*2.3. Power Smoothing Operation*

The target of integrating battery energy storage systems for a PV system is to comply with certain requirements (decided by the transmission system operator) when injecting power into the grid. This paper focuses on the power smoothing and ramp-rate control with the consideration of their impact on the reliability of power semiconductor devices within the PV-battery system. Similar to the previous studies [13,15], in this work, it is assumed that the ramp-rate limit is 10% of the rated power per minute, which is defined by the Puerto Rico Electric Power Authority (PREPA) [25]. Subsequently, the ramp-rate compliant algorithm in [15] is applied to smooth the system output power, size the battery converter/inverter, and obtain the mission profile of the integrated battery system. The power smoothing results for the DC-coupled PV-battery system are shown in Figure 5. The smoothed power is less fluctuating when compared with the PV power before smoothing, as seen in Figure 5a. The difference, as shown in Figure 5b, is the power reference for the BESS. As seen in Figure 5b, in most days, the absorbed/injected power is within 100 kW. Following, Figure 6 presents the ramp rate results of the PV system with and without a 100 kW BESS. As observed in Figure 6, without the BESS, the output power of the PV system can experience even 80% rises or drops in a very short period (e.g., 1 min.), which may affect the overall supply and demand power balance, causing grid stability issues. In contrast, with the 100 kW BESS, the corresponding ramp rate is mainly distributed within the limit (i.e., 10% of the rated power per minute).

**Figure 5.** Power smoothing analysis: (**a**) input power of the PV inverter with and without power smoothing and (**b**) power reference for the DC-coupled BESS.

**Figure 6.** One-year power ramp-rate analysis: the ramp rate of PV output and the ramp rate of PV-BESS (plant) output when energy storage system is deployed for power smoothing.

## *2.4. Battery Energy Storage System*

The reliability analysis that is given in this paper considers two typical energy storage systems shown in Figure 7. For the DC-coupled configuration, as shown in Figure 7a, a simple half bridge-based DC/DC topology is chosen to build an interleaved converter with three stages, interfacing the batteries to the DC side of the 1500 V PV system. While for the AC-coupled configuration, the same topology as for the PV inverter is employed as the interfacing unit between batteries and the point of common coupling (PCC), as shown in Figure 7b. Table 2 presents the designed ratings of the battery systems. Regarding the power modules, three 1200 V/200 A IGBT modules and three 1700 V/150 A IGBT modules from Semikorn are respectively used for the battery inverter and converter shown in Figure 7 [24]. Moreover, the corresponding heatsink parameter, heatsink-to-ambient thermal impedance per module *<sup>R</sup>*th(<sup>s</sup>−<sup>a</sup>), is designed to ensure the junction temperature of the most stressed power devices is below 125 oC during the rated operation with the ambient temperature being 50 oC.

For the battery model, the two BESSs that are shown in Figure 7 are equipped with the same type of batteries as a commercial BESS for 1500 V PV applications [26], while the operating voltage of the two BESSs are different. In the case of the DC-coupled configuration, the voltage of the PV system at the maximum power point (MPP) varies between 1000 V and 1300 V under the Denmark mission profile (see Figure 3). The battery voltage range is 670 V to 870 V, which is lower than the minimum MPP voltage to ensure the converter charges the batteries in the buck mode, and in the boost mode, the batteries are discharged. On the other hand, for the AC-coupled configuration, the battery operating voltage is 860 V to 1120 V, ensuring the battery can be discharged when considering a 10% variation of the grid voltage. For the sake of simplicity, it is assumed that the battery voltage is at the upper and lower limit during the charging and discharging mode, respectively. Detailed sizing of the battery storage is out of the scope of this paper and, thus, the capacity of the battery systems is assumed to be sufficient during operation. Table 2 summarizes the battery specifications.

**Figure 7.** Battery energy storage system: (**a**) for the DC-coupled configuration and (**b**) for the AC-coupled configuration (PCC: the point of common coupling).

**Table 2.** BESS Specifications.

