How to test solar panels

In this post we will illustrate how to do a full performance and diagnostic test of a string of solar panels using the emazys Z200.

Before you do any testing on solar PV systems, please observe the cautions outlined in the “sticker” below.

When the String Tester application is activated, the emazys Z200 Analyzer will set start to test the solar panels and also derive the solar cell impedance. This gives the user a fast overview of the general health state of the PV module string. The following parameters will be checked:

• 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

Please refer to the String Test Application Note for in depth information.

Z200 state machine algorithm

The Z200 PV test instrument has a build in state machine algorithm, that will analyse the data measure in the string test sequence. The user output is the measured parameters as well as a written “conclusion” from the algorithm.

State 1 – voltage overload

If the absolute terminal voltages either individually or the absolute voltage dif- ference between pairs of terminal voltages exceed 1000V, then the instrument will alert the user of overload and disable further measurements. This is also the case if there is some HW problem preventing the measurement of the terminal voltages.

State 2 – external disconnect

If the low frequency impedance of the PV string impedance measured with and without load is high, then typically there is a disconnect not masked by a PV panel bypass diode. Such a system fault needs to be corrected before any other measurements have any real meaning. Symptoms of high impedance both with and without load are also seen at night and if the user connected cables incor- rectly.

State 3 – voltage polarity

This state is described by incorrect voltage polarity, or a PV system ground voltage not within intervals spanned by the positive and negative terminals of the PV string. The State Machine reports this condition whenever these conditions are not fulfilled:
VPV+ >VG >VPV−

State 4 – internal disconnect

The symptom here is a drop in the impedance at low frequencies with an applied load during the measurement. The cause could be the existence of a series fault internal to a module which is masked by the turn-on of an associated panel bypass diode. Another reason could be the presence of a rectifier diode in series with the PV panels. Such a diode is typically used in systems, employing parallel strings of PV panels connected to combiner boxes.

State 5 – low isolation resistance

Low isolation to ground (Riso < 1MΩ). This fault is given lower priority than the ’internal disconnect’ due to the fact that accurate estimation of Riso and the position of a possible leakage point will be influenced by internal disconnects and the user should be encouraged to first identify and correct such faults before the localization of ground faults.

See example of string test reports here:

String test report (normal PV string)

String test report (high impedance case)

Impedance test of solar panels

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. 

Electrical resistance in strings of solar panels

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.

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.