Failure Investigation and Asset Management of Combined Measuring Instrument Transformers

Asset management of instrument transformers has gained momentum in recent years due to their large population in the network and the potential adverse impacts of their increasing number of failures. This paper investigates the failure cause of a family of combined measuring instrument transformers via temperature profile simulations and consideration of moisture migration behaviour in an oil-impregnated paper insulation system. The temperature difference experienced by an instrument transformer, particularly throughout summer, together with a relative saturation hysteresis phenomenon could have caused a lower dielectric strength that led to failures. The paper also offers insights into factors influencing the probability of failures through analysis on the lifetime data from both graphical survival function plots and statistical Cox model. In-service age, operating voltage level and environment appear to have an influence on failure rate.