The Low Voltage Ride Through Technology For Offshore Wind System Connected With Power Grid Using VSC-HVDC

As the flourish of offshore wind power market, the important safeguards to solve theenergy crisis are provided, which includs voltage source converter-based HVDC(VSC-HVDC). VSC-HVDC is very prevalent with its high efficiency and strongcontrollability. As the increasing requirements of the grid interconnection about windfarms with different countries, the low-voltage ride through technology (LVRT) ofVSC-HVDC system becomes the domestic and foreign research hotspot.The LVRT control technology about the VSC-HVDC system of offshore wind poweris mainly analyzed in this paper. The stability problem of DC side overvoltage and ACgrid-connected node voltage are focused on, while network voltage sag sharp depth occurs.By using dynamic voltage restorer (DVR) to assist VSC-HVDC, the control method ofLVRT is achieved. The specific implementation plan is composed of the following twoaspects: on the one hand, when network voltage sag depth is light, using the traditionalreactive power compensation strategy about VSC-HVDC, DVR is in bypass state andwind farm could run steadily. On the other hand, when network voltage sag depth is sharp,the simple compensation strategy has not stabilized DC bus voltage and ACgrid-connected node voltage. At the moment, improved control strategy aboutVSC-HVDC is switched, and super capacitive DVR with power-wasting resistors are used.The excessive energy about DC bus of VSC-HVDC is adsorbed by DC/DC convertor tocontrol super capacitors arrays and power-wasting resistors to maintain the stability of DCbus. Meanwhile, the voltage of power grid fault is compensated by using super capacitorsdischarge to provide DVR inverters with stable DC bus voltage, and the AC bus voltageon the bank of wind power farm is maintained at the normal level. In addition, during thefault duration offshore wind power generators are maintained stable by the excellentrobustness of coordinate control.The VSC-HVDC and DVR simulation model of whole offshore wind power farm arestructured in MATLAB/Simulink. The simulations about three-phase voltage sag sharpdepth, three-phase and single-phase short circuit faults are implemented. The results verify that this strategy not only improves the efficiency of the system, but also shows the verygood effect in compensating the symmetrical and unsymmetrical grid voltage faults andhas some realistic meaning for the reliable operation of offshore wind power system andthe improvement of LVRT capability.