Narendra S. Shiradkar

Evolution of Leakage Current Paths in MC-Si PV Modules From Leading Manufacturers Undergoing High-Voltage Bias Testing

The evolution of leakage currents in photovoltaic modules undergoing outdoor high-voltage bias testing is studied using data from high-voltage bias testing of multicrystalline silicon modules from leading manufacturers. An analysis of the module leakage currents as a function of environmental conditions including temperature, relative humidity, rain, and wetness is carried out. The behavior of the modules was found to be dependent on the module construction and the materials used. The Arrhenius model was used to fit the experimental data and activation energies were computed for various relative humidity values. The effect of dew and rain (wetness) on the front glass was investigated. Changes in the leakage current during dry conditions were studied using the temperature dependence of resistivity of bulk soda-lime glass. Because of the approximately tenfold increase in leakage currents during the wet conditions, it is suggested that the accelerated tests should not be limited exclusively to noncondensing environments but should also be complemented with tests that include wet conditions.

Fire hazard and other safety concerns of photovoltaic systems

Abstract. Photovoltaic (PV) modules are usually considered safe and reliable. But in case of grid-connected PV systems that are becoming popular, the issue of fire safety of PV modules is becoming increasingly important due to the employed high voltages of 600 to 1000 V. The two main factors, i.e., open circuiting of the dc circuit and of the bypass diodes and ground faults that are responsible for the fire in the PV systems, have been discussed in detail along with numerous real life examples. Recommendations are provided for preventing the fire hazards such as designing the PV array mounting system to minimize the chimney effect, having proper bypass and blocking diodes, and interestingly, having an ungrounded PV system.

Device for detailed analysis of leakage current paths in photovoltaic modules under high voltage bias

High voltages used in photovoltaic (PV) systems are known to induce long-term power loss in PV modules due to leakage current flowing through the module packaging materials. It has been difficult to identify the specific materials and interfaces responsible for degradation based on an analysis of only the total leakage current. A detailed investigation of the leakage current paths within the PV modules, under high voltage bias, is carried out by utilizing a device that measures the independent contributions of various paths in real-time. Knowledge about dominant leakage current paths can be used to quantify the physical and chemical changes occurring within the module packaging materials.

The reliability of bypass diodes in PV modules

The operating conditions of bypass diodes in PV modules deployed in the field are considerably harsher than the conditions at which the diode manufacturers test the diodes. This has a potential to significantly reduce the operating life of bypass diodes and has raised concerns about the safety and reliability of PV modules as a whole. The study of modes and mechanisms of the failures encountered in bypass diodes used in PV modules can provide important information which would be useful to predict the module lifetime. This paper presents the review of the failure modes and mechanisms observed in bypass diodes and current work related to reliability testing of bypass diodes. The International PV Module Quality Assurance Task Force has recommended following four potential areas of research to understand the reliability issues of bypass diodes: Electrostatic Discharge, reverse bias thermal runaway testing, forward bias overheating and transition testing of forward bias to reverse bias. As a joint collaborative effort between Florida Solar Energy Center and Solar and Environmental Test Laboratory at Jabil Inc., laboratory testing of bypass diodes on the guidelines provided by the International PV Module Quality Assurance Task Force has been initiated. Preliminary results from this work are presented in this paper.

Finite element analysis based model to study the electric field distribution and leakage current in PV modules under high voltage bias

The maximum system voltage for Photovoltaic systems is 1000 V in US. Some modules are designed to operate even at 1500 V, which is the limit for IEC low voltage systems. The high voltage bias between the cell circuit and frame of the module leads to a leakage current flowing through the insulation of the module to the ground. Over time, this leakage current causes migration of various species to and from the cell circuit, can result in slow degradation of the performance of PV module. It is important to understand the electric field distribution and leakage current pathways in the PV modules in order to study the system voltage induced degradation of PV modules. The leakage current from the PV modules deployed outdoor and under high voltage bias strongly varies with the environmental conditions. The lumped resistance models described in literature that attempt to explain the leakage current flow through the PV module do not provide adequate information about the distribution of leakage current through different layers of insulation present in the PV modules. In this paper, a Finite Element Analysis (FEA) based model for the insulation of PV module is described. It yields useful information about the distribution of electric field, potential and leakage current flowing through different layers of module. The model is also used to predict and analyze the changes in leakage current with changes in module packaging materials and grounding configurations.

High-voltage bias testing of PV modules in the hot and humid climate without inducing irreversible instantaneous degradation

High-voltage bias testing of PV modules is known to be useful for revealing design, material and process flaws in PV modules. It is accepted that the hot and humid climate under high-voltage bias imposes a severely harsh environment on the PV modules and enhances the possibility of degradation. Test location is, therefore, important. A test methodology is being presented here that can provide useful information for testing PV modules at high voltage in natures own laboratory in a relatively short time frame.

Predicting the long term power loss from cell cracks in PV modules

Long term power loss due to cell cracks can become a significant PV wear out mechanism. An experimental framework to assist in predicting the power loss from cell cracks during module service life is presented. The paper is primarily structured around cell crack origin in laminated modules, crack orientation, oriented crack reproduction and climatic testing of custom mini-modules. The results are experimentally verified by a series of accelerated tests involving mechanical load and humidity freeze tests. Periodic characterization using EL and flash I-V are used to study evolution of crack types/categories and performance loss due to cell cracks. Manufacturing modules cell crack data was collected from a population of a week of data post lamination. The data analyzed is for a certain batch of polycrystalline cell modules. Jabil has produced few GW of modules over the past years.

Baseline testing procedures for PV modules beyond the qualification testing

The qualification tests described in IEC 61215 for the c-Si PV modules are essentially pass/fail tests that assist in avoiding infant mortality. This paper reports on the baseline test procedure carried out on PV modules at Florida Solar Energy Center that go beyond the pass/fail criteria of the qualification tests and obtain information about the degradation modes and mechanisms. The importance and limitations of the various characterization techniques are described. Electroluminescence imaging has been used to detect and categorize the faults at the cell level. Indoor infrared imaging has been used to study the quality of electrical interconnects in the module. The infrared imaging carried out on the modules while they are undergoing outdoor exposure has provided information about the presence and distribution of hot spots in these modules. Conventionally, the insulation resistance tester has been used mostly for the dry and wet leakage test. In this study, the importance of the polarization index test and voltage excursion test are described. The use of these tests is essential to provide insight into the modes and mechanisms of degradation, during reliability and durability studies of PV modules. A predictive model for the service life of a PV module may be developed through the results obtained from these characterization techniques in conjunction with long-term exposure and accelerated lifetime tests.

Performance variation of commercially available modules after six months of outdoor system voltage stress testing

System voltage stress has been shown to cause performance degradation in PV modules. This effect has been most prevalent in hot and humid climates such as Florida. Several modules from leading manufacturers have been deployed at the Florida Solar Energy Center for outdoor high voltage bias testing. Following a period of six months of continuous application of negative 600 V, performance analysis has been carried out. A large variation in the performance of different module types was observed. Characterizations included illuminated current-voltage, dark current-voltage, and electroluminescence imaging. A statistical analysis was performed on the performance data to determine the extent and variation of system voltage induced degradation in commercially available modules of similar cost and construction.

Effect of laser marks and residual stress in wafers on the propensity for performance loss due to cracking in solar cells

In this study, silicon wafers were marked with four different kinds of laser marks. Initial characterization of the wafers was performed to measure various parameters such as minority lifetime, residual stress, wafer thickness, Photoluminescence, and Resonance Ultrasound Imaging. The wafers were consequently processed into solar cells. EL, PL and RUV measurements were also performed on solar cells, in addition to the flash I-V measurements to determine solar cell performance parameters. Cells were laminated into custom modules and the modules were passed through static and dynamic mechanical loading tests according to IEC 61215. A procedure to quantify the performance loss due to cell cracking for each cell was developed and applied for each cell that experienced significant cracking after mechanical loading tests. Following correlations were explored: (i) Propensity of power loss due to cell cracks and residual stress at wafer level (ii) Types of laser marks and their effects on parameters measured in various wafer and cell characterization techniques.