基于改进型MMC的风电直流联网系统直流故障穿越协调控制策略

采用双极架空线柔性直流输电技术进行大规模风电远距离外送是其友好型并网的有效手段.针对风电直流联网系统直流故障阻断和功率盈余问题,提出了一种改进电流转移型模块化多电平换流器(M-CT-MMC),使其同时具备直流故障阻断和能量耗散的功能,从而在充分发挥耗散电阻作用的同时实现直流故障穿越.在直流故障阻断方面,通过将M-CT-MMC桥臂吸收支路的引出线互联构造三相中性点,避免了桥臂开关额外承受直流电压偏置导致的成本增加问题,并利用辅助支路间的协调配合,有效阻断了直流故障电流.在盈余功率耗散方面,针对自消纳和非自消纳工况设计了双极M-CT-MMC控制模式切换策略,在提高非故障极功率转带能力的同时自主吸收盈余功率,并基于功率耗散需求设计了耗散电阻分组投切控制策略,避免非故障极M-CT-MMC过载,从而实现不同运行工况下风电直流联网系统的直流故障穿越.最后,基于MATLAB/Simulink仿真平台验证了所提直流故障阻断及盈余功率耗散协调控制策略的有效性和可行性.

DC fault ride-through control strategy of wind power integrated DC transmission system based on modified MMC

MMC-HVDC(Modular Multilevel Converter based High Voltage Direct Current) technology adopted bipolar overhead lines is an effective method for large-scale long-distance wind farm sending system. Aiming at the problems of DC fault blocking and surplus power in wind power integrated through DC transmission system, an M-CT-MMC(Modified Current-Transferring MMC) with the functions of DC fault blocking and energy dissipation to fully exploit the function of dissipative resistors while achieving DC fault ride-through. In terms of DC fault blocking, the cost increase caused by the arm switch being additionally subjected to DC voltage bias is avoided by interconnecting the lead wires of the M-CT-MMC bridge arm absorption branch to construct a three-phase neutral point, and DC fault current can be effectively blocked through the coordination of auxiliary branches. In terms of the dissipation of surplus power, a control mode switching strategy of bipolar M-CT-MMC is designed for self-absorption and non-self-absorption conditions, which can actively absorb power dissipation while improving the power transfer capability of non-fault pole. Based on the power dissipation requirements, a group switching control strategy of dissipative resistors is designed to avoid overloading of non-fault pole M-CT-MMC, so as to realize the DC fault ride-through under different operating conditions. Finally, the effectiveness and feasibility of the proposed DC fault blocking and power dissipation coordinated control strategy are verified based on MATLAB/Simulink simulation platform.