当桥臂电流为对电容充电方向时，选择投入电压最低的n个子模块， 当桥臂电流为对电容放电方向时，选择投入电压最高的n个子模块。(When the bridge arm current is charged to the capacitor, the N sub module with the lowest input voltage is selected. When the bridge arm current is in the direction of capacitance disc