There are various methods to detect failures and defects in a PV system. This article explores the positive and negative aspects of these methods.
Failures & Defects in PV Systems: Typical Methods for Detecting Defects and Failures
Generally,any effect on the PV module or device which decreases the performance of the plant, or even influences the module characteristics, is considered a failure. A defect is an unexpected or unusual happening which was not observed on the PV plant before. However, defects often are not the cause of power loss in the PV plants: they affect PV modules, for example, in terms of appearance (Quater et al.,2014). There are various diagnostic tools and methods to identify defects and failures on PV devices (Golnas, 2013), (Ndiaye et al., 2013) as discussed below.
The visual assessment is a straightforward method and the first step to detect some failures or defects, particularly on PV modules. Visual monitoring allows one to observe most external stress cases on PV devices. Besides, this method can provide an overview of the PV system’s condition. Some visible defects in PV modules are bubbles, delamination, yellowing, browning, bending, breakage, burning, oxidization, scratches; broken or cracked cells, corrosion, discoloring, anti-reflection and misaligning (see Fig. 1).
Visual assessment should be carried out before and after module installation to evaluate effect percentage of electrical, mechanical, and environmental stress on the PV module. In converter and BOS devices, visual inspection allows one to recognize disconnections, burning parts, defect in supporting structure (see Fig. 2) as well as problems regarding dirt and shading.
Real-time analysis &Data Acquisition Systems (DAS)
There are many monitoring systems used in medium-large size PV power plants (nominal power higher than 100kW). Such systems can give us useful information about the general performance of the PV plant, detailed information about the operational status of inverters, transformers, PV arrays and switches thanks to direct measurements performed in the plant by using ad hoc instruments, or collected data by the on-site monitoring system if available. On the other hand, these systems cannot detect problems related to a single module fault or sometimes faults related to a series of modules.
Data acquisition systems (DAS) are applied to store data for evaluation of system performance in high precision. Recently, various DAS was developed to evaluate the PV system’s performance. Performance data presents problems, failures, or malfunction of PV systems in detail. However, the primary purposes of monitoring a system using DAS are to measure energy yield, assess PV system performance and quickly identify design flaws or malfunctions.
Generally, electrical measurement signals in PV array include power, voltage and current in DC and AC sides, which contain rapid fluctuations. These fluctuations affect the accuracy of the data acquisition, and they are not visible by typical monitoring systems. The metrological parameters must be measured as well, for example, solar irradiance on array surface, array planes and ambient temperatures.
Fig. 3 displays the schematic view of the procedure of monitoring a PV power plant by a commercial data acquisition system. The test controller is a high speed, and the precision instrument can be programmed. It can also store the data in internal memory. Data can be transferred over the Ethernet interface; it can also be used as a web client or E-mail.
Thermography inspection is a popular method that can provide enrichment data about PV device status. Typically, it is carried out by infrared radiation (IR) imaging sensor. Thermal vision assessment is a harmless and contactless monitoring technique. It can diagnose some of the defects and failures on PV modules, connectors, AC or DC converter and panels. Furthermore, this method does not require shutting down systems. The main task of thermography measurement is to find the defects or failures under temperature distribution of the device.
Commonly, the thermo-vision assessment is carried out to identify open-circuited modules, bypass diode problems (see Fig. 4); internal short circuits, potentially induced degradation, delamination, complete or partial shadowing, cracks or micro-cracks, broken cell and hot spot. Thermo-vision assessment can also detect the local overheating in connectors, converters, transformers, and some other devices.
PV Module or String I-V Curve measurement
The primary measurement parameters of PV modules consist of open circuit, short-circuit current, fill factor and maximum power point. I-V measurement curve gives sufficient information about PV module's condition. Typically, the I-V curves are measured under Standard Test Condition (Cell temperature = 25°C, Irradiance=1000 W/m², the spectral distribution of irradiance AM=1.5G). Nonetheless, the PV modules do not meet the standard test conditions during the test measurement. Usually, radiation is less than 1000 W/m²; the temperature is higher than 25°C, and the wind blows at average speed outdoors. However, STC can be as an ideal reference for comparison of the PV modules in different conditions (Herman et al., 2012).
Fig. 5 shows I-V, and P-V characteristics reported to STC when some solar cells are subjected to shading. It can be noted the presence of bypass diode in the PV-module to avoid overheating of the PV-module, and also to reduce the loss in power generation due to shading (Alberto Dolara et al., 2013).
Fig. 6 shows a comparison between the measured I-V curves and P-V curves of the 4 PV modules affected by micro-cracks and expected ones (Alberto Dolara et al., 2014), (A. Dolara et al., 2016). Currently, some commercial instruments allow measurement of I-V characteristic in real condition for a string of 20 modules.
Photo-luminescence, Electroluminescence and UV Fluorescence Technique for PV module analysis
Photoluminescence (PL) and Electroluminescence (EL) are recent measurement methods that use luminescence images. The PL and EL measure the irradiative recombination of photons since carriers are excited into the solar cells. If an external injected current obtains this excitation, the technique is called Electroluminescence. If the excitation is created by radiation incident, the technique is called Photoluminescence. Both measurement tests are non-destructive methods for PV module monitoring. Potential induced degradation, hot spot, white spot, cell finger metallization, humidity corrosion, cracks, micro-cracks, soldering, discoloration, snail trails and other defects and failures can be detected by these assessment techniques (Ebner et al., 2013), (Potthoff et al., 2010).
Fig. 7 shows the EL results for an area of a multi-crystalline module affected by microcracks; its thermal and visual images are reported too. Black areas in EL images represent electrically separated sections. The positions of the cell are indicated in terms of coordinate (row, column) within the PV module, e.g. position (1,1) is on the left, top. However, it is possible to identify a link among visual defects, hot parts, and electrically separated areas by comparing these three images (A. Dolara et al., 2016).
Initially, UV fluorescence (FL) imaging technique was performed to detect EVA (Ethylene Vinyl Acetate) degradation. Most of the material degradation is found by using UV fluorescence imaging. FL imaging techniques are useful to detect cell micro-cracks but not the ones along the edge of the cell (see Fig. 8). However, EL technique is more appropriate for monitoring cell cracks on PV modules (Khatri et al., 2011).
As it can be seen from this exploration of typical failure and defect detection methods, each method has its own advantages, disadvantages and more particular uses depending on certain cases. I hope this overview could answer some questions and provide you with a fundamental information about the most common detection ways.
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