单相接地短路故障下构网型并网变流器系统新型无功下垂控制策略仿真分析

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。

The simulation waveforms of the grid-forming inverter system under traditional control strategy and the control method proposed in this paper 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. 'Q+' and 'Q-' are the positive and negative sequence reactive power outputs of the grid-forming inverter system, respectively, where 'i+ d+' and 'i+ q+' represent the positive sequence dq-axis components of the output current, and 'i- d-' and 'i- q-' represent the negative sequence dq-axis components of the output current. Figure 4 shows the simulation waveform under the traditional control strategy, where the fault occurs at 1.5s. It can be seen from the figure that when the grid experiences an asymmetric fault, there is a deviation between the positive and negative sequence reactive power outputs and the set value, indicating that the actual reactive power of the system cannot accurately follow the command value. Figure 5 shows the simulation waveform under the novel reactive power droop control strategy proposed in this paper. It can be seen from the figure that under the condition of grid voltage asymmetry fault, the positive and negative sequence reactive power outputs of the grid-forming 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 power support capability of the system during the fault period. Therefore, the proposed control strategy effectively controls the reactive power output of the grid-forming inverter system, injecting appropriate positive and negative sequence reactive currents into the grid according to grid requirements, and achieving full-process grid-forming control during fault periods as required by the grid.