Characterization of OIP for varying thickness and condition assessment after repeated transients

Abstract

Inter-turn insulation plays a major role in the dielectric integrity of newlineany transformer. Based on the design and rating of the transformer, the newlinethickness of the inter-turn insulation is normally changed. Though the interturn newlineinsulation thickness is designed based on standard Impulse (LI) voltage newlinedistribution, the insulation fails due to various types of overvoltages from newlineSwitching Impulse (SI) to Very Fast Transient Overvoltages (VFTO). To newlineanalyse the effect of thickness of inter-turn insulation the commonly used Oil newlineImpregnated Paper (OIP) is considered for the entire study. The breakdown newlinecharacteristics of OIP under SI, LI, VFTO, AC and Partial Discharge (PD) newlinecharacteristics are obtained for different thicknesses (0.25mm to 0.75mm). As newlinethickness increases the breakdown strength decreases under all the considered newlinewaveshapes. In case of impulse voltages, the breakdown strengths are newlineproportional to the front time of the respective impulses. The percentage newlinedecrease in breakdown strength is more under AC and the PD parameters newlinesuch as, Partial Discharge Inception Voltage (PDIV) and Partial Discharge newlineExtinction Voltage (PDEV) follow the same trend as that of AC. newlineMoreover, the insulation gets degraded due to many factors such as newlineservice-ageing and exposure to repeated applications of overvoltages due to newlinevarious reasons. The effect of the repeated impulses is characterized using the newlineVoltage-Number (V-N) characteristics as recommended by CIGRE Working newlinegroup C4.302. To quantify and compare the impulse breakdown strength and newlinedegradation due to repeated impulses under SI, LI and VFTO, different newlinethicknesses of OIP are considered. Experiments are carried out to obtain the newlineV-N characteristics for all the cases. V-N characteristics are mathematically newlinemodelled by enhancing the existing Exponential model to incorporate the newlinethickness of insulation and are then validated experimentally. newline newline