Protection of the switches in a three-level inverter by using modified switching schemes for the redistribution of losses

Switching devices (IGBT, IGCT, etc.) of Three-level Neutral-Point-Clamped Voltage Source Inverters (3L-NPC-VSI) for high-power applications are typically mounted on dedicated heatsinks, which results in a lack of heat sharing among the switches. Several control strategies have been proposed to enable even distribution of losses among the switches in normal operation, however a local thermal overload can occur due to adverse operating conditions such as: degradation of cooling system, inappropriate placing of semiconductor device, failures in electronics, etc. The thermal overload increases the risk of thermal breakdown of the affected device and reduces consequently the expected lifetime of inverter.

A new fault-tolerant control approach is proposed in the work to deal with this matter. In case of thermal overload, the control method of inverter is immediately adapted, so that the stressed switch is relieved, the heat is distributed to the other devices which are not affected by the overload. Here, redundant states of multi-level inverter are taken advantage of, since redundancies of a voltage space vector deliver the same line-to-line output voltage, but having different impact on the loss distribution among the switches of the inverter. Thanks to the active redistribution of losses among the switches, the temperature of the affected device is kept under the critical limit. In this way, the lifetime of the inverter is maximized even in case of thermal overload.

The proposed strategy will be examined with two control methods for the inverter feeding an induction machine: Fields Oriented Control (FOC) and Direct Torque Control (DTC), which are very widespread in the practice. The implementation of each control method requires different considerations and strategies. Finally, the impact of the optimized switching pattern on loss distribution among the switches will be verified by means of simulation and measurements on a laboratory set-up. During operation of the proposed control methods, the stability of the neutral-point potential of the inverter is ensured without any additional hardware.