Transient Analysis of Power Transformer with FEM Coupled to Circuit

Abstract

Transient overvoltage produces non-linearity in voltage distribution across winding height of transformer due to high frequency. High frequency surges offer high inductive impedance due to which current through inductor cannot change instantaneously, hence inductor behaves as an open circuit during initial stage of impulse. Winding inductance, thus have no or minimal implication on the initial voltage distribution, which imposes non-linearity in voltage distribution across the winding height. Hence voltage distribution is predominantly decided by the capacitances in the network, which makes the winding schematic electrostatic in nature. When applied transient is maintained for sufficient time, appreciable current starts owing through inductor leading to uniform voltage distribution. This non-linearity stresses the line end of winding profoundly, due to high voltage-gradient in that part of winding, thereby creating large mismatch between initial and final voltage distribution. This difference in voltage distribution affects the insulation of initial turns of winding adversely. Studies have been done to investigate the transient behavior of different winding types of transformer. Analysis of initial voltage distribution is a primary focus in switching or lightning impulse. Various methods provide ways to model equivalent circuit and calculate the capacitance and inductance of the winding network. In efficient and accurate equations for lumped-parameter i.e. capacitance and inductance determination are presented, whereas in simulated and experimental results are compared implying that the proposed model is well-suited for modelling of cast resin dry-type transformer. In description about the formation of equivalent circuit of transformer winding is presented. The purpose of this study is to analyze the voltage distribution across each turn of the winding. It then becomes mandatory to calculate the values of various capacitances and inductances of the winding network. The equivalent capacitance and inductance are calculated analytically as well as through Finite Element Method. Equivalent circuit of transformer winding in terms of capacitance and inductance is formulated and analyzed in LTSpice software. Voltage distribution across each turn is thus obtained. Electrostatic simulation of each turn upon impulse conditions is carried out in ANSYS Maxwell soft- ware through which winding insulation withstanding capability is analyzed. Due to the limitation in generic application of this method, the proposed method wherein powerful simulation tools of FEM, are used to calculate the parameter including inter-turn volt- ages, winding insulation withstanding capability without formation of equivalent circuit of winding network.