Optimized Reactive Power Flow of DFIG Power Converters for Better Reliability Performance Considering Grid Codes

If there is no reactive power exchange between a doubly fed induction generator (DFIG) and a grid, the various characteristics of the power converters in a DFIG wind turbine system cause the lifetime expectancy of a rotor-side converter (RSC) to be significantly less than that of a grid-side converter (GSC). In order to fulfill modern grid codes, over-excited reactive power injection will further reduce the lifetime of the RSC. In this paper, the additional stress of a power semiconductor due to the reactive power injection is first evaluated in terms of a modulation index and the current loading. Then, an optimized reactive power flow is proposed in the case where an over-excited reactive power support is applied with the joint compensation from both the RSC and the GSC. Finally, some experimental validations are performed at a downscale DFIG prototype. It is concluded that, among the different combined reactive power support strategies, the best scheme will tradeoff the lifetime between the GSC and the RSC.