*6.1. Configuration*

The original PV system consists on a central inverter grid-tied PV plant, connected to the grid through a standard two level voltage source inverter (2LVSI). The addition of the ESS, formed by a battery pack and a single-stage DC/DC converter as in [35], merged to the DC-link of the PV system, enables clipped energy to be stored and released according to the system requirements and limitations. Figure 6 shows a simplified version of the full configuration; colored arrows illustrate the possible power paths. The grey arrow corresponds to a continuous power flow (from PV to the grid), while the green (PV to ESS) and purple (ESS to the grid) arrows correspond to excluding and non necessarily continuous power flows.

This configuration has a single stage DC/DC converter. However, as shown in Figure 6, the system can be implemented with a seconds DC/DC stage (marked as Optional). The difference between both alternatives lies in the percentage utilization of the capacity of the battery pack. A single DC/DC converter performing a standard battery charging strategy applies constant current (CC) charging mode until reaching a certain State of Charge (SoC), usually around 85%. The addition of a second DC/DC stage allows the control of the output voltage of the converter, hence enabling to transition into constant voltage (CV) charging mode, and therefore allowing full charging of the battery. Storing clipped power requires withdrawing power from the DC-link, since the MPP tracking voltage is imposed by the inverter, thus clipped-storable power must be controlled through current.

The simulation results are focused on the control strategy that enables performing power extraction and injection to the DC-link, without modifying the existing control strategy of the inverter. This strategy depends on the SoC of the battery pack, which was estimated through standard Coulomb counting method [36]. This estimation is independent of the battery model applied since it relies in the output current of the battery pack, enabling the utilization of an ideal circuital model [37], to emulate the behavior of the EST.

**Figure 6.** Central inverter grid-tied PV configuration with additional single or double DC/DC stage ESS showing power flow paths and corresponding data sheet efficiency.
