4.2.1. Monitoring Bypass Diode Voltages

Monitoring the voltage of the bypass diode is effective in detecting the occurrence of partial shading. Under normal operation, the bypass diode is inactive, however during partial shading it will become active, and a voltage drop will appear across it [158]. Conventional (classical) MPPT techniques such as P&O, IC, and HC are implemented during normal irradiation conditions. Global search to track the global peak can be activated once a voltage drop is sensed in the bypass diode. The method works for module-integrated converters where the PV modules are directly connected to the power converters such as DC–DC converters or DC–AC inverters. The advantage of this method is that, unlike other MPPT methods where periodic scanning of the P-V curve is necessary, this technique is activated by sensing the bypass diode voltage. The drawback of this method is that it works for PV systems where the string voltage is accessible for measurement.

#### 4.2.2. Distributed MPPT

Depending on the connection of the DC–DC converter and the DC–AC inverter used to integrate the PV system-generated power into the grid, there are two types of architectures: central and distributed. In central inverter architecture, one highly rated DC–DC converter and one DC–AC inverter are used where the DC–DC converter performs the MPPT, and the inverter is used for grid integration. This type of architecture does not generate the maximum available power in partial shading. On the other hand, distributed architecture alleviates this problem by providing the MPPT converter for each module. This type of arrangement provides greater flexibility, and the power generated from the PV system is better than from central-based architecture. The distributed-based partial shading mitigation technique connects the DC–DC converter and MPPT for each PV system cell, module, or array [159,160]. Each system unit works and tracks the MPP independently as a distributed sub-unit. The distributed and centralized MPPT architecture is provided in Figures 14 and 15, respectively. To reduce the cost and complexity of the MPPT for the distributed architecture, conventional MPPT methods such as P&O and IC are used. The advantage of this method is that the system reliability increases as each unit has its controller, and the failure of one of the sub-units does not affect the entire system.

**Figure 14.** Centralized MPPT architecture.

**Figure 15.** Distributed MPPT architecture.

#### 4.2.3. Differential Power Processing

This method works by placing DC–DC converters between adjacent PV modules [161]. Figure 16 depicts the differential power processing method where the adjacent converters provide the current difference that appears between the current at the MPP of the two modules. The converter will be active when there is a difference in the power generated between the two adjacent modules. Conventional MPPT methods such as P&O and IC are employed for the modules. Compared to the distributed MPPT, this method, where a dedicated converter is connected for each module, minimizes the conversion losses and cost. The differential power processing method also has a better overall conversion efficiency and performs well during the partial shading condition by overcoming the challenges associated with the mismatch MPP current. Moreover, this method tracks the global peak with less power loss as the converter only processes the difference [162].

**Figure 16.** Differential MPPT architecture.

#### 4.2.4. Power Electronics Equalizer

This method works using the power independent principle where series connected cells are operated with different voltages and currents [163]. The power electronics equalizer method works by transferring power from the non-shaded modules to the shaded modules so that all modules work at their respective MPP and exhibit an equal power level across the system. The power electronics equalizer method has a better performance and power harvesting capability as compared to the bypass diode method. The disadvantage of this method is that an extra circuit has to be connected to recover the power from the shaded modules, which increases the complexity of the topology. Energy storage elements such as inductors and capacitors are used to store the power of the non-shaded cells. They will be connected in parallel to distribute the stored power to the cells to have equal power across each cell [164].
