Life estimation of transformer insulation under repeated impulses

Abstract

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 (∆t) 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 (∆t) using exponential model and the results are validated with test waveshape of 0.78/50 μs with ∆t = 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.

Electric stresses on transformer winding insulation under standard and non-standard impulse voltages

“…This degrades the insulation system [2]. The insulation degrades more on repeated exposure to impulses [3,4]. The severity of insulation degradation depends on steepness of the wave, instant of chopping, time to collapse, frequency of oscillations, overshoot near the peak, etc.…”

“…The severity of insulation degradation depends on steepness of the wave, instant of chopping, time to collapse, frequency of oscillations, overshoot near the peak, etc. [1,3,5]. Aschlimann [6] and Sirotinski [7] showed that voltage stresses due to chopped waveforms were higher than those due to full wave of the same steepness.…”

Review of accumulative failure of winding insulation subjected to repetitive impulse voltages

“…Thereafter, in 1996, it was pointed out that oil-paper insulation used in power transformers broke down prematurely when subjected to repetitive impulse voltages [6]. Later, in the 2010s, and following an appeal by Okabe, an increasing number of researchers began to study the accumulative failure phenomenon in oil-paper insulation subjected to repetitive impulse voltages and many useful results were obtained [7][8][9]. The present paper reviews important achievements made in recent decades with regards to the accumulative failure of winding insulation subjected to repetitive impulse voltages and discusses future challenges in this research area.…”