*2.5. Mission Profile Translation*

With the above operation conditions, the thermal loading of all the power semiconductor devices (i.e., the IGBTs and diodes in the PV-battery systems) during one-year operation is investigated. Based on the input power of the PV and battery converters, the thermal loading of the power devices is determined from the loss and thermal models of the components, and this translation is normally achieved through two-dimensional (2-D) look-up tables to process the long-term simulations. More detailed analysis and steps regarding the mission profile translation have been discussed in [27].

Figure 8 presents the resultant junction temperature of the IGBT T1 (see Figure 4) in the PV inverter integrated with the AC- or DC-coupled BESS. It can be seen in Figure 8 that the DC-coupled BESS can considerably reduce the thermal stress of the IGBT T1 during the power smoothing process. This is also true for the remaining devices (e.g., IGBT T2, diode D1, D2, and D5), which is not presented here, as their temperature profiles are similar to the presented one, and they will be considered in the

lifetime and reliability analysis in the following sections. Figure 9 shows the temperature profiles of the power devices within the AC- and DC-coupled BESS converters. The power devices in the battery inverter have a similar temperature profile, as shown in Figure 9a, while they present a significant difference, as shown in Figure 9b, for the battery converter. The variety of these temperature profiles will cause cyclic thermo-mechanical stresses among the different materials inside the power devices, which finally lead to cumulative bond-wire and solder-joint fatigue after a certain number of cycles. The evaluation of the damage will be discussed in the next section. Nevertheless, as seen from the translated thermal loading profiles on the power converters, the coupling configurations will affect the reliability performance of the power converters and, thus, the entire PV-battery system.

**Figure 8.** Junction temperature of the IGBT T1 (see Figure 4) in the PV inverter integrated with the ACor DC-coupled BESS: (**a**) one-year operation and (**b**) one-day operation.

**Figure 9.** Junction temperature of the power devices within the battery converters (referring to Figure 7): (**a**) in the battery inverter and (**b**) in the battery converter.
