Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual windings. Share with your Facebook friends! HV measurement of resistance pdf shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards.
Insulation resistance tests are commonly measured directly in megohms or may be calculated from measurements of applied voltage and leakage current. Short circuit each winding of the transformer at the bushing terminals. Resistance measurements are then made between each winding and all other windings grounded. Solidly grounded winding must have the ground removed in order to measure the insulation resistance of the winding grounded. If the ground cannot be removed, as in the case of some windings with solidly grounded neutrals, the insulation resistance of the winding cannot be measured.
Treat it as part of the grounded section of the circuit. Ref: A Guide to Transformer Maintenance by. Guard the terminals to eliminate surface leakage over terminal bushings. Apply the test voltage and note the reading. Value at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature. The transformer Neutral bushing is to be disconnected from earth during the test. All LV surge diverter earth connections are to be disconnected during the test.
Due to the inductive characteristics of transformers, the insulation resistance reading shall not be taken until the test current stabilizes. Avoid meggering when the transformer is under vacuum. IR between HV and LV as well as windings to earth. For rated voltage below 1KV, measured with a 500VDC Megger.
For rated voltage above 1KV, measured with a 1000VDC Megger. In accordance with IEEE 43, clause 9. 3, the following formula should be applied. For 11KV, Three Phase Motor. 41 MΩ but as per IEEE43 It should be 5 MΩ. IR Value of Motor as per NETA ATS 2007. For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply.