between the FDS curve of high-voltage casing and temperature is not significant, while the FDS curve of medium-voltage casing shifts significantly to the right as the temperature rises, which may be due to the increase of insulation medium conductivity loss caused by the increase of temperature.

Insulation oil test

Oil samples were taken from the casing and tested for oil chromatography, and the results are shown in Table 3.

The oil chromatograms of medium pressure casing Am had similar CO and CO2 contents, indicating the possibility of deterioration of the solid insulation of the casing. At the same time, oil samples were taken and tested for micro water, pressure resistance, volume resistivity and other indicators.


According to the test results, it was found that the insulating oil of the medium-voltage casing Am did not exceed the standard value, but the indexes were significantly worse than those of the high-voltage casing C. In particular, the appearance of the Am insulating oil was obviously yellow.

Both bushings are produced by a French company and belong to the same COT series, so the quality of insulating oil used in general is similar.

Main screen high voltage dielectric loss and capacity

In December 2020 (temperature 5.5℃, humidity 65%) and September 2021 (temperature 33℃, humidity 60%), the high voltage dielectric loss and capacity of the main screen of the casing were measured twice in the plant. The main screen capacity of high-voltage casing C and medium-voltage casing Am showed very little deviation at different temperatures and voltages.

The results of the main screen high voltage dielectric loss (normalized to 20°C) at different temperatures and voltages are shown in Figure 3. From the test results, the difference of high voltage dielectric loss of high voltage casing at different temperatures and voltages is very small and the value is low, which indicates that the insulation of the casing is in good condition; the medium voltage casing dielectric loss increases significantly with the increase of temperature and tends to decrease with the increase of voltage, which indicates the deterioration of medium voltage casing insulation.

The above diagnostic tests show that the medium-voltage casing has the largest dielectric loss test results on site and at the factory, and the dielectric loss decreases significantly with the increase in voltage (10 kV to rated voltage, the dielectric loss decreases by about 50%).

The frequency-domain dielectric response curve is obviously shifted upward in the middle and low frequency bands and shows an upward convex shape, and the frequency-domain dielectric response curve migrates significantly with the change of temperature, which indicate that the casing insulation has a certain degree of deterioration. However, the casing insulation oil did not show obvious degradation characteristics, so it was inferred that the insulation degradation mainly existed in the insulation paper.

Read on Why Does the Power Factor of the Distribution Transformer Become Smaller?

Cause Analysis

The dielectric loss model of oil-paper insulation is established for the main screen of distribution transformer bushing, which can be regarded as an insulation composed of two parts of dielectric oil and paper in parallel, as shown in Figure 4.

Dielectric loss model for oil-paper insulation
Dielectric loss model for oil-paper insulation

Compared with the high-voltage side (5.66 × 1011Ω-m), the volumetric resistivity of the medium-voltage side casing oil (3.34 × 1011Ω-m) has changed significantly and decreased by nearly 30%, so compared with the dissipation loss of the high-voltage side, the tanδ of the medium-voltage side has increased significantly.

The reason for the increase in the volume resistivity of the medium-voltage side casing oil is that the aging of the medium-voltage side casing insulating paper leads to an increase in the degradation of cellulose and a decrease in the degree of polymerization, and the positive and negative ions generated by decomposition are free in the oil, which increases the content of charged particles per unit volume and leads to an increase in the electrical conductivity of the oil, i.e., a decrease in the volume resistivity.

And the increase of water content of insulating paper will lead to the increase of tanδpap, which also affects the value of distribution transformer bushing main screen dielectric loss tanδ. To verify the correctness of the theoretical analysis, the distribution transformer was disassembled and tested to further verify the aging point.

Disassembly test

Disassembly condition

The distribution transformer was disassembled, and there were no obvious abnormalities in the current-carrying structure, seals, and upper and lower porcelain sleeves.

From the disintegration phenomenon, there was a black substance on the surface of the medium pressure casing core, which was located at 75cm-80cm from the bottom of the core, and all three casing cores had local folds, as shown in Figure 5. When the core was disassembled to the innermost layer, no X-wax material was found on the inner insulating paper, indicating that there was no obvious partial discharge inside the casing.

During the disassembly process, the insulation paper was taken from the outermost layer of each step of the core, near the top of the casing, and the water content and polymerization degree were tested.

Polymerization degree of insulating paper

The results of the polymerization degree of insulation paper for each step of the core.

A new insulating paper of the same grade as the disassembled main transformer casing (Finnish TERTRANS N 125/0.80 KRAFTPAPER) was taken from a transformer bushing factory and the polymerization degree was 1427, which was taken as the polymerization degree of non-deteriorated insulating paper.

According to the test results, the polymerization degree of high-voltage casing insulation paper was distributed around 1100, and the relationship with location was not obvious, showing the overall deterioration characteristics, with a decrease of about 23% compared with the initial.

The polymerization degree of medium-voltage casing insulation paper shows an obvious three-stage distribution, with the average polymerization degree of the outer 1-8 layers being about 700, which is more than 50% lower than the initial polymerization degree.

The average degree of polymerization of middle 9~16 layers is about 1200, which is about 16% lower than the initial degree of polymerization; the average degree of polymerization of inner 17~20 layers is about 1000, which is about 30% lower than the initial degree of polymerization.

Overall, the insulation aging of MV casing cores is more serious, and it is closely related to the sampling location. The possible reason is that the deterioration of insulation paper is accelerated by the higher water content of the outer insulation paper and the higher gas content in the oil, and the deterioration of insulation paper is also accelerated by the higher temperature of the inner insulation paper, so the deterioration of outer and inner insulation is more serious.

According to the research results of A.M. Emsley and others, the degree of polymerization of insulation paper is significantly related to the tensile strength of the paper, as shown in Figure 8: it can be seen that when the degree of polymerization is above 600, the tensile strength decreases insignificantly.

However, when the degree of polymerization drops below 600, the tensile strength decreases sharply, i.e., the mechanical strength of the insulation paper is significantly reduced and cannot withstand the mechanical stresses under special working conditions.

Relationship between polymerization of insulating paper and tensile strength

Based on the actual situation of MV casing, the remaining life of casing can be approximated by equation (8): it takes 20 years to reduce the degree of polymerization from the initial 1400 to 700 (commissioned in 1999 and decommissioned in 2019), and it takes about 26 years to further reduce it to below 600.

Considering that the moisture generated by deterioration will further accelerate the deterioration rate, it is considered that the remaining life of MV casing in this substation may be 5 years or even shorter.

Learn more about the 2022 Ultimate Distribution Transformer Losses Guide

Old casing operation and maintenance suggestions

Based on the results of the test and disassembly analysis of old casing in a 500kV substation, the following O&M recommendations are made for old casing that has been in operation for more than 20 years.

(1) For casing with no abnormal routine test data, it can be operated normally.

(2) For casing that may have partial capacitor screen breakdown defects (abnormal electrical capacity and chromatography), replace them in time.

(3) For the casing that cannot determine whether there is partial breakdown of capacitor screen, but the dielectric loss is abnormally high, combine with the field test conditions and carry out high

(a) high-voltage casing

(b) medium-voltage casing

Casing insulation paper polymerization test results pressure dielectric loss, frequency domain dielectric response, insulating oil chromatography test, oiling test.

If the frequency domain dielectric response is significantly shifted upward in the low and middle frequency bands and shows an upward convex shape, it indicates that the insulation is deteriorated. Then combined with the insulation oil test results can be initially judged as oil degradation or paper degradation.

(4) If conditions are available, the FDS test curve history data at different temperatures can be compared. The normal casing FDS curve has a small shift, while the abnormally deteriorated casing FDS curve at different temperatures has a more obvious shift.

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