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.

Solar panel resistance and voltage testing
The Z200 PV Analyzer will automatically measure and calculate a range of solar panel string impedance values. After interpretation, these values helps greatly as characteristic pattern for any type of fault that may be present in the PV array.

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.

Solar panel connector fault
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.
Solar PV resistance fault
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.
Solar panel degradation
Corroded solar PV system cable connectors.

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.