PWM for active thermal protection in three level neutral point clamped inverters

In high power applications, in which the discrete power switches are mounted on separate heatsinks, an overload of a semiconductor device leads to a local excessive thermal stress and eventually to a failure of the system. Despite of a control strategy that ensures the even distribution of losses in the normal operation, a local thermal overload can arise due to failures in the electronics, in the cooling system, due to an inappropriate positioning of the semiconductor device, etc. In this paper, a new fault tolerant control approach is introduced to deal with this matter. When detecting increased thermal stress of one IGBT module, the switching strategy is altered in order to reduce the load of that particular IGBT module and to redistribute the losses and consequently the heat from the affected group or valve to the other switches. Hence, the temperature of the stressed switch is kept under the critical limit and the thermal overload is reduced. In such way the reliability and the lifetime of the converter is maximized even in case of thermal failure. In the proposed novel modulation concept, the balancing of the neutral point potential of the 3L-NPC Inverter is also ensured without any additional hardware.