Insulation coordination among the power system components plays a crucial role in reliable design of the power system. Usually the insulation coordination is effected through the volt-time characteristics of the components under overvoltages of standard waveshapes, like Lightning impulse 1.2/50μs and switching impulses. In practice the system components might be stressed with the overvoltages of non-standard Lightning impulses and the advent of gas insulated substation necessitates the v-t characteristics under VFTO. To incorporate the above facts the CIGRE Working Group C4.302 has discussed the importance of the v-t characteristics for non standard lightning impulse waveforms. As transformers play an important role in power system detailed analysis on v-t characteristics of transformer insulation becomes necessary. In this paper an attempt has been made to analyse the degradation of Oil Impregnated Paper (OIP) for different impulse waveshapes with varying front times from 64 ns to 1.4 μs with same tail time of 50μs and CIGRE representative waveforms. The v-t characteristics are mathematically modelled using hyperbolic model and the corresponding model parameters are extracted. Thus developed models are used for predicting the v-t characteristics for any type of overvoltages of the above mentioned categories and compared with the experimental results.
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