Research On Control Strategy For Integration Of Permanent Magnet Direct-driven Wind Turbines Via HVDC Transmission

Wind energy is one of the most promising renewable resources to be developed.The utilization of offshore wind power attracts more and more attention because offshore wind resources are more abundant and are very suitable for large-scale development.Compared with DFIG-based wind turbines,permanent magnet direct-driven wind turbines do not need gear boxes and have better low-voltage ride-through capability,which makes permanent magnet direct-driven wind turbines competitive in the development of offshore wind power.High-voltage direct current(HVDC)transmission,which is not affected by capacitive currents,is suitable for integrating offshore wind power to the grid over long distances.In this thesis,different HVDC topologies for grid integration of permanent magnet direct-driven wind turbines are studied and corresponding control strategies are designed.The main research works are as follows:(1)The structure and basic control method of permanent magnet direct-driven wind turbines are analyzed.(2)The configuration of modular multilevel converter(MMC)is studied.A control strategy is proposed for enhancing the fault ride-through capability of the MMC-HVDC system connecting permanent magnet direct-driven wind turbines to the grid.A controlled voltage drop in the wind farm AC system and a controlled reduction of wind turbines' output active currents are combined to achieve a fast power reduction.Meanwhile,DC voltage margin control is adopted to limit the voltage increase of the wind turbine DC link.The results of simulations show that the DC side voltages of MMC and wind turbines can keep stable under balanced or unbalanced fault conditions.The whole system can achieve fault ride-through by employing the proposed strategy.(3)A topology for integration of permanent magnet direct-driven wind turbines through a hybrid HVDC system is proposed.The HVDC system,adopting a modular multilevel converter on the rectifier side and a line commutated converter(LCC)on the inverter side,combines the advantages of both MMC and LCC.MMC can transmit power from the passive networks of wind farms without an external commutation voltage;LCC can reduce the investment cost and power losses.By changing the number of the switched-on sub-modules,MMC can change its DC side voltage instantaneously.Based on this characteristic,a method to control the DC current of MMC is proposed,extending MMC control dimensions from AC side to DC side.The results of simulations show that MMC DC current control can reduce the possibility of LCC commutation failures and improve the power recovery performance after a commutation failure,achieving the goal of fault ride-through.