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stepir avatar image
stepir asked

MPPT high voltage

Hi


I've read some topics so I know I'm not in an ideal situation and trying to find the most appropriate solution. If *possible* I'd like avoiding changing my solar charger and remove any solar panel from my config.


I have a 150/100 and 11 solar panels, in occasional conditions I'm hitting maximum 157v. This is the highest I've observed.


Is there a way to reduce the voltage on the circuit? I tried adding some diodes in series to the connection which seemed to reduce a bit, but never really solved completely.


Any recommendations or things I could try?


Thanks

MPPT Controllers
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3 Answers
Elimac avatar image
Elimac answered ·

The best option is if course remove one pannel. And do it ASAP, is not worth the risk to burn the controller.

You coud try also a "workaround" that I think sometimes of, but I never tried.

- Make two assimetric strings, 6+5 pannels. But it will be mandatory to isolate them with blocking diodes.

I've never tried this. You will however suffer performance degradation because both strings will be subject to same voltage and of course, it can be optimal for only one. Also, the diodes will add some losses.

Maybe you will be better off with just removing one pannel.

OR: Buy another similar one (make it 12 total -> 2 strings of 6)

OR Buy 4 more (make it 15 total -> 3 strings of 5)


The added pannels don't have to be same make and even same exact power. Look at the V/ I characteristics, specially:

-Same optimal "I" point for pannels in same string

-Make sure all strings have close similar optimal "V"- add all V from all pannels in same string.

Voc and Isc are good references, but I prefer to look at the optimum operating points.

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stepir avatar image
stepir answered ·

Thanks a lot @Elimac for your answer. I should have made it clear that I'm trying alternative to avoid:

- changing the solar charger

- removing a panel

- restringing (already tried and the drop in efficiency is too high, better removing a panel at that point)


Can I just a component in series to the circuit to create a 7v voltage drop? I tired adding some schottky diode, although minimal the drop it's present. Was more looking for a solution in this way.


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Elimac avatar image
Elimac answered ·

But adding 7V drop is only possible by dissipating power. Imagine the string current is 8A, to lose 7V you will have to cut out 7x8= 56W. And you will create heat - a source of issues... and in my opinion 7V is not sufiicient. Better to have a safe margin. My 250/60 is provided with a string that never ever reached more than 234V, and this is with negative temperatures that very rarely hit my location.

So, I still think is best to just cut one pannel off. Or ADD a number of similar pannels to make it possible to divid in at least 2 strings. For example with 16 pannels you cand create 2 nice strings of 8, and stay well inside the MPPT input range. Of course, this requires space and finding a suitable pannel.


One solution that I thought before, was to use the relay of the MPPT connected to a contactor on the input side. You could make it open based on the voltage. But I disregarded this after a while...

- Need solution to make it switch on again, other than by manual means

- I have found MPPT does not like to have its input switched off at high voltages. It can also create issues (transients are not good for electronics).


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stepir avatar image stepir commented ·

Agree - perhaps I should just remove one panel....

I know the approach of adding a resistor won't work - I was more thinking about a rectifier diode to cap voltage at 150v?

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Elimac avatar image Elimac stepir commented ·

It's the same. In terms of energy, it has to go somewhere... the IV response of a diode is non-linear, but Joule's law still applies.

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stepir avatar image stepir Elimac commented ·

Thanks @Elimac you are definitively right. As this is only happening with low temperature I might be looking for a way to easily remove the panel from the string on the fly. It's installed on the roof of my RV, so at least in summer I will still keep all the panels

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