V-t Characteristics using extended disruptive effect model for impulses of varying front times

Abstract

Overvoltages due to internal and external sources stress the insulation of the power system equipment. Usually the insulation design of any power equipment is tested with standard lightning impulse voltage. In practical cases, the insulations are stressed with overvoltages of non-standard waveshapes like steep fronted, Very Fast Transient Overvoltages (VFTO) and oscillatory impulses also. This demands the study on behavior of insulating materials under non-standard impulses. The prediction of v-t characteristics of any insulating material for such non-standard overvoltages is required for proper insulation design. In this regard, an attempt is made to characterize air insulation using existing Disruptive Effect model for standard lightning impulse (1.2/50 μs) and is found successful only for time to breakdown (tb) greater than 2 μs. Hence, an extended Disruptive Effect model is proposed to cater to tb less than 2μs and the model is verified up to VFTO. The model is also verified for impulses of varying front times between 90 ns to 1.2 μs with oscillations.