Research Of HVDC Transmission Control Strategy For Offshore Wind Power Generation Based On Reduced Matrix Converter

Offshore wind power generation has many advantages, such as little affected byenvironment, abundant wind energy resources, turbines with high annual utilization hours,moreover, the more wind farms farther from the coast, the more larger and stable of the windspeed, and the wind farms output power will be higher and stable. High voltage direct currenttransmission(HVDC) has a higher level of economy, stability and reliability in thelong-distance transmission than high voltage alternating current transmission(HVAC).Offshore wind power generation combined with HVDC will be the future’s developmentdirection of the wind power generation and power transmission technology.Converter is the core areas of offshore wind power generation turbines and HVDC,because of the special of geographical environment, it has high requirements on the size,weight, reliability and efficiency in HVDC for offshore wind power generation converter.Although offshore wind power generation HVDC system based DC grid solved the bulkytransformer problem which exist in the offshore wind power generation HVDC system basedAC grid. But there are still inadequacies: the conventional converter applied in this systemhas too many conversion stages which increasing the energy loss; the use of electrolyticcapacitors which increasing the size and weight, while reducing system reliability. Reducedmatrix converter(RMC) is a novel power converter derived from the two-stage matrixconverter, which became an ideal power converter for offshore wind power generation HVDCsystem because it has smaller size, lighter weight, less conversion stages and higher reliability.Currently, although the study on RMC topology, modulation strategy, loss analysis, etc, ismade certain achievements, but the RMC commutation strategy is still complex and needs tobe improved, and there is no literature for specific wind turbine to provide full RMC-HVDCcontrol strategies, while domestic research in this field is still blank.Firstly, the topology, working principle and bipolar current space vector modulationstrategy(B-C-SVM) of RMC converter is introduced in this paper，and the simplifiedalgorithm of B-C-SVM and expression between input voltage and output voltage is deduced.Then, a new two-step commutation strategy is proposed. The advantages are as follows: noneed additional voltage/current hardware detection circuit, the cost relatively low; eachcommutation state(i.e.,for each vector) for two single-phase switch is turned on, no auxiliaryswith to maintain the driving state, which can preventing the input phase short circuit, andreducing the number of switch conduction and switching losses; commutation strategy issimple, applicable to the entire commutation interval; utilize the characteristics of switch off-time is much larger than switch opening time, the new two-step commutation strategy canbe simplified into one-step commutation strategy. And then, combined with the applicationadvantages of RMC in the HVDC, generator-side converter is constructed by current sourceRMC converter, mathematical model for each module of offshore wind power generationHVDC system based on RMC is established, and also integrated control strategy is proposed,the maximum wind-energy tracking control of DC wind turbine, and independent control ofactive power, reactive power transmitted to power grid and HVDC power balance control canbe implemented. Finally, the system simulation model is established, simulation results verifythe correctness of the proposed control strategy.