Solar Installation with optimisers - Should I change it all to series?

Corkster

Hi all, I've just had a 4.5kw panel only (no battery etc) system installed (11 x 410w panels, Solis S6 Mini 3.6kw inverter) and have noticed the inverter is often clipping at about 2.9kw which is way lower than I'd hoped so would appreciate any thoughts on how to get the best out of the system.

The system was installed professionally and comprises 8 panels facing SE and 3, with optimisers, facing SW. The inverter is single MPPT (although can take 2 strings). There is a roof ridge between the 8 and 3 panels hence the optimisers on the 3 SW panels. The panels are identical and will generate around 30V and 11A each.

I thought they were to be connected in series but looking at the inverter readings it seems they are in two strings (5 SE panels and the 3 SE/ 3 SW panels) in parallel, joined to make a single string into the inverter.

The inverter can take 500V and 19A input so what is happening is when the sun shines late morning the voltage gets up to about 155V but the inverter is clipping at 19A giving a max power output of around 2.9kw. Even this time of year that may be for 2 or 3 hours a day. The absolute highest I've seen the voltage is 169V briefly giving 3.2kw.

I'm no expert but I've run some calcs which suggests it would be better to have them all in a single string in series but I'm not really clear on how the optimisers would affect things.

From the datasheet the optimisers kick in at about 10V so even though the SW panels will be in shade early morning, hopefully they will generate 10V fairly quickly with the ambient light. In that case even though those panels may not contribute much power until later in the day, once the optimisers kick in the panels shouldn't be dragging down the current for the rest of the string? Is that right?

If that is the case, I may lose some very early morning generation which would be more than made up for with the reduced clipping later in the day and at least the inverter should be getting closer to the 3.6kw it's capable of.

Also, I don't think changing to a series only setup would make much difference to the evening generation as the 3 SE and 3 SW panels are on the same string so will presumably stop generating when the SE panels are in shade anyway?

Does that all sound about right or am I missing something there? The next thing will be to get the installer back to change the setup!!

Thanks in advance for any comments or thoughts on this, it's greatly appreciated

idc

When you have a parallel string on one MPPT like that both strings must be identical - eg String 1 = 3x310, 2x360 then String 2 should be the same! So you can't parallel 8 on one string and 3 on the other in parallel (and even optimisers on a parallel string can't fix that)

You should have a single string in series (assuming the 11x410 panels don't exceed limits of inverter

Corkster

Hi idc, thanks for that. I think what they have done is to split the 11 panels into 2 strings which are parallel and join before the inverter so there is only one input into the inverter.

It's the only way I can see that makes sense of the Voltage/ Current readings I'm getting.

Would appreciate any other thoughts you might have on that

bullit_dodger

Hi Corkster.

So your mail (seems) to be contradictory. You say....

I think what they have done is to split the 11 panels into 2 strings which are parallel and join before the inverter so there is only one input into the inverter.

and also you say.....

but looking at the inverter readings it seems they are in two strings (5 SE panels and the 3 SE/ 3 SW panels) in parallel, joined to make a single string into the inverter.

I think I know what you mean, but if you could clarify the situation that would be great. For example, I assume that you have only 2 wires (1 positive and 1 negative) going into PV1 on the inverter?

The problem with 3x panels in parallel, is that you might not have enough "startup voltage" for the inverter. I'd be with idc above, you probably should have the 11 panels on a single string, all setup in series with optimizer on all the panels. Yeah, you may need additional optimizers. Alternatively if you have enough Voc on the 3x panels, you could have them on 1 string and the 8 other panels on the other string, but you'll need to check the "open circuit" voltage for your panels and ensure that 3x Voc is greater than the inverter startup voltage.

graememk

Yeah, photos will be very useful,

They should be all on one string with optimisers, splitting the strings would mean the 3 panels wouldn't have enough voltage to start the second mppt.

Does one wire go to say the 8 panel side, then it comes back in and then goes to the 3 panels side?

Corkster

Sorry guys, looks like I've used the wrong terminology somewhere along the line! I've attached a schematic to hopefully clarify.

Yes there is only one input into the inverter.

Five of the 11 SE facing panels are joined together in series, call it Series 1 generating up to (5 x 30V =) 150V and 11A at mid day

The other 3 SE facing panels and the 3 SW facing panels (with optimisers) are joined together in series, call it Series 2 generating up to (6 x 30V =) 180V and 11A at mid day

Series 1 and Series 2 are parallel with each other and join together before the inverter to form String 1 going into the inverter. This would generate (the lower voltage of) 150V and (11A + 11A =) 22A (which exceeds the inverter limit of 19A so is clipped by the inverter)

That seems to be how the installer has set up the system. I had understood that all the panels would be installed in series and still think that would be the better setup but would appreciate your thoughts on it.

The 3 SW facing panels with optimisers are only intended as a 'bonus' to maximise the mid day generation and extend the generating day. So I am happy that once the SE facing panels are all in shade then the system will shut down so I don't intend adding any more optimisers.

Thanks for your comments, greatly appreciated 😀

panel layout.pdf

graememk

https://www.boards.ie/discussion/comment/120417686#Comment_120417686

Ah, the s6 mini only has one mppt.

And the diagram makes sense. The guys should have just went with a normal 3.6kw dual string inverter and not penny pinched and went for a mini.

Looking at the spec sheet, I see it's paralleled at the inverter, 11 panels would be well under 600v max open circuit voltage.

If you put them all into one string, the SW panels could be hampered by the SE,

There is a tool to make disconnecting mc4 connectors easier, you possibly can just make it all one string.

Corkster

Thanks Graememk, that makes a lot more sense to me.

So for now it looks like all the panels would be best as a single string but if I can get a 3.6kw dual MPPT inverter the wiring is already there to run them as separate strings.

Now to see what the installer has to say about it! Thanks very much for your help 😀

graememk

https://www.boards.ie/discussion/comment/120418453#Comment_120418453

Even a 4 or 5kw inverter would be grand,

Only really over sizing on a 6kw (well soon to be 5.5/5kw due to a change/tightening of esbn regs,) because can't really go any higher on the inverter size