Research on fast solid state DC breaker based on a natural current zero-crossing point

The DC fault characteristics of voltage source converter based high voltage direct current(VSC-HVDC)systems are analyzed in this paper.The phenomenon whereby the capacitor on DC side discharges quickly during a DC fault contributes to a large short-circuit fault current.Neither traditional DC breakers nor DC switches can cut off the fault current under this condition.A fast solid state DC breaker design method is proposed in this paper.This method is based on the fault current characteristics of the inverter in multi-terminal HVDC systems(MTDC),where a fault current appears at the natural zerocrossing point near the inverter.At this point,by coordinating the AC breakers near the rectifier,the DC breaker could reliably cut off the DC fault current and protect the system.A detailed model for this fast solid state DC breaker and its operation sequence are studied,based on this design method.Simulations modeling a five-terminal meshed DC grid and a fast DC breaker were carried out with PSCAD/EMTDC using this design method.The results from the simulations confirmed the validity of the design method.

Research on hybrid multi-terminal high-voltage DC technology for offshore wind farm integration

Multi-terminal high-voltage DC(MTDC)technology is a promising way to transmit large amounts of offshore wind power to the main grids.This paper proposes a hybrid MTDC scheme to integrate several offshore wind farms into the onshore power grids at different locations.A hybrid four-terminal HVDC system comprising two onshore line commutated converters(LCCs)and two voltage source converters(VSCs)connecting an offshore wind farm is constructed in PSCAD/EMTDC.A coordination control scheme based on the VSCs’AC voltage control and the LCCs’DC voltage droop control is designed to ensure smooth system operation and proper power sharing between onshore AC grids.The operational characteristics of the system are analyzed.In addition,a black start-up method without any auxiliary power supply for the VSCs is proposed.The transmission scheme is tested through simulations under various conditions,including start-up,wind speed variation,and the disconnection of one VSC or of one LCC.

Cloud-Data Envelopment Analysis Method Used for Assessment of Restoration Building Block Schemes

The selection of an optimized restoration building block(RBB)scheme among all available schemes is one of the most important factors impacting the power system restoration process after a complete or partial blackout.This paper presents a data envelopment analysis(DEA)model used as an empirical method to assess the RBB schemes.An N-level evaluation scale cloud system is built based on cloud theory to transform qualitative I/O indices of DEA model into quantitative values.Through joint utilization of the CCR(Charnes,Cooper and Rhodes)model and the LJK(Li,Jahanshahloo and Khodabakhshi)model,the established Joint-DEA model makes the newly proposed Cloud-DEA method a more feasible and robust method in assessment of RBB schemes.