4.4.2. Results

Figure 22a shows the response of the system for the ramp rate setting of 1.0 W/s (for both RDR & RUR). Abrupt PV disturbances are manually applied to the PV emulator by switching from PV Curve I to PV Curve II and vice versa as specified in Table 6. When the PV conveter output increases, the battery quickly intervenes to absorb the power. Similarly, the battery supplies power when there is abrupt fall in PV converter power. With the grid voltage of 10 V, the ramp rate of 0.1 A/s in current is obtained. Figure 23 shows the results when the PV power experiences random abrupt disturbances. The study presented in this section confirms that the proposed control system can be successfully implemented in real-time on a typical hardware platform and the desired ramping rates set by the user are achieved by the combination of PV and proposed BES control system.

**Figure 23.** Experimental result: ramp up/down response for random PV power changes.

#### *4.5. Comparisons*

This section demonstrates comparison of the proposed method with conventional methods to execute smoothing of PV power. The comparison is done based on various qualitative parameters such as ramp calculation simplicity, plug-play level of proposed BES, coverage of converter model details, optimal control design, and inclusion of SOC control. The evaluation are done at the levels of poor, fair, average, and good. The comparisons are tabulated in Table 7.


**Table 7.** Qualitative comparison of proposed method with existing methods.

## **5. Conclusions**

PV power experiences abrupt changes due to the fast moving clouds which cause voltage and frequency fluctuations in the weak grid and high PV penetrations. A battery energy storage (BES) is proposed in this paper to address fast power disturbances. Furthermore, the daily profile of the solar irradiance creates sharp ramp in daily load profile, which imposes challenges on the slow conventional generators. The ramping capability of the proposed PV-BES system is utilized to smooth and improve the daily load profile in terms of its steep ramp up and peak demand. Although the concept is developed for a BES system, it can be applied to other behind-the-meter distributed assets such as small gas turbine generators. The optimal computation and communication of the storage ramp rates are the future directions of research. To mention a limitation, the proposed approach of this paper requires the BES to be co-located with the PV and have access to its terminals in order to achieve the desired power ramping of the combined PV-BES.

**Author Contributions:** Conceptualization, R.S. and M.K.-G.; Modeling and simulation, R.S.; Formal analysis, R.S. and M.K.-G.; Investigation, R.S.; Writing–original draft preparation, R.S. and M.K.-G.; Writing–review and editing, R.S. and M.K.-G.; Supervision, M.K.-G.; Funding acquisition, M.K.-G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research work was partially supported by National Science Foundation (NSF 1808368).

**Conflicts of Interest:** The authors declare no conflict of interest.
