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.

10 Practices to avoid for best-in-class solar O&M

Indian solar industry has grown tremendously over the last decade, a trend that is expected to continue in the coming years. India has become one of the cheapest producers of solar power worldwide. While this cost reduction was driven mainly by the falling module prices, increased competition and aggressive bidding had led many developers to cut down on their O&M budget. Lower O&M budgets can downgrade the sophistication of O&M practices.

Here’s a list of 10 items that lean O&M teams should avoid ensuring best-in-class O&M with a limited budget.

01. Improper maintenance of pyranometer 

The angle at which the pyranometer is installed should be the same as the tilt at which solar panels are installed to find the PR ratio (=Specific Energy Output/Radiation per sq m per day). Many times, the angle of the Pyranometer is changed so that less radiation falls on it as a result of which, the input of the Pyranometer reduces, and therefore PR increases. Other than this, pyranometers must be cleaned regularly but this practice is not followed everywhere. As a result, the dust gets settled on the glass and less radiation falls on the Pyranometer. This again results in faulty data on its PR ratio. Abnormally high PR (>85%) is an indication that the pyranometer may not be recording correct radiation.

02. Ignoring safety measures on-site

The site team often ignores safety measures while performing O&M activities and risks their lives. Technicians may not be wearing all safety gear like helmets, shoes, working gloves, working jackets, etc. For instance, if a cable is broken and needs to be fixed, the technician may not use insulated tools or rubber gloves or goggles. If the cables are live, they might injure the technician involved in the process and can even be a cause for death.

03. Lack of proactive testing

SCADA and monitoring software used in O&M have their own limitations and do not give the developer an understanding of the module level defects in the plant. Many times, critical damages like cell cracks, hotspots, PID or even module and string isolations can go unidentified for long periods. Therefore, it is important that the O&M team conducts periodic testing of the modules - including IV curve measurements, EL imaging, and drone-based thermography. Below is an EL image of a module with PID and a power loss of 55.79%.

04. Improper handling of cables

The cables in the solar power plants are located under the modules very near to the ground surface. These plants are in open areas where there’s a lot of vegetation. Over time, these cables hide inside the thick bushes and this causes difficulty in O&M of the cables. 

Many times, it has also been observed that the cables are dangling and not laid properly in the conduits. This can possibly invite multiple ground faults and is a safety risk.

05. Improper labeling of cables

When a plant is established, all the cables are labeled for future reference and data is fed in SCADA accordingly. As time progresses, many labels get damaged or disrupted, as a result of which, whenever a fault occurs on the cables, it becomes quite difficult to identify the cable and resolve the issue. If the cables are labeled correctly, the resolving process takes around 1-2 hours which might extend to 2-3 days if not labeling is not proper. Therefore it is important that the labels be restored periodically. (Labelling is missing - in the caption)

06. Improper cleaning frequency and methodology

Key problems with cleaning

• In some places, cleaning is done during peak hours. This practice is detrimental for the modules because during peak generation hours, the temperature of the modules is very high and the cold water will cause this high temperature to fall quickly, leading to thermal stress. 

• Cleaning frequency and methodology should be derived from the nature of soiling observed on the site. For this, soiling loss measurement can be done at regular intervals. For example; High TDS level could lead to scaling of modules; Sticky dust might need cleaning with IPA solution

• Mopping from the edges to remove dust deposition may not be done properly which reduces transmission

• Uneven soiling occurs if there is no proper cleaning 

• Discoloration might also occur due to cleaning of modules with hard water

• The cleaning of modules may be done with brushes and hard detergents which leave scratches on the modules.

07. Improper module handling

The workers involved in cleaning the modules may damage the modules by kneeling, dropping other equipment, walking or climbing on the modules, leading to micro-cracks.

08. Improper collection of plant data

For preventive maintenance of the plant, certain data sets need to be collected and analyzed at varying frequencies. For instance, daily global horizontal irradiance, maximum DC/AC power, generation start and stop time, DC generated at strings should be collected on a daily basis while net energy generated at the inverter, net energy fed at the meter, normalized energy generated, monthly DC energy loss are calculated on monthly basis. Some data is also collected on a yearly basis like annual average performance ratio, annual average plant availability avoided CO2 consumption, etc. It has been seen that this data is not maintained in plants and therefore many problems may not be detected in a timely manner. This is one of the major practices in O&M that needs to be corrected for the proper functioning of solar power plants and increasing the plant lifespan.

 09. Inadequate inventory availability

Inadequate inventory can prolong the downtime as the site team may not have the right materials to fix a problem. Since most plants are located at remote locations, the restoration of inventory may take a longer time, hence leading to a longer breakdown. Certain equipment and components are a must, however, it is a common observation that these components either may not be present at the site or may not be available in sufficient quantities. Some of the critical inventories are MC4 connectors, string cables,  string fuse, inverter dc fuse, etc.

10. Preventive maintenance not based on proper root cause analysis

When an issue is detected by the O&M teams, it is often resolved on a short-term basis and the O&M teams fail to build a preventive plan based on proper root cause analysis, which sometimes further escalates the problem. An example of this is a plant in Rajasthan where the module mounting structures did not provide a scope of expansion to the modules. As a result, the frame of the modules started bulging due to excessive heat in the region, causing moisture penetration within the modules, reduced insulation resistance and repeated tripping of the inverters. In order to resolve this issue, the O&M teams bypassed the GFDI to prevent the tripping of inverters instead of solving the problem of insulation resistance and module expansion. By-passing GFDI further led to excessive Potential Induced Degradation in the plant leading to power loss. A preventive plan based on proper root cause analysis could have reduced this power loss significantly. 

To conclude, O&M is not just grass-cutting, cleaning, and security. O&M is responsible for maintaining overall plant efficiency and energy delivery along with minimizing losses in the plant. It ensures ease of operations, safety, and reliability of equipment involved in the plant. With certainly added care during O&M, the plant lifespan can be increased significantly.