| The device's losses increase dramatically with the expanded capability of three level converters and the heat dissipation issue has become one of the key problems during the converter developing process. Industrial application indicates that the heat dissipation system affects the converter's reliability and life time directly and some of the power devices could be damaged immediately with a worse heat dissipation system. It is commonly known that the devices are very sensitive to the heat which is generally caused by the losses. The variations of device's junction temperature affect the conduction and switching characteristics and thus the converter's performance as well. So it is very important to get the precise device's losses since the loss information could optimize the heat dissipation system which guarantees the converter's stable and reliable operation.Firstly, as the basis for the loss research, the commonly used carrier based sinusoidal pulse width modulation (SPWM) and space vector pulse width modulation (SVPWM) were analyzed. Secondly, power device's operation principles of neutral point clamped (NPC) three level in different load conditions were analyzed. Based on the loss model of the devices, the loss calculation method which took the junction temperature effect into consideration was proposed. Thirdly, Point to NPC three level converter power device's loss and junction temperature unbalance problem, the operating principle of active NPC (ANPC) three level was analyzed. Based on the analysis of on hand modulation strategies, an improved PWM method was proposed which has better junction temperature balance ability. Finally, taking the advantage of redundancy zero voltage output states, the fault tolerant operation control method of ANPC three level converter under single device open circuit or short circuit fault was proposed. Furthermore, the loss calculation method under fault tolerant operation was also proposed to evaluate the feasibility of the control method.
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