| VSC-MTDC is the multi-terminal HVDC system, which consists of VSC converters and features economical and flexible. However, when multi-terminal system enjoys more economical and flexible than two-terminal system, it also faces more complex situation in operation and control. Different from two-terminal system, MTDC needs master controller to offer every converter the optimized instruction in stable running state. The instruction follows change of load power flow. In abnormal state, such as faults occur in DC or AC side, the master controller will analyze the change of system structure and running parameters and coordinate the whole system to return to a new normal state.Firstly, a static model of VSC converter is developed and a PI controller is designed. Based on PSCAD/EMTDC, the steady state controller model is set up. The correlations between control variables and controlled variables are figured out according to this model and the correlations is expressed by algebra equation .On the base of the adoptive controller, a series of VSC-HVDC controllers are studied and designed, which respectively adapt to active AC grid and passive AC grid then compared two controller modes which is adapt to active AC grids. The simulation results of the two controller modes verify the feasibility of the controller modes, and the capability of the controller is correctness and favorable.Secondly, some operating strategies and master controllers of multi-terminal system are analyzed and studied in this dissertation. There are two operating strategies presented; voltage droop scheme and master/slaver control scheme. The merits of voltage droop scheme are that it does not need the master controllers also does not need communicate and its expansibility is nicer. But the master/slaver control scheme needs both master controllers and communications. Base the limit of the communications it could be made up of unification control scheme and margin control. One of the concerns is the operation and regulation scheme of multi-terminal VSC-HVDC system, which will solve the problem of coordination among VSC converters. Accordingly ,the master controllers of different control schemes of three end VSC-MTDC are studied.Finally according to parallel mode which has less operation problem, design a three end VSC-HVDC system which is connect both active AC system and passive AC system. Simulation analysis its stability operation mode and power flow turn over operation mode. According to this special network mode, and the results prove the controller is feasible and correctness. Every VSC converter can reach the preset capacity value. It has some good reference significances for that MTDC system is designed to supply some important passive loads and the situation of emergent power supported. In addition, this dissertation involves operation and protection of multi-terminal VSC-HVDC system preliminary conclusions.
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