Active thermal control for reliability improvement in photovoltaic single-phase T-type NPC inverters

Single-phase T-type neutral point clamped (NPC) inverters have been extensively employed in small scale photovoltaic (PV) systems due to their outstanding power conversion efficiency. However, it is still necessary to further reduce PV energy costs to successfully replace fossil fuels. To do so, the reliability of inverters needs to be improved, which will result in both operation and maintenance cost reductions and the energy production improvement. On the other hand, the T-type NPC inverter suffers from unevenly distributed power losses among power semiconductor devices, which implies that the devices generating the highest losses are quickly aged and fail sooner than other devices. In this context, it is recommended to reduce the thermal stresses of the highest stressed devices. This work proposes a thermally compensated modulation strategy that enables thermal stress reduction. The capability of the proposed method for controlling power losses is theoretically analyzed, and its impact on lifetime is evaluated. Finally, the feasibility of the proposed method is experimentally validated.