How to test solar panels

The emazys Z200 string test

For an in-depth review of the string test, please download the application note.

String test application note

In this post we highlight the test results that can be achieved, when using the Z200 PV Analyzer String Test. When the string test is activated, the emazys Z200 will collect the following information:

  • PV system open circuit voltage (Voc)
  • PV system short circuit current (Isc)
  • PV system isolation resistance (Riso) and position of any ground faults
  • System impedance spectrum (Z)
  • Impedance norm – measured at Voc and under load (I>0)
  • PV system series resistance (Rs)
  • State machine analysis algorithm

An example of an instrument “output” report can be downloaded here TEST REPORT.

Z200 solar PV test equipment kit no background
The Z200 kit was designed for troubleshooting PV installations. The core technology focused on the service and maintenance tasks. the kit provides fast and seamless fault finding which is becoming essential for the operations of solar energy.

Open circuit voltage and short circuit current 

Photovoltaic (PV) cells generate electricity through the photovoltaic effect when they are irradiated by light. The PV cells are made from semiconductors, that will conduct electricity when exposed to light, and in most cases the cell materials is chosen so that the sunlight spectrum will lead to an effective conversion of energy.

Tiny packets of energy called photons, strikes the surface of a PV cell and the the semiconductor material absorbs these photons. This absorption energizes electrons within the material, allowing them to break free from their atomic bonds.

As electrons become free from their positions within the semiconductor, they leave behind vacant spots called “holes.” so that electron-hole pairs are created. The separation of charge that is essential for generating voltage. now, PV cells are manufactured with a built-in electric field that comes from two different layers of semiconductor. One with an excess of electrons (n-type) and another with an excess of holes (p-type). The interaction between these layers creates a static electric field, that can “pull” electrons and holes in a opposite directions. This movement of charge creates a voltage potential between the two layers, resulting in an electric current that can be harnessed as electricity. The main structure of the pV cell is that of a PN junction diode (see Figure 1). this is an important feature as it can actually explain some ion the most widely used way to test the performance of solar panels i.e. the IV curve.

The maximum voltage generated is measured at the “open terminals” an referred to as Voc (open circuit voltage). Likewise we can measure the maximum current available Isc (short circuit current). Together Voc and Isc are strong indicators of the PV system condition.

For reference, you would typically measure these values close to standard test conditions (STC) in the field and subsequently compare the values to the solar panel manufacturers test results.

Figure 1: Here we see the basic equivalent circuit for a PV cell. It consists of a current source derived form photos hitting the PV cell and being transformed into a current. The internal structure of the solar is that of a PN junction diode. The diode will Strat to conduct current as the voltage goes up, which explains the main curve characteristics of the IV curve. The circuit also have a parallel resistance Rp, and a series resistance RS. The circuit is equivalent to both individual PV cells, PV panels and even strings of PV panels. Link to figure.

PV system isolation resistance (Riso) and position of any ground faults

Electric isolation resistance plays an important role in ensuring the safety and functionality of PV systems. It is crucial to maintain high isolation resistance within these systems, primarily to safeguard individuals from dangerous voltages. PV systems can produce high DC voltages, often exceeding 1000 volts and without proper isolation resistance, these high voltages could potentially lead to severe electric shock hazards. High isolation resistance prevents the unwanted flow of electricity to ground or unintended conductive surfaces.

Isolation resistance also plays a vital role in preventing ground faults within PV systems. Ground faults occur when an unintended electrical path is created between the PV system and the ground, potentially leading to electrical fires or system malfunctions. High isolation resistance helps to maintain the electrical integrity of the system by minimizing leakage currents and ensuring that the current flows only in its intended path, preventing ground faults.

Beyond safety considerations, isolation resistance is essential for the long-term performance of commercial PV systems. A high isolation resistance helps preserve the integrity of insulation materials, preventing moisture ingress, corrosion, and degradation of electrical components. As such, maintaining a high isolation resistance contributes to the overall reliability and longevity of the PV system.

Furthermore regulatory bodies and safety standards, such as the National Electrical Code (NEC) in the US, mandate specific isolation resistance requirements for commercial PV systems. Compliance with these standards is not only essential for legal reasons but also ensures that the system is designed and installed with safety as a top priority.

Peace of mind for owners and operators has also become essential as the global accumulation of solar power keeps growing. Knowing that that a PV system is designed to minimize safety risks reduces liability concerns and contributes to a positive reputation for environmentally responsible practices.

Please read our article about ground fault troubleshooting for in depth information.

Solar system diagram
Figure 2: Illustration of a PV array connected to an inverter (right side) and various conductors that makes up the full PV circuit.

PV system impedance spectrum

the PV system impedance curve is measured at the open circuit voltage of the PV system in a broad frequency range from about 1 – 100 kHz. The test signal amplitude is kept below a few volts, so the testing principle is very gentle on the solar cells.

At low frequencies below 5-10 kHz, we normally do not measure any noteworthy impedance in fully illuminated solar PV panels, and the series resistance of the string dominates the spectrum.

Z200 PV Analyzer impedance curve
Figure 3: This plot shows the Z200 PV Analyzer module string impedance curve plot. The data is shown as the impedance norm |Z| against the logarithm of the frequency. This curve shows a healthy PV panel string with no obvious issues in the main conductor path.

Impedance norm – measured at Voc and under load, at low frequencies close to DC conditions

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.

PV system series resistance Rs

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.

How to test solar panel output and troubleshoot solar arrays
Figure 4. This graphic is an illustration of the main Z200 testing principle. A test signal is superimposed on the string of solar panels as a sinusoidal voltage with a low amplitude. This results in an oscillation in electrical current, which is measured by the Z200. The test is done at a wide range of frequencies, so the end result is a spectrum of impedance values for each test frequency.

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. 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 fault in solar PV systems
The photo show a cable collected in-field. We see a pin holde caused by mechanical tear and wear on the cable since it was not fixed and held into place. The hole opens up for the external atmosphere and lets electrolytes in causing as leakage current and a ground fault.
Ground fault in damaged solar PV installation cables
The photo show a connector + cable collected in-field. We damage caused by rodents which opens up for the external atmosphere and lets electrolytes in a ground fault.
Corroded solar PV connector
The photo show a cable + connector photographed in-field. We see corrosion leaking out of the connector causing an elevated level of series resistance. The leaking electrolytes can further cause an isolation resistance fault.

State machine analysis algorithm

The Z200 conducts a number of calculations based on the assembly of measured data. We call these calculations the State Machine. The State Machine is a systematic approach to the handling of potential PV system faults such as instrument connection problems and internal instrument hardware issues (instrument self check).

The State Machine was developed to provide a clear analysis of measurement data to the user. The assignment of priority to different types of faults, has also been implemented. Fault priority depends on the severity of faults, and the effect the fault in itself, can have on measurement quality and validity. We rank faults with the highest priority at the top is as follows:

State 1 – voltage overload
If the absolute terminal voltages either individually or the absolute voltage difference 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 incorrectly.

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 2 PV string. The State Machine reports this condition whenever these conditions are not fulfilled VP V + > VG > VP V −.

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.

State 6 – high string impedance
By studying the Sandia PV panel data-base, we can conclude that a healthy string of PV panels will always have an impedance of less than 3kΩ, when it is measured under the following conditions:

  • Irradiation > 100 W m2
  • System size Voc < 1000 V
  • Short circuit current Isc,0 > 1A, at Standard Testing Conditions (STC)

In the figures below we see 2 different examples of the state machine output.

String test, emazys state machine conclusion 1
String test, emazys state machine conclusion 2