Photovoltaic PID Testing

Potential-induced degradation or (PID) is a failure mode in solar cells. The degradation is caused by voltage stress on the solar cell. PID can affect both crystalline and thin-film cell, to an extent depending on the material and environment. Often manufacturers promise PID-free solar cells, but this solar type performance and material degradation is frequently 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. It can cause severe reductions in module output 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.
The blog is based on emazys research – please find the full research paper here.

PID testing equipment

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

A PV analyzer from EmaZys (Z100), was used to detect the shunting level of the panels. The instrument ius indeed able to measure the shunting resistance of individual modules in a string. Please find the test principle described in detail here: single module test.

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 Z100 PV Analyzer to point out PID infected modules.

The laboratory Electroluminescence (EL) imaging and I-V characterization 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 show EL measurements, confirming the suspected degradation in this case. The pattern is consistent with PID – see FIGURE 2 and 3 below.

The shunt resistance determined by Impedance Spectroscopy in the lab can be seen in FIGURE 4. It shows a good correlation with the light I-V measurement results sees 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.

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.

PID testing — Figure 1: The plant setup is composed of 36 PV panels connected in 3 equal strings to an 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.

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 infected panel (60608) pointed out in the field using the Z100 PV Analyzer.

PID solar — 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 — 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.