Doctor of Philosophy

dissertationThree major catastrophic failures in photovoltaic (PV) arrays are ground-faults, line-to-line faults, and arc faults. Although the number of such failures is few, recent fire events on April 5, 2009, in Bakersfield, California, and April 16, 2011, in Mount Holly, North Carolina 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. A fault prediction and detection technique for PV arrays based on spread spectrum time domain reflectometry (SSTDR) has been proposed and was successfully implemented. Unlike other conventional techniques, SSTDR does not depend on the amplitude of the fault-current. Therefore, SSTDR can be used in the absence of solar irradiation as well. However, wide variation in impedance throughout different materials and interconnections makes fault locating more challenging than prediction/detection of faults. Another application of SSTDR in PV systems is the measurement of characteristic impedance of power components for condition monitoring purposes. Any characteristic variations in one component will simultaneously alter the operating conditions of other components in a closed-loop system, resulting in a shift in overall reliability profile. This interdependence makes the reliability of a converter a complex function of time and operating conditions. Details of this failure mode, mechanism, and effect analysis (FMMEA) have been developed. By knowing the present state of health and the remaining useful life (RUL) of a power converter, it is possible to reduce the maintenance cost for expensive high-power converters by facilitating a reliability centered maintenance (RCM) scheme. This research is a step forward toward power converter reliability analysis since the cumulative effect of multiple degraded components has been considered here for the first time in order to estimate reliability of a power converter

PV faults: Overview, modeling, prevention and detection techniques

pre-printRecent 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

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

pre-printA 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

Output impedance modeling of a multilevel modular switched-capacitor converter to achieve continuously variable conversion ratio

pre-printThe multilevel modular capacitor clamped dc-to-dc converter (MMCCC) topology is completely modular and belongs to two-phase switched capacitor converter group. The conversion ratio of an ideal MMCCC converter in step-up mode is an integer and depends on the number of modules used. For a k-module MMCCC, the maximum up-conversion ratio is (k+1), and it has already been shown in literature that different integer conversion ratios can be achieved by changing the active number of modules of an MMCCC. In this paper, different methods are proposed for MMCCC in order to achieve fractional conversion ratios (CR) in step-up mode without changing the complementary two-phase switching orientation. Fractional CRs can be obtained in several switched-capacitor circuits at the cost of significantly lower efficiency. However, MMCCC with the aid of a new pulse dropping technique can produce fractional CR while maintaining high efficiency. The variation in efficiency and equivalent resistance as a function of frequency has been analyzed in this paper. Simulation and experimental results using a reconfigurable 5-module MMCCC prototype have been used to validate the new control scheme

Transfer function mapping for a grid connected PV system using reverse synthesis technique

pre-printMathematical modeling of power electronic converters is a historical problem. Numerous efforts have been documented in literature to model different converter topologies and their control schemes since the 1940s. Traditional modeling approaches avoid transfer function derivation due to high degree of nonlinearity involved with the power converter's switched operation. In this paper, a simple transfer functions for a grid connected PV system is derived. First an ideal transfer function is derived from the steady state input and output relationship of ideal converters. Then, it is compared with the non-ideal model of a practical converter. Using this technique, it is possible to find generalized modules to represent a complex power converter, and it is possible to deduce new converter topologies using the reverse mapping

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

pre-printThe 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

Interconnection and optimization issues of multijunction solar cells - A new mitigation approach using switching power converters

Journal ArticleA multijunction solar cell can extract higher solar energy compared to a single junction solar cell using the spectrum splitting technique. Extensive research on efficiency enhancement of the solar cells to achieve near theoretical limit is in place. However, there are limited research activities to identify the optimum interconnection methods and necessary power electronics solutions for multijunction solar cell systems. A detailed study to identify the optimum interconnection method for various multijunction solar cells is revealed in this paper. The authors believe that the conducted research in this area is very limited and an effective power electronics solution could substantially improve the efficiency and utilization of a 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. Both Simulation and experimental results have been attached to show the practicality and the potential benefit of the proposed concept

Efficiency characterization and impedance modeling of a multilevel switched-capacitor converter using pulse dropping switching scheme

pre-printApulse 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

Determination of Carbamate and Organophosphorus Pesticides in Vegetable Samples and the Efficiency of Gamma-Radiation in Their Removal

In the present study, the residual pesticide levels were determined in eggplants (Solanummelongena)