PID Testing

Potential-induced degradation (PID) is a failure mode in solar cells caused by voltage stress on the PID module. PID can affect both crystalline and thin film modules, to an extent depending on material and environmental. Often manufacturers promise PID free solar cells but still this solar degradation is observed in the field.

In this post only the shunting type PID (PID-s) will be considered. PID-s is a common degradation that can develop rapidly and can cause severe reductions in module performance, due to the decreased parallel (shunt) resistance (Rp) and a reduction of fill factor (FF). PID can be reversed to some extent if detected in time, so it is crucial to find cheaper methods for detecting PID.

PID testing equipment

Finding low cost practical means of PID detection can help O&M companies and PV power plants owners in reaching a better return on investment. We therefore chose to develop such a practical solution at EmaZys Technologies.

Detection of solar panel PID – a case study

A residential PV plant located in Southern Denmark, with an total installed capacity of 9.36 kW, was inspected for faults, since the plant experienced a dramatic loss in power generation compared to the installed power.
The plant consists of a total of 36 c-Si panels connected in 3 strings with each having 12 modules in series. The strings are connected to a SMA inverter, limited to 6 kW due to grid connectivity rules. The converter has two maximum power point tracker (MPPT) inputs, thus one string is connected to MPPT2 while the other two are paralleled at MPPT1. See the layout of the PV system in FIGURE 1 below.

PID testing
Plant setup is composed by 36 PV panels connected in 3 equal strings to a SMA converter. The inverter has 2 MPPT channels hence two of the string are parallel on one input while the third string is connected to the second channel.

A PV analyzer from EmaZys (Z100), was used to detect the shunting level of the panels. The best performing panel and worst five performing panels have been selected for laboratory testing with the purpose to evaluate the module health state, and evaluate the robustness of the EmaZys PV Analyzer PID detection capabilities.

The laboratory Electroluminescence (EL) imaging and I-V characterisation tests confirm that this is a case of PID of shunting type. The maximum power has dropped significantly from the datasheet rating, as well as the fill factor. The figures below shows EL measurements, confirming the suspected degradation in this case. The pattern is consistent with PID – see FIGURE 2 and 3 below.

Check PID
FIGURE 2. EL image of a good panel (60406) pointed out in the field by the Z100 PV Analyzer
PID module
FIGURE 3. PID degraded panel (60608) pointed out in the field using the Z100 PV Analyzer.

The shunt resistance determined by Impedance Spectroscopy in the lab can be seen in FIGURE 4. It shows good correlation with the light I-V measurement results see in FIGURE 5. Clearly panel 60406 was almost not degraded, while the 5 other panels pointed out by the Z100 were severely infected by PID.

PID testing
FIGURE 4. Decrease of STC Pmax of the modules relative to their datasheet value. It can be observed that the best performing module has an expected loss in power of around 1% per year from the rated value, while the other worst performing modules have an indisputable degradation problem.
PID free solar cells
FIGURE 5. Modules light I-V characteristic accentuate the advanced state of degradation of the PV panels.

Conclusion

In this short technical overview we saw how the Solar PV Test Equipment from EmaZys Technologies (Z100 and Z200) may be used for field detection of solar modules suffering from Potential Induced Degradation. To download the full article and learn more about EmaZys PV Analyzers, please click below.

The Z200 PV Analyzer from EmaZys Technologies is a PV tester based on impedance measurements. It is an unconventional but highly cost effective testing method for solar PV Array Troubleshooting. Our aim is to bring down the cost of solar electricity by lowering the service and maintenance cost. We do this by offering innovative testing equipment for any solar PV technician, solar O&M etc.

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