Coordination Control Strategies Of Multi-terminal DC Transmission System Based On VSC

VSC-MTDC (Multi-terminal DC Transmission System based on VSC), which has the wide application in renewable energy generation, high voltage DC distribution system and distributed generation is the trend of VSC-HVDC. Compared with two-terminal system, VSC-MTDC is more felxisible and relaible. In this thesis, operation issues relevant to VSC-MTDC are investigated by means of preliminary theoretical analyses and simulation validations. The research on mathematic model, coordination control strategies and protection strategies against DC faults are the key work.Firstly, VSC-MTDC in parallel and VSC-MTDC in Series are compared in Operating characteristics. Furthermore, three kinds of converters including two-level converter, three-level diode-clamped converter and MMC (Modular Multilevel Converter) are compared. The results show that Multi-terminal DC Transmission System based on MMC in parallel has the wide application prospects, so it is selected as the study object. The working principle of MMC are presented and the mathematic model of Multi-terminal DC System based on MMC is deduced.Based on the mathematic model, the converter-level control strategies for the converters linked to differernt kinds of AC system including strong AC system, passive AC system, large-scale wind farm are desiged. What's more, a multi-point DC voltage control strategy based on DC voltage margin method applied to VSC-MTDC is then proposed and the main controller and auxiliary controller are designed.Finally, the impact of different DC faults of the system is analyzed and then the corresponding control and protection strategies are carried out. The DC faults include single-line to ground fault, line to line fault and line break fault.Relative simulation models are built in PSCAD/EMTDC to validate the feasiblity and effectivity of the coordination control strategies and the protection strategies against t DC faults proposed by this thesis. The simulaiton results show that multi-point DC voltage control strategy based on DC voltage margin method can realize automatic switching between active power and DC voltage control modes, so that improve the stability of the system. The system under fault can achieve fast recovery by using the protection strategies against DC faults. The protection strategies improve the fault ride-through capability of multi-terminal DC transmission system based on MMC.