The Inner Voltage Component Analysis and Testing System for Converter Transformer Valve Side Bushings

The epoxy/paper composite insulation of the valve side bushings of converter transformer in HVDC transmission system withstand complex electrical stress during operation, which consists of DC, fundamental AC and harmonic components. Test voltage applied in tolerance test and partial discharge (PD) detection do not take harmonic component into consideration, which can not accurately reflect real insulation condition of converter transformer valve side bushings during operation. Based on the PSCAD simulating and on-site measuring analysis of voltage waveform withstand by converter transformer valve side bushings in ±800kV DC transmission system, the distinguishing features of the inner voltage component of converter transformer valve side bushings are summarized, and the Flourier transformation calculation results of the voltage waveforms show that these characteristics are both regularity and complexity. As for valve side bushings of converter transformer at different positions, which withstand a large number of DC and 50Hz AC voltage components, as well as harmonics of 150, 250, 300 and 350Hz, of which the 150Hz is the highest. The component ratios of 50Hz and DC voltage of YY and YD bushings in high and low valve are 1:7, 1:5, 1:3 and 1:1. For further study, DC combined AC and harmonics voltage generating system are designed and set up, which can output similar voltage waveform compared with real valve side bushing. This investigation can be a foundation to evaluate the real insulating status of the valve side bushings of converter transformer.