Solar panels in series / parallel

[Hyena]

Hey all,
Has anyone got any experience / advice in running used panels in a combination of series and parallel ?
I've got an off grid property (no AC available) and I'm looking for a way to maximise the cheap used panels that are readily available.
I'm planning to build a 'solar carport' - using the panels as the actual roof. If for no other reason, these used panels are essentially he same price as using tin roofing!

I've got 10x 250w panels that will be used as my main source of power but these will be on my my roof in a fashion that isn't ideal (around 11 degrees and facing roughly east).
In summer they should provide enough power for my needs (charging a 20 kwhr battery) but based on what I've been seeing from my home 6.6kw system lately now in winter (usually around 16-18 but a few bad days where I've only pulled in 10kwhr) this would leave me well short if I only bank around 5kwhr in the day from my sub-optimally set up 2.5kwhr system.

So I'm trying to think of a way to use the supplementary solar carport if needed. This could potentially be only be when I'm physically there so it could be manually switched / plugged in.
Can I run 2 separate arrays into the one input in the inverter ? I'm not sure how this would go with slight differences in voltage / current output. Though potentially it's not much different to having a lower voltage system with parallel strings if one is in the shade and putting out a little less current or voltage ?

The real cowboy in me wonders about having a 2 in series bank pumping 60v odd straight into a 48v pack for a few hrs if push came to shove.

[jonescg]

Hey Jay,

If you get an inverter with two MPPT inputs, they can be at different voltages, or number in series. So one on the east at 8 s, one on the north at 10 s.

As to the inputs of those MPPTs, generally there's no problem with putting two separate arrays of equal voltage in parallel, but any more than three strings in parallel will need fuses on each string. I think that's right @weber ?

[T1 Terry]

If you have multiple strings of series connected panels in parallel, you will need blocking diodes at the point the series strings join together in parallel to stop the panel series strings in full sun back feeding into the shaded strings. I know some here argue that blocking diodes are not required, but after seeing many cooked panels and very poorly operating systems coming back to life after I've added the blocking diodes, I know which way I prefer to set a system up.
If you have a method of feeding the battery voltage directly into the MPPT controllers, you could use multiple smaller MPPT controllers in parallel, one for each string. That can get rather expensive if using the better known brands that have Bluetooth network capability so the BMV monitor supplies the reference voltage for all the controllers on the network, or using the VE direct cables into a central management hub.
We fit a lot of second hand panels to off grid systems, no one else seems to want to touch them because they don't get the certificates or some such reason and use the Clean Energy Council as their reason for not reusing solar panels that had been removed for things like roof repairs etc. A total pile of B/S if you actually read through the regulation, it is perfectly legal to reinstall a system under the maintenance section of the regulations. Anyway, if you are building an off grid system, none of them have any say in what you can or can't do because you are not feeding into the grid

Rant over

T1 Terry

[weber]

As to the inputs of those MPPTs, generally there's no problem with putting two separate arrays of equal voltage in parallel, but any more than three strings in parallel will need fuses on each string. I think that's right @weber ?

No. Any more than 2 strings in parallel requires a fuse in each string.

Any given string should be made up of panels of the same type (all monocrystalline or all polycrystalline), whose short circuit currents are within 5% of each other. Strings having different short circuit currents can be connected in parallel provided their open circuit voltages are within 5%. In addition, all strings should have the same number of cells in series, and the strings should all be of the same type (all monocrystalline or all polycrystalline).

@T1 Terry, did those systems with cooked panels have a fuse in each string?

[jonescg]

So that means: 1 string - no fuse. 2 strings in parallel - no fuse. 3 strings in parallel - fuses on all strings.
Because in the case of three strings in parallel, if one string had a failed diode in a panel, the other two strings would have enough potential to push current back through the failed string?

[weber]

So that means: 1 string - no fuse. 2 strings in parallel - no fuse. 3 strings in parallel - fuses on all strings.

Correct.

Because in the case of three strings in parallel, if one string had a failed diode in a panel, the other two strings would have enough potential to push current back through the failed string?

Yes. Although there may be other faults besides bypass diodes failing short-circuit, that reduce the voltage of one string and allow the other string(s) to push current through it backwards.

And current can be pushed backwards even in the two string case. It's just that a panel can always take the same current in reverse as it can produce forward, without causing a fire. But the current of two other strings being pushed through it backwards is usually dangerous. It depends on the fuse rating of the panels, on its datasheet.

Of course this assumes the strings all have the same short-circuit current. If you had one strings of modern panels whose short circuit current was 10 amps (and fuse rating 15 amps), in parallel with a string of old panels whose short circuit current was only 6 amps (and fuse rating 10 amps), then you'd need to actually put in a 10 amp fuse for the lower-current string, even though there are only two strings.

[T1 Terry]

As to the inputs of those MPPTs, generally there's no problem with putting two separate arrays of equal voltage in parallel, but any more than three strings in parallel will need fuses on each string. I think that's right @weber ?

No. Any more than 2 strings in parallel requires a fuse in each string.

Any given string should be made up of panels of the same type (all monocrystalline or all polycrystalline), whose short circuit currents are within 5% of each other. Strings having different short circuit currents can be connected in parallel provided their open circuit voltages are within 5%. In addition, all strings should have the same number of cells in series, and the strings should all be of the same type (all monocrystalline or all polycrystalline).

@T1 Terry, did those systems with cooked panels have a fuse in each string?

I use double pole DC circuit breakers rather than fuses rated at 40 amps each, less issues with false open circuit and not near as much of a pain the reset. Fuses between the solar controller/s and battery to protect that end of the system. All cabling used is rated at 50 amps plus and basically, a fuse or a circuit breaker is there to protect the cable, not the actual individual unit be it an appliance or a solar panel. The system with fried/arced panels did have fuses and mechanical switches and a few did have blown fuses, but the effected panels were already arced and dead. Because of the voltage they were all HRC fuses, but I can't say one way or the other if the higher current required to trip the fuse caused the demise of the panel, but probably the other panels in that string finished it off because the current path was interrupted so the next easiest path was taken.

Not sure about your reasoning about all poly or mono panels, not that I'd mix them on any array but rather dedicate the strings of the other type panel to their own controller.

T1 Terry

[Hyena]

If you get an inverter with two MPPT inputs, they can be at different voltages, or number in series. So one on the east at 8 s, one on the north at 10 s.

Yep that would be the ideal solution. Though it seems like pretty limited options in the non-grid tied, budget end hybrid 48v 5kw inverters.

Any given string should be made up of panels of the same type (all monocrystalline or all polycrystalline), whose short circuit currents are within 5% of each other. Strings having different short circuit currents can be connected in parallel provided their open circuit voltages are within 5%. In addition, all strings should have the same number of cells in series, and the strings should all be of the same type (all monocrystalline or all polycrystalline).

Ah ok. I was hoping to have a 190w string and a 250w string in parallel (with a diode ?) and that the inverter would just see that ~4A and 6A as 10A combined at whatever voltage they averaged out to. It sounds like I need to source another 10x 250w panels with near enough the same voltage and current.

Anyone got a link to recommended diodes between strings ?

[antiscab]

I use these:
https://www.aliexpress.com/item/3296779 ... hweb201603_

and fuse:
https://www.aliexpress.com/item/3295363 ... hweb201603_

[antiscab]

Ah ok. I was hoping to have a 190w string and a 250w string in parallel (with a diode ?) and that the inverter would just see that ~4A and 6A as 10A combined at whatever voltage they averaged out to. It sounds like I need to source another 10x 250w panels with near enough the same voltage and current.

Anyone got a link to recommended diodes between strings ?

when using different sized panels:
match Imp and Isc inside a string (in series)
match Vmp and Voc when paralleling strings

you can use a string of 190w and a string of 250w panels, just get the Voc and Vmp of the whole strings to be close

[T1 Terry]

The last 48v system we installed had 2 different capacity solar arrays but also ran 2 PIP inverters in parallel. We conected one array to one inverter and the other array to the second inverter. Each show a different harvest but they both go towards supplying the load and recharging the lithium battery bank.
An alternative if you don't want to run 2 inverters in parallel because you do not require that much output, you could run the PIP MPPT controller that can handle 60 amps output for one string and the inverter for the higher output string as it can handle 80 amps output.

T1 Terry

[weber]

If the 190 W panels have 72 cells and the 250 W panels have 60 cells, as is usually the case, that's a 6:5 ratio. That means you will only be able to match voltages if you have a string of 5 x 190 W panels in parallel with a string of 6 x 250 W panels. Or a string of 10 x 190 W with 12 x 250 W. If you don't match voltages to within the previously mentioned 5%, some MPPTs can get stuck in a local maximum, pulling power only from the string with the higher voltage, and missing out on the global maximum at a lower voltage.

This is a case where you may actually do better with a simple switching (or PWM) type of charge controller. With such a controller, and a nominally 48 V battery, you'd have strings of 2 x 190 W, in parallel with strings of 3 x 250 W, and you'd definitely need diodes as well as fuses in this config. But you'd only be using about 80% of the available voltage from the 250 W panels, so they would effectively only be 200 W panels in this configuration.

[Hyena]

I use these:
https://www.aliexpress.com/item/3296779 ... hweb201603_

Thanks - I was expecting they'd be something more... substantial!

when using different sized panels:
match Imp and Isc inside a string (in series)
match Vmp and Voc when paralleling strings

you can use a string of 190w and a string of 250w panels, just get the Voc and Vmp of the whole strings to be close

Cool - though I didnt realise as weber pointed out below it seems there's often a difference in the cell counts which makes the voltage a little too far different to work.
Sounds like I need to hang out for higher powered panels to make things match up.

This is a case where you may actually do better with a simple switching (or PWM) type of charge controller. With such a controller, and a nominally 48 V battery, you'd have strings of 2 x 190 W, in parallel with strings of 3 x 250 W, and you'd definitely need diodes as well as fuses in this config. But you'd only be using about 80% of the available voltage from the 250 W panels, so they would effectively only be 200 W panels in this configuration.

I think by the time I mess around with getting a second charge controller I'd be better off just spending a bit more to get more matched 250w panels so it's all equal specs across the board. I'll start off with just to roof mounted panels and then add the second carport bank later (possibly 6-12 months later once I get my act together)

Thanks guys

[Roman khan]

Hi every one
I have installed 5kw inverter having 120 to 450 vdc, i have installed 12 solar panels of 440wp in two strings,my inverter mppt blown away ,the supplier said that this inverter has mppt of 18A max and your solar produces amperes than 18amp(10+10),is this ampere increase ,the reason of this?

[antiscab]

Not likely, the mppt just moves away from maximum power point if the actual power gets too high.

More likely that unit wasn't well designed from a temperature for back perspective

[coulomb]

Hi every one
the supplier said that this inverter has mppt of 18A max and your solar produces amperes than 18amp(10+10),is this ampere increase ,the reason of this?

I've wondered about this 18 A limit too. I had a bit more of a poke into the PIP-MG / Axpert MKS II firmware (is that the model that you have?) and it's hard to pinpoint a current limit. I believe that I finally found one, but the constant is 1460, which you'd assume represents 14.60 A (they often use decimal scaling like that). That's the limit for the set point; with overshoots, the current could go higher than that. 14.60 V does sound a bit low though; you'd need 274 V to make 4000 W (though the specifications are ambiguous; maybe it's supposed to be 250 V x 18 A = 4500 W). Still, there is a maximum current set point, and it seems to be of the order of 15 or 18 A. Perhaps MG owners can tell me how their machines behave (what maximum PV power they see, and what PV voltage and current that happens at).

I don't have any experience with these higher voltage solar charge controllers, so I have no idea what the margins for error are. I would guess that the SCC control system is just as prone to under- and over-shoots, and with higher than designed panel current, it's possible that an overshoot brought the system past a limit, either current, voltage, or temperature rating of the semiconductors (which I assume are IGBTs to withstand 450 V).

The damage to the PCB seems to include overheated tracks, [ edit: I've changed my mind; see next post ] and not by 10% or so, these seem to have become incandescent. So I suspect that the tracks blistering was a result of some semiconductor failing shorted, rather than overheated tracks causing the damage directly, perhaps finally causing a semiconductor failure in the end.

[coulomb]

I found an image to compare the burned board with; see below. On closer inspection, what I initially thought were blistered PCB tracks was actually just the melted transparent plastic cover they put over the PCBs, presumably to prevent flashover to the metal case, or perhaps to direct the air flow, or both. So something got really hot, and ground zero seems to be the connection from the brown wire (which I believe runs off to the boost inductor) to the PCB. There is a lot of soot near the top right of the photo, but that could be from the plastic actually catching fire.

It might have been as simple as a loose connection; say ten ohms of resistance times 18 A of panel current would allow 180 V to drop there, and dissipate over 3 kW of power. Ten ohms is a very high resistance, but it could have started with a few extra milli-ohms of resistance causing local heating, which might have tarnished the plating on the lug, adding more resistance and more heat, in a vicious cycle. Perhaps something insulating got trapped under the lug, so the current was carried through the bolt, which has poor conductivity, and that started the cycle.

If it's really rated for 18 A of PV current, then overloading it with 20/18x100% = 111% of rated panel current, even with occasional short term overshoots, doesn't seem to me likely to cause this sort of damage. But I'm only guessing.

Axpert MKS II internal rotated.jpg (249.89 KiB) Viewed 889 times

The above image [ rotated ] is from https://powerforum.co.za/topic/4721-axp ... ment=65766 ; thanks user APV for posting it.

[coulomb]

Yet more thoughts. The brown wire and lug don't seem to be badly affected; if the connection was the source of the problem, I would expect more damage to the lug, wire, and heatshrink plastic. The damage seems to be more just north (in the first photo) of the connection. I have no idea what's on the other side of that board; there doesn't seem to be much on the side of the board we can see.

[birdibird]

I've had 3 x 9 of 220Wp panels = 5940 Wp connected to my 5048MG from the beginning, both with and without batteries connected, and never had any issues.

[coulomb]

I've had 3 x 9 of 220Wp panels = 5940 Wp connected to my 5048MG from the beginning, both with and without batteries connected, and never had any issues.

Your panels might be less than 6A Imp, meaning 3 strings won't exceed 18A. It seems to be the 18A figure that is the main problem.

[birdibird]

I've had 3 x 9 of 220Wp panels = 5940 Wp connected to my 5048MG from the beginning, both with and without batteries connected, and never had any issues.

Your panels might be less than 6A Imp, meaning 3 strings won't exceed 18A. It seems to be the 18A figure that is the main problem.

They are 60 cell poly 220W 36,9 VoC / 30,2 Vmpp 7,85 Isc / 7,28 Impp

[coulomb]

They are 60 cell poly 220W 36,9 VoC / 30,2 Vmpp 7,85 Isc / 7,28 Impp

Thanks. That's over 21.8 A Imp. Hopefully the burned SCC was an isolated case.

[weber]

The damage looks like it was from arcing.