基于内模控制的MMC-HVDC海上风电场不对称电网下的保护策略

Internal model control based protection strategy for MMC-HVDC offshore wind farm under unbalanced grid

为解决基于MMC-HVDC的海上风电场在不对称电网故障下普遍存在的低电压穿越问题,提出了一种基于PI电压外环和内模电流内环的新型控制策略及辅助保护方案。首先分析了不对称电网故障下MMC换流器运行特性,在αβ两相静止坐标系下建立了三相不对称电网下MMC换流器的数学模型。然后以抑制负序电流为控制目标在αβ两相静止坐标系下基于内模控制器设计了系统控制策略,同时针对两相故障时由于MMC限流所导致的母线电压抬升问题,提出了基于直流泄放回路的系统辅助保护方案。最后在Matlab/Simulink仿真平台上对单相接地和两相接地短路两种不平衡故障工况下的控制系统性能进行仿真分析。仿真结果表明,所提的控制策略和辅助保护方案可以保证基于MMC-HVDC的海上风电场在电网不对称故障下实现低电压穿越运行,有效地提高了海上风电场的低电压穿越能力。

To solve the universal low voltage ride-through problem of the offshore wind farm based on MMC-HVDC under unbalanced grid fault,a new control strategy by combining PI voltage outer loop and internal model current inner loop is proposed along with the auxiliary protection scheme.First,the performance characteristic of Modular Multilevel Converter(MMC)under unbalanced grid fault is analyzed,the mathematical mode of MMC inαβtwo-phase static coordinate is established.Then,the system control strategy based on Internal Model Control(IMC)method is designed in the aforementioned coordinate to suppress negative sequence current,meanwhile,aimed at the busbar voltage rise problem caused by MMC current-limiting feature in the case of two-phase grounding fault,the system auxiliary protection scheme based on DC discharge circuit is developed.Finally,the simulation model is established in Matlab/Simulink under the unbalanced operating condition of one-and two-phase grounding fault.Results indicate that the proposed control strategy and auxiliary protection scheme can ensure the offshore wind farm based on MMC-HVDC to implement the low voltage ride-through running even under the unbalanced grid fault,the low voltage ride-through capability of offshore wind farm is improved effectively.