A Study Of Coordinated Control Strategy For Offshore Wind Power Generation RMC-HVDC System

Offshore wind power generation-high voltage direct current is the development directionof wind power generation and power transmission technology. With the high cost of maritimetransport, installation, maintenance, so it requires the central part of offshore wind powergeneration-high voltage direct current transmission system—the converter, possessed of thecharacteristics of higher power density, higher dependability and higher workpiece ratio, andalso had to consider the new converter topology and control strategy to reduce the investmentand operation costs. Reduced matrix converter which is a kind of new power converterderived from the conventional matrix converter topology. The RMC high frequency linkconverter possess the advantage of less stages of conversion, higher power density, higherworkpiece ratio and higher dependability, so it has a big application foreground in theoffshore wind power generation-high voltage direct current.In recent years, with the application background of offshore wind power generation-highvoltage direct current transmission, the scholars have studied the topologies and modulationstrategies of the RMC high frequency link converter, and has obtained certain achievement.The volatility of wind speed leads to the instability of the active power output from windfarms, and the ac system faults can limit the active power output from the onshore inverterwhich will endanger the safety of the equipment running. Yet, the research about all kinds ofworking conditions and fault cases of offshore RMC and onshore voltage source convertercoordination control strategy, which to ensure the balance of the active power in the seed andreceived end of the high voltage direct current transmission system, haven’t seen any relevantreports in the domestic and foreign. In addition, the series multi-terminal offshore wind powergeneration-HVDC system based on RMC high frequency link converter can boost the voltagelevel without offshore step-up transformer, reduced the size and weight of the system, andalso improved the power density and efficiency of the system.Firstly, a voltage source reduced matrix converter bipolar voltage space vectorpulse-width modulation is analyzed, and the mathematical relationship between the inputvoltage and output voltage is deduced. Secondly, the control strategies of direct-drivenoffshore wind power generation-HVDC system based on RMC are proposed, which canrealize the maximum power point tracking, stabilize DC voltage and decouple control ofactive and reactive power transmitted to the power grid. In order to improve the low-voltageride-through capability of the system, control strategies based on the active power instruction revise of the RMC combined with variable pitch control are proposed, which implement theHVDC system’s coordinated control of offshore RMC and onshore VSC and keep the balanceof active power transmission when the grid voltage sags. Thirdly, the topology structure ofseries multi-terminal offshore wind power generation-HVDC system based on RMC highfrequency link converter and its coordinated control strategies are proposed. The controlstrategies are consisted of wind farm supervisory control, offshore wind turbines control andpower grid side control and put to use the optimal dc current reference algorithm. The systemcan realize the independently maximum power point tracking control, dc current control andactive and reactive power decoupling control transmitted to the power grid during normaloperation of the system. When wind turbines have fault and grid voltage fluctuates, thesystem can also operate safely and high efficiently. Lastly, the simulation models of offshorewind power generation-high voltage direct current transmission system based on RMC arebuilt and the emulation proofs are done. The simulation results verify the correctness andfeasibility of the proposed topologies and its control strategies.