• Airprobe

Severe Hotspots clustered on the modules! Something you would definitely want to avoid.

Updated: Jul 29

Quite an unusual thermal image right?

Is it normal hotspots? cracked cells? bypass diode issues?

Thermal imaging professionals will jump to a conclusion its a short circuited string!

But is that true?

We have researched on what causes this kind of hitch in the PV sites through years of experience and we intend to share our knowledge with you so that you can take preventive actions to avoid such glitch.

Let us deep dive into the visual inspection AirProbe team performed to diagnose this mystery. Following were few of the visual observations and measurements done at site:

1. Few Bypass diodes in the string were short-circuited.

2. The junction boxes recorded high temperatures.

3. The Voc rating of the module was lower than its rating.

4. Most of the modules in this string were physically isolated.

On further investigating the issue we found that the site had experienced heavy rain and thunder lightning a few days back. This string was located in one of the corners of the site and it was out of the area of influence of the installed lightning arresters which may have been the primary reason for it to get affected by the thunder and lightning.

When such a lightning strike occurs over a solar module mounting structure, the metallic parts get exposed to very high voltage. It causes high current to flow and that results in damaging the bypass diodes which makes it a permanent short circuit.

But what caused these high-temperature hotspots in the modules?

In general, when the bypass diode gets is shorted, it leads to a short circuit current flow across the 24 cells in the module. In this situation, any small non-uniformity on the module caused due to slightest of dust particle (even on a clean module), causes a mismatch of solar radiation falling on the cells resulting in severe hotspots.

What about the variation in Voc?

The module’s expected Voc rating was 45 V but in on-site testing, it was recorded to vary from 26.3V to 0V for few modules. The shorted bypass diode was one of the major reasons for reduced voltage or 0V.

Why was the string physically isolated from the system?

The bypass diode short circuit had also resulted in reduced string voltage. To avoid string voltage mismatch losses, it is recommended to keep such strings isolated. Thanks to O&M team associates of the site, for on time isolation of affected string to avoid system losses.

Based on all our findings the key takeaways are:

1. The site layout should be checked thoroughly for trustworthiness while installation – The lightning arrester should be strategically placed in the site such that it is able to avoid a lightning strike on the modules.

2. A sound O&M team is very eminent for quick remedial actions at the sites – Let’s not ignore the thoughtfulness of the site’s O&M team which disconnected the modules expected to have been affected by the short-circuited bypass diode issue.

3. Thermal inspections are a great way to inspect the sites - Thermal inspection is a great way to derive insights that can be backed by visual inspection which are guided by thermal irregularity.

We love sharing our experiences and contributing our bit to the solar PV industry worldwide!

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