Fault Characteristics Study of VSC Converter and Protection Design in DC Microgrids

DC microgrid is an effective way for Grid-connected with distribution generators. However, its development is constrained by the lack of precise fault analysis and valid fault protection scheme. In this paper, cable pole-to-pole and pole-to-ground faults are analyzed in stages with fault circuits and relative equations. The current waveform and process of diodes in voltage-source converter (VSC) is focused on because of its vulnerability. It is demonstrated that diodes conduct simultaneously at the voltage of VSC dc side crossing zero point with inrush current, which may damage VSC severely. Moreover, the impact generated by ac source current to VSC dc-side voltage is studied. It is found that the fault distance, leading to VSC dc-side voltage crossing zero, is shortened. Consequently, the diodes will not conduct simultaneously for terminal faults, which is advantageous for protections designing. Then, the simulation system is established through PSCAD/EMTDC and the correctness of all the analysis is verified. Based on the fault characteristics, non-unit current differential protection combining with directional unit protection is proposed, which provides the possible that both speed and selectivity of protection can be satisfied.