Solar string test

PV string testing

When the String Tester application is activated, the Z100 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
  • 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 flow of PV system current


Solar PV module string series resistance

Series resistance derive 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 modules we find resistance in the junction box connections and and bypass diodes. The solar cells in the PV module, represents 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 more complex in nature, we also find series resistance in the emitter and base parts of the solar cells. 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.

A new solar string tester


The main impact of series resistance is to reduce the solar PV system fill factor “FF”. But note that when a high series resistance exist in a solar PV system, there is a danger of electrical power dissipation in the areas with high resistance. Such power dissipation is a cause of burn marks and disconnections in solar PV strings. Often cabling and module connectors turns out to be the actual problem, when solar string test troubleshooting real solar PV systems in the field – it´s not always the PV cells. Below we see an example of some solar PV system cable-connectors with series resistance caused by wear, tear and moisture. The samples are found using the Z100 PV Analyzer.

Corroded solar PV system cable connectors
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