Low temperature PV string bypass relay/switching

[Symbioquine]

I'm curious whether anyone has worked out a circuit that bypasses one of the panels in a PV string at low temperatures?

It seems like such a device could be very simple and allow normal operation with larger strings than the temperature coefficient and absolute lowest expected temperature might otherwise dictate.

[NiHaoMike]

Easier would be a shunt regulator that connects a dummy load to clamp the voltage.

[Symbioquine]

Sounds interesting, but I guess I'm nervous about trying to "respond to the high voltage" in the moment with any active components since (at least naively) it seems like the response time might not be good enough to protect the solar charge controller (SCC). My understanding is that the first sunlight on the panels on a frosty morning is often the death knell of many a good SCC.

That's why I thought a mechanism to take one or more of the panels out of the string preemptively when the temperature is dangerously low might be safer/simpler.

[NiHaoMike]

The rise time is going to be very slow due to the capacitance of the panel. It wouldn't be hard to have an "amplified Zener" circuit that responds plenty fast.

[Symbioquine]

Interesting! Why wouldn't all the mid to high end SCCs do that then?

I've only heard of Midnite Solar's equipment having provisions (HyperVOC) to survive extreme over voltage scenarios and, even then, the approach they took disables charging during those conditions.

I was assuming that if there was something simple and reliable that could be done at the SCC end of things it would be generally available in the consumer market - maybe not in every SCC, but at least in the likes of Victron, Schneider, etc.

That's why I thought something that modifies the panel series topology might be called for.

[NiHaoMike]

The dummy load costs a fair amount to implement and the resistance needed depends on the panels. A good first step is to use a resistor value of about Vmp/Imp for the string. (That will be fairly large physically, probably easiest to use multiple resistors, most likely heating elements.) Use a MOSFET or IGBT to switch in the resistor when the voltage nears the charge controller's rated maximum and out when the voltage dips a bit below that as sort of a simple PWM.

[gf]

A shunt regulator across the string would need to dissipate a lot of power when the charge controller cannot consume power due to full battery, or when the panels are overprovisioned wrt. to the maximum power of the charge controller. For a string with say 10x 400W panels this could be up to 4kW (worst case).