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Solution for panels slightly over Voc but operate fine otherwise?

adirondack_wanderer

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Aug 12, 2023
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Adirondacks
I had my first over-voltage issue today after roughly a year without any problems. My charge controller is limited to 150V and I use 4 PV inputs of 3 strings of 100W panels with 6 in series (total 12 arrays and 72 panels) and each panel's Voc is listed at 24.3V. The arrays are house roof-mounted and paralleled with Y-adapters. This has not been a problem for roughly a year but today I flipped on the PV disconnect and the voltage was reading roughly 154V and of course the charge controller stated over voltage. Normally the PV disconnect is on and as the sun comes up the voltage slowly approaches 150V and usually doesn't go above an operating voltage of 130V but yesterday I had to turn them off for maintenance with my batteries - nothing to do with the arrays. Yesterday there was some snowfall so this morning I got all the panels cleared and today's weather is roughly 10°F with snow-covered grounds and the sun is beaming bright and I'm getting huge production from other strings without issue. Is there any workaround for this scenario, like something that will throttle down the voltage slightly instead of me doing something like removing 1 panel as a temporary mitigation? Ultimately I can deal with this as it just seems to be two separate arrays that are going over-voltage and the other 10 are doing fine, but I'm interested in any suggestions anyone may have.
 
You already received the orthodox response.

If you really want to make this work, you could devise a clamp circuit that diverts power when the voltage reaches, say, 149 volts to a heat sink.

Manually, you could switch in a load resistor on the PV input to get under 150 volts, then once the MPPT is working, switch it out again. An automatic circuit might oscillate or it would need a large hysteresis, say on at 149 volts, off at 135 volts.

So this isn't trivial to do, but possible.

Mike C.
 
Easy solution is to wait a few hours until it is dark.fyi: you may have problems when running grid down and the batteries are near full. Pv will throttle, raising voltage.
 
Easy solution is to wait a few hours until it is dark.fyi: you may have problems when running grid down and the batteries are near full. Pv will throttle, raising voltage.
Yep - this appears to be a one-off and I only had the PV disconnected for maintenance but I'll flip it back on tonight and I expect tomorrow it will slowly ramp up to ~140V and not hit that Voc of 150V+. I was hoping there was a device I could add to mitigate this if it ever comes up again but aside from @mciholas 's suggestion I think I'll just leave it. Thanks anyway!
 
The most practical solution to this is a voltage clamp or a progressive load dump that cuts in just below max safe voltage.
The amount of power diverted need not be that great, just sufficient to pull the voltage down under conditions of high sun, cold, and minimal panel loading.

What is needed is a shunt voltage regulator that starts drawing power, as much power as is required, to hold the voltage at some safe maximum.
There is a thread here about a simple home made MPPT constant voltage solar controller that works exactly as required.

https://diysolarforum.com/threads/simple-home-made-analog-mppt-contoller.47057/

This is NOT intended to replace your existing MPPT controller, but to connect to the panels in question along with your existing controller, to act as a maximum voltage limiter.
The output of this second controller could be fed into a suitable load dump resistor, which would only need to be of relatively low power.
 
Yep - this appears to be a one-off and I only had the PV disconnected for maintenance but I'll flip it back on tonight and I expect tomorrow it will slowly ramp up to ~140V and not hit that Voc of 150V+. I was hoping there was a device I could add to mitigate this if it ever comes up again but aside from @mciholas 's suggestion I think I'll just leave it. Thanks anyway!

Have you considered that you're extremely lucky that your MPPT wasn't destroyed?

Have you considered the next time this happens, your MPPT may be destroyed?

Are you still willing to be careless about it?
 
Have you considered that you're extremely lucky that your MPPT wasn't destroyed?
Hmm, 154 volts on a 150 volt rated input is unlikely to damage anything, less than 3% over. So the risk is quite minimal given there is design margin in the inverter.

If 154 volts would damage the inverter, then the ever so slightest impulse from a lightning strike moderately close would be an issue.

Are you still willing to be careless about it?
There are times to be preachy about it, I don't think this is one of those times.

Still, it is obviously correct to stay under the input rating.

A novice learns the rules.
A journeyman follows the rules.
An expert writes the rules.
A guru knows when they can break the rules.

Mike C.
 
No need to reinvent the wheel.
Just fix the mistake.
Mistakes happen, it's how we deal with them that matters.
 
Yep - this appears to be a one-off and I only had the PV disconnected for maintenance but I'll flip it back on tonight and I expect tomorrow it will slowly ramp up to ~140V and not hit that Voc of 150V+. I was hoping there was a device I could add to mitigate this if it ever comes up again but aside from @mciholas 's suggestion I think I'll just leave it. Thanks anyway!
I think you're misunderstanding Voc. The voltage measurement with the disconnect open is the correct open circuit number. There is still likely some rather high impedance load when the MPPT is off and the disconnect is closed.

You're taking a calculated risk and you need to have a game plan for frying the MPPTs. Like having spares ready to go.

Since you hit exceeded Voc within 1 year, I would say the chances of exceeding it every year is probably high (granted, it's a polar vortex year, but we're having 100 year floods more regularly these days)

EDIT: I think going over Voc spec, with pre-positioned spares (or accepting the risk of however long the outage will be to locate and get one sent to you) is reasonable.

Going over VoC spec and using hope, is not reasonable. I don't think there is enough information about the margins to go this route.
 
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Hmm, 154 volts on a 150 volt rated input is unlikely to damage anything, less than 3% over. So the risk is quite minimal given there is design margin in the inverter.

On what do you base 3% being okay? 1%? 10%? Where's the line? What's the rating of the component that can't tolerate the over-volt? What's its tolerance?

If 154 volts would damage the inverter, then the ever so slightest impulse from a lightning strike moderately close would be an issue.

Isn't that to be expected? Lightning fucks things up from a distance. Unless you have specific lightning protection for a site, it should be no surprise when equipment is damaged even if you've done everything correctly.

There are times to be preachy about it, I don't think this is one of those times.

Still, it is obviously correct to stay under the input rating.

Feels like you kinda can't make up your mind here.

Talk about preachy:

A novice learns the rules.
A journeyman follows the rules.
An expert writes the rules.

A guru knows when they can break the rules.

Please identify the guru. Personally, I don't see one in this thread, but it almost seems like you're claiming the title as you're encouraging breaking the rules, thus implying you know when it's okay... but then you waffle... so, are you the reluctant guru? I don't see that defined on your list.

The OP has clearly indicated he intends to take no action whatsoever even after overvolting his MPPT.

I think it's okay to be preachy to keep others from developing such a cavalier attitude.

Sure. He'll be fine. Until he isn't.

I made the same mistake. 3S 72 cell panels on a 150V MPPT. I learned the lesson before I over-volted it and paid tuition via a 250V controller and left my panels 3S. The highest I've seen logged SINCE replacement is 152V.

I discourage others from making mistakes, especially the ones I've made.
 
Do your temperatures ever go below 10ºF? VOC goes higher as temperatures go lower. If you are getting overvoltage warning at 10ºF, consider yourself lucky and fix the problem otherwise you will blow something up when temp goes lower.
 
So it appears that the system was set up with about a 2% margin between the printed Voc and the max the controller is rated for. With three parallel strings to boot. Is it common to push the boundaries like that? To me, that seems like asking for trouble. I like all of my equipment to have at least a little bit of headroom. Things seem to last longer that way, and also generally seem to be more robust as a system.
 
I'm going to continue to be passive aggressive, and point out that the OP in post 1 did not say anything about an ante factum VOC temperature compensation calculation, and there appears to be some target fixation on the chosen path.

"each panel's Voc is listed at 24.3V " -> This is STC, or at best NOCT.

While I may not agree with mciholas's position... his more YOLO stance than mine likely assumes someone did calculations like that up front.
 
Hmm, 154 volts on a 150 volt rated input ...

If 154 volts would damage the inverter ...

If nothing else, it is certain to damage your warranty.

A novice learns the rules.
A journeyman follows the rules.
An expert writes the rules.
A guru knows when they can break the rules.

Which one are you?

A wise man doesn't break rules unless there is a damn good reason.

Is this you?


Is this your equipment in question? (you didn't give us model numbers)


Maybe you are accidentally complying with the rules.
Or maybe by "charge controller" you meant not the built in MPPT.

I'm going to continue to be passive aggressive, and point out that the OP in post 1 did not say anything about an ante factum VOC temperature compensation calculation, and there appears to be some target fixation on the chosen path.

"each panel's Voc is listed at 24.3V " -> This is STC, or at best NOCT.

While I may not agree with mciholas's position... his more YOLO stance than mine likely assumes someone did calculations like that up front.

If OP is experiencing the warmest winter on record ...
(Oh, that was last year)

 
There was even an EG4 18kPV user who got too close to the Voc limit (was still below) and had to have parts replaced. Midnite solar warns you of damage if you go over the Voc rating.

And as @Hedges warns with regard to warranty, Victron has this for their RS charge controller at least:
Screenshot 2025-01-20 at 8.53.44 PM.png
 
On what do you base 3% being okay?
Experience. I design electronics. I know what margins are typically used for ratings over the spec sheet, and then what margins exist beyond the part ratings.

Nothing can be designed to within 3% of a failure point. Over unit variation and temperature, there would be field failures.

10% is almost always fine, BTW. Start getting above 30% then things can go boom. I regularly see parts survive double their voltage ratings.

Sure. He'll be fine. Until he isn't.
At 3% over, he will never experience a failure due to over voltage. If the device will work reliably at 150 volts, it won't be damaged at 154 volts.

Mike C.
 

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