A Study on Overvoltage Distribution Across the High Voltage Winding of an Electric Power Transformer

The main objective of this paper is to represent effects of overvoltage on a transformer winding by analysis and modelling with special attention given to the voltage distribution across the winding. The authors have considered both approaches in modelling the winding: windings with distributed electrical parameters, and secondly disk coils with concentrated parameters. All known models are assembled in a general model based upon distributed parameters, while the excitation voltages display sinusoidal variation in time (commutation) or step. Both induced and commutation voltages, applied across the transformer winding, will generate free oscillations which are analyzed further on. According to the model, the transformer’s windings are divided in several disk coils with concentrated known parameters. This results in a complete electrical network used for simulations. All simulations have been performed using the software package SYSEG (SYmbolic State Equation Generation). Using SYSEG package, from the state equations assembled in terms of the disk coils voltages, one can obtain the overvoltage across the transformer winding as function of time. If the frequency of the commutation voltage and the frequency of the free oscillations are in close range, then the voltage across the disk coils shows a non-uniform distribution. An important aspect of this paper is accounting for asymmetry of the transformer by modelling the reinforced insulation of the first turns of the disk coils of the transformer’s high voltage winding. This affects the value of the inter-turn capacitance of these coils, and is an aspect which is treated in our simulations.