A modular multilevel converter (MMC) is a circuit configuration that uses a switching element such as Insulated Gate Bipolar Transistor (IGBT) for which ON/OFF control can be performed, and is capable of outputting voltage equal to or higher than withstand voltage of the switching element. This circuit configuration is expected to be applied to a DC transmission system (HVDC), a static synchronous compensator (STATCOM), and the like.
Conventional electric power conversion devices are applicable to the MMC, and are composed of a plurality of cell converters connected in cascade. A main circuit of each cell converter is a bidirectional chopper circuit composed of a high-voltage-side switching element, a low-voltage-side switching element, and a DC capacitor. Each cell converter has a cell control circuit and a self-feed power supply. The self-feed power supply is connected in parallel to the DC capacitor, and is supplied with power from the DC capacitor.
A short-circuit switch is connected in parallel to output of the cell converter, and drive power for the short-circuit switch is supplied from the self-feed power supply. The short-circuit switch is means for short-circuiting output of the cell converter when the cell converter has failed, and is a NORMALLY-ON switch which short-circuits output of the cell converter when no drive power is supplied from the self-feed power supply. When the self-feed power supply has stopped, output of an abnormal cell converter is short-circuited by the short-circuit switch. Therefore, even if a cell converter has failed, the system can continue its operation (for example, Patent Document 1).