Shading occurs when objects, such as trees, neighboring buildings, and overhead power lines, cast shadows on PV modules [41,42]. Shading in PV arrays could be homogeneous, where there is a balanced reduced irradiation across the PV panels or non-homogeneous, where there is an unbalanced reduced irradiation across the panels [39].

A frequent high-power discharge of electricity through an air gap between conductors causes this type of fault [43,44,45]. The two forms of arc faults include first, the series arc faults that usually originate from solder separation, connection corrosion, cell damage, rodent damage or abrasion from numerous sources. Second, parallel arc faults that result from insulation failure in current-carrying conductors [43,44].

A line-line fault is an unintentional short-circuit between two points with differing voltage potentials [46,47,48]. These faults are more difficult to detect than other faults and are frequently misinterpreted as short-circuit faults in grounded PV systems, since the fault current is determined by the voltage differential between two fault spots [39]. The most common types of line-line faults are intra string faults, which are short-circuit faults between two locations on the same string, and cross string faults, which are short-circuit faults between two places on separate threads [39].

To protect users from a possible electric shock, it is common practice that the metallic parts of the PV array are grounded using earth-grounding conductors (EGC) [39]. The term “ground fault” refers to any unintentional connection between a current-carrying conductor and an EGC that results in a current flow to the ground [45,46,49,50].

This refers to any fault that cannot be categorized under any of the previously discussed faults. These types of faults include MPPT and inverter faults that mostly occur due to inverter components failure, such as IGBTs, capacitors, and converter switch failure [51,52,53]; bypass diode faults resulting from a massive reverse current flow during faults, which leads to short-circuits [54,55]; blocking diode faults, also as a result of a reverse current flow [39]; open-circuit faults caused by items falling on PV panels, physical failure of panel-panel cables or joints, and sloppy termination of cables, plugging, and unplugging connectors at junction boxes [56]; faulty connections damage of connecting cables or a wrong connection of panels [57]; battery bank failures due to abnormal charging conditions [39]; and blackouts caused by natural disasters, such as a storm and lightning [58]. Most of these mentioned faults are usually due to an after effect of the other faults [39].

References:

39. Pillai, D.S.; Rajasekar, N. A comprehensive review on protection challenges and fault diagnosis in PV systems. Renew. Sustain. Energy Rev. 2018, 91, 18–40. [CrossRef]
40. Ghosh, R.; Das, S.; Panizrahi, C.K. Classification of Different Types of Faults in a Photovoltaic System. In Proceedings of the 7th IEEE International Conference on Computation of Power, Energy, Information and Communication, Chennai, India, 28–29 March 2018; pp. 121–128.
41. Nguyen, X.H. Matlab/Simulink Based Modeling to Study Effect of Partial Shadow on Solar Photovoltaic Array. Environ. Syst. Res. 2015, 4, 20. [CrossRef]
42. Patel, H.; Agarwal, V. MATLAB-Based Modeling to Study the Effects of Partial Shading on PV Array Characteristics. IEEE Trans. Energy Convers. 2008, 23, 302–310. [CrossRef]
43. Spooner, E.; Wilmot, N. Safety issues, arcing and fusing in PV arrays. In Proceedings of the 3rd International Solar Energy Society Conference—Asia Pacific Region (ISES-AP-08) Incorporating the 46th ANZSES Conference, Sydney, Australia, 25–28 November 2008.
44. Xia, K.; He, Z.; Yuan, Y.;Wang, Y.; Xu, P. An arc fault detection system for the household photovoltaic inverter according to the DC bus currents. In Proceedings of the 2015 18th International Conference on Electrical Machines and Systems (ICEMS), Pattaya City, Thailand, 25–28 October 2015; pp. 1687–1690. [CrossRef]
45. Chen, L.; Li, S.; Wang, X. Quickest Fault Detection in Photovoltaic Systems. IEEE Trans. Smart Grid 2018, 9, 1835–1847. [CrossRef]
46. Zhao, Y.; Lehman, B.; de Palma, J.-F.; Mosesian, J.; Lyons, R. Challenges to overcurrent protection devices under line-line faults in solar photovoltaic arrays. In Proceedings of the 2011 IEEE Energy Conversion Congress and Exposition, Phoenix, AZ, USA, 17–22 September 2011. [CrossRef]
47. Zhao, Y. Fault Analysis in Solar Photovoltaic Arrays. Ph.D. Thesis, Northeastern University, Boston, MA, USA, 2010.
48. Yi, Z.; Etemadi, A.H. Line-to-Line Fault Detection for Photovoltaic Arrays Based on Multiresolution Signal Decomposition and Two-Stage Support Vector Machine. IEEE Trans. Ind. Electron. 2017, 64, 8546–8556. [CrossRef]
49. Appiah, A.Y.; Zhang, X.; Ayawli, B.B.K.; Kyeremeh, F. Review and Performance Evaluation of Photovoltaic Array Fault Detection and Diagnosis Techniques. Int. J. Photoenergy 2019, 2019, 6953530. [CrossRef]
50. Zhao, Y.; De Palma, J.-F.; Mosesian, J.; Lyons, R.; Lehman, B. Line–Line Fault Analysis and Protection Challenges in Solar Photovoltaic Arrays. IEEE Trans. Ind. Electron. 2013, 60, 3784–3795. [CrossRef]
51. Hua, C.-C.; Ku, P.-K. Implementation of a Stand-Alone Photovoltaic Lighting System with MPPT, Battery Charger and High Brightness LEDs. In Proceedings of the 2005 International Conference on Power Electronics and Drives Systems, Kuala Lumpur, Malaysia, 28 November–1 December 2005; pp. 1601–1605.
52. Wang, Y.; Li, Y.; Ruan, X. High-Accuracy and Fast-Speed MPPT Methods for PV String Under Partially Shaded Conditions. IEEE Trans. Ind. Electron. 2016, 63, 235–245. [CrossRef]
53. Chan, F.; Calleja, H. Reliability: A New Approach in Design of Inverters for PV Systems. In Proceedings of the 2006 IEEE International Power Electronics Congress, Puebla, Mexico, 16–18 October 2006; pp. 1–6. [CrossRef]
54. Mellit, A.; Tina, G.M.; Kalogirou, S.A. Fault detection and diagnosis methods for photovoltaic systems: A review. Renew. Sustain. Energy Rev. 2018, 91, 1–17. [CrossRef]
55. Davarifar, M.; Rabhi, A.; El Hajjaji, A. Comprehensive Modulation and Classification of Faults and Analysis Their Effect in DC Side of Photovoltaic System. Energy Power Eng. 2013, 5, 230–236. [CrossRef]
56. Akram, M.N.; Lotfifard, S. Modeling and Health Monitoring of DC Side of Photovoltaic Array. IEEE Trans. Sustain. Energy 2015, 6, 1245–1253. [CrossRef]
57. Schimpf, F.; Norum, L.E. Recognition of electric arcing in the DC-wiring of photovoltaic systems. In Proceedings of the Intelec 2009—31st International Telecommunications Energy Conference, Incheon, Korea, 18–22 October 2009; pp. 1–6. [CrossRef]
58. Omer, A. Renewable energy resources for electricity generation in Sudan. Renew. Sustain. Energy Rev. 2007, 11, 1481–1497. [CrossRef]