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

For 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 several control strategies that ensure the even distribution of losses in the normal operation, a local thermal overload can arise due to degradation of the cooling system, inappropriate placing of the semiconductor device, failures in the electronics, etc. In this paper, a new fault tolerant control approach to deal with this matter is introduced and verified by measurements on a laboratory setup. When detecting increased thermal stress of one IGBT module, the switching strategy is altered in order to reduce the thermal load of that particular module and to redistribute the heat from the affected group or valve to the other switches. Hence, the temperature of the stressed module is kept under the critical limit and the thermal overload is reduced. In this way the reliability and the lifetime of the converter is maximized even in case of thermal overload. 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.