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Active Lifetime Extension for a 3L-NPC inverter with Direct Torque Control

The redundant states in the output voltages of a Three-level Neutral Point Clamped Voltage Source Inverter (3L-NPC VSI) can be used to control the neutral point or to reduce the common mode voltage. However, the use of the redundant states to redistribute the losses among the switching devices of 3L-NPC VSI is a further possibility that was reported in previous papers. The proposed method for active redistribution of losses among the switching devices can become necessary in the practice to relieve stressed switches, which suffer excessive thermal stress due to malfunctions. In this paper, a new fault tolerant control approach is introduced to deal with this matter by modifying the switching table of the DTC (Direct Torque Control) scheme for a 3L-NPC VSI. An increased thermal stress in one IGBT module can be detected by sensing the temperature of the heatsink; consequently the switching patterns are immediately altered to redistribute the losses of the stressed IGBT module to the other cooler switches. Hence, the temperature of the stressed switches is kept under the critical limit and the thermal overload is reduced. In such way the reliability and the lifetime of inverter are maximized, even in case of thermal failure. This modulation scheme is called “Active Lifetime Extension” (ALE) and was presented in previous works for the case of field oriented control. In this paper the procedure is extended to the case of DTC.