The main measured parameters are:
- PV module voltage – test module performance
- Impedance at open circuit conditions – test bypass diode functionality
- Impedance under operation – test for degradation such as PID
This application is based on measuring various parameters, when 1 module in a string of modules, is shaded during measurements. This particular approach has the peculiarity that it can reveal latent and complex faults, while still being rather simple to carry out.
The main measured parameters are:
During the Z100 PV Analyzer “PV module test” a baseline is first measured under irradiation. Then each PV module is shaded fully one by one, and measurements are carried out while the PV module under investigation is shaded. This procedure lead to valuable data, that are normally only possible to obtain when PV modules are examined in the PV test laboratory.
The module voltage drop caused by shading modules, one by one, is measured in the string of modules under test. Under normal circumstances, the string voltage drop thus corre- sponds closely to the voltage of the shaded module. The result of this testing procedure, is an overview of the individual modules voltages in bar diagram form. Modules with a low voltage, relative to the majority of the string modules, may then be identified in a convenient manner.
If a single module voltage is found to be about 2/3 or 1/3 of the normal open circuit mod- ule voltage, the problem could be caused by 1 or even 2 bypass diodes, that are in a short circuit state. Short circuited bypass diodes are often seen as a damage following lightning strikes, but please note that many types of damage could lower the voltage of a solar PV module installed in the field.
The low frequency impedance norm, measured in the open circuit state. In the case of fully illuminated modules, the low frequency impedance is normally very low; approximately around the RH value. A shaded module will however normally show a much raised impedance value, even when it is placed in a string showing a significant voltage. The high impedance appears, since the test signal must travel through the shaded solar cells in the module. The impedance is caused by a phenomenon normally referred to as shunting resistance (RSH also called parallel resistance RP), which hinders the flow of return-currents within the solar cell PN junction. In this way a low value of RSH indicates degradation in the module i.e. a condition where generated current is not harvested externally. Especially a gradually falling value of module-RSH toward a string terminal is an indication of Potential Induced Degradation (PID).
The low frequency impedance norm, is measured while loading the string with a weak load i.e impedance is measured while a small electric current is allowed to flow in the string. When shading a module, while the instrument is transmitting the test signals (during a current flow) it is thus possible to determine, if the module bypass diodes function as intended. If the diodes do not ”open”, the instrument will measure a much raised impedance value. The impedance shows up in the measurement, because the test signal must pass shaded solar cells. If the impedance does not change, the current flows in the diodes, and the instrument will conclude that there is no risk.
The result of the measurement is a Module Risk Factor (MRF), that is assigned to each module. The higher the MRF is for a module, the more likely it is, that electric power will be dissipated in the module in case of long-term shading or internal cell damages. This will in most cases lead to so-called ”hot spots” and burn marks, which causes significant irreversible damage to the system.
The shunting resistance RSH can be quantified by measuring the low frequency impedance of the whole PV string, while the PV module under test is being 100% shaded. Simply cover the PV module, and measure the impedance of the whole string. If the “low frequency impedance” has a relatively low value when compared to other PV modules in the same string, be aware that a low shunting resistance is indicated, and hence the module could be degraded e.g. by Potential Induced Degradation – PID.