A Unified Approach for Analysis of Faults in Different Configurations of PV Arrays and Its Impact on Power Grid

: Fault analysis in photovoltaic (PV) arrays is considered important for improving the safety and efficiency of a PV system. Faults do not only reduce efficiency but are also detrimental to the life span of a system. Output can be greatly affected by PV technology, configuration, and other operating conditions. Thus, it is important to consider the impact of different PV configurations and materials for thorough analysis of faults. This paper presents a detailed investigation of faults including non-uniform shading, open circuit and short circuit in different PV interconnections including Series-Parallel (SP), Honey-Comb (HC) and Total-cross-Tied (TCT). A special case of multiple faults in PV array under non-uniform irradiance is also investigated to analyze their combined impact on considered different PV interconnections. In order to be more comprehensive, we have considered monocrystalline and thin-film PV to analyze faults and their impact on power grids. Simulations are conducted in MATLAB/Simulink, and the obtained results in terms of power(P)–voltage(V) curve are compared and discussed. It is found that utilization of thin-film PV technology with appropriated PV interconnections can minimize the impact of faults on a power grid with improved performance of the system.


Introduction
Rapid growth in the deployment of renewable energy has been witnessed over the past few years despite it being less efficient and more susceptibile to unexpected faults [1]. Advancement in PV technology is still restricted due to high cost arising from its low efficiency and high probability of fault occurrence during photovoltaic system operation. The most commonly used crystalline silicon PV material has up to ~27% efficiency under normal operating conditions [2]. The efficiency of a PV system can be greatly reduced due to various factors such as degradation of the PV module and wiring losses including open circuit; short circuit faults; and severe shading through obstruction of trees, buildings, bird dropping, and heavy dust layer accumulation on PV panel [3].
A study on monitoring of PV arrays in 2010 showed that faults can reduce the generated power of solar systems annually by about 18.9% [4]. The faults in large PV systems are difficult to spot and can remain undetected, leading to higher risks of arc faults. Many fire accidents have been reported in the past [5]. One such event happened in Bakersfield, California in 2009 due to undetected ground Table 1. Comparative analysis of shading condition in PV with respect to interconnections.

Interconnections
Increase in Power Generation Ref.
Partial shading

Series (S) Series-Parallel (SP) Honey-Comb (HC)
Total-Cross-Tied (TCT) 11-17% power increase in TCT than other 4% power increase in TCT 10% power increase in HC than SP 10-20% power increase in TCT than other 5-10% power increase in TCT than SP [21] [22] [23] [24] [25] In this research paper, a unified approach is adopted for comparative analysis of faults such as open circuit, short circuit and severe shading in different interconnections of PV arrays including SP, HC and TCT interconnection. A special case of multiple faults in PV array under non-uniform shading is also investigated to analyze the combined impact of short circuit and open circuit fault under severe shading on P-V characteristics. It aims at enhancing power generation through optimizing the maximum power point (MPP) of the P-V characteristic curve and improving the performance of the power grid during severe faults. Therefore, monocrystalline and thin film (amorphous silicon) PV materials are considered for a more detailed analysis of faults. Thin-film PV technology optimizes the performance of PV systems in all fault scenarios. The utilization of suitable interconnection improves the obtained results which have been compared in terms of P-V curves. The main contributions of this paper can be listed as follows: (1). An increase in power generation is achieved through the utilization of suitable PV interconnection and PV material under different fault scenarios; it should be noted that in the literature, faults impact on PV arrangement and PV material is not studied thoroughly. Only the impact of shading and high temperature on PV arrangements is studied in the literature. The performance of thin-film PV under short circuit and open circuit faults is also investigated in this paper, which has not been done so far. (2). A special case of multiple faults under non-uniform shading is also investigated in this paper for an analysis of the combined impact of all developed faults in PV array under severe shading conditions.The impact of all faults on the power grid is also investigated. It is found that the utilization of suitable PV interconnections and PV material also helps minimize the fault impact on power grids, which has also not been thoroughly studied in the literature. i. The rest of this article is organized as follows: Classification of PV array faults based on location is presented in Section 2. The system model and interconnection schemes of PV array under different fault scenarios are explained in Section 3. Simulation results of PV array and its impact on power grids are presented in Section 4. A comparison and discussion of results are included in Section 5. Distintinctive features are presented in Section 6, and a conclusion is drawn based on results in Section 7.

Classification of PV Faults
Typical faults in the PV system are given in Table 2. and are classified on the basis of type and location [1]. PV module mismatch faults due to partial shading and wiring losses due to short circuits within a PV module string and among different PV strings, line-to-line, and line-to-ground faults are common faults in PV systems. These faults may result in low power generation and can lead to permanent damage of a PV system. Cracks of PV cells [14] Defects of PV cell degrade performance MPPT faults Charging issue [29] Charging problem in controller A.C side

Invertor faults
Problems in inverter [30] Problems in any component of invertor leads to failure of inverter

System Model
In this section, a PV array model which is under study is presented for evaluation of the performance of a PV system under various fault scenarios. The system model is described briefly in the flow chart of the proposed methodology which mainly contains modeling of PV arrays and interconnection schemes of PV arrays under various fault scenarios. Mathematical modeling is also presented before a description of the proposed procedure for understanding its non-linear characteristic equations and the impact of faults on parameters of the PV model.

Mathematical Modeling
The equivalent circuit of a PV cell [31] is shown in Figure 1 in which the PV cell is represented as a current source in parallel to a diode. The photocurrent is represented by Iph and shunt resistance is represented by Rsh. Both five parameters and seven parameters of single and double diode respectively can be used for analysis of electrical behavior of PV cells. The five parameter model [32] is considered in this research due to its higher accuracy for fault analysis and faster convergence of numerical methods than the seven parameter model.

Iph
Io Rsh Rs Figure 1. The equivalent circuit of PV cell.
The shunt resistance 'Rsh' denotes the flow of leakage current whereas 'Rs' shows series resistance.The photocurrent of the PV module Iph or IL is a light-generated current and calculated using Equation (1).
Where sc I denotes short-circuit current in Amperes, i k is the coefficient of sc I , T is the working temperature, r T is the reference temperature i.e., 25°C, and G is the solar irradiance of 1000W/m 2 .
Reverse saturation current is measured using Equation (2).
Where, the 'q' charge of the electron is 1.
The output current and voltage is represented by 'I' and 'V' respectively as found in Equation (4).
The thermal voltage of the diode is represented as Vt, and shunt current Ish is found by Equation Voc and Isc are open-circuit voltage and short-circuit current respectively and can be calculated mathematically [8] as follows in Equation (6) and Equation (7) [33].
Whereas, NS and NP are the number of series and parallel connected PV cells, respectively, ISTC is the short-circuit current at STC with irradiation =1000 W/m 2 and TSTC = 25 0 C, whereas VSTC is the opencircuit voltage of the module at STC. GR is received irradiance in W/m 2 . kV is the temperature coefficient of open-circuit voltage.

Current and Voltage as Indicator of Fault
Any fault occurrence in the PV array affects the value of output voltage and output current due to their dependency on irradiance and temperature coefficients. Imp and Vmp are the maximum current and maximum voltage, respectively, which can be calculated mathematically in Eq. (8) and Eq. (9) as follows: The proposed procedure verifies the variation in physical quantities Imp and Vmp after fault occurrence through P-V curve analysis.

Algorithm for P-V Curve Analysis
Many interconnected PV cells are packaged in the same PV modules and have the same irradiance value. Therefore, modeling and simulation of PV modules is a key step for the analysis of the P-V curve under normal and fault conditions. PV modules take values of received irradiance and temperature as input parameters to find a solution for current 'I' for sending this value to the controlled current source. The P-V curve is plotted after sending measured values of dc voltage and dc current through output parameters of PV modules including Isc, Voc,Vmp, and Imp as given in the algorithm for the P-V curve in Figure 2.

Flow Chart of Proposed Methodology
The flow chart of the proposed methodology is given in Figure 3. A Simulink model of a 6x6 PV array under electrical faults including module disconnection, short circuit, and module mismatch faults is developed to study the performance of faulted PV array. A special case of multiple faults is also analyzed to study the impact of short-circuit and open-circuit faults under low irradiance conditions on peak power. All faults are thoroughly analyzed on crystalline and thin-film PV technology in all adopted interconnections. The impact of the proposed procedure on performance of a power grid is also analyzed for performance optimization of the PV system.

Modeling of PV Array
A 6 × 6 PV array is modeled in MATLAB with a single PV module of 150 Watts. The specifications of the considered PV module are given in Table 3.

Interconnection Schemes of PV Array
There are three configurations which are designed for the analysis of fault impact on PV array including SP in which PV modules are interconnected in series and parallel, HC in which PV modules are interconnected with more interconnections than series-parallel interconnections in H patterns, and TCT interconnection in which all modules are closely tied together with more interconnections than SP and HC interconnections, as shown in Figure 4.

Introduced Faults in PV Array
The following developed cases of faults are analyzed in adopted configurations of PV arrays as shown in Figure 5. All fault scenarios are analyzed in monocrystalline and thin-film types of PV material. The bypass diodes are connected with PV modules for protection from shading to shunt the current around them under shading condition. These diodes act as reverse bias in the normal operation of PV modules. The blocking diodes are connected with the PV modules to prevent the current flowing back into them. Short-circuit fault in the PV array (F2): An unintentional short circuit in a PV string can be referred to as line-to-line fault. A short-circuit fault is developed between PV modules in the second PV string in this fault scenario.
Non-uniform shading fault (F3): Non-uniform insulation develops a PV module mismatch fault. In this study, a complicated partial shading pattern is observed in this study for mismatch fault as shown in Table 4. 'n' represents PV modules connected in series, and 'm' represents parallelconnected strings of the PV module. Different values of irradiance are received by each interconnected PV module. For example:n=1, m=1 i.e., the first PV module in the first PV string received 1000 W/m 2 irradiance.  n=1  1000  500  1000  1000  500  500  n=2  1000  500  1000  1000  500  500  n=3  1000  500  1000  1000  500  500  n=4  400  700  400  400  700  700  n=5  400  700  400  400  700  700  n=6  400  700  400  400  700  700 Multiple faults under non-uniform irradiance (F4): A special case of multiple faults including short-circuit and open-circuit fault in PV array under non-uniform shading is introduced in this scenario for the analysis of the combined impact of faults on interconnections. The same non-uniform shading pattern is analyzed as observed in F3 in this case. This is a special case of faults and this situation can be developed in real life when faults occur during the day to night transitions.
Simulation results of all developed faults are presented in the next section.

Simulations
In this section, simulation results of the proposed methodology are presented. The obtained results are compared in terms of the P-V curve, and the impact of all fault scenarios on the power grid is also investigated for a detailed comparative analysis of faults.

Comparative Analysis of P-V curve
P-V curve is analyzed for comparative analysis of faults in all adopted PV configurations with respect to power generation. Firstly, fault-free operation is analyzed for the comparison of faults' impact on output parameters of the PV model. The parameters of the PV model in fault-free operation are given in Table 5.   Open-circuit fault (F1) reduces peak power from 5 kW to approximately 4.39 kW and 4.44 kW in the SP and HC interconnection of monocrystalline PV array, respectively, as shown in Table 6. The interconnection scheme of TCT optimizes peak power from 4.39 kW in simple SP interconnection to 4.9 kW. Thin-film PV technology performs better than monocrystalline through the optimization of MPP in all adopted interconnections. The increase in MPP from 4.8 kW to 4.94 kW in TCT, 4.5 kW to Power (Watts) 4.66 kW in HC, and 4.39 kW to 4.53 kW in SP is achieved with minimization of multiple MPP's in the P-V curve as shown in Figure 7 and Figure 8.  Figure 7 and

Power (Watts)
The parameters for short circuit fault (F2) are shown in Table 7 for the comparative analysis of short circuit fault (F2) in monocrystalline and thin-film PV, respectively. Peak power increases from 3.06kW to 3.36kW in SP, 3.01kW to 3.29kW in HC, and 2.6kW to 2.82kW in TCT interconnection.  Figure 9 and Figure 10 show a comparative analysis of short-circuit fault (F2) in monocrystalline and thin-film PV respectively. The MPP gets reduced due to the occurrence of shortcircuit fault as shown in Figure 9 and Figure 10. Short circuit fault reduces power from 5 kW to approximately 3.06 kW and 3 kW in SP and HC interconnections. TCT interconnection reduces MPP as compared to SP and decreases peak power from 3.15 kW to 2.6 kW. The adoption of TCT and HC interconnection fails to increase power generation during short-circuit fault. Thin-film PV technology optimizes the performance of all adopted interconnections.
Severe non-uniform shading fault (F3) reduces peak power from 5 kW to approximately 2.59 kW and 2.63 kW in SP and HC configurations, respectively, as shown in Table 8. The reduction in peak power and appearance of multiple MPPs in the P-V curve is due to the sudden decrease in current values. TCT configuration minimizes multiple power peaks and increases power from 2.57kW in SP to 2.98kW. TCT configuration optimizes MPP with the minimization of multiple peaks as shown in Figure 11 and  Thin-film PV performs much better than monocrystalline PV in severe shading conditions and increases peak power from 2.57 kW to 2.77 kW in SP, 2.63 kW to 2.9 kW in HC, and 2.98 kW to 3.19 kW in TCT interconnection. The utilization of thin-film technology optimizes the P-V curve and multiple MPP's almost diminish in TCT configuration Multiple faults during severe non-uniform shading fault (F4) reduce peak power from 5 kW to approximately 1.94 kW and 2.1 kW in SP and HC configuration, respectively, as shown in Table 9. The reduction in peak power and appearance of multiple MPPs in the P-V curve is due to a sudden decrease in current values. HC configuration performs better than other interconnections under multiple faults including short circuit and open circuit with severe shading and minimizes multiple power peaks as shown in Figure 13 and  The increase in peak power from 1.89 kW in TCT to 2.1 kW in HC is observed with monocrystalline PV. Thin-film PV performs better than monocrystalline PV in the multiple fault scenario and increases peak power from 1.89 kW to 2.0 kW in TCT, 2.1 kW to 2.5 kW in HC, and 1.94 kW to 2.31 kW in SP interconnection. The utilization of thin-film technology optimizes the performance of the PV system in all fault scenarios.
Thin-film PV material performs better in all fault scenarios than monocrystalline PV material according to P-V curve analysis with respect to maximum power generation. The impact of thin film and monocrystalline PV in adopted interconnections on the performance of the power grid is also analyzed for a more detailed analysis of all fault scenarios.

Impact of Faults on Power Grid
The 6 × 6 PV array is integrated with a 25 kW grid via a DC-DC boost converter and inverter. The block diagram is given in Figure 15. Vpv and Ipv from the PV array are sent to the MPPT controller for tracking of MPP through controlling of the duty cycle. MPPT algorithm [19] tracks MPP by adjusting the operating voltage to track MPP. The voltage changes in the same direction if power increases, and voltage variation reverses its direction if power decreases. The algorithm of perturb and observe (P&O) [18] is easy to implement but the performance of this algorithm is not efficient enough to accurately track MPP during developed faults. The DC-DC boost converter is an important part of the PV system which boosts the voltage level after receiving control pulses 'D' through the MPPT algorithm. It consists of a capacitor, resistor, inductor ,and high-speed switching device 'IGBT' with a diode. The circuit [34] is shown in Figure 16. A 6x6 PV array is integrated with a 25 kW power grid for an analysis of the faults' impact on the power grid through observing the sequence of events on scope. Firstly, fault-free operation of the PV system is analyzed as shown in Figure 17. The duty cycle of the boost converter is fixed from the start to 3000 ms, and control pulses 'D' start changing after enabling MPPT. The MPPT is not enabled from the start which is evident from the disturbance, as shown in Figure 17  This MPPT algorithm improves power generation but it is not efficient enough to track global MPP in a dynamic changing environment and tracks reduced power in all fault scenarios. Overcurrent protection devices (OCPD) are usually installed in PV systems to clear faults, but developed faults in the PV array cannot be cleared by OCPD due to operation of the MPPT algorithm which results in a small current. The decrease in grid current due to multiple faults in PV array under severe shading is shown in Figure 18. All faults developed in PV array and non-uniform shading decrease the current in the presence of MPPT and make the fault's impact less severe. The impact of all faults is also investigated in all PV interconnection schemes integrated with a grid for the analysis of impact on the power grid. Faults are introduced from the start in all simulations. The grid power shows disturbance from the start but after MPPT is enabled at approximately 3000 ms and starts tracking reduced MPP less than 5. respectively, after enabling MPPT at 3000 ms and continues to track the reduced MPP till the end as shown in Figure 19. TCT interconnection minimizes this power loss and increases tracked MPP from 4.09 kW to 4.5 kW. Thin-film PV further minimizes power loss by increasing tracked MPP from 4.5 kW to 4.8 kW in TCT interconnection as shown in Figure 20. The interconnection of SP and HC in thin-film type PV array increases tracked MPP to 4.14 kW and 4.23 kW, respectively, and increases power generation in comparison to monocrystalline PV.  Short circuit fault (F2) reduces tracked MPP from 5.2 kW to 2.27 kW and 2.51 kW in TCT and HC interconnection, respectively, after 3000 ms and continues to track the reduced MPP till the end. SP interconnection performs better than other interconnections and produces tracking power of a 2.62 kW in monocrystalline PV array, as shown in Figure 21. Thin-film PV optimizes the PV performance through increasing tracked MPP from 2.62 kW to 3.01 kW in SP, 2.51 kW to 2.8 kW in HC, and 2.275 kW to 2.75 kW in TCT as shown in Figure 22. TCT interconnection cannot optimize performance in this fault case. Only power loss compensation through TCT interconnection can be achieved under those faults, which results in a sudden decrease of current due to more internal connections in TCT and more current flow in TCT than other interconnections as observed in opencircuit fault (F1) and shading condtions (F3).  Severe shading in mismatch fault 'F3′ reduces tracked MPP from 5.2 kW to 2.8 kW, 2.19 kW and 2.1 kW in TCT, HC and SP, respectively, in monocrystalline PV as shown in Figure 23. TCT interconnection minimizes power loss in a grid-integrated PV system. Thin-film optimizes PV performance in shading fault and increases power from 2.8 kW to 3.2 kW, 2.19 kW to 2.55 kW, and 2.2 kW to 2.51 kW in TCT, HC and SP respectively as shown in Figure 24.  Multiple faults under severe non-uniform shading (F4) reduce tracked MPP from 5.2 kW to 1.4 kW,1.8 kW and 1.62 kW in TCT, HC and SP, respectively, in monocrystalline PV as shown in Figure  25. HC interconnection optimizes the performance of the power grid by minimizing the losses of SP and TCT interconnection. Thin-film PV material increases tracked peak power from 1.4 kW to 1.71 kW, 1.8 kW to 2.01 kW and 1.6 kW to 1.7 4kW in TCT, HC, and SP, respectively, as shown in Figure  26. HC performance is better due to the combined impact of all faults in this case.  Thin-film PV technology performs better than monocrystalline in all developed faults and optimizes the performance of the PV system through an increase in peak power as shown in Table  10.

Discussion
In this section, the obtained results are compared and discussed in detail. An increase in power generation during all developed faults is calculated by comparing the MPP of designed

Type of interconnection
interconnection, which produced reduced peak power, with the considered interconnection as shown in Eq. (10), and a detailed comparison of results is given in Table 11.  Table 11. Comparative analysis of results as represented in Table 10.

Topolog y
Increase in max P (Monocrystalline PV) Increase in max P (Thin-film (a-Si) PV) comparison of increased power generated is shown in Table 11 for both monocrystalline and thinfilm PV material. Thin-film PV material performs better in all developed fault scenarios and shows more efficient performance than monocrystalline as shown in  Thin-film PV material maximizes power generation and shows higher efficiency than monocrystalline according to P-V curve analysis. Thin-film PV material also improves the performance of the power grid and achieves higher efficiency according to simulation results. Only the power generation of a defective cell is affected after the occurrence of a fault in thin-film PV material, but the functioning of other connected cells is not affected by the occurrence of a fault. The functioning of the whole PV module is affected through the development of a fault in one PV cell of monocrystalline. Thus, thin-film PV has higher power generation during fault conditions.

Distinctive Features of Proposed Methodology
(1). This paper presents a detailed investigation of faults including non-uniform shading, open circuit and short circuit in different PV interconnections. A special case of multiple faults in PV array under non-uniform irradiance is also investigated to analyze their combined impact on utilized PV interconnections.
(2). The impact of all faults on the power grid is also investigated for a detailed analysis of faults on a PV system.  analysis which has not been done before. The utilization of thin-film PV with an HC interconnection optimizes the system performance by 11% and 25% power increases in P-V curve analysis and by 28% and 17% power increases in a grid-connected PV system during multiple faults under severe shading (F4), respectively. (6). The unified approach of utilizing both-thin film PV and monocrystalline PV technology in three different arrangements of PV including TCT, HC and SP for detailed comparative analysis of their impact on a power grid is a major contribution of this research.

Conclusions
In this research, a detailed comparative analysis of faults in different PV interconnection schemes including SP, HC, and TCT in terms of a P-V characteristic curve is presented. It is investigated that TCT can enhance power generation in open circuit and severe shading fault due to more internal connections in comparison to SP and HC where a sudden decrease in current develops multiple MPPs. HC interconnection performs better than other interconnections during the case of multiple faults under severe shading due to the combined impact of all faults. In addition, a comparison of monocrystalline and thin-film PV material with respect to power generation is also presented. The performance of thin-film technology has only been investigated under shading in the literature. The performance of thin-film PV under all developed electrical faults is investigated in this research. It is found that thin-film PV material achieves higher peak power than monocrystalline in all developed faults. Thus, the thin-film PV technology with suitable interconnection topology optimizes power generation during developed faults and also minimizes the severe impact on the power grid during all fault scenarios through an increase in tracked MPP.
Simulation results show the possibility of enhancing the power generation of PV arrays under severe faults though proposed methodology, and the investigation of various fault scenarios in different PV interconnections and materials can compensate power loss and improve the efficiency of the PV system. It would be useful to further investigate the impact of many different faults on different interconnection schemes and different PV technologies, and its practical implementation can also be considered as future work. The development of techniques and procedures is also needed to further improve the efficiency of thin-film PV technology for the safe and efficient operation of a PV system under developed faults.