## 2 Faults in a PV System
### Photovoltaic Generator Faults
#### 2.1.6 Mismatch/Shading Defects  
A mismatch fault occurs when a group of photovoltaic cells has different electrical characteristics [62]. This type of fault can be permanent, like an open-circuit fault, or temporary, like partial shading. Partial shading is a specific type of mismatch fault and is one of the main causes of failures in a PV system. The shading phenomenon can be classified as uniform or non-uniform [62]. The source of uniform shading can be adjacent buildings, passing clouds, trees, other signs, bird droppings, dirt and so on. Non-uniform or partial shading defects occur when some cells or modules receive direct irradiation and temperature in a non-uniform manner. On the other hand, uniform or total shading occurs when all cells or modules receive uniform but reduced exposure, resulting in a constant reduction in the output current and voltage of individual cells in a string. It is important to note that technical abbreviations should be explained the first time they appear [63]. Figure 3 shows a partially shaded and fully shaded module.  
The setting of the sun causes shading of the photovoltaic (PV) module, which reduces its power output. It is important to note that although shading has a negligible impact on the PV module's overall performance, it should still be avoided. Shaded cells can become reverse polarized, consuming energy instead of producing it, leading to a drop in power and the hotspot phenomenon [64]. The hotspot phenomenon can accelerate the aging process of the PV system and may even lead to an open-circuit fault or fire risks [65]. Figure 4 provides an illustration of the various faults described above.  
Figure 4: Illustration of ground faults \(F_{1}\), short-circuit faults \(F_{2},\ F_{3}\), open-circuit faults \(F_{4}\) and arcing faults (\(F_{5},\ F_{6}\) in a PV array).  
In the case of a short circuit, the output voltage drops significantly while the output current slightly increases. Short-circuit faults can affect cells, modules, and bypass diodes [66]. Bypass diodes are protection devices against shading and are connected in parallel to each group of cells as show in Figure 5. However, these diodes can be damaged during factory electrical discharge and high reverse voltage due to any fault [67]. If the bypass diode is faulty, there will be a sudden drop in power due to the absence of the voltage chain. The fault may be caused by non-functioning diodes, diodes reversed during assembly, poor diode connection, disconnection, or corrosion of the junction boxes. A bypass diode fault can cause damage such as hot spots, electric arcing, and the risk of fire if the diode is in an open circuit [68].  
PV modules can degrade in several ways, including discoloration of the encapsulant due to exposure to UV rays, which causes the PV cells to turn brown or yellow. Another form of degradation is delamination or the separation of different layers of the PV module. There are also two specific types of degradation to be aware of. Potential Induced Degradation (PID) occurs when there is a potential difference between the metal frame of the PV module and the solar cell, which can significantly degrade the electrical characteristics of the PV cell. Light-induced degradation (LID) is a loss of performance that occurs when the boron-oxygen effect and the boron-iron effect are activated after the PV modules have been exposed to sunlight [5]. In addition to the breakdowns observed at the PV generator level, the photovoltaic inverter is also a vulnerable component with unreliable performance [68]. Therefore, it is necessary to have knowledge of the common faults associated with this component.