The number of solar power plants has been steadily increasing over the past years. Photovoltaic systems are known for:
Nevertheless different kind of faults in PV systems can occour:
FAULT DETECTION
Even if manufacturer of panels promote their products as: Non-PID, No Cell crack, no hotspot possible, of course the above mentioned problems occour at any type of PV panel.
Mismatch losses occur at serial or parallel connection of PV
panels due to differing electrical characteristics. The reasons for
mismatch can be: different panels, different elevation, shading,
hotspots, PID, any other faults. The following picture gives an
explanation of the losses due to serial (left) and parallel (right)
connection:
In solar farms usually a combination of series and parallel connection of PV panels is used in order to use the full MPP input range of inverters. Via series connection panels will be connected to a PV-String. Connecting this PV strings together via parallel connection will represent a PV-Array. If now one string of the PV-array will reduce it’s output power due to any defective module or tempory shading, not only the power of this string will be reduced. The whole system voltage (parallel connection of voltage sources) will decrease and the power of the whole array decreases. In the example below the output power of the array will be reduced by 8 kW (30%) instead of 3kW (10% reduction at string) due to this Mismatch losses.
Inspection of solar PV parks are divided in several disciplines. First of all the system needs to be checked for any safety issues. Leakage currents and isolations faults can be dangerous people and can also affect other equipment like pipelines. Further performance limiting faults like PID, Hotspots, broken, bypass diodes etc. should be detected and the performance of the whole system according to IEC62446-2 analysed.
Visual inspection
Visual inspections are important to detect major problems, like broken fuses, obvious panel issues, etc. Nevertheless this method only allows detecting few issues. As an example after a hailstorm only a few modules were exhanged at a solar power plant. Detailed investigations showed that a lot more modules were affected by the hailstorm.
Thermal Imaging
This technique is most used for inspection of PV plants. It requires Know-How for executionand analysis of the measurements. Often drones or airplanes are used and the power plant needs to be in full operation (heat). It allows detection of different kind of faults and to find broken components.
Drawbacks:
Electroluminescene
This technique is mainly done in laboratories. Modules will get activated by current injection. Measurements are done without solar radiation (by night). It allows detailed analysisof PV panels. Nevertheless mobile measurement systems to analyse whole PV strings are not available yet. The systems require high power for signal injection.
This technique will record the voltage and current profile (IV curve) of PV panels starting at the open-circuit voltage (Voc) to the short-circuit current (Isc) by applying a load. Depending of the shape of the curve, the different possible faults can be detected and differentiated. Further it’s the only method to detect mismatch losses.
The following picture shows an example of a simultaneous measurement of a PV array with 17 strings.
While using single-channel IV tracing systems will take a lot of time (days) for inspection of solar farms (e.g. 400 strings for 3 MW) the muti-channel IV-curve tracing system will allow inspection of solar power plants within very short time (~4 hours per MW) and will give valueable information about the mismatch losses.
The patented diagnostic system automatically shows the condition of each string. Different colours outlines the string performance and shows different PV module faults.
Fault detection: Mismatch, PID, Hotspot, Bypass diode breakage, Shading, Soiling, Glass breakage, Delamination, Cell Crack etc.
The following table gives an comparison of different PV / solar inspection methods:
Analysing the effects of Supraharmonics asks for Power Quality Analyzer that cover a frequency range up to 500 kHz. Classical PQ Analyzers present problems because of their limited bandwidth. Typically they can measure up to the 50th Harmonics (2.5kHz), but cannot detect higher emissions of voltage and currents. By design, they are not able to be used in the troubleshooting process at all.
In this Application Note, we outline the complexity and shortcomings of traditional measurement approaches we often see out in the field. Fortunately, all these problems do not concern engineers with the appropriate measurement equipment. Our customers appreciate the PQA8000H in troubleshooting Supraharmonic emissions, where its strengths really come into play ..
