Solar panel back-bone. ??? Anyone?

[Frank in Thailand]

Does anyone know:

I used to have 3 sets of 9 panels, each connected to "own" MPPT inverter.
Together they are connected to the same battery pack by "back bone"

Is there a reason not to make the 3 x 9 panels to one "solar back-bone"?

I understand that it a few panels are partly shaded (by cloud or so) their voltage drops.
Having 9 in a series, I already have this "problem" (now 335v, can do max 450v)
and even if it happens, i have lower voltage, but still higher amps.

Watts are watts.
they don't just get lost, right?

If one set of the 9 suddenly have 100 watts per panel and lower voltage, it doesn't stop the other 18 for giving 275 watts at high voltage
total voltage will be little lower.
That's where the MPPT does its work

I have enough left over 4-AWG copper cable (+ 80 meters) to make a back-bone to the solar panel groups.
(they now use 10- AWG)

I need to re-do them anyway.

is it good or bad to do??
and if it is, why ??

 

[LiFer]

Ignoring temperature and other caveats...
All panels in series must have the same current. See Kirchoff's current law for more details.
So the problem is that if one panel in the string gets some shade its voltage will drop (as you mentioned), and because the panel's internal resistance remains constant Ohm's law tells us the current will decrease. The drop in current on one panel will force the same drop in current on all other panels. There is no escaping this.

So, Watts are Watts. Sure. Just don't forget to think about current issues when talking about power.

 

[Frank in Thailand]

I didn't know they need to have the same current!

So if I put 100 watt, 200 watt and 300 watt panels in series (all have 36v)
I don't have total of 600, but the lowest, that is 3 x 100 watts??

As result also, having 9 in series, if one panel get a problem and gives only 50a% the other 8 automatically get reduced to 50% also?

 

[Frank in Thailand]

Interesting!!

I "hate" single point of failure.
(Unless I'm absolutely sure it's is "undistricutable"

My 27 panels are now 3 parallel sets where 9 panels per set go in series.

So if I make use of 3 x own line to the MPPT, maximal one set of 9 is effected if one panel gets shade (or defect). 8 are fine, but need to match the weakest link.

If I make one set, 26 panels need to match the weakest link?

 

[Frank in Thailand]

Next consequential thought..
MPPT /PWM..
Why do people want / sell / advertise high amps??

I read that the voltage of the panel needs to be higher then the voltage of the setup.
For 48v setup you should use minimal 2 x 36 v panels.

In theory you should make step-up the voltage from 36 to 48, but apparently no one does this.

What setup can be made to have the least dependency of the panels??

Current doesn't always have to match.

2 batteries in Parallel one of 20 ah and one of 80 ah are together 100 Ah, not 2 x 20.
I can understand that the series have problems, like the above batteries would be 2*20 in series

how it could be effecting the parallel sets, I don't understand.

Just brainstorming here.

 

[RobinWMartin]

Hey i'm not really technically up to speed on the subject but I had a similar issue with different wattage panels and this article helped me out...to basically use matched panels https://solarpanelsvenue.com/mixing-solar-panels/

There is also the issue (and please correct me if i'm wrong) that even a current matched 100w panel will drag the current down for a 300w even further in low light due to it's different low light performance. It looks like a bad show all round for mixed panels in series.

 

[Frank in Thailand]

It looks like a bad show all round for mixed panels in series.

It looks like better to avoid series at all.
Not sure how to accomplish this.

It shouldn't be that difficult to find step-up per panel to boost the voltage up to required.
(And with it lower the thickness of the cables)

Only parallel is the best way to have least impact on performance??

In my situation 27 X 300 watt at 36v..
Lol

My MPPT won't like it.
It needs minimal 120v, best 350v

 

[RobinWMartin]

Surely not all your panels are different ratings. You can series them by similar current ratings then parallel the groups.

In my mind stepping up is just going to be a dramatic waste of current. Stepping up as you increase the voltage the current falls. You'll have the right voltage for your mppt but nothing to play with at the end of the day. Easier to add more panels so you have +volts then you'll never go hungry again.

 

[RobinWMartin]

Funny you mentioned the 48v system and only 2 panels. It's exactly the situation I had as I was originally going for a 24v system. Changed my mind to 48v, found out the same panels are no longer available so got one very similar but just slightly over spec'd to the others and the 3 now produce enough for the system to function with only a fraction of a percent loss on the 3rd panel. But as i'm in the UK i'll need at least 12 panels for the other 360 days of the year

 

[Frank in Thailand]

All 27 panels are 330watt.

25 are Poli crystalline, 2 are mono.
The 2 might be 300w , not sure.

They are going to replaced by 330 Poli.

My father in law needs a few panels for water pump.

As I'm buying, better give him the old ones
(3 years old)

Then there are 36 X 330watt, all 36 volts.

27 of them go on the noon sunny side of the roof, 9 are already at the afternoon sun side.

The 27 will be quite identical for performing, except if there is a small cloud that shades half the roof.

The 9 on the side, they have different angle, and will perform different.
On purpose.

On some months the "sunny side of the roof" seems to be on the wrong side of the house

(In Thailand the sun is almost 90 up, straight above our heads at noon.)

Then the set on the side really pays off.
Giving alone more energy then the 18 now on the sunny side.

Most months, they are not optimal producing.

As I was thinking to put all series onto a parallel back bone...
That set really would mess up things if it really would lower the output of all panels.

But I don't really understand why it would.

For the series, I can understand.
It's the same with batteries, match to the lowest part / weakest link.

But parallel??
Isn't that the setup that should fix this issue??

 

[RobinWMartin]

I'm a little lost when you say 'parallel back bone', does that means essentially having the 9 panels in series and the 18 in series then paralleling them together into 1 x mppt/pwm? I would I suggest that will be bad as the whole system voltage will be dragged down to the side with the least sun.

In this situation 2 x mppts would sort out this difference in output from the 2 groups whilst giving you the best performance.

Kinda hoping someone else is going to jump in here to confirm this. It makes sense in my mind but then again!

 

[Frank in Thailand]

3* 9 panels in series, together each set about 335 volt.
The idea is to add a 4th set of 9 panels in series to this line.

The 3 * MPPT all take what they can use from this line / back-bone
Convert it, use it or store it in the shared battery.

 

[Frank in Thailand]

According to earlier post parallel does the same as series ??

So if you would make 3 x 3 series, 2 x 330 and one set of 330,330 and 150, again you end up with 9 X 150 watt??

That I don't understand

I would guess you make 3 x 330 is 990 watt, + 990 watt and one 450 watt set.

Together 2430 watt

But apparently the 990 watt sets are being reduced due to the one panel and all become 450 watt sets??

Together only 1350 watts??

And this happens every time one or more panels have reduced production.
This can be by cloud, or dust, or bird sh!t, or.. or... And naturally damage or being different wattage to even start with.

Please explain!

 

[Frank in Thailand]

@RobinWMartin , interesting read, but you need pop-up killer to be able to read the story..

I like the way they make the drawings.
What would have been good is they would have showed the total production without adding the panel.
there might be a x-% in performance drop of the other panels, at the end it's the total power you care about.

Yes, natural and logic that you would use same or higher rated panels to add to a string.
if you only have lower rated, your total output might even be lower then without this panel!

According to the site, for parallel the current is NOT an issue.




(https://solarpanelsvenue.com/mixing-solar-panels/)

The voltage might be.

Now, luckily, that's not a real problem.
All panels are rated at 36 volt, and they reach this voltage really fast.
when one panel have 16 watt production or more (5%), it already is at 36v.

Thanks to https://www.solarpowerworldonline.com/2014/02/dual-mppt-defined-understanding-mppt/

The quarter falls..
they talk about dual mppt, vs one. (i have 3)


and a few pictures that give more insight





Naturally..
Actually even stupide for me to think otherwise.

MPPT actively changes the voltage of a section (its controlled string of series / parallel)

I have 2 different angles who need totally different control pattern.
making them into one back-bone, is letting the 3 MPPT controllers fight each other to control the panels.

It is good to keep 3 lines.

I have to find a way to divide the power production of the new total of 27 panels, who have the same angle, between to 2 x 4500w MPPT
They don't have to be exact same size, but need to stay close to 4500 watt. (MPPT have peak of 5200w)

The third one needs to do the set that have different angle.

 

[Hedges]

Put fuses in each string of panels.

You can put two similar strings in parallel without fuses but for 3 or more strings each should have a fuse. That way a short in one string doesn't carry all the current produced by the other strings (well, it will but only up to the current level needed to blow the fuse. Three, 7 A Isc strings probably won't blow the 20A fuse shown as max on the label so long as this is a separate fuse in each string)

Fuse should be no larger than maximum recommended for panel, and not less than 125% (or is that 150%?) of Isc in order to avoid nuisance blowing.

There are touch-safe fuse holders. I also see MC-4 fuse holders but those don't appear touch-safe to me. I think the instructions even say to disconnect the fuse holder from the PV string before replacing the fuse.

 

[Hedges]

I have strings of several different panels in parallel, and multiple orientations.
Some strings are 24, 12V 120W Astropower single-crystal.
Some are 12, 24V 165W Sharp polycrystaline.
Some are 8, 36V 327W SunPower single crystal.
The strings have various orienations.

This may be slightly sub-optimum vs. having separate MPPT for each, but only a couple % less according to tests and analysis by some vendors.

I don't have a series diode per string. I could, but its voltage drop would waste a similar percentage of power.

Reason I'm doing this is the inverter I selected has just one MPPT, but enough watts for several strings. The key to this is that the strings have very similar open-circuit voltages. Because they have different types of cells, their voltage for optimum power may differ more at different temperatures than if the strings had been homogeneous but I haven't analyzed how much.

Because I installed a box with touch-safe fuse holders it is easy to isolate and check string voltages. I disconnect DC with a switch (or AC; these are grid-tie inverters) before using the fuse holders because they aren't meant to make/break current. I can check voltage of all strings, then parallel them by closing the fuse holders. Very little current has to flow for an illuminated and a shaded panel to match their voltage.

Now I just want to get a clamp DC current probe so I can measure and calculate watts per string.

 

[CoryBaker]

So reading all this has me wondering if what I have in mind for my system needs is ok or not... Wanting one controller with 3 series strings run through a combiner box then all on same circuit (wires) to mppt. I want the panels in 3 different positions. Morning, solar noon, evening! Those voltage changes going to a single line will do what with my system? Any help would be great!

 

[Hedges]

So reading all this has me wondering if what I have in mind for my system needs is ok or not... Wanting one controller with 3 series strings run through a combiner box then all on same circuit (wires) to mppt. I want the panels in 3 different positions. Morning, solar noon, evening! Those voltage changes going to a single line will do what with my system? Any help would be great!

It works pretty well. Voltage doesn't differ much, just current. At first, manufacturers said multiple strings in parallel had to be identical and oriented the same direction. But examining PV panels curves, current or power vs. voltage, shows that if you take two strings with different orientation and force them to the same voltage, you get almost full power from each. Here's a paper by SMA where they confirmed that with tests:

http://www.smainverted.com/files/2017/06/The-Performance-of-PV-Inverters-with-Strings-at-Different-Azimuthal-Orientations.pdf

I think two orientations, morning & evening, may present a suitable area for noon-time sun. The third orientation could be for a different season. Maybe morning and afternoon summer + noon winter. You can optimize it for which season you need more power. Note that when sun is low in the sky due to time or season, the amount of light is reduced due to longer path through atmosphere.

Do make all strings the same number of panels (assuming identical panels.) Or, same open circuit voltage if some strings have different make/model. Avoid having partial shade on the strings.

Because you're paralleling three or more strings, use a fuse on each string. MC-4 fuse holders are available as are "Y" connectors, an alternative to combiner box.

 

[Frank in Thailand]

I don't know.
Really.

If one panel that is shaded, or defective or simply just lower Wattage then the rest, apparently that is the one that determines the output for all other panels.

9 300 watt panels, and one shaded to 100 watts will give (in theory) 1.000 watts (10*100)
And not 2800 watts (2700+100)

That is what I've been reading uptill now.

This document says actually the opposite!!

I can't call it proof, there is no research reference.

I guess we need to try and find out!!

With a little bit of luck I can to in about 21 days from now.(Corona transport times)

I'm waiting for some parts to arrive.

Just by quick test, the output doesn't seem to be the total sum of the 3 sides!!
It's a LOT lower.

To make more accurate test, I need my parts.

 

[CoryBaker]

It works pretty well. Voltage doesn't differ much, just current. At first, manufacturers said multiple strings in parallel had to be identical and oriented the same direction. But examining PV panels curves, current or power vs. voltage, shows that if you take two strings with different orientation and force them to the same voltage, you get almost full power from each. Here's a paper by SMA where they confirmed that with tests:

http://www.smainverted.com/files/2017/06/The-Performance-of-PV-Inverters-with-Strings-at-Different-Azimuthal-Orientations.pdf

I think two orientations, morning & evening, may present a suitable area for noon-time sun. The third orientation could be for a different season. Maybe morning and afternoon summer + noon winter. You can optimize it for which season you need more power. Note that when sun is low in the sky due to time or season, the amount of light is reduced due to longer path through atmosphere.

Do make all strings the same number of panels (assuming identical panels.) Or, same open circuit voltage if some strings have different make/model. Avoid having partial shade on the strings.

Because you're paralleling three or more strings, use a fuse on each string. MC-4 fuse holders are available as are "Y" connectors, an alternative to combiner box.

Thank you for the information. I know the combiner boxes have fuses or breakers as well as diodes and such.. Is just using the cables fine as well? I know you lose a touch of output going through some combiner boxes. But I figured it would be a good idea... If it changes nothing, I can just put a switch at the charge controller and inline fuses. No need to buy the box?