question

duckman avatar image
duckman asked

evaluation of mppt wiring

Hi, new here, but not new to off grid, i remember what a trace dr15/12 is

I Have been running a Victron Multi II with 6 units of pylon us3000c, 3 x mppt, grid and generator for a year

My site is agricultural, my arrays are a combo of fixed and manual tracking, total approx 3500w

In my quest to dump the grid, or be as tight as possible i have purchased an additional 450/100 mppt.

I have an already designated load for No1 of the 2 inputs,

I am trying to evaluate the pros and cons of the wiring for No2, I understand well enough the parameters of the VoC/Amps etc for the 450/100, its the use of "string series parralel" i need to know

My site requires fairly long lengths between the mppt and the arrays, I have a heavy 50mm2 cable running 30m, to which i either want to attach:

Option 1

2 arrays each of 6 panels, series approx VoC 42v/panel, issue is the 2 arrays will be receiving differing amounts of sunlight (one pointed S/E, one S/W) , they will each require approx 8m of wiring before meeting the common 50mm2 cable running back to mppt,

Q Will they conflict? I`m so used to 1 string/1 mppt that i have no knowledge of 2 strings "series parallel" but with differing shade/orientation

Option 2

Wire 10 panels series approx VoC 42v/panel facing due south, the more "conventional" method

However, in my situation i find that a steady solar input throughout the day (achieved by arrays that are trackable/fixed with differing angles to South) is vastly preferable to just a huge charge midday, the Ah of pylons (imho) being vastly overstated once discharge exceeds region C20, thats coming from a longtime Exide solar classic user, I would also mention i rate that it uses 2 Kwh of Solar or Grid to insert 1Kwh into pylons once below 50% d.o.d, which further guides me toward more continuous output in daylight hours,


Image shows a manual tracker with 1850w, i have yet to develop a form of hamster power and child labour is very un-pc, but working at home means i can nudge them around during the day and as i intend to oversize to meet winter needs im also able to push them out of production on high yield days, kwh/day yields exceed 150% of previous figure when the frames were static before being mounted onto gearboxes.

When the advantage of continual output and the ineficiencies of pumping kwh into pylons are added in, for me the advantage is in excess of 200% over static arrays.

In an ideal world i would construct 2 more trackable arrays with 6 panels each, however cost of materials is now crazy and my time is taken surviving the dire state of the world, economically and otherwise, hence i propose a dual axis static array S/E and S/W

Any guidance re wiring alternatives gratefully received or steering toward an advanced (ish) published work that covers such evaluations most welcomed

I tried to keep this short, failed, appologies

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4 Answers
JohnC avatar image
JohnC answered ·

Hi @duckman

The 6S2P option into 1 tracker won't 'conflict' as such, you'll just get a 'weighted average' of Vmp from each string. If both have full sun (not at the same angle) then you won't probably notice any difference. With one in shade it will possibly drag the other's Vmp down somewhat from it's optimum, but even then would be be better than a 10S string.

You'll have to watch the Amps though. The RS mppt is designed for long strings and low Amps, with just 18A Imp and 20A Isc limits. With largish panels could be close. And then your wires..

Your Pylons' 2kW to get 1kW registered doesn't make sense. Sounds like they're not set up correctly on the Pylon side.


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duckman avatar image
duckman answered ·

Many thanks for those thoughts, in an ideal world id construct another 2 manual trackers and be able to keep the strings fairly equal.

Cabling will be 8m of 16mm2 meeting 30m of 50mm2, in duct

the panels i have are Canadian solar 380w, data sheet attached, i am unsure how to calculate the Imp and Isc limits, I have 12 panels in total to play with.

It is high time i upped my education and read some up to date books, any pointers GLADLY recieved, i have found most publications too basic and at the other end its full blown tech level stuff i have little need of

Re the recharge on the pylons i have noticed that they have a "sweet spot", with charges being more easily acheived for less kwh above 50% d.o.d, though i appreciate the % of charge isnt set in stone.

Suffice to say with some observation i notice that some times they are "greedy" and less linnear in their receptiveness to kwh thrown at them 2x is an exageration

measuring this kind of thing back in exide solar classic days was never really possible, so its merely an OBSERVATION that pylons dont really deliver the Ah stated and do not charge efficiently 100% of the time, but compared to what ive had in the past Li ion is very nice and the price was ok

A duck is a dangerous thing to be these days with not just rambos and survivalists to be wary of, now government health inspectors and their oh so precise "pcr" testing now have their sights on the poor sods too...h5n1...not to mention global "software" magnates foisting bugs and fake meat on an already toxic population... i digress :0/

But, hey a starved human population will accept restrictions and conditions on its existence that others would shun....Canadian Solar 380W Solar PV Datasheet.pdf

Thanks and all the best


1 comment
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JohnC avatar image JohnC ♦ commented ·

@duckman

From that specsheet, Imp=11.02A, Isc=11.68A. For 2S strings, double that. So it exceeds the RS mppt limits. Not by all that much though, and that's at STC conditions, which you may never experience. Your longish wires might help keep it down a little too.

So you might just get away with it, although can't be officially recommended. Maybe a test with a single 6x string could help with a practical assessment, especially if you can crack it for some cloud-edge-effect conditions.

I doubt your 10S option will make it though. I'll let you run that yourself thru this..https://mppt.victronenergy.com/ Don't try to fudge this one though, V is a different animal to A. And there's also a limit here.. 1676353607126.pngSo I reckon your max string length is 9S.

For educational reading there's myriads of it on the Victron website. Much is Victron related of course, but that's where you are. Sign in to Victron Professional for the full treatment.. even get yourself 'certified'. :)

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duckman avatar image
duckman answered ·

Many thanks John, the purchase of 450/100 is forcing me to do some well needed education, single string smartsolars were not challenging, there are aspects of the 450 i had not fully taken on board...

Primarily becuase i was focussed on earning the money to buy it, 12x can solar and 2 more usc3000`s, not pocket money

As with most things i do there will be plenty time spent in the planning stage, i prefer to over engineer

the calculator wasnt something id seen before, doh! that helps alot

if i run the 2x6 strings and back the Imp down to 9 it works, which is what id encounter with arrays pointed 1x S/E and 1xS/W ?

https://mppt.victronenergy.com#djM7ZXlKdVlXMWxJam9pUlhobGRHVnlMQ0JWU3lJc0lteGhkQ0k2TlRBdU56STJNRE0yTmprNU9UazVPVGtzSW14dVp5STZMVE11TlRJM05EZzRPWDA9O2V5Sm1aV0YwZFhKbGN5STZXMTBzSW5SbGNtMXBibUZzSWpwMGNuVmxMQ0poYkd4dmQwOTJaWEp6YVhwbFpDSTZkSEoxWlgwPTtjdXN0b20sNiwyLDUxLC0xMCw3MCw0OCw5LDAuMDQsMTEuNjgsMzgwLDM0LjUsLTAuMzUsNDEuMiw0LDY=

likely then i will build array 1, test and see if array 2 needs to go on tracker or 2 or can be added to No 1

Given the ideal of 2 differing angles toward due S it seems impossible to acheive max power?

My dealer feels that 2 arrays with differing angles to due S on the same tracker would conflict, has this subject ever been discussed in depth?

Your take that it might drop certain parameters but overall function quite well seems more considered, it could be viewed like "partial shading"?

The absolute ideal is to have each 6 panel string manually trackable, its the time and money involved that makes this problematic.

However,

If perfection is what we require, we landed on the wrong planet

Many thanks again for your input, ive learnt alot in a short time

Nick,

Certifiable? i have to dodge the white coats as it is...

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JohnC avatar image
JohnC answered ·

I don't recall any particular discussion on conflicting arrays. Bear in mind though that Victron have made 100A mppts for some time, and until recent years topped out at 150V. So it's common practice to have multiple strings on one tracker, and even preferable with shading on some strings.

I have seen mention in sales stuff that you can sometimes get two peaks in the Vmp curve and Victron mppts can recognize the best one. I've seen this on my own system, when one (of 4) strings comes out of shade mid-morning. The panel V steps up, the A goes down, yet the W continues rising without hardly a bump in the curve. Even strings of different V spec might be expected to exaggerate this effect.

In the washup, it's likely a matter of try it and see. Your own experience with existing directional trackers should help, as the modern way is just to brute force it with more (cheaper) panel.

You're not alone when every design direction you look at has a limit. That's what it's all about. And why Victron has such a wide range of mppts. You have advantages over my own 100 Amper - an extra tracker and panel options. I'm sure you can make it work very well.

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