Solar PV panel electrical 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 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 many parameters are measured fast and easy.
Solar Array Performance Test - IEC 62446 "steady state PV Riso"
The following panel string parameters are measured when the String Test mode of the Z200 PV Analyzer is activated:
- Solar Irradiance
- PV module temp.
- Ambient temp.
- Impedance curve – measured at VOC
- Open circuit voltage – VOC
- Short circuit current ISC
- Low frequency impedance norm – measured at VOC
- Low frequency impedance norm – measured during flow of PV system current
- PV system series resistance RS
- Isolation resistance RISO
- Estimated position of RISO
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PV Test: solar module series resistance Rs
Increased series resistance 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 also. Such power dissipation causes burn marks and disconnections in solar PV strings. Often cabling and module connectors turns out to be the actual problem. 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 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. Z200 PV Analyzer can substantially reduce troubleshooting times by measuring the position of the damaged components fast and convenient.
PV Testing scenarios
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 very different from the expected series resistance value, we thus detect abnormalities.
The “low freq. norm with load” should also be low when the solar PV modules are fully illuminated. The load means that a small current is drawn in the Z200 instrument, meaning that we measure the impedance during a flow of DC current.
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.
Understanding the Z200 PV Analyzer String Tester report
In solar PV modules, the impedance is dominated by a frequency dependent combination of resistance and capacitance. Inductance may also be observed when there is long cable lines between the testing instrument, and the actual solar PV modules under test. The effect is observed at higher frequencies.
The basic information comprises the information which is entered into the Settings menu by the user, as well as a time stamp and information about the instruments serial number and the installed software version.
The first part of the results is the impedance curve. More precisely the orange curve shows the measured impedance Norm on the y-axis, and the Z200 testing frequency on the x-axis. Note that the frequency axis is exponential and not linear. The concept of the impedance norm is described above.
When the frequency impedance norm curve corresponds nicely to an ideal functional solar cell, the instrument will calculate a fitted value that corresponds to the series resistance of the solar cell module string. The series resistance curve is plotted as a grey line. In the curve below, the series resistance is 8 Ohm.
The open circuit voltage is the full voltage of the string, and the short circuit current is the maximum DC current of the module string.
The low freq. norm was Ohm, and the low freq. norm with a load was 14 Ohm in this case. These values are very close to the series resistance value of 8 Ohm. In this way all 3 values are relatively low, which means that DC current can flow almost unhindered through the PV system. If one or more of these values increase, it usually means that some electro technical issue is present in the module string. A disconnection e.g. will quickly be indicated by very high norm values towards the lowest frequencies. In such a case, the Z200 will not estimate a series resistance value, since the measured data no longer correspond to a healthy solar cell module string.
In the Z200, the measurement of the isolation resistance Riso is also carried out by the String Tester function. Should the value be less than 3 MOhm, the instrument will search for the position of a singular ground fault and report this to the user. If the Riso is above the 3 MOhm threshold, the instrument will just report the value and not look for a fault.
The trick is to understand the relation between the different parameters measured with String Tester, and use this as quick guide to the overall health status of the PV module string.
When the parameters measured indicate a certain fault state of the module string, the build in software (STATE MACHINE) will estimate some well known electrical fault scenarios that can explain the measurements. Keep in mind that PV system electrical faults are not always singular and well defined issues. Often we see more than one issue present at the same time, and this can make the troubleshooting job more complicated. With the Z200 PV Analyzer, it is possible to analyze the complex solar PV module string circuit and more easily determine what should be done to optimize the safety and production of the PV array.