Power transformers Insulation Resistance & Dielectric Loss Analysis

1 Introduction

Power transformers are among the most critical equipment in power systems, and it is essential to maximize prevention and minimize the occurrence of transformer failures and accidents. Insulation failures of various types account for more than 85% of all transformer accidents. Therefore, to ensure safe transformer operation, regular insulation testing of transformers is necessary to detect insulation defects in advance and promptly address potential accident hazards. Throughout my career, I have frequently participated in transformer testing work, accumulating extensive knowledge in this field. This article provides a detailed introduction to comprehensive insulation testing of transformers and the insulation conditions reflected by test results.

2 Measurement of Insulation Resistance and Absorption Ratio

2.1 Measuring Insulation Resistance

During measurement, a megohmmeter should be used according to standard specifications to sequentially measure the insulation resistance between each transformer winding and ground, as well as between windings. The terminals of the winding under test should be short-circuited, while the terminals of non-tested windings should all be short-circuited and grounded. The measurement locations and sequence should follow the table below.

When comparing insulation resistance values, they should be converted to the same temperature using the following mathematical expression:

In the formula:

R1 represents the insulation resistance value (in megaohms) measured at temperature t1

R2 represents the insulation resistance value (in megaohms) calculated at temperature t2

The measured insulation resistance values are primarily judged by comparing the results of successive measurements of each winding. Compared with the previous test results, there should be no significant change, generally not less than 70% of the previous value. During commissioning tests, the value should generally not be less than 70% of the factory test value (at the same temperature).

When no reference values are available, the standard for insulation resistance values is generally as listed in the table below.

2.2 Measurement of Absorption Ratio and Polarization Index

The absorption ratio is the ratio of insulation resistance values measured with a megohmmeter at 60 seconds and 15 seconds after voltage application. The absorption ratio is highly sensitive to moisture in the insulation. When the temperature is between 10°C and 30°C, the absorption ratio should not be less than 1.3.

For transformers rated 220kV and above or 120MVA and above, the polarization index should be measured. This index is the ratio of readings taken at ten minutes and one minute, with the polarization index not less than 1.5.

Measuring insulation resistance and absorption ratio is a simple and universal method for checking the insulation condition of transformers. This test can effectively detect insulation moisture and local defects, such as cracked porcelain bushings, grounded leads, etc. If the measured insulation resistance and absorption ratio do not meet the specified values, certain defects of the aforementioned types definitely exist in the insulation.

3 Leakage Current Test

During testing, a DC high voltage generator and microammeter are used. The voltage application points are as shown in the following table:

Test voltage application standards are shown in the following table.

After raising the voltage to the test voltage, read the DC current passing through the tested winding at one minute; this value is the measured leakage current.

The leakage current test essentially measures insulation resistance. However, because a higher DC voltage is used for measuring leakage currents, it can uncover insulation defects that a megohmmeter cannot detect, such as partial breakdown defects in transformers and lead bushing defects. When analyzing and judging the measurement results, comparisons are primarily made with similar transformers and between different windings, as well as against previous years' test results, without significant changes expected. If the values increase year by year, attention should be paid since this often indicates gradual insulation deterioration. If there's a sudden increase compared to previous years, it may indicate serious defects that need investigation.

4 Measuring the Tangent of the Dielectric Loss Angle

Since the transformer casing is directly grounded, the QS1 type AC bridge with reverse wiring is used for measuring the tangent of the dielectric loss angle. The measurement locations are as shown in the table below.

Note: The actual table contents were not provided in the text, so they're mentioned here in general terms. If you have specific details or data for the table, those could be included in the translation for more precision.

This translation covers the technical procedure for testing the dielectric loss angle and the rationale behind using certain equipment due to grounding considerations. It also reflects the importance of comparing current test results with historical data to identify potential issues within the transformer's insulation system.

During measurement, the two terminals of the winding under test should be short-circuited, while all non-tested phase windings must be short-circuited and grounded. This prevents measurement errors caused by winding inductance.

The standard values for the tangent of the dielectric loss angle of transformer winding insulation (at 20°C) are shown in the following table:

The tangent of the dielectric loss angle should not show significant changes when compared with historical values (generally not exceeding 30%). The test voltage is 10 kV when the winding voltage is 10 kV or higher, and is equal to the rated voltage (Un) when the winding voltage is below 10 kV.

When measuring, the tangent of the dielectric loss angle should be converted to the same temperature using the following mathematical expression:

In the formula:

tgδ1 and tgδ2 represent the tan delta values at temperatures t1 and t2, respectively.

Measuring the tangent of the dielectric loss angle of transformer winding insulation is primarily used to check for transformer moisture ingress, insulation aging, oil deterioration, sludge accumulation on insulation, and severe local defects. If the measured tangent of the dielectric loss angle does not meet the specified values, certain defects of the aforementioned types definitely exist in the insulation.

5 Power Frequency AC Withstand Voltage Test

Equipment for power frequency AC withstand voltage testing typically requires a test transformer, voltage regulator, high-voltage electrostatic voltmeter, and sphere gap. When necessary, an AC ammeter and water resistance may also be connected in series on the high-voltage side. During testing, test equipment should be properly selected based on the test voltage and capacity requirements of the test specimen.