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Numerical analysis of lightning transient voltage distributions in HV converter transformer winding

High voltage (HV) converter transformer is one of the most important power equipment in DC transmission system. The determination of transient voltage distribution along the winding plays an important role in the insulation design of converter transformer. In this paper an equivalent circuit model for the single-phase converter transformer with the double wound windings has been established, which is mainly composed of longitudinal capacitance, longitudinal inductance and radial capacitance. Both finite element and analytical methods were applied in the calculation of winding capacitance parameters. A combined twelve-pulse rectifier equivalent circuit model, consisted of six single-phase converter transformer and twelve converter valves, was established by means of the PSCAD/EMTDC software. The maximum transient voltage on the terminals of six converter transformers caused by a lightening stroke to the DC transmission line was analyzed. And therefore, the transient voltage characteristics along six converter transformer windings, such as maximum potential distribution and the gradient voltage of each winding disk, were obtained, respectively. The calculated results show that voltage and gradient voltage corresponding to the conducting phase and the non-conducting phase transformers have the same variation trend. The maximum voltage occurs at the end of the winding next to the neutral point of the transformer, while the maximum gradient voltage locates at the head of the winding.