Significance of solar power plant Diagnostics
With the rise of the demand for renewable power sources, one of the front-runners is solar power. Its installation has increased 10 fold in the last decade itself. This exponential increase in solar plant installations makes it imperative to ensure the best quality of modules and other BoS components are installed in the plant. This is essential to achieve optimum plant performance. Since each of the components has its own set of technical parameters, they need to be tracked and assessed periodically. The tests listed below cover all the major components in a solar power plant and their purpose. Let’s get into it.
Thermal Imaging
Thermal testing provides the thermal signature of the modules and BoS components which further helps in identifying the possible issues due to considerable temperature differences. This temperature difference is visual from the hotspots captured in the thermal images. This is a non-invasive technique to test the module’s health.
Inferences possible from the testing
Identification of possible uneven soiling
Modules with shadow loss or bird droppings
Nut bolts or connecting points with corrosion in BoS components
Loose connections
Technology, its advantages, and challenges
Thermal image of a thin film module
This testing can be easily conducted on mono or poly-crystalline modules. In thin-film modules, the chances of hotspots are very less due to the less current generation. Also, due to their shiny texture, it is difficult to distinguish hotspots from glares.
I-V Curve Measurements
The current-voltage (I-V) curve shows the relationship between the output voltage and current of a pv module/string. This testing is primarily conducted to understand the relative health of the strings and/or modules. Since IV curves capture all the operating points of current and voltage, the difference in these operating points (after translating it to STC) from the datasheet values signifies under-performance.
Inferences possible from the testing
Relative estimation of underperformance in the strings and modules
The shape of the IV curve gives a gist of possible defects in the modules and/or strings which can be further validated by other tests like EL Imaging.
Technology, its advantages, and challenges
IV measurement can be conducted in the following ways:
In a stationary lab
In a mobile lab
On-site with a portable IV tracer tool
Even though on-site testing is less accurate, it is a very good indicator of module performance when coupled with EL and/or thermography.
EL Imaging
Electroluminescence (EL) testing is used to identify inherent defects of the modules which might have come at the production, transportation, installation, or operations stage of the plant’s life. This is a fast and non-destructive technique with a measurement time of approx. 1s per module. The testing is done in the dark so that IR from the sun does not interfere with the IR luminescence from the module.
Inferences possible from the testing
Some of the major defects identifiable with EL testing are Cracks (micro, partial, full, spider, point, compounds, etc.), bad soldering, snail trails, moisture ingression, finger interruptions, PID, diode failures, etc.
This test helps in the identification of manufacturing defects from other defects since manufacturing defects can be insured at a later stage as per the QAP of the manufacturer.
Technology, its advantages, and challenges
This test can be easily conducted on mono and poly-crystalline modules but not on the thin film. The reason behind this is the weak luminescence signature and low current which makes it difficult to distinguish the module signal from other outside signals (noise). Amorphous Si and CdTe technology testing can be managed indoors.
EL Image of a mono/poly-crystalline module
EL Imaging of a thin film module
Inverter testing
This test helps in determining the health quotation of the inverter. There are 2 major tests conducted:
Efficiency test - It determines the dc to ac energy conversion efficacy of the inverter.
THD analysis - This analysis helps in determining the amount of DC wasted as harmonics due to energy conversion from DC to AC
Another important test conducted on inverters is Thermal analysis which identifies any loose connections or corrosion on the nut bolts or other connections in the inverter.
Inferences possible from the testing
Efficiency of inverter
Operational or manufacturing defect identification
Harmonic content
Technology, its advantages, and challenges
In the solar power plant, there are majorly two types of inverters:
Central inverters - These inverters are usually of high capacity and accommodate multiple SCBs (String Combiner Box).
String inverters - These inverters are smaller in capacity and strings are directly connected to them. Therefore, there is no need for SCBs in the case of the installation of string inverters.
Major challenges in the case of the central inverter is that they have large bus bars and there is a challenge to attach the clamps to conduct the testing. Also, since the inverter needs to be opened during testing, it sometimes voids the inverter warranty. This is true for both central and string inverters.
PR analysis
On-site pyranometer recordings are compared with another pyranometer which is installed for a few days on the site to ensure that the radiation recorded by the site pyranometer is correct. This further helps in determining an accurate Performance Ratio.
Inferences possible from the testing
Comparison between on-site and other reference radiation
Determines accurate Performance Ratio of the plant
Technology, its advantages, and challenges
There are majorly 3 types of pyranometer - Thermophile, silicon cell, and silicon photodiode.
The thermophile type pyranometer gives a similar response to all the different spectrums, while the other two are dependent on the spectral response. Due to the dependency on spectral response, silicon cells and silicon photodiode are less accurate.
The response time of silicon cell and silicon photo-diode (in milliseconds) is less as compared to thermophile (in seconds)
Silicon cells and silicon photodiodes are cheaper than thermophiles. For utility-scale plants, thermophile is preferred while for small plants, silicon cell/ silicon photodiodes are preferred.
PV Diagnostics has conducted many health check-ups of solar power plants in which all these tests are conducted strategically to target the main issue in the plant. This allows us to identify the main causes of underperformance and helps the client in resolving them at the earliest. You can contact us at rohit@pv-diagnostics.com or call us at +91 85275 31188 for more details.