Research On Several Issues In MTDC Transmission System

Multi-terminal DC(MTDC) electrical power transmission system, which can implement power transmission with multi power sources and multi receiving ends, has more flexible operation and better economic efficiency, comparing with the traditional two-terminal HVDC transmission system.Firstly, this paper summarizes the key technology of MTDC transmission system in terms of converter equipment, transmission line, grounding electrode, control and protection system, main electrical wiring scheme, operating mode, techno-economic analysis, etc. Then, it compares parallel MTDC with series MTDC system in term of the power loss, reactive power demand, economic factors, recovery speed from system failure, power reversal, system expansion, and comes to a conclusion that parallel MTDC system has more advantages than the series one. Under this conclusion, a four-terminal bipolar HVDC system and its control-protection system is built in PSCAD/EMTDC, the simulation results show a stable operation without significant disturbances and fast response during the start-stop, steady-state and control-mode switch under the control strategy this paper presents. Finally, the paper presents the main differences theoretically between MTDC system and traditional two-terminal HVDC system in terms of system protection, especially under the common failures, shows DC line and AC system fault-characteristics and their protection scheme in the simulation, comes to some conclusions.