# A Novel Cascaded Modular Photovoltaic Energy Storage System for Partial Shading Conditions
## 3 Experimental Verification  
The hardware-in-loop (HIL) experimental platform based on the OPAL-RT OP5600 developed to verify the effectiveness of the proposed topology and control strategy is presented in Figure 7. A KEVSIGHT DSO9104A oscilloscope is used to record the experimental results. A PV/ES system composed of four PV/ES modules is established according to the topology and control structure shown in Figure 6, and its parameters are presented in Table 1. Furthermore, the light intensities of the four PV modules, which are different in order to emulate the partial shading conditions caused by the slow movement of clouds, are presented in Figure 8.  
Figure 6: Block diagram of the control scheme using the proposed energy optimization strategy. Figure 7: Hardware-in-loop (HIL) experimental platform. Figure 9a-d shows the experimental powers of the PV modules, the compensation currents of the DC/DC converters, and the _SOCs_ of the ES modules in four PV/ES modules in the proposed system, separately. \(P_{\text{MPPI}}\), \(P_{\text{MPPI}}\), \(P_{\text{MPP3}}\), and \(P_{\text{MPPI4}}\) are the theoretical maximum powers of the individual PV modules at their own maximum power points, depending on their light intensity, respectively. \(P_{\text{PV1}}\), \(P_{\text{PV2}}\), \(P_{\text{PV3}}\), and \(P_{\text{PV4}}\) are the actual powers generated by the four PV modules, respectively. According to the figures, all of the powers generated by the PV modules are very close to the theoretical maximum values. Therefore, the topology and control strategy of the PV/ES module effectively realizes the independent MPPT control for each PV module, ensuring the PV/ES system can obtain a maximum solar energy under partial shading conditions. From Figure 9a-d, \(i_{\text{com1}}\), \(i_{\text{com2}}\), \(i_{\text{com3}}\), and \(i_{\text{com4}}\) are the compensation currents generated by the DC/DC converters in the four PV/ES modules, respectively, which compensate for the current difference between the individual PV module and DC bus. As the current of the DC bus is the same for all of the modules, there is an inverse relationship between the light intensity and the compensation current, i.e., the PV module with a lower light intensity should be compensated by a larger current than the ones with a higher light intensity. For example, at 1440 s, the light intensity decreases one by one from the first PV module to the fourth PV module, while the corresponding compensation current for each PV module increases sequentially, which is approximately \(-\)20 A, \(-\)10 A, 0, and +6 A, respectively. Considering the case at 3840 s as another example, the light intensity gradually increases from the first PV module to the fourth PV module, while the compensation current in the corresponding modules decreases from positive to negative. According to the current direction defined in Figures 3 and 6, the positive compensation current represents that the ES module is discharging, and the negative compensation current represents that the ES module is charging. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Value** & **Parameters** & **Value** \\ \hline \(L_{\text{f}}\)/mH & 8 & N & 10 \\ \(C_{\text{f}}\)/mF & 5 & M & 100 \\ \(L_{\text{l}}\)/mH & 2 & \(b\) & 100 \\ \(I_{\text{rated}}\)/A & 80 & \(w\) & 0.5 \\ \(C_{\text{es}}\)/Ah & 40 & \(c_{1}\) & 1 \\ \(U_{\text{grms}}\)/V & 380 & \(c_{2}\) & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the PV/ES system and control strategy. Figure 8: Light intensity of four PV modules. The _SOCs_ of the ES modules are also presented in Figure 9a-d, as \(SOC_{1}\), \(SOC_{2}\), \(SOC_{3}\), and \(SOC_{4}\). According to Figure 9a, when the compensation current is negative, the \(SOC\) is rising as the ES module is charging; when the compensation current is positive, the \(SOC\) is declining and the ES module is discharging.