Surge distribution in the Oil Impregnated Paper (OIP) bushing of 400KV transformer to determine failure initiated by draw lead

Failure of high voltage bushing leading to partial or complete destruction of power transformer has been considered as severe event in the electrical power system. Apart from power distribution it has led to human tragedy in a number of sites. Transformer industry is concerned with such problems and hence several committees have discussed the phenomena. Amongst other reasons attributed to it, the failure of draw lead causing complete failure of bushing and insulation breakdown in paper insulation between aluminum foil and the core has been major cause at power plant. The present paper aims at formulating an electrical equivalent network comprising inductances, capacitances and resistances representing the bushing and draw lead. The voltage distribution in the annular region of draw lead and aluminum pipe supporting core of HV bushing is calculated using Electro Magnetic Transient Programme [EMTP] programmed for standard and fast impulse voltage surge. The analysis shows that higher stress leads to the failure of draw lead and paper insulation of the core. Photographs of failed bushing along with initiating spots are given in the paper.