Insulation Resistance in Transformer

Insulation Resistance in Transformers

Insulation Resistance (IR) Test of transformer reveals the condition of insulation (i.e. degree of dryness of paper insulation), presence of any foreign contaminants in oil and also any gross defect inside the transformer (e.g. Failure to remove the temporary transportation bracket on the live portion of tap-changer part).

IR measurements shall be taken between the windings collectively (i.e. with all the windings being connected together) and the earthed tank (earth) and between each winding and the tank, the rest of the windings being earthed. Before taking measurements the neutral should be disconnected from earth.

Acceptable Limits: Unless otherwise recommended by the manufacturer, the IR Values of 500MΩ may be considered as the minimum satisfactory values at 30°C (one minute measurements) at the time of commissioning as a thumb rule for all transformers above 66KV voltage rating. The ratio of 60 second insulation resistance to 15 second insulation resistance value is called dielectric absorption coefficient or Index (DAI). For oil filled transformers with class A insulation, in reasonably dried condition the absorption coefficient at 30°C will be more than 1.3

The polarization index test is performed generally by taking mega ohm readings at 1min and 10min insulation resistance value. The polarization index is the ratio of the 10 min to the 1 min mega ohm readings.

A PI of more than 1.25 and DAI of more than 1.3 are generally considered satisfactory for a transformer when the results of other low voltage tests are found in order. PI less than 1 calls for immediate corrective action. For bushings, an IR value of above 10,000 MU is considered satisfactory.

Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual windings. Insulation resistance tests are commonly measured directly in mega ohms or may be calculated from measurements of applied voltage and leakage current.

The recommended practice in measuring insulation resistance is to always ground the tank (and the core). Short circuit each winding of the transformer at the bushing terminals. Resistance measurements are then made between each winding and all other windings grounded.

Insulation resistance testing:- HV – Earth and HV – LV

Transformer windings are never left floating for insulation resistance measurements. 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.

We need to test winding to winding and winding to ground ( E ).For three phase transformers, We need to test winding ( L1,L2,L3 ) with substitute Earthing for Delta transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N ) for wye transformers.