Study On Overvoltage Control Strategy For Series-Parallel Topology Based On Wind Farm

The wind farm of Series-Parallel topology based on voltage source converter-high voltage direct current(VSC-HVDC)is the best potential solution to realize the expansion of wind farm capacity and the long distance transmission of wind energy,because of the advantages of low load requirements,high transmission capacity and low cost.However,the coupling between the Wind Turbines in the cluster is strong,and the randomness of the wind energy ca uses the DC bus overvoltage.The over-voltage of DC bus has caused the temperature of motor rises too high,damages the motor insulation,reduces the middle of the DC loop filter capacitor life and other hazards,is the main factor affecting the development of Series-Parallel Topology.Therefore Series-parallel structure of offshore wind farm based on VSC-HVDC,which was proved to be the best scheme will benefit offshore wind farm capacity expansion and long distance transmission.The mathematical model of wind turbine,Direct-drive Permanent Magnet Synchronous Generator(D-PMSG),PWM converter and the operation characteristics of series-parallel D-PMSG wind farm is introduced in this paper.For series-parallel topology wind farm,adopting traditional maximum power point tracking(MPPT)control strategy will lead to over voltage when the Wind turbines in same series branch under high wind speed.To deal with the problems,two novel control strategies of over-voltage has been proposed for series-parallel topology Direct-drive Permanent Magnet Synchronous Generator(D-PMSG)based wind farm,which named overvoltage control strategy of the energy storage and coordinated overvoltage control strategy of power-voltage.The overvoltage control strategy of the energy storage,which needs the super-capacitor energy storage system as hardware accessory equipment to achieve wind turbines power equally,avoids to over voltage and over modulation.And coordinated overvoltage control strategy of power-voltage avoids to over voltage and over modulation by controlling the maximum output power of the wind speed in the series clusters to adjust the proportion of output power for the wind turbines within low wind speed in the total power.At last,the result of MATLAB/Simulink simulation proved the two over-voltage strategies can achieve the purpose of maximum wind power capture in a safe range.And coordinated overvoltage control strategy of power-voltage is the best,which may become the research hotspot of over-voltage control strategy for future offshore DC wind field.