# A Novel Cascaded Modular Photovoltaic Energy Storage System for Partial Shading Conditions
); [PERSON]AIL_ADDRESS] (B.L. );  
[EMAIL_ADDRESS] (J.H. ); [PERSON]AIL_ADDRESS] (S.L.) Huang  
1 State Grid Fujian Electric Power Research Institute, Fuzhou 350007, China;  
[PERSON]AIL_ADDRESS] (Y.F. ); [PERSON]_ADDRESS] (H.W. ); [PERSON]ESS] (B.L. );  
[PERSON]SS] (J.H. ); [PERSON]RESS] (B.L. );  
[PERSON]@fj. regions both work at revers bias, the brighter regions are treated as loads, which are heated up as a result of consuming the power generated by minor revers current. This hot spot phenomenon may cause irreversible cell destruction and accelerate the rate of thermal degradation, leading to considerable energy loss, reduced efficiency, and potential harm for PV power generation systems [6,7]. To address the partial shading effect on PV power generation systems, two types of methods, namely software and hardware methods, are proposed in the existing literature. To avoid the hot spot phenomenon, some PV modules connect bypass diodes to pass through a larger current, as shown in Figure 1a. In this string structure, the output characteristic curve of PV array presents a global maximum power point (GMPP) and few local maximum power points (LMPP), in which the software method are widely used [8,9]. These methods are mainly based on modified MPPT strategy referred to GMPP algorithms, which can be further classified into two groups including natural inspired optimization algorithms and two-stage scanning-based algorithms. Furthermore, the various intelligent algorithms, such as particle swarm optimization (PSO), simulated annealing algorithm, fuzzy algorithm, and firefly algorithm, are studied and implemented to track the GMPP for PV power generation system under partially shading conditions [10, 11, 12, 13]. However, the software method contains some drawbacks, such as the difficulty in initial point selection, lack of flexibility and adaptability, and the complexity in implementation. Another solution to eliminate the partial shading effect on PV power generation systems is hardware methods, mainly used for the reconfiguration of connection schemes of PV modules. This method requires extra switches and involves a corresponding control algorithm [14,15]. One of the hardware solutions is string structure configuration, as presented in Figure 1a. The PV modules are connected in series to increase the output voltage and are connected to a DC bus through a DC/DC converter, and the MPPT of PV system is realized by the operation and control of the converter. Furthermore, to avoid the hot spots and potential damage to PV modules under a partial shadow condition, each PV module is connected paralleled with a diode so as to provide a bypass current path. However, this scheme cannot solve the problem of the MPPT control falling into the local optimum point, resulting in an efficiency reduction in PV power generation, and its efficiency is always lower than 90%. Moreover, this scheme needs a complex control method and increases the difficulty in application [16, 17, 18, 19]. The distributed circuit  
Figure 1: Conventional configurations of a photovoltaic (PV) power generation system dealing with partially shading conditions. (**a**) String structure configuration; (**b**) distributed circuit configuration; (**c**) the hybrid structure. configuration combined scheme provides another solution, as shown in Figure 1b. One DC/DC converter is set for each PV module, and the cascading multiple structures satisfies the voltage level requirement and can achieve a high efficiency of about 97%. However, the output power of each PV module flows entirely through the respective converter, leading to large current stress, power loss, and construction cost [20, 21, 22, 23]. Combining these two solutions, the dual MPPT inverters with string modules are also widely applied, and the problems, including the output characteristic curve of the PV array with multiple peaks and an excessive loss of converter need to be considered simultaneously.