Life estimation of transformer insulation under repeated impulses

Oil Impregnated Paper (OIP) is commonly used dielectric material in transformer windings. The most of the transformer winding failures are due to breakdown of dielectric material. The breakdown mechanism of composite dielectric undergoes gradual degradation in nature. OIP is subjected to rapid degradation when it is subjected to overvoltages. The severity of the degradation depends on steepness of the waveshape. It is necessary to analyze the characteristics of OIP for varying wavefronts (t_f) ranging from lightning to Very Fast Transient Overvoltages (VFTO). V-N characteristic provides detailed information on degradation with respect to repeated overvoltages. The time interval (At) between the successive impulse overvoltages plays a vital role in determining the V-N characteristics. In this paper, an attempt has been made to predict the V-N characteristics for any (t_f) and (At) using exponential model and the results are validated with test waveshape of 0.78/50 μs with Δt = 2s. Apart from the V-N characteristics of OIP, the remaining life of the transformer can be estimated from degree of polymerization (DP) using Ekenstam second order equation.