The insulation resistance standards for power transformers vary according to their voltage levels and operating conditions. The specific requirements are as follows:

1. For transformers in operation:

• For 10kV and below: The insulation resistance should be greater than 300 megaohms (MΩ).
• For 35kV: The insulation resistance should be greater than 400 megaohms (MΩ) (measured at an ambient temperature of 20°C).

By measuring the insulation resistance of power transformers, it is possible to identify insulation issues or other transformer problems, such as winding contact with the casing, the iron core, or short circuits between coils. This is why insulation resistance measurements are typically conducted during routine maintenance or after major repairs.

2. Measuring transformer insulation resistance:

When measuring transformer insulation resistance, it is common to test:

• The insulation resistance between the high-voltage winding and the casing.
• The insulation resistance between the low-voltage winding and the casing.
• The insulation resistance between the high-voltage and low-voltage windings.
• If a major overhaul involves core lifting, the insulation resistance between the through-core bolts and the iron core should also be measured.

For most insulation resistance measurements, a 2500V megohmmeter is used. For measuring the insulation resistance between the through-core bolts and the iron core, a 1000V megohmmeter is typically employed.

3. Influence of humidity and temperature:

The insulation resistance of power transformers is greatly affected by humidity and temperature. When humidity increases, the surface and internal components absorb moisture, causing a rise in leakage current and a reduction in insulation resistance. Similarly, when the temperature rises, the movement of charged particles becomes more intense, resulting in an increase in leakage current and a decrease in insulation resistance. Therefore, the measured insulation resistance varies with temperature, with higher temperatures corresponding to lower resistance values.

Transformer Insulation Resistance Requirements

1. For new installations or after major repairs: Transformers that have been out of operation for more than three weeks should have their insulation resistance measured before being put back into service.
2. For windings with voltage levels above 1000V: Use a 2500V megohmmeter, while for windings below 1000V, use a 1000V megohmmeter.
   • Insulation resistance values (at 20°C):
     • 3-10kV: ≥ 300MΩ
     • 20-35kV: ≥ 400MΩ
     • 63-220kV: ≥ 800MΩ
     • 500kV: ≥ 3000MΩ
   • For transformers with voltages of 0.4kV or below, a 500V megohmmeter should be used, and the resistance should be ≥ 0.5MΩ.

4. Absorption ratio:

When the insulation resistance value drops to less than 50% of the previous measurement, maintenance should be notified. It may also be necessary to measure the dielectric loss and absorption ratio of the transformer. The absorption ratio R60/R15R_{60}/R_{15}R60​/R15​ (resistance measured at 60 seconds divided by resistance measured at 15 seconds) should not be less than 1.3.

Insulation Resistance Types:

• High to low and ground: Insulation resistance between the high-voltage winding and both the low-voltage winding and the casing.
• Low to high and ground: Insulation resistance between the low-voltage winding and both the high-voltage winding and the casing.

Insulation Resistance Acceptance Criteria:

1. The insulation resistance value measured this time, when converted to the same temperature as the previous measurement, should not be more than 30% lower than the previous value.
2. The absorption ratio R60/R15R_{60}/R_{15}R60​/R15​ should be ≥ 1.3 at temperatures between 10°C and 30°C.
3. For transformers with a primary voltage of 10kV, the minimum acceptable insulation resistance depends on the temperature.

Temperature Impact on Insulation Resistance:

The insulation resistance of transformers changes with temperature. A useful rule of thumb is: "For every 10°C increase in temperature, the resistance decreases by half; for every 10°C decrease, the resistance doubles."

5.For new installations or after major repairs: 

The insulation resistance value must meet the specified standards. For transformers in operation, the insulation resistance should not fall below 10MΩ.