How much can I practically/legally DIY, and other newb questions

[nowty]

So, your first paragraph is totally correct, but what's the point of the lower voltage string then ?, it will sit there and do nothing, even in full sun as its voltage will still be lower than the shaded string.

The point is this: I would have thought you could connect the E and W arrays to the one MPPT. The arrays face in opposite directions so to first order only one is generating at a time.

Your second paragraph is a common misconception, we had a massive argument about this many years ago on the old forum. An identical array with less light does not pull down the voltage, in fact a shaded string will still contribute a little to the one in sunlight. Yes it will still work with blocking diodes, but it just adds another point of failure for no benefit.

I've done many experiments with this over the years with different types of inverters.

Thinking about this again a photodiode is reverse biased in normal operation. So if you just connect them in parallel the array in the sun will generate slightly more voltage so will bias the other one slightly more in the reverse direction and it will contribute less current than it would on its own, or none at all. Maybe you don't need the diodes after all.

If you can draw me the combined load line on the output curves of two typical panels connected in parallel, one in 5x the irradiance of the other it might help my understanding as to why there are multiple possible maxima. I will look at it myself when I have more free time.

In any case I thought any decent MPPT does a global search periodically, Victon certainly claim to. Of course there is a trade off because during the search the panels are by definition not at the optimum operating point.

If the two strings are of similar working voltage then absolutely, they can be connected but they don't need to be E / W, each can be ANY orientation and ANY slope angle. They don't even need to be the same panels as long as each string voltage is similar.

A decent MPPT may well do a global peak function, my SMA ones do but you have to turn the function on and set the scanning interval. Its meant for shade management.

I refer you my post from ages back below which shows two of my strings which are in parallel and there aint no blocking diodes. Been working fine like that for many years.
https://camelot-forum.co.uk/phpBB3/view ... tore#p9562

[Oldgreybeard]

I remember watching a YouTube video a while ago where a chap compared his system with and without diodes. He concluded as you have, Nowty, that they made no difference. I'll try and see if I can remember who it was and post a link.

[nowty]

@sharpener, if you want load lines, just look at any solar panel datasheet.

The max power point is roughly at the knee of the curve just as it starts to drop. As you can see even at 1/5'th of the max illumination the voltage hardly changes, its the current that does the reduction which in turn reduces the amount of power.

[sharpener]

@sharpener, if you want load lines, just look at any solar panel datasheet.

Yes, I have a good few saved now from all the different installers I have contacted in vain. That's the picture for one panel, nothing there to suggest connecting two with different light levels in parallel will result in more than one peak though.

Because the Vmpp is somewhat dependent on light level the one in the sun will pull the working point of the other to the right, away from its own optimum (not sure if the reverse occurs too). Still can't figure out how/why this behaviour results in more than one maximum overall as you suggest.

If the two strings are of similar working voltage then absolutely, they can be connected but they don't need to be E / W.

I was trying to help the OP not embark on a wide ranging discussion.

[nowty]

@sharpener, if you want load lines, just look at any solar panel datasheet.

Yes, I have a good few saved now from all the different installers I have contacted in vain. That's the picture for one panel, nothing there to suggest connecting two with different light levels in parallel will result in more than one peak though.

Because the Vmpp is somewhat dependent on light level the one in the sun will pull the working point of the other to the right, away from its own optimum (not sure if the reverse occurs too). Still can't figure out how/why this behaviour results in more than one maximum overall as you suggest.

Because you made a comment about having an unbalanced set of strings in parallel which would have two different max power points which could be a long way from each other, thus requiring a global peak MPPT which not all inverters have.
https://camelot-forum.co.uk/phpBB3/view ... =90#p23269

I would have thought you could connect the E and W arrays to the one MPPT using blocking diodes. The higher voltage will then win so you need not have the same # of panels in the two arrays provided each is within the MPPT spec. If they are facing opposite directions you won't lose any appreciable output by doing this, when the sun is in the S they will both be at glancing incidence anyway.

[dangermouse]

Some sort of isolator - I was think of using another Mersen fused disconnect, the same as I bought to isolate the batteries.

Standard way to isolate DC from panels is one of these: https://www.cef.co.uk/catalogue/product ... lator-ip65

I've bought one of these DC isolators, waiting for other stuff to arrive. Have actually ordered panels! Exciting times

But a question about the isolator, if anyone can help - it's a 4 pole switch, but comes with 2 link wires that effectively make it 2 banks of 2 switches wired in series. The diagram from the instruction book below shows what I mean (bottom left, "provided pre-wired linking").

My question is - is this a requirement to use more than one switch in series to meet the specs of the isolator? For example if I just used one pole for +ve, and one for -ve between the panels and the inverter, would I be exceeding the rating of the switches?

The specs say "4 pole 16A 450V" but is it the case that each individual pole is not rated at 16A 450V?

[Oldgreybeard]

The linking is for increased reliability when switching DC, as DC can create problems with switches welding shut when switched under load.

[dangermouse]

The linking is for increased reliability when switching DC, as DC can create problems with switches welding shut when switched under load.

I suspected as much ... but was expecting to see the rating given per pole, I suppose.

I'll just use it as supplied and be sure it meets the spec.

[Oldgreybeard]

The 16A rating will apply if just using a single pole of the switch, the series connection just allows the switch to handle the full 450V it's rated at, I suspect. If you're only running, say, 225V DC maximum then the switch should be fine without the links..

[sharpener]

The 16A rating will apply if just using a single pole of the switch, the series connection just allows the switch to handle the full 450V it's rated at.

Because there is no zero-crossing as with AC it is much more difficult to extinguish the arc at the contacts when breaking a DC circuit.

IIRC these isolators were originally designed for AC and they have had to use 4 contacts in series to reliably disconnect 450V DC. (The Kraus + Naimer ones are very similar, for the same reason.)

Hence it is good practice to switch off the inverter at the AC isolator first, then when you switch the DC isolator off there is no DC flowing so the contacts don't get eroded.