构网型并网变流器系统不对称故障穿越方法研究

图4和图5分别给出了单相接地短路故障后并网点电压正序分量跌落至0.7pu时，采用传统控制策略和采用本发明控制方法时构网型并网变流器系统的仿真波形对比图。Q+和Q-分别为构网型并网变流器系统输出的正序无功功率和负序无功功率，i+d+和i+q+为构网型并网变流器系统输出电流正序dq轴分量，i-d-和i-q-为构网型并网变流器系统输出电流负序dq轴分量。图4为采用传统控制策略时的仿真波形图，图中1.5s时为电网发生故障。由图中可以看出当电网发生不对称故障时，系统输出的正负序无功功率与无功功率给定值之间存在偏差，即系统的无功功率实际值无法准确跟随指令值。图5为采用本发明所提新型无功下垂控制策略时的仿真波形图。从图中可以看出，电网电压不对称故障条件下，构网型并网变流器系统输出的正负序无功功率能够准确跟随无功功率设定值，有效地按照电网导则要求向电网注入了正负序无功电流，提高了故障期间系统的动态无功支撑能力。因此，采用所提控制策略有效的控制了构网型并网变流器系统输出的无功功率，能够按照电网导要求向电网注入合适的正负序无功电流，实现故障期间全过程构网型控制。

综上，本发明所述的适用于构网型并网变流器系统的不对称故障穿越方法，可以保证构网型并网变流器系统在电网发生不对称故障时稳定运行，具有如下有益效果：1)电网不对称故障情况下，能够准确控制系统输出的无功功率，保证输出满足电网导则要求的正负序无功电流，实现全过程构网型控制；2)在充分考虑构网型并网变流器容量基础上，将系统的有功电流设为0，可最大程度抑制故障期间的故障电流，提高系统的稳定运行裕度。

The simulation waveforms of the grid-forming grid-connected inverter system using traditional control strategy and the control method proposed in this invention are compared in Figure 4 and Figure 5, respectively, when the positive sequence component of the grid voltage drops to 0.7pu after a single-phase grounded short circuit fault. The positive and negative sequence reactive power output by the grid-forming grid-connected inverter system, as well as the positive sequence dq-axis components of the output current (i+d+ and i+q+) and the negative sequence dq-axis components of the output current (i-d- and i-q-), are shown in the figures. Figure 4 shows the simulation waveform when the traditional control strategy is used, and the fault occurs at 1.5s as indicated in the figure. It can be seen from the figure that when the grid experiences an asymmetric fault, there is a deviation between the actual reactive power output and the set value, indicating that the actual reactive power value of the system cannot accurately follow the command value. Figure 5 shows the simulation waveform when the novel reactive power droop control strategy proposed in this invention is used. It can be seen from the figure that under the condition of grid voltage asymmetrical fault, the positive and negative sequence reactive power output by the grid-forming grid-connected inverter system can accurately follow the set value, effectively injecting positive and negative sequence reactive currents into the grid according to grid requirements, and improving the dynamic reactive support capability of the system during faults. Therefore, the proposed control strategy effectively controls the reactive power output of the grid-forming grid-connected inverter system, enabling the injection of appropriate positive and negative sequence reactive currents into the grid according to grid requirements, and realizing full-process grid-forming control during faults.

In summary, the method for traversing asymmetrical faults in grid-forming grid-connected inverter systems disclosed in this invention ensures stable operation of the system when the grid experiences asymmetrical faults, and has the following beneficial effects: 1) Accurate control of the reactive power output of the system under grid asymmetrical fault conditions, ensuring the output meets the grid requirements for injecting positive and negative sequence reactive currents and realizing full-process grid-forming control; 2) On the basis of fully considering the capacity of the grid-forming grid-connected inverter, setting the active current of the system to 0 can effectively suppress fault currents during faults and improve the stability margin of the system.