Insulation Resistance or IR Test is carried out by maintenance engineers to ensure the healthiness of overall insulation system of an electrical power transformer. It reflects the presence or absence of harmful contamination, dirt, moisture and gross degradation.
IR value will be usually high (several hundred Mega ohms) for a dry insulation system. Maintenance engineers use this parameter as an index of dryness of the insulation system.
This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer.
This test should be conducted before and after repair or when maintenance is performed. The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes.
The general rule of thumb that is used for acceptable values for safe energization is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ. Sample resistance values of good insulation systems are shown in Table.
Recommended Minimum Insulation Resistance for Transformer Winding
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Rated Voltage
( kV)
|
IR value in MΩ
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30°C
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40°C
|
50°C
|
60°C
|
|
|
66 and above
|
600
|
300
|
150
|
75
|
|
33
|
500
|
250
|
125
|
65
|
|
11 & 6.6
|
400
|
200
|
100
|
50
|
|
Below 6.6
|
200
|
100
|
50
|
25
|
|
Note: As a rough guide, the Megger values should be 2 meg. Ohms for every rating at 60°C for 11 KV and above. The Megger values double itself for every 10°C fall in temperature.
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Procedure of IR Test:
The procedure of insulation resistance test are given below
- First disconnect all the line and neutral terminals of the transformer.
- Megger leads to be connected to LV and HV bushing studs to measure Insulation Resistance IR value in between the LV and HV windings.
- Megger leads to be connected to HV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the HV windings and earth.
- Megger leads to be connected to LV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth.
NB: It is unnecessary to perform insulation resistance test of transformer per phase wise in three phase transformer. IR values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta.
Measurements are to be taken as follows :
- For Auto-transformer
- HV-IV to LV, HV-IV to E, LV to E
- For Two winding transformer
- HV to LV, HV to E, LV to E
- For Three winding transformer
- HV to IV, HV to LV, IV to LV, HV to E, IV to E, LV to E
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