Development of an IEC test for crystalline silicon modules to qualify their resistance to system voltage stress

Abstract

IEC 62804 Ed. 1, System voltage durability qualification test for crystalline silicon modules, is being developed. First, two module designs are compared in chamber and in the natural environment of Florida (USA). From these results, a stress level of 60°C, 85% relative humidity, a bias of nameplate system voltage, 96 h dwell, and a pass/fail limit of 5% relative power degradation at 25°C and 1000 W/m 2 irradiance is initially proposed for the draft protocol. This paper next focuses on one of the main controversies within the development of this standard-the use of damp heat in an environmental chamber versus a conductive foil to complete the circuit to ground during the test. Conventional 60-cell multicrystalline silicon modules with (i) a standard aluminum frame, (ii) a modified frame, and (iii) a rear rail design were tested for potential-induced degradation (PID). These three module designs were stressed at the draft protocol conditions stated above and outdoors, applying negative system voltage bias during hours of daylight to simulate array voltage. The damp heat environmental chamber tests run according to the protocol distinguish the relative resistance of five module designs to PID in the field and correctly rankorder the durability in the field to the extent tested (up to 28 months). Finally, the degradation rate is determined at 25°C using a foil to ground the module face on a subset of modules susceptible to PID, and the results with respect to measured field performance of the modules are discussed.

Interlaboratory Study to Determine Repeatability of the Damp-Heat Test Method for Potential-Induced Degradation and Polarization in Crystalline Silicon Photovoltaic Modules

“…However, capability of good reproducibility within a given lab has previously been shown [10]. To further determine the effect of possible inadvertent variation in the stress levels applied by the laboratory to yield different outcomes of the test, the median degradation for each module type measured in the various laboratories was calculated and then added to the individual degradation data points.…”

“…In many instances, this mimics the behavior in the natural environment [12]. Alternately, foil placement on the module surface contacts the whole module face [10] and may be desired to approximate the situation of a very highly conductive soiling layer, or a continuous water layer pooling for extended periods of time on the whole module face that is connected to ground.…”

“…Testing at 60°C and 85% RH appears to primarily actuate the PID mechanism, so this condition was used as the condition for comparison. The choice of 96 h comes from a study showing that designs that pass this damp heat with voltagebias stress-test level for this duration with less than 5% relative degradation also do not degrade in the natural environment in Florida (USA) for a period of about 3 years, the extent tested [10].…”

“…Application of heat and humidity in an environmental chamber promotes ionic conduction in module packages [9]. In comparisons to the performance of modules in the natural environment, the finite conductivity provided by the damp heat has been found to correctly evaluate PID in the presence of module frame-based mitigation techniques that impede leakage current flow to ground [10]. The method would be expected to represent the interactions between poor-quality glass and water (such as sodium leaching), and it has the ability to make electrical connections in declivities and pores on the module surface.…”

Photovoltaic Module Qualification Plus Testing

“…The test proposed here has been examined in a round robin study [47] and validated in fielded modules. [48] Light exposure during application of the bias stress may be appropriate in order to more closely simulate the field conditions, but data on the degradation observed in this sort of test is not available. Metal foil may also be used to apply the voltage bias, but the basis for the pass criteria needs to be established, so this option is not included here, but will be included in the IEC test method.…”