Thermal Condition Equalization of Power Modules in a Low-voltage Cell of Modular Multilevel Converter

Medium voltage multilevel converters contains serial connected low-voltage cells, which normally have full-bridge composed of two power modules. The particular design of a low-voltage cell for a transformerless static synchronous series compensator has unequal thermal conditions for the power modules because the flowing cooling air is heat up by the first half-bridge, and temperature under the second power module is 10 degrees centigrade higher than for the first one. In this paper it is suggested to redistribute the switching losses between power modules by choosing proper transition between positive or negative voltage and the zero-voltage state. This allows to shift losses to the coldest power module and decrease the maximum silicon temperature of the hottest one. The proposed method helped to equalize temperatures of the hottest diodes and decrease the maximum temperature by 10 degrees centigrade under rated load conditions.