One-pulse control for modular multilevel converter based STATCOM

This paper reports an investigation results for one-pulse control methods for modular multilevel converter (MMC) based static synchronous compensator (STATCOM). MMC-STATCOM has cascaded full-bridge inverter cells in each phase arm, and the phase arms are configured with delta connection. In proposed MMC-STATCOM, an one-pulse control is applied. With one-pulse control, every cell outputs one positive and one negative pulse during a system voltage cycle. Since switching frequency is very low, extremely low switching losses can be provided. Cells have a capacitor, and the capacitor voltages should be balanced. Average voltages of the capacitors in each phase arm can be balanced by injecting zero sequence current which flows through delta connection loop. Individual capacitor voltages among arm cells are balanced by sorting algorithm. Cells are sorted by the capacitor voltage and pulse-order (PO) is determined. According to PO, the output of the cells turns ON and OFF, and one-pulse voltage is output. Lower voltage cell is charged more and higher voltage cell is discharged more, therefore the capacitor voltages are balanced. However, the capacitor voltage cannot be controlled in the cells which are not involved with the output. The uncontrolled cells have the last order or previous ones in PO. The capacitor voltage of the uncontrolled cell decreases because of self-discharge. Conventional sorting algorithm is insufficient. In proposed control method, to solve this problem, put the last order cell in a controllable order. This control allows all the capacitor voltages to be balanced including the uncontrolled cells. The proposed control methods were confirmed by experimental model MMC-STATCOM. The STATCOM with one-pulse control operated under steady state and the unbalanced system voltages. The STATCOM kept stable operation under the both conditions. And all the capacitor voltages were balanced even it has the uncontrolled cells.