250 100 MPPT, using a relay to avoid PV over voltage

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chrisgru asked Jan 10, '22

250 100 MPPT, using a relay to avoid PV over voltage

Hi there. I have 5 48v VOC volt panels in series that I wish to use with my Victron 250 100 unit. I get a reading of 234 volts when I check the panels with an ohmmeter, but I am still worried that the VOC is too close for comfort to the 250 volt limit on the controller. It does get cold at this location, down to perhaps -7C a few times a year.

I was reading the relay programming options in the manual and I notice a mode where you can set the relay switch to trip when PV voltage exceeds a stated amount, and that it the trip condition will clear when the PV voltage comes down to a lower level which you can also set. This option is referred to as the "PV high voltage" mode for the relay.

Two questions, has anyone out there used this feature? It sounds pretty neat, those Victron guys seem to think of everything! I'd like to try this mode with the relay, but would really like to hear from anyone that has used it before with a few tips or any extra info.

Also, I'm running a 48v system, and this controller will have under 20 amps of PV of power from the two strings of 5 panels that are connected. Could anyone suggest a specific relay where the coil can take 48v DC and the switch is safe for the DC voltage and amperage ranges stated above?

Thanks for any comments,

Chris

MPPT SmartSolar

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klim8skeptic ♦ commented · Jan 10, 2022 at 01:39 AM

Buy another panel and string panels, 3 series x 2 parallel.

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chrisgru klim8skeptic ♦ commented · Jan 10, 2022 at 02:11 AM

Adding more panels is impossible, as the panels are on a trailer with a fixed amount of space available - one of those solar power generator trailers. I would have added a panel and run more strings already if that was possible. Thanks for your "help"

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Answer 1 · 2022-01-10T07:02:51Z

klim8skeptic answered · Jan 10, 2022 at 07:02 AM

I was reading the relay programming options in the manual and I notice a mode where you can set the relay switch to trip when PV voltage exceeds a stated amount, and that it the trip condition will clear when the PV voltage comes down to a lower level which you can also set. This option is referred to as the "PV high voltage" mode for the relay.

This does not exist.

Please use the Victron online MPPT calculator.

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Answer 2 · 2022-01-10T16:08:45Z

chrisgru answered · Jan 10, 2022 at 04:08 PM

Please see page 25 of the Victron MPPT controller manual here, the "PV Voltage High" option is discussed there:

https://www.victronenergy.com/upload/documents/Manual_BlueSolar_MPPT_150-70_up_to_250-100_VE.Can/MPPT_solar_charger_manual-en.pdf

The option exists at least in the manual from Victron on their website.

1641830876399.png (170.3 KiB)

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Matthias Lange - DE ♦ commented · Jan 10, 2022 at 04:30 PM

What do you want to do with that relay?

If you want to cut off the PV from the MPPT the voltage at the MPPT will fall below the "clear panel high voltage" and will reconnect the PV to the MPPT what is even worse because you have the Voc in that moment.

The best would be to design the system that you never reach the 250V limit.

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chrisgru Matthias Lange - DE ♦ commented · Jan 10, 2022 at 05:18 PM

Hi Mathias, you make a good point, after the relay disconnects the array due to high voltage, how would the controller know when the voltage is safe to reconnect? Hopefully someone will comment here that has actually used the feature or is knowledgeable about it. There would have to be some means for the controller to monitor the voltage after a relay disconnects it, that makes sense. Maybe it reconnects periodically and continues to disconnect until the level is safe? It would be nice if the manual explained it all in more detail.

I tried to find other discussion of this feature in the victron forum but was unable to.

Chris

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Answer 3 · 2022-07-05T21:40:35Z

I have a similar problem with my Vic MPPT 250/100. I have 13 panels I would like to connect in series each of which have Voc=23v Isc=6.5a Vmp=18v Imp=7.5 ==> ~132w driving a 24v system. Given they are of Chinese origin and nothing in their documentation is ever actual or accurate that implies that the open load series would be at 23v*13 ==> 299v against the MPPT's limit of 250v. On the other hand under load they drop to a pretty consistent 18v (actually 17.9) which again 18v*13 ==> 234v and with only one series the panels' 6.5a to 7.5amps are well within the amperage limits of the MPPT. I really don't want to split this into two strings since 13 is a odd number and hence one string would have a wasted panel under even sun on both strings. In addition under lower light and with their bypass diodes of 1000v a single string makes more sense as they will mostly operate near their 18v loaded voltage and not the Voc of 23v.

So with that background I am thinking that using either MOV or Zener circuit to limit the input to 250v and dump should it ever go over. Makes more sense then the relay concept discussed above. And yes, once you pull disconnect the reconnect cannot occur. (stand on a bucket and kick it while standing on it - uh, no). But the Zener circuit load dumping set at the 250v limit will do exactly what you/we are looking for. Basically the limiting load is only applied when the voltage exceeds the limit of the Zener voltage limit (250v) and the dump load would force that access voltage down by dumping power into a heat coil dummy load. That would only happen when the MPPT cannot utilize the excess voltage, as it typically would load the panels pulling them down to the 18v level under charging or utilization load. See: https://electronics.stackexchange.com/questions/391045/what-is-a-simple-voltage-limiting-circuit OR https://www.eetimes.com/clamping-circuit-tames-automotive-voltage-transients/ ; In our case we need a circuit that is for more than transient. https://www.circuitlab.com/circuit/9sj5kb/zener-mosfet-regulator/ shows a 48v sample.

In my case, with such a circuit, I would lose some power under very full light when all the cells were fully and equally exposed to sun AND the MPPT did not have sufficient load to pull the panel string voltage down to the individual cell 18v Vmp level. That excess voltage loss would be acceptable in my mind as it wouldn't be used or stored anyway. The exchange for such a limiter is that the full series panel string produces much more power under lower light, or which is more likely, when a few of the panels are shaded by the mast and sail and the bypass diodes basically make the string appear as 6-8 panels generating power and the shaded ones aren't.

Why Victron has not thought about offering this in their design is a bit surprising to me as it makes sense for may battery based boat or RV usages.

Anyone have other thoughts on this?

-Rich

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