Extended analysis on Line-Line and Line-Ground faults in PV arrays and a compatibility study on latest NEC protection standards
Introduction
In modern era, electric power generation from photovoltaic resources has conceived a huge market in micro grid environment. On the other hand, despite all benefits and research advancements that have been made so far, photovoltaic (PV) systems are highly vulnerable to fault occurrences that drastically reduce the efficiency and safety [1]. For instance, undetected faults in PV systems recently provoked severe fire hazards in California and Bakersfield [2]. Nevertheless, faults in PV systems and the necessity of advanced protection standards are less addressed and investigated. Thus, fault analysis in solar PV arrays becomes a fundamental requirement to increase the reliability of PV systems.
Very few standards are developed and recommended for protection in PV systems against faults and electric shocks; at international level, IEC 60364–7-712 [3], IEC 62,548 [4] and IEEE standard 1374 [5], at national level, NEC (National Electrical Code) article 690 [6] and Spanish Royal Decree 1663 [7]. Meanwhile, protection standards such as IEC 61,140 [8] and IEC 60364–41 [9] that are suitable for conventional power systems are not compatible with PV systems since PV systems are relatively new and posses unique operating characteristics that are no way comparable to conventional power generating units [10], [11], [12], [13].
NEC article 690 covers numerous aspects of PV protection and is widely practiced around the globe. Unfortunately, NEC 690 also lacks features that address important practical issues related to PV system safety [1], [14]. In this regard, limitations of NEC 1998 and 2008 standards in Line-Ground (LG) fault detection are studied in [15], [16], Line-Line (LL) fault detection challenges involved with NEC 2011 are investigated in [17] and the adverse effect of Maximum Power Pointing Trackers (MPPTs) towards LL fault detection is discussed in [18]. It is important to note that, the experimentations carried out in aforesaid works [15, 16 and 18] were too short, and the analysis was restricted to standard PV operating conditions only. While in [17], the compatibility of NEC standards has been evaluated for various PV operating conditions with different fault impedances. However, as for the earlier works, the findings were not generalized since the study was constrained to a single PV system configuration. Also, the objective of all aforementioned works was only to showcase LL/LG fault detection challenges without providing any suggestions for improvement. It is important to note that, though researches available till date in literature attempts to analyze the incompatibility of NEC standards, conclusive results with respect to various practical operating conditions of PV systems are yet to be arrived. Moreover, regardless of these short investigations, even the latest NEC 2017 standards are not revised to counter act the protection challenges involved.
On this note, the proposed work endeavors to provide much detailed investigations to understand the unique behavior of LL and LG faults in PV arrays. Furthermore, new mathematical equations based on the equivalent circuit of a faulty PV array are derived to determine the fault current levels; that enhances PV array fault analysis possibilities. In addition, in order to set accurate benchmarks, extensive fault analysis in various unique PV operating conditions is performed and the compatibility of latest NEC 2017 protection standards for LL/LG detection is evaluated. Moreover, to generalize the findings attained with respect to fault current magnitudes, the proposed study is extended to two different PV configurations. Further, based on the critical implications attained from the analysis, some suggestions to achieve reliable LL/LG fault detection are also presented. On the whole, to be versatile from the existing scientific contributions, this research attempts to: (1) Provide a brief overview on various fault occurrences in PV systems, (2) Perform a detailed behavioral analysis on LL and LG faults in PV arrays, (3) Analyze the effectiveness of latest NEC standards for LL/LG fault detection with respect to: (a) PV array size, (b) Variation in fault mismatch levels, (c) Application to MPPT and non-MPPT based PV systems and (d) Adaptability in varying irradiation levels, and (4) Suggest some valuable facts that are expected to be meritorious to accomplish dependable LL/LG fault detection for PV arrays.
Section snippets
Faults in PV arrays
PV arrays are frequently challenged by two common fault scenarios: (1) Electrical faults and (2) Shade faults. Electrical faults occur due to abnormalities that arise in the internal electrical configuration of the system; while, shade faults are induced by change in insolation levels and the external environment. Further, both these faults are bound to have significant impact on the output characteristics of any standalone and grid connected PV system. Typical PV fault scenarios are depicted
Line-Line and Line-Ground fault analysis
With the clear understanding on different PV faults and protection standards, it is imperative to provide a general fault behavior analysis with protection devices installed in a PV array. To exemplify the evolution of fault current, LL and LG faults are extensively studied and its impact on the system performance is perceived. For examination, an ‘’ PV array equipped with OCPD and GFDI as shown in Fig. 3(a) is considered and the evolution of faults is analyzed. Since, the severity of the
Suitability analysis on NEC 2017 standards for LL and LG fault detection
The signature of faults on the PV array is unique and the protection challenges primarily rely on three major factors [1, 14 and 17]: (1) Mismatch impedance determined by fault location, (2) Presence of MPPT controller and (3) Instantaneous irradiance falling on the PV array. Hence, any proposed protection standards for PV systems must have the following inherent and inevitable capabilities: (a) Irrespective of varying fault impedances, protect the PV system, (b) Detect faults even in low
Experimental validation and discussions
To validate the simulation results discussed in the previous sections, a small scale laboratory prototype has been established and tested. Since the challenges in LL and LG fault detection in PV systems are primarily influenced by fault mismatch levels and MPP tracking technology, extensive experimental evaluations have been carried out in this section with respect to different PV array sizes to exclusively demonstrate the practical limitations involved in LL/LG fault detection.
Energy loss and income analysis
Any undetected fault in a PV array will reduce the power output of the system. Hence, the considered 1.375 kW PV system has been tested to analyze the energy lost per annum, if a fault remains undetected in the PV array. The test results obtained are shown in Table 12. In normal operating conditions, the considered PV configuration can generate 3555.7 kWh of electricity considering 7 h of peak sunshine per day. However, under faulty conditions, the units generated per annum significantly reduce
Suggestions to improve LL and LG fault detection in PV arrays
The protection standards defined in NEC 2017 focuses only to extend the applicability and flexibility of previous NEC standards. Besides, the conventional protection challenges still exist in PV systems; particularly due to the inconsistencies in the selection of device ratings. The analysis carried out in this research points out the following challenges in the NEC standards: (1) OCPD standards are not sensitive enough to protect the PV array from high mismatch impedance LL faults, (2)
Conclusion
This paper has presented a fundamental analysis on Line-Line and Line-Ground faults in PV arrays. The basic fault behavior, its evolution and its impact on the output parameters have been investigated. In addition, as NEC standards proclaimed to be the best protection standards available for PV systems, the compatibility of latest NEC 2017 standards has been critically analyzed with respect to numerous LL and LG fault cases under different operating conditions. Further, the challenges posed by
Declaration of Competing Interest
The authors declare that there are no known conflicts of interest.
Acknowledgment
The authors thank Vellore Institute of Technology (VIT) - Vellore, India for providing ‘VIT SEED GRANT’ for carrying out this research work. This work is carried out at Solar Energy Research Cell (SERC), School of Electrical Engineering (SELECT), Vellore Institute of Technology (VIT) - Vellore. Further, the authors also would like to thank the reviewers for their valuable comments and recommendations to improve the quality of the paper.
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