Three-level power conversion circuit system

A three-level power conversion circuit system for outputting potential at levels selected from among three absolute values during conversion of power from two series-connected DC power supplies to alternating current, comprising:

• a plurality of one-phase switch circuits that receive power from the two DC power supplies, each of the one-phase switch circuits including
• a semiconductor switch series circuit in which first and second semiconductor switches are connected in series and the series is connected in parallel to the DC power supplies,
• diodes that are connected in anti-parallel to the first and second semiconductor switches;
• a bidirectional semiconductor switch circuit in a circuit path between a series connection point of the semiconductor switch series circuit and a series connection point of the DC power supplies, and
• opening means in the circuit path for opening the circuit path if the bidirectional semiconductor switch circuit fails,
• wherein the bidirectional switch circuits in all of the one-phase switch circuits are placed in a constant off-state, if the bidirectional semiconductor switch circuit in any of the one-phase switch circuits fails, and
• wherein after any of the bidirectional switch circuits fails, operation of the power conversion circuit system is continued as a two-level output inverter system using the semiconductor switch series circuits of the one-phase switch circuits.