Photovoltaic single module test

Z200 PV Analyzer: PV Module Tester

• The Z200 PV Analyzer may be applied for individual PV module tests and measurements, even when the instrument is connected to the string terminals. The procedure is to first measure a baseline, and then subsequently shading the PV modules one by one, while impedance data is collected by the Z200.
• The module voltage drop caused by shading modules, one by one is measured. The result is a containing individual module voltage.
• The low-frequency impedance norm measured in the PV string open circuit state. In the case of fully illuminated modules, the low-frequency impedance is about Rs. A shaded module will however normally show a much-raised impedance value, even when it is placed in a string showing a significant voltage. If the module is shunting e.g. due to Potential Induced Degradation (PID), it is detected here.
• 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. This will easily detect if bypass diodes are functional and if there is a risk of hot spots and power dissipation in components

Solar panel shunting resistance – a PID indicator

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 shunting resistance R_SH 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 likely be caused degraded e.g. by Potential Induced Degradation – PID if there is a trend towards a terminal or a grounding point.

During the “PV module test,” a baseline is first measured under irradiation. Then each PV module is shaded one by one, and measurements are carried out while the PV module under investigation is shaded. This procedure leads to valuable data, that is normally only possible to obtain when PV modules are examined in the PV test laboratory away from the installation.

Solar module voltage measurement

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 corresponds 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 module 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 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.

PV module impedance under operation

The low-frequency impedance norm is also 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.

Impedance at open circuit condition

The low-frequency impedance norm is measured in the open-circuit state. In the case of fully illuminated modules, the low-frequency impedance is normally very low; approximately around the RS 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 the generated current is not harvested externally. Especially a gradually falling value of module-R_SH toward a string terminal is an indication of Potential Induced Degradation (PID).