A circuit design for clamping an overvoltage in three-level GTO inverters

Three-level GTO inverters seem to be best suited for high power and high voltage AC applications. The circuit design of GTO power conversion systems is determined by the snubber circuits, which are necessary to release the GTOs from turn-on and turn-off switching losses and to keep the overvoltages to be caused by GTO switching low. However, the overvoltage snubber for the GTOs in a three-level inverter has not existed until now. In three-level inverter topology, the overvoltage across the inner GTOs can give rise to the larger blocking voltage than half that of the DC-link voltage, so that the power dissipation at turn off may be much larger than expected, and also switching failure of the GTOs may happen. This paper presents a circuit design for clamping the overvoltages across the GTOs in three-level GTO inverters. The proposed circuit has two roles as follows; one is to minimize the power dissipation in each GTO. It can be achieved by clamping the overvoltage to half that of the DC-link voltage as exactly as possible. The other is to get blocking voltage balancing between the inner GTOs and the outer GTOs