What is the qualified value of transformer high and low voltage insulation resistance?
The qualified standard for insulation resistance of power transformers in operation is: for 10KV level and below, the insulation value is greater than 300 megohms; for 35KV level, the insulation value is greater than 400 megohms (ambient temperature is 20℃).
The insulation resistance measurement of power transformers can effectively detect certain insulation problems and other problems of transformers, such as windings touching the shell, touching the iron core, short circuits between coils, etc. Therefore, insulation resistance should be measured during regular maintenance or after overhaul. When measuring the insulation resistance of a transformer, it is generally necessary to measure the insulation resistance between the high-voltage coil and the shell, the low-voltage coil and the shell, and the high-voltage coil and the low-voltage coil. When the core is overhauled, the insulation resistance of the through-core screw to the iron core should also be measured. A 2500V megohmmeter is generally used to measure the insulation resistance of a power transformer, but a 1000V megohmmeter is generally used to measure the insulation resistance of the through-core screw to the iron core.
The insulation resistance of power transformers is greatly affected by humidity and temperature. When humidity increases, the surface and the interior absorb moisture, the leakage current increases, and the insulation resistance decreases; when the temperature rises, the charged particles are easy to conduct electricity due to the enhanced thermal motion, the leakage current increases, and the insulation resistance decreases. Therefore, the insulation resistance values measured at different temperatures are different. The higher the temperature, the lower the insulation resistance.
Transformer insulation resistance standard
- 1. The insulation of a newly installed or overhauled transformer or a transformer that has been out of service for a long time (three weeks) should be measured before it is put into operation.
- 2. For windings with voltage levels above 1000V, use a 2500V megohmmeter, and for windings below 1000V, use a 1000V megohmmeter. The resistance value is specified (20°C) as 300MΩ for 3-10KV, 400MΩ for 20-35KV, 800MΩ for 63-220KV, and 3000MΩ for 500KV. For transformers with voltages of 0.4KV and below, use a 500V megohmmeter with a resistance of ≥0.5MΩ.
- 3. When the resistance value is lower than 50% of the previous value, notify the maintenance process, and measure the dielectric loss and absorption ratio of the transformer if necessary. The absorption ratio R60 “/R15”, the ratio is not less than 1.3.
Name of insulation resistance:
High to low and ground: (primary winding to secondary winding and shell) the insulation resistance of high voltage winding to low voltage winding and shell; low to high and ground: (secondary winding to primary winding and shell) the insulation resistance of low voltage winding to high voltage winding and shell .
Standards for qualified insulation resistance values
1. The insulation resistance value measured this time is compared with the value measured last time at the same temperature. This value shall not be lower than the last value by 30%;
2. The absorption ratio R60/R15 (the ratio of insulation resistance at 60 seconds and 15 seconds in telemetry) should be 1.3 or above at 10-30℃:
3. For transformers with a primary voltage of 10kV, the minimum qualified insulation resistance value is related to temperature. Relationship between transformer insulation resistance and temperature during test
Transformer insulation resistance calculation formula: Use the formula to calculate the insulation resistance at each temperature: “a rise of ten is halved, a decrease of ten is doubled, and a good one is multiplied by one and a half” Absorption ratio: R20 = Rt × 10t-20/40 For every 100℃ increase in temperature, Rt × 2/3 times. For every 100℃ decrease in temperature, Rt × 1.5 times.
4. The insulation resistance qualified value of newly installed and overhauled transformers should meet the above regulations. The insulation resistance qualified value of operating transformers should not be less than 10 megohms.
