Heating up? Better not...

After explaining the influence of UV, let′s take a look into another "killer" of the cables - HEAT.

If the temperature is controlled, and the design of the installation is aware of the heat exposure, it is OK. But there are plenty of situations where the heat is not expected (unpredicted) and this can lead into various problems.

Mr. Arrhenius & his diagram

Svante August Arrhenius was a Swedish scientist, awarded with the Nobel Prize in 1903. He also contributed to the fundament of the whole science of physical chemistry.

Nowadays, he is more popular because of his studies of the Greenhouse Effects, but except for this and other important inventions, there was one, which was really important to recognize in our today′s topic.

Based on his proposal, the Arrhenius equation has been determined. As it is a very complex definition, I will wrap it into a very simplified explanation - there is a measurable reaction rate (in our case material ageing) in relation to the temperature.

Arrhenius Diagram/ Temperature Index

See the blue line, which indicates a temperature index/thermal endurance of radiation-crosslinked polymeric insulation material of the cable insulation. The line would be different for various materials, but the compounds used for PV cable Studer BetaFlam defines the Operating Temperature for conductor temperature at the conductor at 120°C (blue-dotted line). However, it is very important to combine the temperature indication with a matching period of lifetime - in this case it is defined at 20.000hours (green-dotted line).

The red-dotted line indicates an extrapolated life time at 90°C conductor temperature. And in contrast with the 120°C / 20.000hours - take a look onto the yellow-dotted line - it lays by 130°C, but 10.000hours only - means 1/2 of the 120°C!!

Just to make the information complete - the cable ageing method is described in the Standard IEC 60216-1.

Lesson learned: Importance to observe the possible heat traps within the PV installation. Even a seemingly niche temperature increase can rapidly decrease the lifetime of the cable!

Temperature traps in the real life

Now comes the question - where and how to avoid the temperature traps. There are plenty of risky spots, but just few examples....

Junction box:

Due to the constant price pressure, most of the junction boxes do not have any heat-sink anymore, which means that in case of the (over)heating of the diodes, all the heat escapes through the terminals, and - of course - cable!

As the diodes operate quite frequently, the cable length of 5-15cm after the j-box entry is enormously exposed to the heat. And this part of junction-box cable is extremely sensitive to ageing then. The O&M teams shall pay special attention to the cable outlets at junction boxes, as those are frequently affected.

The picture above shows a frequently happening symptom - greyish/white surface discoloration as a result of high temperatures. Bear in mind that the insulation of cross-linked cables does not melt, but the first indication which you can see is a discoloration of the surface.

Attention - the same what you see on the junction box side of the cable, the same can be observed on the side of assembled connector. However, important to say that in case of some assembly failure of the connector, typically you see the damage of the connector even sooner than the cable.

Cable Management/Cable-sizing

The way of laying the cable also influences the maximum current load to the cables. The more amps flowing through the cable - the higher temperature. The higher temperature - the higher risk.

It is important to pay attention and make sure that the required current transmission is aligned with the cable cross-section and cable-management method. Especially, when grouping cables together in a closure (conduit, duct), the situation becomes tricky and the solution shall be well designed & calculated. Underestimating of the above also leads into unnecessary overheating of the cables!

I have already shown this picture in one of my previous posts, but this is an excellent example of how to negatively affect the heat-trap within the cable management.

And again, see the discoloration of the surface (greyish/white/greenish) color appearing on the cable surface. Another frequently seen phenomenon is that the cables are becoming sticky.

Wrong Installation

Overheating of the conductor is not only a matter of string cables, but also main cables are frequently affected. Not following the instructions will lead into higher resistance. Higher resistance (in better case) means lower output of the PV plant (lower performance, lower earnings). In the worse case, higher temperatures lead into faster degradation of the cables, fires, damages....

Wrap-up

Watch the specification of the cable. Besides the operating temperature range, the indication of the time which can be considered for the defined temperature range shall be provided by the manufacturer. Otherwise the information is incomplete and can be misleading.

Temperature index as noted in the IEC60216 describes the performance of the plastic in the long-term run. It defines the ageing temperature, at which the material still has an elongation at break of 50% after 20.000 hours.

The examples listed above were showing hyperbolic situations, but remember to learn & understand the Arrhenius curve, and do not underestimate the heat management within the PV installation, and keep in mind:

Each 10°C in addition = cutting the lifetime by 50%!!