Research On Hybrid Multi-terminal HVDC Technology For Large Offshore Wind Farms Integration

Offshore wind power has gained more and more attention around the globe. Alongwith the increasing installation capacity and transmission distance of offshore wind farms,HVDC technology will be more and more widely used in offshore wind farms integration.Compared with the Line Commuted Converter (LCC) technology based HVDC, the VoltageSource Converter (VSC) based technology shows its priority in offshore wind powertransmission. However, LCC-HVDC is still dominant nowadays. To utilize the existingHVDC technologies effectively in large offshore wind power transmission with longdistance, this thesis proposes a suitable hybrid multi-terminal HVDC scheme.Firstly, describe the topology of the hybrid-MTDC system, and then construct a typicalfour terminal HVDC system, mainly composed of two VSCs connecting a respectiveDoubly Fed Induction Generator (DFIG) based offshore wind farm, two long DCtransmission cables and two onshore LCCs connected to an AC system respectively. Thecoordination control scheme was designed, where VSC controls farm side AC voltage whileLCC controls DC voltage and share DC power with DC voltage droop control method.Secondly, investigate the operating characteristics of the system. Controlling orders ofthe converters were acquired by DC power flow optimization. The black start-up processwas designed and verified. Based on steady-state stability analysis, the systemcharacteristic under wind power fluctuation was simulated. The flexibility of systemoperation was verified under simulations with a main part dropped out from the system.Finally, design the onshore system’s frequency support method by controlling DFIGand LCC. With an extra power control loop in rotor side converter the DFIG couldparticipate in temporary frequency regulation. By adding a frequency feedback loop toLCC’s droop coefficient, the primary frequency control capability of other AC systemscould be utilized. Simulation results under frequency disturbance in one onshore systemshowed that the proposed strategy improved the frequency regulation capability.