Research On The Controller For Vsc- Multi-terminal Hvdc

The traditional PCC (Phase Change Converter) based HVDC (PCC-HVDC) has great development since 1960s and continued to develop AC-DC interlinked network, which has promising future. In contrast to AC transmission, PCC-HVDC has unparalleled advantages in some areas, such as long distance power transmission or asynchronous interlink of large capacity system. However, the PCC-HVDC also has limits as fellows: produces large low frequency harmonics and absorbs large reactive power; expensive in investment of PCC converter station; unable to supply for the small capacity system and the load without revolving motors. However, as the operation and control technology have developed rapidly since 1990s and some practical projects have been put into commercial operation, this dissertation focuses on the operation and control of an innovative VSC based multi-terminal HVDC system. The capacity of VSC-HVDC has reached hundreds of MW. It not only can be applied in the connection to the AC system and long-distance transmission, but also can improve the power quality and connect the passive network to supply power to the isolated system.However, if one of the two VSCs is broken, the VSC-HVDC can not work any longer. Since the VSC-MTDC(VSC- Multi-Terminal HVDC) comprizes more than two VSCs, it has more advantages compared with VSC-HVDC. VSC-MTDC has promising future in the field of distributed generation, renewable energy generation, transmission and distribution with high/low voltage class and power market. But the research on the VSC-MTDC is still at the phase of theory study and simulation experiment. The control system of VSC-MTDC has been studied in this paper, and for the system with or without communication, the control strategies are separately presented.VSC-MTDC is the expansion of VSC-HVDC and VSC is the basis of VSC-HVDC (or VSC-MTDC). So the mathematical model of the VSC was analyzed particularly first in this paper. The topology applied to VSC-HVDC (or VSC-MTDC) and the controls of the VSC were studied. In addition, the effect of different topologies on the power quality also was analyzed. After the analysis of the VSC, the control strategies for VSC-HVDC were presented for different control targets.For the control of VSC-MTDC, there are two kinds of control ways in general. The one is with communication mainly including principal-subordinate control mode with the coordination of the upper controller. The other is without communication mainly including multi-terminal DC voltage control scheme. Both of them were analyzed in this paper. Then, a new multi-terminal DC voltage control strategy without communication was presented.The structure of controller without communication is simple, but it can't guaranty the stability and optimal control of VSC-MTDC ultimately when there is a fault or a great disturbance. However, the limit of the computing speed and the spending of the communication time between upper controller and local controller restrict application of principal-subordinate control mode on the long-distance transmission or complex transmission. In view of these limits, the Multi Agent System(MAS)was applied on the control system of VSC-MTDC in this paper, which could improve the computing speed and require less for communication. Then the special functions and control strategies for each agent were further discussed as well: coordinated control system for MAS was proposed, which can improve the efficiency and stability of VSC-MTDC.The application of VSC-MTDC for wind farm was discussed at last. The proper control strategies with the batteries were proposed to restrict the fluctuation of the output of wind power.