MODEL PREDICTIVE CONTROL ON THERMAL STRESS REDUCTION FOR GRID-CONNECTED INVERTERS RELIABILITY ENHANCEMENT

Thermal stress has been identified as one of the major failure causes in the power module. It is generated from the mechanical strain by severely varying temperatures at different loci in the power module and the different coefficients of the thermal expansion of materials, where the varying temperatures result from the real-time power loss across the power converter. This thermal stress accelerates the degradation of semiconductor devices, downgrades the system quality and efficiency, and eventually causes catastrophic system breakdowns and extensive economic losses. Therefore, this research is dedicated to investigating both local control level methods and system level strategies to ameliorate the real-time power loss in order to reduce the thermal stress in the power module, thereby extend the component lifetime and enhance the system reliability. A finite-control-set model predictive control (FCS-MPC) is introduced and deductively investigated from the local control level. Its variable switching frequency property is derived through the geometry analysis on the voltage vector space. It realizes the switching frequency variation autonomously by the loading power. By taking advantage of this property, the power loss is leveled in the real-time operation by FCS-MPC, and a more mitigated thermal profile is acquired compared with the one by the conventional controller. Furthermore, a centralized thermal stress oriented dispatch (TSOD) system level strategy is proposed for multiple paralleled distributed energy resource systems, which helps to reduce the thermal stress in the power module of paralleled converters. It is thermal stress oriented and takes effect according to the real-time junction temperature variation, the health condition of the individual converter, and the system operation. Two local control level methods, the switching frequency variation and the reactive power injection, are imported separately as the dispatch algorithm to generate the expected power loss. Dealing with the varying mission profile, the more mitigated thermal profiles are achieved for all converters with the assistance of the proposed TSOD strategy.