Electric Field Optimization of Cast Resin Dry-Type Transformer under Lightning Impulse

Lightning impulse voltage is one of the main causes of transformer breakdown in power system and also the key factor in determining its insulation structure. The optimization of electric field distribution under lightning impulse is the key point of insulation design of dry-type transformer. In this paper, the electric field distribution of 110 kV dry-type transformer was analyzed and optimized based on a field-circuit coupled way. First, considering the symmetry of the dry-type transformer structure, a two-dimensional axisymmetric finite element analysis model was established. The equivalent circuit of high voltage winding was built up in MATLAB Simulink toolbox by using the circuit parameters calculated according to the structure of transformer. Then the voltage distribution can be obtained by applying the lightning impulse waveform with a peak value of 480 kV to the circuit. After importing the voltage data, the electric field distribution can be calculated by finite element analysis software. Finally, on the basis of the initial scheme, the electric field of air channel were further optimized by increasing air channels and adjusting winding arrangement, so as to make the insulation design safer and more reliable. The results show that due to the existence of distribution parameters and the characteristics of short duration and high frequency of impulse voltage, its distribution in windings was characterized by nonuniformity, hysteresis and oscillation. For the initial scheme, the maximum electric field of the air channel of transformer appeared at 127 gs with a value of 5.84 kV/mm. After optimization, the electric field of transformer air channel was lower than 3 kV/mm in most time, and the maximum value was 3.46 kV/mm, which appeared at 10 gs.