Study On The Control Strategies Of S-MTDC Transmission Systems

Multi-terminal direct current (MTDC) transmission systems, which include parallel MTDC (P-MTDC) and series MTDC (S-MTDC) transmission systems, will be the development direction of conventional two terminal high voltage direct current (HVDC) transmission due to MTDC’s superiority in non-synchronous connecting power systems than several of two terminal HVDC transmission. After discussed the characteristics of S-MTDC transmission systems, the control modes of normal shunt-down and emergency switch off (ESOF) in S-MTDC transmission systems are key analyzed and simulated in the paper. The research results include that both phase-shifted shut-down control mode and bypass pair (BPP) shut-down control mode are suitable to the S-MTDC transmission systems. The S-MTDC transmission system of simulation model is built by the electro-magnetic transient simulation software PSCAD/EMTDC. According to the simulation results, phase-shifted shut-down control mode is better than bypass pair (BPP) shut-down control mode when S-MTDC transmission systems are in normal shunt-down or ESOF. The only one converter may use constant current control in S-MTDC transmission systems, the paper analyzes the control characteristics and current margin of four converter. Simultaneously, while with the same power capacity on each converter station, a rectifier station is normal shut-down simultaneous with a inverter station in S-MTDC transmission systems, but in P-MTDC transmission systems, both of a single converter station or a rectifier station simultaneous with a inverter station shut-down are reasonable.