Research On Coordination Control And Optimizing Operation Of Multi-terminal HVDC Flexible System

Multi-terminal HVDC flexible System based voltage source converter(VSC) can achieve multi-power supply, multi-infeed and isolated power supply. It is more flexible and reliable than two terminal system. Thus, it’s better for distributed energy system and grid interconnection of wind farms. Multi-terminal HVDC flexible System has several DC terminals. In DC system, its voltage should maintain a certain level in order to keep the power sources and the loads run normally, or else the whole system would crash. Plus, while one of the converter stations quits operation due to failure, the other converter stations should help the system sustain certain disturbances and transit to a new stable condition. Therefore, cooperative control of different DC terminals of the Multi-terminal HVDC flexible System becomes one of the most important research subjects in DC network construction. In this paper, an in-depth study on Multi-terminal HVDC flexible System, multi-terminal coordinate control strategy and system’s optimized operation is presented, which also provides theoretical basis for the construction of Multi-terminal HVDC flexible system.Voltage source converter control is the core part of Multi-terminal HVDC flexible technology, which is also the key difference from traditional HVDC. In this paper, firstly, through analyzing two major VSC control methods: direct control(indirect current control) and vector control(direct current control), both the inner-loop and external-loop control methods are determined. Secondly, a thorough study on Multi-terminal HVDC flexible system control strategies is carried out, such as centralized control of slack bus, voltage margin control, and voltage droop control. Then, a new droop control method is given, which is based on“multiple feeding and multiple receiving”operation mode. The droop slope adjusting method is also given in this part. The above control strategy can make the power of converter station proportional to its capacity.An important precondition of the stability of Multi-terminal HVDC flexible System is that the DC voltage should maintain in allowable range. That is to say DC voltage of the system shouldn’t be too high, or insulation margin of the system devices would be decreased; neither too low, or direct current of the system would increase rapidly, thereupon the total system power loss would increase. Lastly, based on traditional droop control, this paper presents a voltage droop control strategy, using slope self-adaptive adjustment. This strategy brings in a common reference DC voltage. Then automatically adjust the DC voltage droop slope according to deviation value between the actual direct voltage and the preset reference value. By doing so, the droop controller can regulate the feeding converter’ DC power and direct voltage in real time. Therefore, the error between actual DC voltage of the feeding converter and its preset reference value can be well managed in order to make the system’s DC voltage maintain in safe zone.The last part of this paper is the conclusion of the research, including the inadequacy and further improvement that can be done, which also shows future direction of this research.