How to test Photovoltaic Ground Fault
Photovoltaic Module Test
June 1, 2016 by Anders Rand Andersen
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.PID test – scientific background
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 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-RSH toward a string terminal is an indication of Potential Induced Degradation (PID).https://www.youtube.com/embed/e5IHgCkkt2M?start=4&feature=oembed
Photovoltaic String Test
Z200 PV Analyzer – string test mode
When the String Tester application is activated, the Z200 Analyzer will set up a series of measurements based on deriving the basic solar cell impedance, which gives the user a fast overview of the general health state of the PV module string under test. The following is measured:
- Open circuit voltage – Voc
- Short circuit current – Isc
- Isolation resistance Riso
- PV system series resistance Rs
- Impedance curve – measured at Voc in a broad frequency range
- Low-frequency impedance norm – measured at Voc
- Low-frequency impedance norm – measured during the flow of PV system current
- If Riso < 3 MΩ, a ground fault position is returned
- A build-in algorithm returns a conclusion about the ” fault state” of the PV system
Solar PV panel electrical resistance
Series resistance in PV panels derives from different components of solar power installations. In the exterior of the PV system, we find series resistance in cables and worn connectors. Within the PV module, we find resistance in the junction box connections and bypass diodes. The solar cells in the PV module represent the most complex source of series resistance.
The silver busbar and “fingers” on the cell surface have series resistance, and we also find resistance in the front and back contact materials. Although the many series resistance components are complex, the general understanding is that high resistance is problematic, and low series resistance is desirable in solar PV systems.
With the Z200 PV Analyzer PV testing becomes easy and the build-in troubleshooting features help the operator to quickly solve problems in the field.See test data example
Increased series resistance reduces the solar PV system fill factor “FF”. But note that when a high series resistance exists in a solar PV system, there is a danger of electrical power dissipation in the areas with high resistance also.
Such power dissipation causes burn marks and disconnections in Solar PV strings. Often cabling and module connectors turn out to be the actual problem. Below we see an example of Solar PV system cable-connectors with series resistance caused by wear, tear, and moisture.
The samples are found in the field during PV testing with the Z200 PV Analyzer.
Mechanical damage to the PV cabling can cause a loss of electrical isolation and increased series resistance. This kind of problem can be difficult to locate using conventional PV array testers, but the Z200 PV Analyzer does it quickly.
Mechanical damage on the PV cabling, causing a loss of isolation. This kind of problem can be difficult to locate since the grounding faults are periodic and often appear only when the surrounding environment is moist.
Cable samples found in the field show the damage created by rodents. The cables and connectors are penetrated, and the cable is no longer isolated, which creates both serious performance and safety issues.
Corroded solar PV system cable connctors.
Solar panel efficiency over time
The Z200 test signal frequency is in the interval 1 Hz to 100 kHz, and the test signal amplitude is quite low. At low frequencies, we normally do not measure any noteworthy impedance in illuminated solar PV modules, and the series resistance of the string dominates the spectrum.
The “Low freq. norm” will be close to the series resistance value when the PV modules are fully illuminated, and when there is no resistance problem in the string. If we measure something different from the expected series resistance value, we thus detect abnormalities.=
If the “Low freq. norm” and the “Low freq. norm with load” are both high values, it could mean that e.g. a cable is broken. If the two values are very different e.g. if the “Low freq. norm with load” is low compared to the “Low freq. norm” the DC current is making the difference. This implies that some fault internal to the cells and modules is present.
