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How much can I overpanel a Victron 250/70


PJJ

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Hello everyone

I think at @plonkster will be able to confirm this for me, and then we would all know :)

I currently have 5x JA Solar 330W panels in series connected to a Victron 250/70 giving me 1650Wp of panels

I would like to keep the 5S configuration, but that would mean with 3 strings I will have 4950Wp of panels

The 250/70 is rated as a nominal power of 4000W at 48V so at 4950W I am over sizing around 25% which if I remember correctly is still ok?

Thanks guys!

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44 minutes ago, PaBz0r said:

https://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/

You can play around with that, will show you if the configuration is acceptable or not :)

 

 

That doesn't really answer my question.

It just advises me to buy the 250/85 (which makes sense since I would be able to use more of the available power) but buying panels is much cheaper than buying MPPT's

Also the array in question is very west facing so it very rarely makes nominal power, but makes power brilliantly into the late afternoon.

Otherwise I would just drop it down to 4S and make the total power would then be 12 x 330W = 3960W

EDIT: Nvm, I should learn to read...

I see now I clicked on the wrong calculator.

Edited by PJJ
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Ok so it seems like the Max. PV short circuit current according to the Victron spec sheet is 35A for the 250/70

Max VOC is not a problem, I am safely under 245V

So it seems like the only thing is the Max PV short circuit limit of 35A, but that would mean under normal conditions the panels are operating at 165V I could do 165V x 35A = 5775W

Of course that is short circuit territory, but I think that means I am safe?  

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14 hours ago, PJJ said:

So it seems like the only thing is the Max PV short circuit limit of 35A, but that would mean under normal conditions the panels are operating at 165V I could do 165V x 35A = 5775W

Correct. As long as you are under the maximum PV current, nothing will blow up. The 250/70 controller also has a hardware current clamp, as opposed to some of the cheaper controllers that handles overcurrent in software.

Overcurrent: Suppose you are running close to the maximum DC output current and changing conditions suddenly causes the current to overshoot. This is known as an overcurrent event. The cheaper controllers handles this in software, pretty much by turning off and starting from zero. Therefore large amounts of oversizing is not adviseable on the smaller controllers (eg 100/50, 150/35, and so on).

But with the 250/70, you're fine.

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I don't have Victron MPPT's, I have Outbacks ( 60A and 80A versions), that said I think my practice would work with either.

I have matching string configurations, by that I mean strings of 8 in a 2S4P configuration of identical panels, The thing is different strings although uniform in the string are at different slopes and facing different directions. (I am a proponent of staying well clear of the voltage limit).

All the strings are brought down to a junction box with MCB'c before they are wired to the MPPT's.

Initially, I made my best guess at maxing out the MPPT current capacity, whilst trying to minimize clipping, by combining E, W & North strings to a CC. ( I used the conventional thinking as advised above in this thread).

Then I watched, at noon on peak production days.

What I have learned is when you combined different aspect strings, you can get a far better MPPT/power ratio, because you lower the peak and at the same time you lengthen the solar day.

The other thing I've learned is there is no magic % over-paneling formula, it's a guideline at best. I have found I could easily add another string when I had theoretically already over-paneled according to conventional practice.

I still have 20+ Amps headroom on my 80A CC's which peak at 57A.  So I can actually transfer their strings on my 60A CC's and they will be fully utilized, and not clip. ( There is an E and N string on one and a W & N one the other, both with a theoretical peak of 97 A, working purely on panel power, so if I transferred these strings to my 60A CC's, they would be over 60% over-paneled without a bother).

So what I am saying is, your panel slope and direction is particular to you, decide on a particular standard string layout. They more aspects and directions, the merrier.

Bring the wiring down to ground level and mix and match the string until you've optimized your CC, so that it is fully utilized in terms of capacity, for as long in the day as you can make it.

 

Edited by phil.g00
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On 2019/11/27 at 3:10 PM, PJJ said:

The 250/70 is rated as a nominal power of 4000W at 48V so at 4950W I am over sizing around 25% which if I remember correctly is still ok?

Remember that the output current (into the battery) is limited to 70A, so strictly speaking the max charging power (@48V) is 48V x 70A =  3360W.  The battery will most likely go above 48V so you do get a bit more than 3.36kW, but 4kW is a bit on the optimistic side.

Assuming your batteries are okay with the higher charge current, I would personally opt for the 85A version so that one can take advantage of hazy/cloud-edge conditions where your panels can generate over 120% of their rated power.  The MPPT will also then spend less time running at max output - possibly improving long-term reliability.

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8 minutes ago, NigelL said:

max charging power (@48V) is 48V x 70A =  3360W

Well to be precise its 52V x 70A = 3640W

The system in question only has 2x US3000 batteries connected to it, so I have a maximum recommended charge current of 74A anyway.

8 minutes ago, NigelL said:

Assuming your batteries are okay with the higher charge current, I would personally opt for the 85A version so that one can take advantage of hazy/cloud-edge conditions

But I already have the 250/70 I am not going to sell it just to buy the 85A version,.

You are right I will have an entire 1.4KW of excess PV which will be useless on a clear day, but I will still have the advantage on a cloudy day of the over sized array.

If its a cloudy day and the array only makes 70% of its nominal power great my MPPT is still working at 100%

Edited by PJJ
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42 minutes ago, PJJ said:

You are right I will have an entire 1.4KW of excess PV which will be useless on a clear day, but I will still have the advantage on a cloudy day of the over sized array.

If its a cloudy day and the array only makes 70% of its nominal power great my MPPT is still working at 100%

What i am saying is, non-optimal slope and direction, dirty panels, and working temperature + losses will probably see you around that level (70%) on a sunny day.

The clouds came over today my production dropped 90%. (see below).

Put you panels up, see what you get and then add more panels. 

PS. Don't go for 5 panels in series, go for 4. Prime numbers limit options and you're pushing the 250V limit.

 

Clouds.JPG

Edited by phil.g00
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18 minutes ago, phil.g00 said:

PS. Don't go for 5 panels in series, go for 4. Prime numbers limit options and you're pushing the 250V limit.

The install is located in a very hot/humid area in the lowveld, it never reaches 0 here, the highest VOC according to the MPPT is 223V

Also I forgot to mention, the panels are a good 40-50M away from the SCC, so I am gunning for the higher voltage to make that distance less of a problem.

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8 minutes ago, PJJ said:

The install is located in a very hot/humid area in the lowveld, it never reaches 0 here, the highest VOC according to the MPPT is 223V

Fair enough, I haven't got that guarantee in the Drakensberg.

What's the peak current?

 

Edited by phil.g00
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6 minutes ago, phil.g00 said:

Fair enough, I haven't got that guarantee in the Drakensberg.

What's the peak current?

 

Today was a hot-ish day, but not blistering, and we had clear skies.

MPPT.png.5145331c60731481f694283ccfc1e06f.png

I see the peak PV current was around 6.8A

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13 minutes ago, PJJ said:

I see the peak PV current was around 6.8A

At about 177 volts, I see. That's about 1200W peak, out of your 1650W of panels (72%). And you've got about 3640W of MPPT, so if you put two more strings at the same tilt and direction, you'll be fine. Perhaps minimal clipping when all the planets align.

13 minutes ago, PJJ said:

Today was a hot-ish day, but not blistering, and we had clear skies

You'll make less power on a hot day.

Edited by phil.g00
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14 hours ago, VisN said:

Would anyone have a problem with this configuration or words of advice perhaps? The Victron MPPT calculator accepts it.

My concern is the Isc value for 5 strings would be very close to the 50A max PV current of the controller, so it is accepted but only just barely. You'll probably be fine... these things are designed and tested to that value... but I suspect you already know it is close, which is why you asked 🙂

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50 minutes ago, plonkster said:

My concern is the Isc value for 5 strings would be very close to the 50A max PV current of the controller, so it is accepted but only just barely. You'll probably be fine... these things are designed and tested to that value... but I suspect you already know it is close, which is why you asked 🙂

😉 I did know it was very very close to the limit.

Strangely though changing the Isc figure to 11 or 12 or anything that should make the configuration unacceptable with 5 panels in parallel does not break the configuration, it remains acceptable. However, upping the Imp number to above 10 with 5 panels does make the configuration unacceptable, throwing up this error.

Is that a bug in the spreadsheet?

image.png.4910e4a67eb7f76c7ce2cd77dd99b2a3.png

 

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5 hours ago, VisN said:

Strangely though changing the Isc figure to 11 or 12 or anything that should make the configuration unacceptable with 5 panels in parallel does not break the configuration, it remains acceptable. However, upping the Imp number to above 10 with 5 panels does make the configuration unacceptable, throwing up this error.

Is that a bug in the spreadsheet?

No bug, the MPPT makes the panels run at the maximum power point, so the current (Imp, not Isc) is current the MPPT has to contend with.

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There are two limits, when determining the maximum array size that can be connected to an MPPT:

  1. The Maximum PV open circuit voltage (Voc at STC)
  2. The Maximum PV short circuit current (Isc at STC)

Determining the maximum PV short circuit current

Get the maximum PV short circuit current from the PV Panel datasheet. Multiply by the number of panels in parallel in the array. Having more panels in series does not change the number.

The result of the calculation may not exceed the Max PV short circuit current as specified in the MPPT Datasheet.

The above snippets are from https://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/ - Matching solar modules to MPPT charge controllers. It seems to contradict what's in the spreadsheet. 
 
 
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3 minutes ago, VisN said:

There are two limits, when determining the maximum array size that can be connected to an MPPT:

  1. The Maximum PV open circuit voltage (Voc at STC)
  2. The Maximum PV short circuit current (Isc at STC)

Determining the maximum PV short circuit current

Get the maximum PV short circuit current from the PV Panel datasheet. Multiply by the number of panels in parallel in the array. Having more panels in series does not change the number.

The result of the calculation may not exceed the Max PV short circuit current as specified in the MPPT Datasheet.

The above snippets are from https://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/ - Matching solar modules to MPPT charge controllers. It seems to contradict what's tested for in the spreadsheet. 
 
 

 

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I would definitely observe the voltage threshold limit.

I am not sure short circuit current  is a current that would be seen by the MPPT in practice. -maybe, but I can't think of the circumstances that an MPPT would initiate power transfer with a zero impedance to the panels in full sun. I don't even know if a zero impedance, taking cabling and MPPT innards into account is even possible.

Whereas, presenting a high impedance and therefore testing the voltage threshold would be a daily occurrence.

So, it would seem they have stated a more conservative guideline practice, but had their spreadsheet use the more realistic current variable.

 

 

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4 hours ago, VisN said:

However, upping the Imp number to above 10 with 5 panels does make the configuration unacceptable, throwing up this error.

I'd think because Imp would be where it spends the bulk of the time, so they used that. But the MPPT also has a feature where it protects itself if the input voltage starts to approach the maximum (by clamping it), and in that case the Isc needs to be below 50A. Yours is on the brink. But again, I think you will be fine.

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1 hour ago, plonkster said:

But the MPPT also has a feature where it protects itself if the input voltage starts to approach the maximum (by clamping it), and in that case the Isc needs to be below 50A.

Just for clarity, clamping a voltage still infers an impedance, just a lower impedance to the max power point. 

Short circuit current will only flow when that impedance is zero, and the voltage is clamped to zero volts. Which is a condition far away from the self-protecting case you are describing.

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