基于混合直流的受端电网黑启动方法及协调恢复策略

作为直流输电的新方向，混合直流结合了传统以及柔性直流系统的优点，可在电网黑启动中发挥重要作用。为明确混合直流参与黑启动的技术条件与控制方法，针对两端电网换相换流器–模块化多电平换流器（line commutated converter–modular multilevel converter，LCC–MMC）混合直流输电系统，提出了受端电网大停电情况下基于混合直流的黑启动方法及受端电网协调恢复策略。根据黑启动不同恢复阶段特征，首先，明确了在起始阶段受端系统全黑情况下混合直流的启动方法，包括送端LCC换流器和MMC受端换流器的启动及控制方法。然后，针对黑启动初期的弱交流系统阶段，采用可增强系统稳定性的混合直流虚拟同步协调恢复策略；在系统达到一定强度后，提出了相应的控制策略及不同控制方式间的平滑切换方法。最后，在PSCAD/EMDTC软件中进行了仿真验证，结果表明：黑启动初期LCC采用耗能电阻启动能有效满足最小启动电流限制；MMC经限流电阻启动后采用无源网络控制可实现无源端交流电压的建立，并维持系统电压和频率在稳定范围内；在非黑启动电源机组和负荷并网后，交流系统强度改变，所提的3阶段控制切换策略能够有效实现各阶段平滑过渡，且在弱交流系统阶段采用虚拟同步控制可保证弱交流系统阶段的系统稳定性，3阶段控制策略与两阶段恢复策略相比具有显著的优势，从而验证了所提黑启动方法和协调恢复策略的有效性。

Black Start Method and Coordinated Restoration Strategy of DC Receiving-end Power Grid Based on Hybrid Direct Current

Hybrid DC, as a new direction of DC transmission technology, combines the advantages of traditional and flexible HVDC systems and plays an important role in the power grid black start. To clarify the technical conditions and control methods of hybrid DC participating in black start, a black start method based on hybrid DC and a coordinated recovery strategy of the DC receiving-end power grid were proposed in the case of major power outage for LCC-MMC hybrid DC transmission system. According to the characteristics of different recovery stages of black start, firstly, the start-up method of the hybrid DC transmission system under the condition that the receiving-end system is completely black in the initial stage were investigated, including the start-up and control methods of the sending LCC converter and the receiving MMC converter. Then, for the weak AC system at the early stage of black start, a virtual synchronization coordination recovery strategy of hybrid DC were adopted to enhance the system stability. Furthermore, after the system reaches a certain strength, corresponding control strategies and smooth switching methods between different control modes were proposed. Finally, the simulation verification were carried out in PSCAD/EMDTC software. The results show that with the energy consuming resistor at the initial stage of black start, LCC converter can effectively meet the minimum starting current limit. The MMC converter can establish the AC voltage at the passive end by using passive network control and maintain the system voltage and frequency within a stable range. After the non-black start power unit and load are connected to the grid, although the AC system strength changes, the proposed three-stage control switching strategy can effectively achieve smooth transition of each stage, and in the weak AC system stage, virtual synchronous control can ensure the system stability in the weak AC system stage. The three-stage control strategy has significant advantages over the two-stage recovery strategy, thus, the effectiveness of the proposed black start method and coordinated recovery strategy are verified.