Simulation Analysis of Surge Behaviour of Power Transformer Model Winding Represented by Large Number of Sections

Surge voltage behaviour of HV power transformer windings are of great importance to electrical engineers for design of insulation. Behaviour of HV power transformer winding insulation for fast rising surge voltages such as lightning overvoltages, is dependent upon square root of the ratio of the total ground capacitance to total series capacitance of the winding which is termed as alpha["alpha"]. Few recent investigations have been reported which have considered HV transformer winding as a model winding consisting of less than ten sections. However an analysis by representing a HV power transformer model winding by large number of sections can provide more accurate results and other technical information also. In the present investigations computer simulation of HV power transformer model winding represented by 8, 16 and 24 sections are analyzed. The different alpha values of winding selected are 5, 10 & 20. Computer simulations have been carried out using software PSPICE version 10.0. Unit step voltage and standard lightning impulse voltage [1.2/50 mu sec wave shape] have been considered for surge voltage analysis of model winding. The results show that the maximum difference between electrical stress computed for 24 sections representation and 8 sections representation is about 8% for higher alpha value windings corresponding to zero initial time instant. However there are large differences between voltages appearing across sections of winding at other instants of time.