Diagnostics – An Effective Approach for Solar industry

Diagnostics of a solar power plant can be an important tool to evaluate the “health” of a solar plant and identify measures to improve generation and optimize O&M. A typical diagnostics study would entail complete power flow mapping of a solar plant by measuring module degradation and damages, O&M efficacy, inverter efficiency, breakdown and plant availability etc. to understand where the developer is losing generation.

Problems observed in Solar Power Plants

Solar power plants have many major components like modules, structures, BoS components, etc. Each of these components needs specific maintenance and planned strategy for its optimal performance. We have observed various issues in these components, listed below:

1. Module-Level Defects

Module level defects can be identified through EL Imaging, Thermography and IV analysis. Defects identified through EL imaging include micro cracks, soldering defects, finger interruptions, Snail Trails, moisture ingression, solder flux interaction with busbars and PID. Following are some examples of EL images showing these different kinds of defects.

Fig. Finger interruptions

Fig. Snail Trails

Fig. Moisture ingression

Fig. Solder flux interaction with bus bars

Fig. PID

At a 21 MW plant in Rajasthan, we observed that approx 0.28% modules were severely damaged (>11.6%) and 4.6% modules were moderately damaged (8.8-11.6%). Apart from this around 95.13% of modules were observed with minor damages (>8.8%). When 1829 modules were taken for Thermal Imaging from the same plant, 115 modules were observed with hotspots.

2. Issues related to O&M

The O&M team is responsible for healthy lifespan of the plant as well as for addressing fatal issues immediately. Some of the specific issues are discussed below:

• Improper maintenance of Pyranometer: Often, the angle of the Pyranometer is changed or the pyranometer is not cleaned properly to reduce irradiance reading and show bloated PR, misleading the developer regarding the health of the plant. Abnormally high PR (>85%) is an indication that the pyranometer may not be recording correct radiation.

• Improper cleaning frequency and methodology: At some sites, cleaning is done during the peak hours. This practice is detrimental for the modules because during peak generation hours, the temperature of the modules is often very high and the cold water causes this temperature to fall quickly, leading to buildup of thermal stress.

Cleaning methodology and frequency is barely adjusted as per the site conditions. For example, a high TDS-rated water could lead to scaling on modules, whereas areas with sticky dust might need an IPA solution to clean the modules. If the cleaning is not done properly, there might be uneven soiling present on the modules which might lead to hotspots.

• Maintenance of Cables: If the cables are dangling and not laid properly in conduits, they can cause multiple ground faults and introduce the safety risk for the O&M team.
• Labeling of cables: If the cables are not labeled properly, the fault associated with the cables could take days to get resolved, which ideally should have taken just 2-3 hours maximum.

3.Other BoS Defects

• Poor inverter efficiency: At a 5.75 MWp plant in Rajasthan, thorough analysis for inverter efficiency was carried out. Data was collected on a daily basis for 18 consecutive days. Out of the 7 inverters, two were reportedly underperforming. One peculiar observation was that the inverter having the highest normalised generation was the least efficient. This variation was quite unusual. Thus the inverter was replaced and a successful claim was raised with the manufacturer. A performance gain of 1% was recorded for the entire plant.

Corrosion:

• Improper maintenance and lack of knowledge about geographical conditions can accelerate corrosion in different components of a solar plant which can impact the longevity of the solar plant. Corrosion in earth strips can cause continuitybreakage of the grounding system which can turn into a serious hazard. The resistance will increase and the plants will be exposed to natural calamities like lightning.

At a 50 MW plant, located in the basin of a salt lake in Rajasthan, we observed severe corrosion in the structures due to the high salt content in the atmosphere coupled with water-based cleaning of modules twice a month. Even alternate waterless cleaning cycles could have significantly reduced the rate of corrosion while maintaining low soiling levels on the modules.

1. Module level solutions

• Identification of diode failures, severe hotspots and isolations Drone based thermography can help in identifying modules with diode failures, severe hotspots and isolated modules. These modules should be replaced immediately as they could impact the generation from the entire string in which they are connected. E.g. at a 21 MW plant in Rajasthan, we found that 130 modules were broken and still connected to the strings. The cumulative capacity of these 130 modules was 144 kWp, however, due to the reduced generation from the entire string, their net impact on the overall capacity of the plant was 18.05 kWp.
• PID reversal (Potential Induced Degradation)
• PID is another major issue existing in plants which do not have a functional negative grounding at inverters. The degradation due to PID can be reversed by subjecting the string to a high positive voltage in the evening. At a 15MWp plant in Gujarat, a commercially available PID reversal tool was installed for a period of 126 days. The results from the exercise are given below:

Results of PID reversal tool deployment are shared below

We observed the Voc to improve by 12.55% and Pmax to improve by 29.91% in the module above. Average improvement in power was 9.03% with respect to the Pm of the module before PID reversal.

• Re-binning of modules to reduce mis-match loss:

If the modules in a plant are highly degraded and the variation in degradation of individual modules is high, then re-binning of modules can be conducted to reduce mis-match losses. A combination of re-binning along with PID reversal or module replacement can lead to much higher increase in generation from the plant.

At a 8MW site in Gujarat, we were able to achieve an increase in generation by ~1% due to re-binning of modules.

2. O&M level solutions

Most of the O&M issues can be avoided with proper awareness and training. The solutions related to O&M are situation specific. For instance, if there is under-performance because of soiling in the system, the easiest and most effective solution is to conduct periodic cleaning. Many times, the cleaning itself poses a threat to the system causing moisture ingression and corrosion. Hence, novel O&M practices like robot-driven mopping and reducing the frequency of washing if water salinity is high, should be adopted according to the site conditions.

At a 15MWp plant in Gujarat, the above practices were enforced and an overall average power gain of 2% was observed. And not just soiling, a lot of loss-causing defects can be checked in time if close attention is paid to the overall condition of the plant components. That being said, it’s also imperative that thorough inspection must be carried out by the O&M personnel, consistently.

3.BoS level solutions

• If the inverters are not working optimally, it is always recommended to take this up to the manufacturer. For this, the efficiency of the inverters needs to be regularly monitored.
• Wherever loose connections or terminations are identified, severe hotspots have been found. Periodic thermography is recommended. If the fasteners are properly tightened, the reduction in generation losses is significant.
• Improper Labelling and Dressing of cables is a major issue found across several plants. Whenever the cables and connectors are incorrectly labeled, or if their layout is flawed, it can pose a serious risk to the O&M operations on site.
• Challenges with Seasonal tilt-type of structure need particular mention. Quite a number of sites were reported to be having cases of misalignment of panel tables, leading to inter-shading of modules and torsional stresses being developed in the mounting structures. Regular inspection of affixations and inclinations is vital for the plant.
• Corrosion and related maintenance was also a common yet crucial BoS challenge encountered. Several sites have poorly maintained earth pits, severe corrosion across the earthing electrodes, GI coating of strips exposed, etc. These require immediate redressal or might result in rapid deterioration of the plant.

Conclusion

With the solar industry growing fast, it is necessary to address the issues related to module defects, O&M practices, inverter and other BoS component’s performance to ensure optimum performance of the plant. It is extremely important to keep the health of these components up to the mark as they all contribute to the optimum performance of the plant. If you have reasons to believe that the plant is underperforming, ensure that you conduct frequent diagnostics to evaluate the reasons behind this underperformance and take appropriate measures.