Field experience with return voltage measurements for assessing insulation condition in transformers and its comparison with accelerated aged insulation samples

There has been growing interest in the condition assessment of transformer insulation by the return voltage (RV) method. RV measurements have been applied to a number of power and distribution transformers. The RV parameters are found to vary significantly and consistently with the ageing condition of insulation systems. Among RV parameters, a central time constant has been found to be most sensitive to ageing and is independent of rating of transformers. However, the peak maximum return voltage and initial slope are dependent on the geometry of the insulation, and hence it was difficult to compare these for different ratings of transformers. For the same rating transformer, the initial slope was found to be very sensitive to ageing as well. Peak maximum return voltage has been found to increase with rating (perhaps due to the effect of different insulation resistance). The initial slope and central time constant can therefore be used for assessing the condition of aged insulation in power transformers. A good replication of ageing condition was achieved by the ageing experiments in the presence of air.