Lightning impulse testing of three phase combined instrument transformer

Instrument transformers are designed to transform voltage or current from high values in the transmission and distribution systems to low values that can be utilized by low voltage metering and relay devices. The three phase combined instrument transformers are used in distribution systems up to 33kV level due to techno economic feasibility. Lightning impulse test is a type test, and is performed to evaluate instrument transformer insulation strength to withstand lightning impulse voltages. This paper discusses about the different lightning impulse testing practices of the three phase combined instrument transformer and their relevance to the procedures stipulated in the National and International standards.