## 2 PV Power Systems
### Grid-Connected PV Systems
In reference [43], one PV string within the array operated as an auxiliary string to implement the MPPT algorithm and determine the available power. The remaining PV strings worked together to maintain constant power generation, meeting the AC power demand. However, this method faces challenges: variations in shading levels across strings can lead to inconsistencies, and the communication required among strings may increase operational complexity. In [44], the desired power was determined through offline calculations based on PV module datasheet values. This method enables a fast frequency response and high accuracy, though it requires additional sensors to precisely measure temperature and irradiance. In reference [45], a method based on the estimation of the power losses of electronic power converters that connect the PV panel to the AC grid was proposed, avoiding mismatches between the generated and delivered power. Table 1 summarizes the main characteristics of the mentioned PV power curtailment methods. * _Integration of energy storage_  
Another approach to reduce the power system's frequency fluctuations is the integration of energy storage with grid-connected PV systems [46; 47; 48], as shown in Figure 3 (with the dashed lines). The power fluctuations, which may be short-term ranging from seconds to hours, caused by the PV power variation, are absorbed by the energy storage. Thus, the power fluctuations are not transferred to the AC grid, increasing the stability and reliability of the power system, especially in the case of power systems that do not have a sufficient spinning reserve [49]. Moreover, in the case of low-voltage conditions in the grid, large-scale PV systems are required to remain connected to the power system, but the amount of power that can be injected into the grid is limited. This causes energy accumulation in PV systems, increasing the voltage in intermediate stages, and instabilities and stress on the power converter system. This instability makes it difficult for PV systems to remain connected during the recovery period. In this case, the energy storage can significantly contribute to keeping the system stable with the capability to absorb energy during low-voltage periods as well as to avoid energy waste [27]. Compared to the PV power curtailment method, the power fluctuation pressure on grids with energy storage-integrated grid-connected PV systems is lower and the PV power is used in a more efficient way [50; 51]. In recent years, many energy storage technologies, such as superconducting magnets, pumped hydro, ultracapacitors, and BESSs, with their control methods, have been proposed to smooth PV power output [52; 53]. For superconducting magnetic storage systems \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multirow{2}{*}{**Estimation Output**} & **Operation** & **Controlled** & **PV** & **Converter** \\  & & **Side** & **Variable** & **Configuration** & **Losses** \\ \hline
[40] & MPP & Left & Duty cycle & Single-stage & Neglected \\
[41] & MPP and P-V curve & Left or right & PV voltage & Single-stage & Neglected \\
[42] & MPP and P-V curve & Right & PV power & Double-stage & Neglected \\
[43] & MPP & Left & PV voltage & Double-stage & Neglected \\
[44] & MPP & Right & PV voltage & Single-stage & Neglected \\
[45] & MPP and P-V curve & Left & PV voltage & Double-stage & Considered \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Different types of photovoltaic power curtailment methods. and ultracapacitors, the energy densities are low, and therefore, they are usually used in pulsed-power and system-stability applications. For pumped hydro, it has topographical limitations, and therefore, it is not suitable for large-scale PV systems in flat areas [26]. Since BESSs have the advantages of high power density, long cycle life, and a fast response time, they are widely used in PV systems as energy storage [27].