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Ayush Mahajan 5/3/21 Ayush Mahajan 5/3/21

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. Based on some of the diagnostics studies conducted by PV Diagnostics, we have identified the key problems observed in solar power plants and some of the solutions that have helped in improving 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:

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.

**Solder flux interaction with bus bars**

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.

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.

Tilt angle of Pyranometer is changed

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.

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.

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.

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 continuity breakage 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.

Corroded earthing strip and pipe

Corroded GI structure and Joint strips

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.

EL images before and after reversal

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

Re-binning of modules to reduce mismatch 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 mismatch losses. A combination of re-binning along with PID reversal or module replacement can lead to a much higher increase in generation from the plant. At an 8MW site in Gujarat, we were able to achieve an increase in generation by ~1% due to the re-binning of modules.

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.

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.

Conclusions

With the solar industry booming at its peak, 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.

Ayush Mahajan 4/30/21 Ayush Mahajan 4/30/21

Issues of Balance of System

BoS of a solar PV plant constitutes all the components of a PV system other than the modules. These components makeup roughly 15-20% of solar purchasing and installation costs and account for the majority of maintenance requirements. It is through the balance-of-system components that we control cost, increase efficiency, and modernize solar PV systems. It has been observed in the industry that BoS care is sometimes ignored in comparison to the modules. We are sharing some of the similar cases in which owners and developers learned this in the hard way that BoS holds as important a place in the system as the solar modules.

Design level issues

At a 12.5 MWp plant in Telangana, we diagnosed serious issues with the tracking system and other BoS components. Some of these issues and suggested recommendations are described below:

• Driveline slip due to slotted joints: The driveline is supposed to act as a rigid member to rotate all the torque struts identically. Since a single driveline is not possible in a plant, it is usually broken into segments and then connected. It was observed that there were long slots in these drivelines which resulted in slipping of slotted joints and improper operation of the whole tracker block. We recommended welding these slotted bolts and creating a permanent connection.

• Lack of sufficient Lightning Arrestors: According to the LA drawings, there were 4 lightning arrestors installed in the plant which were not able to cover all the plant area for protection (the circled area shows the approach of LAs). It was evident from this LA drawing that in any severe lightning episode, the installed lightning arrestors will not be capable of safeguarding every module. After analyzing these drawings, we recommended the installation of 4 additional LAs in the region.

LA diagram showing unprotected plant areas

• Health Safety Environment (HSE): HSE protocols are made to ensure the safety of workers as well as visitors in the plant but these protocols were not followed in this plant. For instance, the protocol specifies to install safety signs on all the high voltage areas, under construction areas and other watchful locations to alert the workers/visitors in advance of the risks around them.

Similarly, fire protocols state the strict availability of fire extinguishers near transformers, switchyard, inverter station, control room, and other fire-prone areas which were nowhere to be found.

We recommended following the HSE protocols and brief everyone the precautions needed to be taken until HSE protocols are installed in the plant.

• Drainage issues: The soil in the location was clayey. Since clayey soil consists of very fine particles and not many organic materials, it’s usually sticky and compact. As a result, this soil is incapable of draining well and increases the water-logging issues wherever present. We suggested a drainage system that included culverts, internal drains of 9.8 km, and peripheral drains of 5.5 km drainage and Hume pipes to connect the drainage path to overcome this.

Water Logging Due to the nature of the soil

• Module cleaning system: The plant had unavailability of a proper water storage system and therefore water tankers were brought from outside for cleaning modules. The water brought in tankers was hard (TDS value was >500 PPM) which escalated the scaling on the modules and might reduce the generation output in the long run. Also, the majority of water in the tank was wasted as it was handled by unskilled workers. We suggested installing a module cleaning system (MCS) in the plant along with RO and borewell systems to eliminate this problem permanently.

• Plant approach road: The approach roads in the plant were in a dreadful condition and the situation was even worse during the rainy season. This created issues in providing basic care to the plant. The O&M teams faced trouble in reaching the site and took them a considerable amount of time to clear the faults than necessary. We suggested a proper road construction strategy along with the layout of the most efficient road for the plant.

2. At another 104 MWp plant in Maharashtra, we observed the below-listed issues in the plant.

• Electric poles in between the modules rows: This location had some electric poles in between the rows of the modules which cast a shadow on the modules and resulted in a lower generation. We suggested removing these electric poles to reduce the shading losses.

• Issues in the structure: We observed white rust on several braces which might lead to early deterioration of the structure members.

One brace member was found missing for most of the structures running along the East-West direction. This brace was ensuring stability in EW direction and therefore it was essential to add these braces as per the drawings.

Drawing showing missing brace in one of the tables.

Operation-level issues

AC cables attached in place of DC cables: In a 2 MW plant in Maharashtra, some DC cables were clipped from the side of the modules and robbed by the locals. The local team attached the AC cables of 2.5 mm2 in place of the DC cables of 4 mm2 as an immediate solution. Since the size of the AC cables were lesser than the DC cables, it reduced the current carrying capacity of the cables. At the same time, the voltage drop was increased (because length remained the same and diameter was reduced) which changed the maximum operating point of the system and the complete system started underperforming. Another issue with this remedy was that as per standards, double insulation cables are used on the DC side but they replaced the cables with single insulation cables which increased the risk of insulation failure, fire, or shocks as AC cables have less insulation.

Another major issue was using insulation tape to make the connections instead of proper MC4 connectors.

2. Higher temperature differences on the cable end-termination:
At a 5.75 MW plant in Maharashtra, high temperatures were observed at the terminations during the thermography of the BoS (IGBT section of the inverters and SCBs). It was observed that the cables were of copper, and lugs used for end-termination were of aluminum which caused this high-temperature difference. We suggested changing these aluminum lugs with bimetallic lugs to reduce the temperature difference.
Also, in most of the SCBs, the WAGO terminal block of strings was found with a high-temperature difference (approx. 20-30℃), for which we suggested replacing them as the terminal block deteriorated. In a few cases, SCB to Inverter outgoing DC cable and heat sink were also found with hotspots, for which we suggested tightening loose connections and replacements of rusted end-termination lugs.

DC section of the inverter: The temperature difference between 2 DC cables at termination end is 29.5℃ and 40.1℃ in 1st and 2nd image respectively

Aux. transformer: LV side - Temp diff = 27.1℃

Inverter- transformer: LV side - Temp diff = 40.9℃

In the plant-side switchyard, we observed one phase (out of three phases) of the isolator had a maximum temperature of ~180℃. After proper visual inspection, it was found that the panther conductor end-termination was not done properly due to which there was a loose connection. Therefore, we suggested immediate rectification of the end-termination.

3. Burnt SCB due to improper isolation - In a site in Rajasthan, a technical team was running IR measurements on SCB’s strings, completely unaware that one of the strings had ground fault because of which one of the SCB got burned. There were 2 main principle reasons behind it.
(i) Before the measurements, the technical team did not open the DC isolators which is a basic rule before taking any measurement.
(ii) While opening the fuse, they opened it very slowly which caused a spark and burnt the strings along with the SCB.

There was one more problem on the site which worsened the situation further. When the technical team reached for the fire extinguisher nearby, it was empty and therefore it took considerable time to get a hold on the fire. Until then, much of the cables were burnt which could have been saved if the fire extinguisher was filled. We suggested the field team to check all the fire extinguishers and ensure that they are filled to tackle any future episode like this.

4. SCB’s SPD issue due to earthing terminal not earthed: Surge Protection Device (SPD) is designed to limit transient overvoltages and divert current waves to the earth, to limit the amplitude of this overvoltage to a value that is not hazardous for the electrical installation and electrical switchgear and controlgear. This SPD is located inside the SCB and has 3 terminals: positive, negative, and earth. We have observed that positive, negative, and earth terminals of SPD are connected with the SCB but many times, technical teams ignore connecting the earth terminal of SPD to the ground. As a result, when these surges occur, SPD doesn’t have a path to divert the current waves to the earth and can’t protect the electrical installations from overvoltages. We observed one such fault in a 15 MW plant in the state of Gujarat and recommended to earth all the SPDs in the plant.

To conclude, the BoS system is responsible for the optimum working of the plant. A little ignorance can prove hazardous in the long run and therefore it is pivotal to look for little details while designing and operations of the BoS to ensure the best performance.

The big picture is - Attention to small details always shows the best result.