Research On Control Strategy For Multi-terminal HVDC Transmission System Based On VSC

VSC-MTDC is a new generation of HVDC. Compared with the traditional DC transmission technology, the voltage source converter (VSC) being composed of the power electronic devices in advanced, can flexibly and efficiently change the power transmission direction and size according to the demand of power network, and provide better quality of electric energy. Compared with the two terminal VSC-HVDC transmission technology, the VSC-MTDC transmission system can flexibly access to multiple sites of solar, wind, small hydro, geothermal and other clean energy, and convey to a plurality of load center of cities by means of a large capacity, long-distance power transmission channel. VSC-MTDC transmission system is an important part of the construction of smart grid.It solves the transmission problem multiple power supply or multiple power consumptione, and is widely applied in renewable generating integration, large area power grid interconnection and power supply the fields of city center. Therefore, the study of VSC-MTDC transmission technology has important scientific theory value and engineering application value.Firstly, the thesis analyzed the working principle, the topology structure and mathematical model of the voltage source converter (VSC), the core component of VSC-MTDC transmission system. Then, the main circuit parameters, wiring mode and control systems of VSC-MTDC transmission systems, were discussed and analyzed. The controllers were designed. The controllers were validated by simulation, using the electromagnetic transient simulation software PSCAD/EMTDC to build the two terminal VSC-HVDC transmission systems. At last, this thesis introduced and analyzed the principle and subordinate control stagy and voltage droop control stagy of VSC-MTDC transmission system. Base on summering the strengths and weaknesses between the principle and subordinate control stagy and voltage droop control stagy, a new hybrid control stagy was put forward, and then was validated by simulation, using the electromagnetic transient simulation software PSCAD/EMTDC to build the four terminal VSC-MTDC transmission systems. The simulation results proved that the system was able to switch the control stagy freely between the principle and subordinate control stagy and voltage droop control stagy according to system disturbance strength. This control stagy has a great improvement in reducing the system reserve capacity and the ability of anti disturbance of the system, and can maximize the stable operation of the system.