Mohammed Khorshed Alam

A Comprehensive Review of Catastrophic Faults in PV Arrays: Types, Detection, and Mitigation Techniques

Three major catastrophic failures in photovoltaic (PV) arrays are ground faults, line-to-line faults, and arc faults. Although there have not been many such failures, recent fire events on April 5, 2009, in Bakersfield, CA, USA, and on April 16, 2011, in Mount Holly, NC, USA, suggest the need for improvements in present fault detection and mitigation techniques, as well as amendments to existing codes and standards to avoid such accidents. This review investigates the effect of faults on the operation of PV arrays and identifies limitations to existing detection and mitigation methods. A survey of state-of-the-art fault detection and mitigation technologies and commercially available products is also presented.

Efficiency Characterization and Impedance Modeling of a Multilevel Switched-Capacitor Converter Using Pulse Dropping Switching Scheme

A pulse dropping switching technique (PDT) has been presented in this paper to accomplish variable conversion ratio (CR) in a multilevel modular capacitor-clamped dc-dc converter in the step-up conversion mode. The switching pattern is generated by comparing a triangular wave with a rectangular wave, and a proper output voltage regulation can be obtained by controlling the relative frequency and amplitude of these two waveforms. A state-space modeling technique has been applied here to estimate the variation in equivalent output resistance (EOR) for different operating conditions of the PDT. The EOR can be varied in a wide range without changing the operating frequency of the converter, and thereby the PDT enhances the degrees of freedom to accomplish voltage regulation in a two-phase switched-capacitor converter. Slow-switching limit of the converter has been derived to define the boundary of the EOR. Different challenges and limitations of the proposed modulation scheme has been discussed in detail, and the proposed analysis has been verified by comparing the analytical expressions with the simulation and experimental results for different switching frequencies, modulation indices, and number of active modules. In addition, variations in the CR, efficiency and ripple voltage for different number of active modules and switching conditions have been described in detail.

Reliability Analysis and Performance Degradation of a Boost Converter

In general, power converters are operated in closed-loop systems, and any characteristic variations in one component will simultaneously alter the operating point of other components, resulting in a shift in overall reliability profile. This interdependence makes the reliability of a converter a complex function of time and operating conditions; therefore, the application may demand periodic replacement of converters to avoid downtime and maintenance cost. By knowing the present state of health and the remaining life of a power converter, it is possible to reduce the maintenance cost for expensive high-power converters. This paper presents a reliability analysis for a boost converter, although this method could be used to any power converter being operated using closed-loop controls. Through the conducted study, it is revealed that the reliability of a boost converter having control loops degrades with time, and this paper presents a method to calculate time-varying reliability of a boost converter as a function of characteristic variations in different components in the circuit. In addition, the effects of operating and ambient conditions have been included in the reliability model as well. It was found that any increase in the ON-state resistance of the MOSFET or equivalent series resistance of the output capacitor decreases the overall reliability of the converter. However, any variation in the capacitance has a more complex impact on the converters reliability. This paper is a step forward to the power-converter reliability analysis because the cumulative effect of multiple degraded components has been considered in the reliability model.

PV ground-fault detection using spread spectrum time domain reflectometry (SSTDR)

A PV ground-fault detection technique using spread spectrum time domain reflectometry (SSTDR) method has been introduced in this paper. SSTDR is a reflectometry method that has been commercially used for detecting aircraft wire faults. Unlike other fault detection schemes for a PV system, ground fault detection using SSTDR does not depend on the amplitude of fault-current and highly immune to noise signals. Therefore, SSTDR can be used in the absence of the solar irradiation as well. The proposed PV ground fault detection technique has been tested in a real-world PV system and it has been observed that PV ground fault can be detected confidently by comparing autocorrelation values generated using SSTDR. The difference in the autocorrelation peaks before and after a ground-fault in the PV system are significantly higher than the threshold set for ground-fault detection.

PV faults: Overview, modeling, prevention and detection techniques

Recent PV faults and subsequent fire-hazards on April 5, 2009, in Bakersfield, California, and April 16, 2011, in Mount Holly, North Carolina provide evidence of a lack of knowledge among PV system manufacturers and installers about different PV faults. The conducted survey within the scope of this paper describes various faults in a PV plant, and explains the limitations of existing detection and suppression techniques. Different fault detection techniques proposed in literatures have been discussed and it was concluded that there is no universal fault detection technique that can detect and classify all faults in a PV system. Moreover, this digest proposes a transmission line model for PV panels that can be useful for interpreting faults in PV using different refelectomery methods.

Optimization of Subcell Interconnection for Multijunction Solar Cells Using Switching Power Converters

A multijunction solar cell can extract higher solar energy compared to a single junction cell by splitting the solar spectrum. Although extensive research on solar cell efficiency enhancement is in place, limited research materials are available to identify the optimum interconnection of multijunction solar subcells using power electronic circuits. Multijunction solar cells could be grouped into two main categories: vertical multijunction (VMJ) solar cells and lateral multijunction (LMJ) solar cells. In this paper, a detailed study to identify the optimum interconnection method for various multijunction solar cells has been conducted. The authors believe that the conducted research in this area is very limited, and an effective power electronic circuit could substantially improve the efficiency and utilization of a photovoltaic (PV) power system constructed from multijunction solar cells. A multiple input dc-to-dc boost converter has been used to demonstrate the advantage of the proposed interconnection technique. In order to ensure maximum power point (MPP) operation, a particle swarm optimization (PSO) algorithm has been applied needing only one MPP control for multiple solar modules resulting in cost and complexity reduction. The PSO algorithm has the potential to track the global maxima of the system even under complex illumination situations. A complete functional system with the implementation of the proposed algorithm has been presented in this paper with relevant experimental results.

Quantifying Device Degradation in Live Power Converters Using SSTDR Assisted Impedance Matrix

A noninterfering measurement technique designed around spread spectrum time domain reflectometry (SSTDR) has been proposed in this paper to identify the level of aging associated with power semiconductor switches inside a live converter circuit. Power MOSFETs are one of the most age-sensitive components in power converter circuits, and this paper demonstrates how SSTDR can be used to determine the characteristic degradation of the switching MOSFETs used in various power converters. An SSTDR technique was applied to determine the aging in power MOSFETs, while they remained energized in live circuits. In addition, SSTDR was applied to various test nodes of an H-bridge ac-ac converter, and multiple impedance matrices were created based on the measured reflections. An error minimization technique has been developed to locate and determine the origin and amount of aging in this circuit, and this technique provides key information about the level of aging associated to the components of interest. By conducting component level failure analysis, the overall reliability of an H-bridge ac-ac converter has been derived and incorporated in this paper.

An efficient power electronics solution for lateral multi-junction solar cell systems

Compared to a single junction solar cell, a multi-junction (MJ) solar cell can extract higher energy from sun by splitting the solar spectrum. Depending on the spectrum splitting techniques, two different structures of MJ solar cells are possible: vertical multijunction solar cell (VMJ) and lateral multijunction solar cell (LMJ). Both of these structures have their own advantages and limitations. LMJ solar cell has the potential to emerge as an effective solution for solar energy conversion although the availability of research materials on LMJ is limited compared to that of VMJ. In this paper, a complete photovoltaic (PV) power system constructed from lateral multijunction solar cells is proposed along with a new interconnection technique. The I/V characteristics of the solar cells have been matched in the proposed interconnection using a multi-input dc-dc converter. In order to ensure maximum power point (MPP) operation, particle swarm optimization algorithm is applied that requires only one maximum power point control for four solar modules resulting in cost and complexity reduction. Particle swarm optimization algorithm has the potential to track the global maxima of the system even under complex illumination situations. A complete functional system with the implementation of the proposed algorithm has been presented in this paper with relevant experimental results.

A high-efficiency modular switched-capacitor converter with continuously variable conversion ratio

The multilevel modular capacitor clamped converter (MMCCC) topology overcomes the difficulties of the multilevel switched capacitor (SC) based dc-to-dc converters in high conversion ratio applications. MMCCC is completely modular and has many other advantageous features. Like most other SC converters, MMCCC suffers from limited voltage regulation. The conversion ratio of an ideal MMCCC converter in step-up mode is an integer, and this integer conversion ratio depends on the number of active modules. The maximum conversion ratio in step-up configuration for a k-module MMCCC is (k+1). It has already been shown in literature that different integer CRs can be achieved by changing the number of active modules of an MMCCC. Achieving voltage regulation by lowering the operating frequency is another well known technique for switched capacitor converters. However, the output voltage ripple increases in inverse proportion of the frequency. In this paper, a new switching scheme is proposed for MMCCC to achieve continuously variable CRs. The proposed switching scheme requires introducing a small inductor in each module of the MMCCC without altering the modular structure of the converter. This additional inductor can be realized using the stray inductance distributed in the circuit or small external inductors. It has been shown that continuous CR variation with lower output ripple can be achieved without lowering the operating frequency of the converter. This proposed method introduces another degree of freedom in order to achieve variable CR using MMCCC. Simulation results and experimental results obtained from an MMCCC prototype have been used to validate the new control scheme.

Reliability analysis and performance degradation of a Boost converter

In general, power converters are being operated in closed-loop systems, and any characteristic variations in one component will simultaneously alter the operating point of other components resulting in a shift in overall reliability profile. This interdependence makes the reliability of a converter a complex function of time and operating conditions; and therefore, the application may demand periodic replacement of converters to avoid downtime and maintenance cost. By knowing the present state of health and remaining life of a power converter, it is possible to reduce the maintenance cost for expensive high-power converters. This paper presents a reliability analysis for a boost converter although this method could be used to any power converter being operated in closed-loops. Through the conducted study it is revealed that the reliability of a boost converter with control loops degrades with time, and this paper presents a method to calculate time varying reliability of a boost converter as function of characteristic variations in different components in the circuit. In addition, the effects of operating and ambient conditions have been included in the reliability model as well. It was found that any increase in the ON-resistance of the MOSFET or equivalent series resistance (ESR) of the output capacitor decreases the overall reliability of the converter. However, any variation in the capacitance has more complex impact on the converters reliability. This conducted research is a step forward to the power converter reliability analysis because the cumulative effect of multiple degraded components has been considered in the reliability model.