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diy solar

PV array voltage significantly higher than expected

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donsolarak

New Member
Joined
Aug 23, 2023
Messages
23
Location
Fairbanks AK
  • 18x Renogy 100w panels wired in series
  • 24.3voc per panel
  • 20.4vmp @ 25C
  • voltage temp coefficient -0.28%/C
  • current outside temp +12F/-11C
  • coefficient 0.28x36 = 10.08%
  • panel voltage 22.45v at -11C
  • string voltage 404vdc at -11C
  • also string voltage 434vdc at -40F/-40C which is what I spec'd for as that's my lowest expected temps with output
  • power is running through 600v 10g wire
  • EG4 supports 500voc
  • EG4 supports 120v-450v operating voltage
With all that said I'm seeing voltage today as high as 443vdc with the array at only generating about 25% rated output at +12F/-11C. This voltage is significantly higher than I was expecting to see. 443v vs the 404v. I verified the line voltage with a volt meter which also showed 443vdc.

Oddly enough I'm often seeing array voltage in the 300v range (much lower than expected) a lot of the time as the array is often only generating about 10-20% of rated output. Much of this is due to the fact that I live in a boreal forest with lots of trees which creates lots of small shadows on the panels. I'm waiting for the snow to melt so I can go cut down a few hundred birch trees to go green to save the planet by using solar.

Any thoughts why I'm seeing voltage this high?

Pictures were taken at time of very high voltage. Was only able to catch 433v at the charge controller. Witnessed 443v max. -45F temps for reference to spec.IMG_20240402_140101.jpgIMG_20240402_140020.jpgIMG_20240402_140047.jpgIMG_20240402_140210.jpgIMG_20240402_140236.jpgIMG_20240402_142529.jpgIMG_20240127_135646.jpg

Attachments: sun/shadows/specs/array/temp/voltage
 
Unless I missed something, 18 panels in series x 24.3 Voc = 437.4V
and that is at 25 deg C, the voltage goes up when it is colder.

Need to use the specification on the label, not what you measure.
You are too close to the 450V limit, probably safe from the 500V. Edit: Not safe!
 
Last edited:
Wow thats cold!

24.3 x 18 = 437.4 @ stc rating, voltage goes up as temps go down. Id expect much higher in full sun at those temps for sure.
 
ricardocello said:
Unless I missed something, 18 panels in series x 24.3 Voc = 437.4V
and that is at 25 deg C, the voltage goes up when it is colder.

Need to use the specification on the label, not what you measure.
You are too close to the 450V limit, probably safe from the 500V.
Click to expand...
That's open circuit voltage. I'm not measuring open circuit voltage. I'm measuring the voltage with a load connected.
 
donsolarak said:
That's open circuit voltage. I'm not measuring open circuit voltage. I'm measuring the voltage with a load connected.
Click to expand...
Yes, but the MPPT in the SCC goes from open circuit to dead short by varying resistance to determine the optimum power point!
So at some point it will see that voltage.
 
donsolarak said:
That's open circuit voltage. I'm not measuring open circuit voltage. I'm measuring the voltage with a load connected.
Click to expand...
You must design for Voc max at coldest temperature. Usually in the morning after a cold night. Without any load on panel, the panel will approach Voc when illumination level generates current that exceeds the inherent shunt leakage current of panel. This is common in morning sunlight levels.

During MPPT searching a controller will approach Voc. Also before it starts up in the morning it will have to initially take Voc when is first fires up DC to DC converter.
 
ricardocello said:
Yes, but the MPPT in the SCC goes from open circuit to dead short by varying resistance to determine the optimum power point!
So at some point it will see that voltage.
Click to expand...
Ok, the EG4 manaul says

Max PV array open circuit: 500vdc
PV array: 120vdc-450dvc

It's not clear why the voc and array max voltages are different then.
 
I don’t think you will damage it, but since EG4 is not clear about the meaning, what is the point of those upper 50V?
I assume it means you will not get more than 450V out of it, but it is ok if Voc goes up to 500V.

EDIT: See Voc calculation below, you are at 517V!!


IMG_6623.jpeg
 
Last edited:
donsolarak said:
Ok this makes sense. Sort of. Is it likely that above 450vdc the mppt controller shuts off like it does below 120vdc?
Click to expand...
It will smoke above 500

Operating voltage above 450 likely means VOC will be above 500, so it’s a dangerously academic question. To ponder after you disconnect your string.

And by operating I mean the voltage you measure when it’s running and use for various calculations
 
donsolarak said:
Ok this makes sense. Sort of. Is it likely that above 450vdc the mppt controller shuts off like it does below 120vdc?
Click to expand...
It will try to load the circuit enough to pull the voltage down below 450v.
But it's limited by its amperage rating.
VOC is a hard limit. Do not exceed it, unless you are planning to replace the equipment, anyway.
Watching things burn, can be entertaining sometimes.
 
zanydroid said:
It will smoke above 500

Operating voltage above 450 likely means VOC will be above 500, so it’s a dangerously academic question. To ponder after you disconnect your string.

And by operating I mean the voltage you measure when it’s running and use for various calculations
Click to expand...
Ok,

So what is really want then is to use Voc multiplied by the temp differential at -40C/-40F multiplied by the number of panels and keep that value below 450vdc not 500vdc to be safe.
 
ricardocello said:
View attachment 206665
Click to expand...
I was multiplying Vmc to the temp differential times the number of panels and keeping that value below 450vdc which it is. In reality though I want to multiply Voc times the temp differential times the number of panels and keep that voltage below 450vdc instead of 500vdc.

Now it makes sense.
 
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