The Parallel Mode Of VSC-MTDC And Voltage Control

In recent years, with power grid expands, the limitation of ac power transmission is obvious. HVDC system, as a new transmission technology, has significant advantage. HVDC transmission has faster response, better system stability and less waste energy. It is in a wide range of applications such as wind power generation, distributed power generation, offshore power supply, urban power distribution, long-distance transmission and interconnection of synchronized AC system.This paper focus on the radial parallel multi-terminal HVDC system based on voltage source converters. In the first place, the mathematical model of VSC is established in different coordinate system. According to the mathematical model, constant voltage controller, constant power controller and voltage droop controller are proposed. This paper introduced the unified and voltage droop type control strategy. Based on this, the new hybrid control strategy is put forward. This new control strategy is better than the old ones in controllability and stability. This paper also carried out the dc power flow calculation method for the radial parallel multi-terminal HVDC system. A Five-terminal HVDC system is built in the electromagnetic transient simulation software PSCAD/EMTDC.The simulation results show that the parallel multi-terminal HVDC system with new hybrid control strategy has excellent reliability and controllability. The new hybrid control strategy can be split into two stages according to the load condition of the voltage regulator. Stage one is when the voltage regulator is in light load condition. System voltage is set by the voltage regulator. Stage two is when the voltage regulator is in full load condition or cut off from the system. The voltage regulator works as a normal power dispatcher. System voltage is set by voltage droop converters. So the reliability and controllability of the new control strategy is superior to the existing control strategies. Combining the proposed dc flow calculation method, it is pretty easy to predict the stable operation of the multi-terminal system. And through the reverse operations, it is also very convenient to control the stable operation of the multi-terminal system.