Analysis of lightening impulse response of cast resin dry-type transformer

Lightning impulse has the biggest influence on the insulation structure of power transformer, which may lead to transformer breakdown. When the transformer is subjected to lightning impulse, the complex electromagnetic process will be produce on the winding. The overvoltage distribution along the winding is a very important issue for the designers of insulation structure. In this paper, a 35kV cast resin dry-type transformer is taken as an example to establish the FEM simulation model. The inductance and capacitance distribution parameters of the high voltage winding are analyzed and calculated. Then, according to the actual structure of the transformer, each section of coil is divided into one unit for the equivalent circuit of transformer. The transient overvoltage distribution of each section under the action of lightning voltage is analyzed by using the Simulink simulation toolbox of MATLAB. For more detailed research, each section is remodeled as a sub model that each turn conductor is divided into a unit. The ground voltage of each turn and turn to turn voltage under lightning impulse are calculated. The results show that due to the existence of the distributed inductance and capacitance, there is a hysteresis and oscillation in the potential of each section. The maximum potential exceeds the peak of the lightning impulse voltage, and appears at Section 3, 4, 5, which near the low voltage windings. At about 10μs, potential of Section 3, 4, 5 rises to the maximum, and electric field is concentrated in the air channel between the high and low windings, which is about 3.5 kV/mm. The front and the end sections have maximum voltage between sections and maximum electric field between turns.