Research On Simulation And Modeling For MMC-HVDC Connected To Wind Farm

With the development of wind power，the size of wind farm become more and more big.But the wind power is intermittent and uncontrollable，a large amount of reactive powercompensation are needed if transmit by AC，while transmit by traditional HVDC will requirea lot of filtering devices and it’s easy to occur commutation failure，so it is important to find asuitable way to transmit wind power. The MMC-HVDC have many advantages，such aslarge capacity，independent control of active and reactive power，stabilize voltage of AC bus，fault isolation，constitute multi-terminal DC system and so on，the technology is very suitablefor large wind farm and will be widely used.This paper has and compared AC、HVDC and VSC-HVDC technology. Research statusand engineering examples are introduced at the same time. Then LVRT technology of windfarm were reviewed.The main works of this paper are described on the basis of these aspects.In order to study the MMM-HVDC，its topology，principle，modulation method andvoltage balance method of MMC are firstly analyzed.The mathematical model is studied，theauthor has analyzed the direct current control strategy on the base of model. ThenMMC-HVDC connected the wind farm and AC grid is designed， by the way， controlstrategies for MMC at both side of wind farm and AC grid are proposed.A squirrel-cageinduction wind turbine models and permanent magnet direct drive wind turbine model arebuilt，then form a wind farm using equivalent techniques. After the modeling，the paper hassimulated the starting of MMC-HVDC and the process of speed fluctuation. Putting thestart-up strategy and control strategy into Multi-terminal MMC-HVDC，a three terminalsystem was designed，the simulation shows the feasibility of the strategies.At the end of paper，the characteristics of different types failures occurred at PCC of ACsystem are analyzed. Circuit composed by chopper resistor was proposed to maintain thevoltage stability of wind farm when serious three-phase fault occurs. To restrain the negativesequence component at asymmetric fault，a control strategy combined of positive and negativesequence component is put forward. Finally，simulation of voltage disturbances has verified the isolation of MMC，and simulations of three faults have proven the good feature of controlstrategy.