A Novel Monitoring Technique Using Common-mode Voltages for the Transformer Energized by VSCs.

Converter transformers are operated under high switching stresses and face a severe problem of insulation ageing. Traditional monitoring approaches usually suffer from invasiveness and may fail to diagnose the insulation defects under harsh operational conditions. Effective technique to forewarning insulation ageing of converter transformers is highly desired. To realize online non-invasive insulation monitoring, this paper proposes a novel monitoring technique for transformers using inherent common-mode components in the voltage source converter system. Firstly, the common-mode model of converter transformer is built to clarify the insulation characteristic and the effects of leakage flux and core loss. Then, the common-mode-based response using the neutral-end voltages is put forward to evaluate the change of insulation capacitances. The consideration for the frequency-dependent effect on the response is also discussed. To validate the effectiveness of the proposed common-mode model and monitoring technique, experiment tests in the laboratory are conducted on a 1-kW 380/220-V three-phase transformer. Experimental results demonstrate that the proposed response has a satisfactory performance of evaluating the ageing level of different insulation components.