*6.2. Control Schemes*

The central inverter was considered to be controlled through a standard voltage oriented control strategy with MPPT reference, as depicted in the control scheme in Figure 7 [38]. Since the objective is to retrofit an existing PV system, the addition of the ESS is performed at the DC-side of the inverter and does not consider modifying the PV system or its control strategy. It must be noted that the addition of an ESS at the point of common coupling would not allow harnessing the power loss caused by power clipping, since the inverter power limitation would remain an issue, which is not the situation in other similar cases that can benefit from the same ESS sizing strategy, such as power curtailment, where the power limit is imposed by the control strategy and not by the rating of the converter.

Retrofitting forces the ESS control strategy to extract or inject power to the DC link without modifying the existing MPPT strategy of the PV inverter. For this purpose, a tailored ESS control strategy, shown in the flow chart in Figure 8, was designed. *<sup>P</sup>*pv, *P*inv, *P*ess, *<sup>P</sup>*mpp, *<sup>P</sup>*clip and *<sup>P</sup>*pre correspond, respectively, to PV plant output power (*P*pv = *<sup>i</sup>*pv ·*<sup>v</sup>*pv), inverter input power (*P*inv <sup>=</sup>*i*inv·*<sup>v</sup>*pv), ESS power (*P*ess = *i*ess ·*<sup>v</sup>*pv), estimated MPP according to Equation (1), clipping level, and pre-clipping level. This latter parameter is used as margin to perform power injection or subtraction to/from the DC link (*P*pre − *P*mpp). *P* ˇ ess (< 0) and *P*ˆess (> 0) correspond to the minimum and maximum ESS power. Cases I–V match those in Figure 9. This control strategy can be adapted to handle any EST. For this purpose, estimation of the ESS SoC value (*SoC*) in the control scheme of Figure 8 must be replaced by an estimation of the available energy in the applied EST.

The ESS output current reference (*i*∗ess) is provided to a standard PI controller, generating the modulation index and switching pattern according to the power flow direction (storing or releasing energy). The SoC estimation was performed through standard Coulomb counting method [39].

**Figure 7.** Configuration control schemes: (**left**) Perturb and Observe MPPT and Voltage Oriented Control (VOC) scheme applied to the central inverter; and (**right**) CC charging mode controller scheme applied to the ESS.

**Figure 8.** ESS control strategy to enable storing and releasing of PV clipped power.
