PID Reversal Process for On-Site PID
PID stands for Potential Induced Degradation which occurs when the modules are exposed to high negative voltage with respect to the ground. PID happens in ungrounded PV systems and is more prominent near modules towards the negative end of the string, exposed to negative potential with reference to ground.
The main causes of PID are:
Migration of positive ions from the glass plate through the EVA and ARC to the cell. The presence of these ions causes an effective shunt path across the cell and reduces the output.
The buildup of the negative potential of a cell w.r.t encapsulation and support structure due to earthing of the frame.
The main promoters of PID are temperature and humidity.
As specified above, PID is observed when mono or poly-crystalline modules are subjected to large negative potentials with respect to the ground. The small magnitude of the current is induced with positive ions, gradually accumulating in the semiconductor. The presence of these excess positive charges near the junction imbalances the EMF in the depletion layer (mainly responsible for the flow of current). This accumulation, if not monitored closely, can result in the loss of even 3/4th of the full power of the susceptible modules during its initial years itself. If you compare this with the normal expectation of up to 80% power retention till 25 years, you’ll understand that PID prevention is a crucial matter.
In this article, the main focus is on mono/poly-crystalline technologies.
Why so? Because PID has different effects on different technologies. In poly-crystalline or mono-crystalline technology, this effect is reversible to a great extent by applying an external positive potential b/w PV- and ground, thereby reversing the migration of ions and the PID effect itself.
While in thin-film technology, this effect becomes irreversible because PID changes the chemistry of the elements (also known as TCO corrosion).
PID can be approximately inspected by monitoring module Vmpp (voltage at maximum powerpoint). If the module Vmpp decreases from the positive to the negative end of a string, it is an indication of PID. More accurate and robust detection methods include IV curve tracing and Electroluminescence (EL) Imaging.
Prevention against PID
There are a few methods to prevent these effects as listed below:
Using certified PID resistant modules
High Impedance negative grounding
Using PID reversal equipment
PID resistant modules
The term PID first materialized in the year 2010, but people started realizing its severity around 2013-14 when the industry observed high levels of generation loss due to PID. The manufacturers enhanced their fabrication processes by adding an oxidation barrier (creating resistance for the flow of ions). Since this protection is available in newer modules only, you can still observe PID in older installations.
This enhancement doesn’t ascertain PID-free modules but just makes the modules more resistant to PID. Therefore, this is still a need for other protection methods to ensure protection against PID.
High Impedance Grounding
In this method, the negative terminal of the PV array is grounded through a high impedance. This is a cheap and effective way to avoid PID but it can’t reverse the PID process, it can only prevent it from further happening. Also, there is an additional requirement of ground fault detection and interruption (GFDI) equipment, thereby resulting in additional investment. It is suggested to use this solution in the new installations so that there is PID prevention from the beginning. For the old installations having undergone PID, the use of PID-reversal equipment would be a better option.
High Impedance Grounding is possible with central inverter configuration but not with the string inverters due to its topology. This is because central inverter configuration usually has galvanic isolation through an inverter transformer which is not the case with string inverters. Float controller (PID reversal tool) can be used in these particular cases.
Connection diagram of a PID reversal or float controller equipment
PID-reversal Equipment
Using PID-reversal equipment is one of the best and efficient ways to prevent and reverse PID in old installations. This equipment indeed asks for additional investment, but after a point, it will payout its cost by improvement in the plant’s generation through PID reversal. This equipment is known as a float controller, or simply PID-reversal tool
Giving context to the above statement: Consider the cost of a float controller as 3 lacs. With even a 2% improvement at a 1 MW inverter level, the increase in the generation will be approx 40,000 units per year.
If we consider INR 3/unit cost, then there will be an increment of ~1.2 lacs per year. Therefore, the simple payback period will be around 2.5 years.
Connection and working: This box is installed between the PV array and the inverter. It senses the PVarray voltage. If the voltage is higher than a set threshold, the voltage is not applied. At night, when the PV array voltage falls, then a DC bias voltage is applied between the array and ground, and controlled depolarization current flows from the semiconductor to the ARC, glass encapsulation, the frame, and the mounting structure.
This way, the PID effect is reversed gradually. Float Controllers can be used with any crystalline-Si modules which have been diagnosed to be suffering from PID.
Connection diagram of a PID box
Practical observation
PV-Diagnostics has worked on many diagnostics projects where we have observed a generation loss of even 10-12% at the plant level. To resolve this, we installed a PID-reversal tool to reverse the effect of PID. One such case study is discussed below.
The plant under consideration is 15 MW located in Gujarat. During our diagnostics study, we observed that the overall average module degradation was about 17% (Expected degradation was approx. 8%). We recommended installing one PID reversal tool for ~3-4 months. The results are shared below:
%Voc improvement = 6.90%; %Pmax improvement = 12.56%
% Voc improvement = 12.55%; %Pmax improvement = 29.91%
Since PID mainly affects the Vmpp and FF of the module, it also affects the curvature of the IV curve. For the modules affected by PID, the average improvement in Voc and Pmax is 5.19% and 9.03% respectively, with respect to the module before PID reversal. The Voc and Pmax improvements are visible in the IV curve snapshots, while the difference is visible via EL Images too.
To conclude, PID causes a significant effect on a module's power output. The new modules usually are less prone to PID due to improved fabrication processes, but it is always a good practice to monitor this defect closely. If not, it could result in higher generation loss and irreversible conditions as in the case of thin-film modules.
You can contact us at ayush@pv-diagnostics.com or schedule a call with us at +91 9757051472 for more details.