Katherine A. Kim

Differential Power Processing for Increased Energy Production and Reliability of Photovoltaic Systems

Conventional energy conversion architectures in photovoltaic (PV) systems are often forced to tradeoff conversion efficiency and power production. This paper introduces an energy conversion approach that enables each PV element to operate at its maximum power point (MPP) while processing only a small fraction of the total power produced. This is accomplished by providing only the mismatch in the MPP current of a set of series-connected PV elements. Differential power processing increases overall conversion efficiency and overcomes the challenges associated with unmatched MPPs (due to partial shading, damage, manufacturing tolerances, etc.). Several differential power processing architectures are analyzed and compared with Monte Carlo simulations. Local control of the differential converters enables distributed protection and monitoring. Reliability analysis shows significantly increased overall system reliability. Simulation and experimental results are included to demonstrate the benefits of this approach at both the panel and subpanel level.

Photovoltaic Hot-Spot Detection for Solar Panel Substrings Using AC Parameter Characterization

Hot spotting is a problem in photovoltaic (PV) systems that reduces panel power performance and accelerates cell degradation. In present day systems, bypass diodes are used to mitigate hot spotting, but it does not prevent hot spotting or the damage it causes. This paper presents an active hot-spot detection method to detect hot spotting within a series of PV cells, using ac parameter characterization. A PV cell is comprised of series and parallel resistances and parallel capacitance, which are affected by voltage bias, illumination, and temperature. Experimental results have shown that when a PV string is under a maximum power point tracking control, hot spotting in a single cell results in a capacitance increase and dc impedance increase. The capacitance change is detectable by measuring the ac impedance magnitude in the 10-70 kHz frequency range. An impedance value change due to hot spotting can be detected by monitoring one high-frequency measurement in the capacitive region and one low-frequency measurement in the dc impedance region. Alternatively, the dc impedance can also be calculated using dc operating point measurements. The proposed hot-spot detection method can be integrated into a dc-dc power converter that operates at the panel or subpanel level.

A Dynamic Photovoltaic Model Incorporating Capacitive and Reverse-Bias Characteristics

Photovoltaics (PVs) are typically modeled only for their forward-biased dc characteristics, as in the commonly used single-diode model. While this approach accurately models the I-V curve under steady forward bias, it lacks dynamic and reverse-bias characteristics. The dynamic characteristics, primarily parallel capacitance and series inductance, affect operation when a PV cell or string interacts with switching converters or experiences sudden transients. Reverse-bias characteristics are often ignored because PV devices are not intended to operate in the reverse-biased region. However, when partial shading occurs on a string of PVs, the shaded cell can become reverse biased and develop into a hot spot that permanently degrades the cell. To fully examine PV behavior under hot spots and various other faults, reverse-bias characteristics must also be modeled. This study develops a comprehensive mathematical PV model based on circuit components that accounts for forward bias, reverse bias, and dynamic characteristics. Using a series of three experimental tests on an unilluminated PV cell, all required model parameters are determined. The model is implemented in MATLAB Simulink and accurately models the measured data.

Converter Rating Analysis for Photovoltaic Differential Power Processing Systems

When photovoltaic (PV) cells are connected in series, they experience internal and external mismatch that reduces output power. Differential power processing (DPP) architectures achieve high system efficiency by processing a fraction of the total power while maintaining distributed local maximum power point operation. This paper details the computational methods and analysis used to determine the operation of PV-to-bus and PV-to-PV DPP architectures with rating-limited converters. Simulations for both DPP architectures are used to evaluate system performance over 25 years of operation. Based on data from field studies, a PV power coefficient of variation can be estimated as 0.086 after 25 years. An improvement figure of merit reflecting the ratio of energy produced to that delivered in a conventional system is introduced to evaluate comparative performance. Converter ratings of 15-17% for PV-to-bus and 23-33% for PV-to-PV architectures are identified as appropriate ratings for a 15-submodule system (five PV panels in series). Both DPP architectures with these ratings are shown to deliver up to 2.8% more power compared to a conventional series-string architecture based on the expected panel variation over 25 years of operation. DPP converters also outperform dc optimizers in terms of lifetime performance.

Fault impacts on solar power unit reliability

This paper introduces a generalized reliability model of a solar power unit (SPU) based on physical characteristics including material, operating conditions, and electrical ratings. An SPU includes a photovoltaic panel, power converter, control and sensing. Possible faults in each component of the unit are surveyed and their failure rates based on physics-of-failure models are formulated. PV panel faults include possible installation faults, environmental effects, and material degradation. Power electronics faults are developed in depth for different components of a dc-dc boost converter. A systemlevel simulation model is developed and verified experimentally, and then used to define the survivor function of the SPU. Results show that it is important to include panel faults for accurate reliability values. The developed model is flexible and can be tailored for various SPU operating conditions, panel designs, and electrical ratings. The proposed reliability model can be extended to parallel and series interconnected topologies of multiple SPUs.

Reexamination of Photovoltaic Hot Spotting to Show Inadequacy of the Bypass Diode

Hot spotting is a reliability problem in photovoltaic (PV) panels where a mismatched cell heats up significantly and degrades panel performance. High temperatures due to hot spotting can damage cell encapsulant and lead to second breakdown; both cause permanent damage to the PV panel. Although bypass diodes are used for protection and qualification tests are used to reduce cell mismatch, these strategies are shown to be insufficient for hot spot prevention. This paper reexamines the hot spot problem in PV strings through simulation and load-line analysis. Results show that cells in typical panel string lengths are susceptible to hot spotting because of reverse bias behavior. A number of existing and emerging solutions aimed at hot spot prevention are discussed and evaluated. Commercially available active bypass switches are an improvement over passive diodes but do not prevent hot spotting. Cells with low breakdown voltages limit power dissipation but are not fully vetted as a long-term solution. A combination of hot spot detection and open-circuit protection is a complete solution to hot spotting.

Photovoltaic hot spot analysis for cells with various reverse-bias characteristics through electrical and thermal simulation

Hot spots result from localized heating in a string of photovoltaic (PV) cells due to mismatch that is often caused by partial shading or uneven degradation. Over time, this localized heating can result in permanent damage and degrade string performance. Bypass diodes are commonly employed in PV panels to mitigate this problem, but it does not eradicate the problem-hot spots can still form. This study investigates how the reverse-biased I-V characteristics and number of cells in series affect the potential for hot spotting. Using an electrical and thermal model in Matlab Simulink and Simscape, three distinct PV cells are modeled and simulated in various string lengths. Simulation results confirm that shorter strings reduce hot spot risk, but none of the cell types were immune to hot spotting, particularly in bypass. Bypassing even a short string can lead to hot spots and the temperature rise worsens as string length increases.

Photovoltaic differential power converter trade-offs as a consequence of panel variation

Photovoltaic (PV) elements have inherent variation between cells and panels due to manufacturing tolerance, degradation, and situational differences. This variation increases over system lifetime and creates maximum power point current mismatch that reduces output power when PV elements are strung in series. Traditionally, mismatch loss is addressed using cascaded converters. However, this research examines a differential converter architecture that achieves higher efficiency by processing a fraction of the total power. The effect of PV maximum power point (MPP) current variance on output power is modeled and examined using Monte Carlo simulation for the series string architecture with and without bypass diodes, and the PV-to-Bus and PV-to-PV differential power processing (DPP) architectures at various power ratings. Hot spotting can be a problem that significantly reduces output power. PV elements at fault can be bypassed, passively or actively, to reduce power loss. Simulation results show that both DPP architectures employing active bypass are able to compensate mismatch over the 25-year lifetime of a PV system with converters sized at approximately 10-20% of the panel ratings.

Hot spotting and second breakdown effects on reverse I-V characteristics for mono-crystalline Si Photovoltaics

Hot spots are a common problem in photovoltaic (PV) panels that accelerate cell degradation and reduce system performance. Hot spots occur when a cell is reversed biased, sinks power, and heats the cell. At a certain threshold, the PV p-n junction goes into second breakdown and heats a small portion of the cell to very high temperatures. This study experimentally tests mono-crystalline Si cells as they hot spot at different power levels. Heating effects on the I-V characteristics during hot spotting and permanent changes after seven days of hour-long hot spot tests are observed and analyzed. I-V characteristics are significantly affected under second breakdown, which is observed when cells are reverse-biased above two times the rated maximum power level.

Laboratory emulation of a photovoltaic module for controllable insolation and realistic dynamic performance

In this paper, a high fidelity, easy-to-implement photovoltaic (PV) module emulator is presented. The proposed emulator can replicate the electrical behavior of a sunlight illuminated PV module in an indoor environment. The construction of this emulator requires only a PV module and basic laboratory equipment, while still providing dynamic performance that closely matches that of an illuminated PV module in an outdoor environment. The output I-V characteristics of the PV module under real sunlight and that of the proposed emulator were experimentally obtained and compared, such that the functionality of the proposed emulator was verified. An in-depth analysis of the PV module output small-signal impedance is also presented to illustrate the dynamic performance of this emulator.