Study of Insulation Material Properties Subjected to Nonlinear AC–DC Composite Electric Field for Converter Transformer

The nonlinear ac-dc composite electric field in the winding area of the converter transformer was studied by considering the frequency and temperature dependence of the insulation materials in this paper. First, the relative permittivity and conductivity of vegetable oils, mineral oils, and insulation papers were measured at different frequencies and temperatures. Then, the oil-paper insulation structure model of one 500 kV converter transformer valve-side winding was established. The nonlinear composite electric field of the converter transformer insulation with vegetable oils and mineral oils was analyzed. The maximum voltage of the converter transformer with mineral oils is 524.9 kV, at -20 °C and that with vegetable oils is 497.1 kV. The results show that the calculation method proposed in this paper is more accurate than the traditional superposition method.