Coordinated Control Strategy Of Multi-terminal HVDC Transmission System For Offshore Wind Farms

The development of offshore wind power has a huge potential in China. Use theflexible HVDC technology (Voltage-Sourced Converter Multi-Terminal HVDC,VSC-MTDC) for offshore wind farm to the grid. Wind power transmitted over a longdistance to the shore power grid system. The VSC-MTDC is anticipated as an attractivesolution to the large scale wind power transmission bottleneck.This paper focuses on thecontrol strategy of VSC-MTDC for wind farm grid interconnection, as well as themethod of fault ride through control strategy when the AC grid failure or fault of dcsystem’s own.1. To analyze VSC-MTDC network model, established the mathematical model ofVSC-MTDC.The double-loops and direct-current control strategies of VSC-MTDC isstudied, the current inner loop controller based on the current state feed-forward anddecoupling theory is designed, and then the outer loop control strategy (e.g. constant DCvoltage control, active and reactive power control and frequency control).The controlstrategy for wind farm side converter and grid side converter is detailed studied. Also thedoubly-fed wind power generator control system is analysed and the model of largecapacity wind farm is derived.2. The control strategy is developed based on the voltagemargin method and iscustomized to comply with different dispatch schemes possibly be applied in the futuretransnational network. Consider access weak grid system, designed the auxiliaryfrequency controller, improve the frequency of weak power system stability.3. To avoid the high-speed communication between multi-terminal DC(VSC-MTDC) transmission system and converter stations and to implement theautonomous coordinated control among converter stations, based on the voltage-currentcharacteristic of voltage source converters and the idea of reducing the wind powerinjected into power grid during faults, a coordinated control strategy applied inlarge-scale offshore wind farm to outward transmit wind power via VSC-MTDCtransmission system is proposed. When VSC-MTDC transmission system is operatedunder normal condition, not only the stability of DC voltage can be controlled byconverters at power grid side, but also the wind power transmitted to power grid can beflexibly controlled. When VSC-MTDC transmission system is operated under abnormalcondition, the stability of DC voltage can be sustained by converters at wind farm side.Matlab/Simulink based simulation verification for dynamic performance of the proposedcontrol strategy is performed, and simulation results show that using the proposed controlstrategy the DC voltage can be kept relative stable under large disturbance such as faultsoccurred in AC or DC system to sustain normal operation of VSC-MTDC transmission system.4. VSC-MTDC system is built; the grid-connection control strategy proposed in thispaper has carried on the experimental study. Experimental results show that, this articlestudies the VSC converter station control strategy can realize the steady operation.