Adding a third aspect to a two MPPT inverter?

bobbyy gee

mppt inverters


https://forum.solar-electric.com/discussion/352411/sharing-pv-input-with-two-different-mppt-inverters

unkel

air wrote: »

I'd recommend instaling a diode in series with the East & West strings before the inverter, you can buy them pre installed with MC4 connectors so plug & play.

Any link to one?

air

unkel wrote: »

Any link to one?

https://m.aliexpress.com/item/4001189061502.html

unkel

air wrote: »

Put the 2 strings of 7 panels in parallel into 1 input, put the 4-5 panel South array into the other input.
I'd recommend instaling a diode in series with the East & West strings before the inverter, you can buy them pre installed with MC4 connectors so plug & play.

Bought a couple of those. Currently don't have any parallel strings but might come in handy. Thanks for the link!

So if you want to parallel 2 strings (different orientation or not), do you recommend only to do that with strings of the same number of identical panels or would slightly different panels (with slightly different max output) still be ok?

air

air wrote: »

This is incorrect, the shaded panel voltage will not be pulled down, but it's current output will be.

thanks air. i was confusing issues. so we could benefit from optimisers [i'm thinking of contactors myself...or move the array...hrrmm]


...but we don't need diodes because the voltage will not be pulled down?

air

Sir Liamalot wrote: »

...but we don't need diodes because the voltage will not be pulled down?

I think we've been through this before.
There's virtually no downside to using diodes, barring the very tiny efficiency hit.

They protect against one string reverse biasing the other & potentially damaging it in a freak circumstance like a sheet flying off a line onto the array (totally shading it) or something equally unlikely.

You could very well run for decades without ever encountering an issue, up to the user whether they think it's worthwhile or not.

air

unkel wrote: »

So if you want to parallel 2 strings (different orientation or not), do you recommend only to do that with strings of the same number of identical panels or would slightly different panels (with slightly different max output) still be ok?

You really want to match the same number and type of panels in parallel strings to get the best tracking performance.
If you're mixing panel types in different strings you ideally want the total VMP of each string to be as close as possible.
Diffferent current ratings between parallel strings is not an issue.
On the contrary mixing panel types in a single series string rarely makes sense.
The max current of the string will be clipped to that of the lowest rated panel.

unkel

air wrote: »

On the contrary mixing panel types in a single series string rarely makes sense.
The max current of the string will be clipped to that of the lowest rated panel.

Damn, didn't think of that. That makes a significant difference now I have upgraded some of my panels to 355W that have a current at max power of 10.6A vs my older 290W panels that have a current at max power of just 9.2A

Thanks for your comment. I have choices to consider and changes to make now...

air

Sir Liamalot wrote: »

It's generally considered poor form to link a video without any commentary or explanation of relevance.

To save anyone else the bother, the guy is making the point that optimisers are pointless for series strings with common orientation & shading where they can only effectively compensate for differences in panel output due to manufacturing tolerances.

I don't think this is really news to anyone & the reason that string inverters rightly dominate the market.

He makes a further point that optimisers are not much better than panel diodes for landscape panels (a minority of installations) where the shade moves from bottom to top.
I would expect that optimisers are far more likely to be installed to deal with shading from chimneys & other irregular objects which don't create this type of shading.
Overall not much useful information presented in the video.

air

air wrote: »

I think we've been through this before.
There's virtually no downside to using diodes, barring the very tiny efficiency hit.

why are they needed when the effect they protect against has an opposing voltage at zero current and that current is traveling through conducting bypass diodes? how do you reverse bias an opposing conducting diode?


optimisers kindov make sense in retrospect in regards to moving shadows in that they reduce string loss and accommodate multi-aspect mounting. i dunno...i think i'll find a simplier solution myself.


unkel just derate the array in accordance to applying the lowest panel lmp to the array lmp. l reckon you're still ahead on price per watt.


i have two 60v 5a lmp panels in parallel in my array making a 60v 10a in series with a loada 8.5ish lmp 30v - 40v jobbers.


my array is oversized to the inverter. l still hit the peg anytime it's considerably sunny.

air

I don't think you fully understand how bypass diodes work, they don't provide any blocking protection when a panel is reverse biased.

Sir Liamalot wrote: »

why are they needed when the effect they protect against has an opposing voltage at zero current and that current is traveling through conducting bypass diodes?

The unlikely scenario I proposed (total coverage of an individual panel) means that the shaded panel would produce zero (or very low) voltage and the unshaded string would then reverse bias it.

Sir Liamalot wrote: »

how do you reverse bias an opposing conducting diode?

Not sure what you mean, bypass diodes are fitted in the same direction as the cells, they only conduct when some or all of a panel section is shaded - thus "bypassing" the current past the shaded section.
They don't provide any alternative conduction path to protect a panel when a panel is reverse biased as they are reverse biased themselves in this scenario!

Sir Liamalot wrote: »

optimisers kindov make sense in retrospect in regards to moving shadows in that they reduce string loss and accommodate multi-aspect mounting. i dunno...i think i'll find a simplier solution myself.

They should be a last resort ideally but in certain circumstances are the best option. A bit like having an additional MPPT inverter input for every optimiser that is added.

air

air wrote: »

I don't think you fully understand how bypass diodes work,

:pac: probably not

air wrote: »

they don't provide any blocking protection when a panel is reverse biased.

conceivable...i haven't thought about it to be honest

air wrote: »

The unlikely scenario I proposed (total coverage of an individual panel) means that the shaded panel would produce zero (or very low) voltage

i have measured this myself and shaded panels produced vmp give or take at zero current give or take

air wrote: »

and the unshaded string would then reverse bias it.

if it had no voltage but i've not measured the phenomenon despite trying to contrive it

air wrote: »

Not sure what you mean, bypass diodes are fitted in the same direction as the cells, they only conduct when some or all of a panel section is shaded - thus "bypassing" the current past the shaded section.

so they are conducting a forward voltage when the panel is shaded, the panel next to it is forward biased, with a forward voltage through a diode feeding it, where and how can it be reverse biased?

air wrote: »

They don't provide any alternative conduction path to protect a panel when a panel is reverse biased as they are reverse biased themselves in this scenario!

as i say i've never seen a reverse biased panel. cells yes, cell strings with bypass diodes no.
i'd love to see a demonstration of them helping.
all well and good having small losses on a 400v circuit and a low risk added failure point.
in elv land diodes are a very significant waste of power.

air wrote: »

They should be a last resort ideally but in certain circumstances are the best option. A bit like having an additional MPPT inverter input for every optimiser that is added.

i've half an idea involving a chainsaw, some vehicle mounted panel bartering, the second mppt inverter input and a spdt contactor maybe with a photocell.


i'll worry about it later. i've another 12kwh i pulled outtov solar storage and 6kva in the post to be getting on with.

air

Sir Liamalot wrote: »

i have measured this myself and shaded panels produced vmp give or take at zero current give or take

Covered I said, not shaded.
Dark sheet blows off the line, can of paint spilled while painting the chimney etc.
All highly unlikely, but not impossible.

Sir Liamalot wrote: »

if it had no voltage but i've not measured the phenomenon despite trying to contrive it

As above.

Sir Liamalot wrote: »

so they are conducting a forward voltage when the panel is shaded, the panel next to it is forward biased, with a forward voltage through a diode feeding it, where and how can it be reverse biased?

as i say i've never seen a reverse biased panel. cells yes, cell strings with bypass diodes no.
i'd love to see a demonstration of them helping.
all well and good having small losses on a 400v circuit and a low risk added failure point.
in elv land diodes are a very significant waste of power.

Take 4 panels, 2 x 2 series strings.
Cover one panel completely, compare output voltage of both strings. The one with the covered panel should be about half the other one (bypass diodes conducting)
Now (if you dare risk it) connect the two strings in parallel. The output from the unshaded string will flow backwards through the two with the shaded panel.
* caveat, I can't be sure sure of the reverse breakdown voltage required for reverse flow to start, it might take more than 2 panels in series depending on the panels / cell type, light intensity etc.

Sir Liamalot wrote: »

i have two 60v 5a lmIp panels

Have you an Irish supplier for these by any chance? I have 1 but need a few more.

Sir Liamalot wrote: »

in elv land diodes are a very significant waste of power.

Nobody is talking ELV here though and at the low current levels typical in Irish conditions the forward volt drop is going to be <0.5V nearly all the time in practice.

You should really be going at least 24V and preferably 48V for your vehicles at this stage mind you.
Running 32V and 48V in lithium myself, 12V/24V down converters for loads that need it, I would never use lead in anything mobile.
Drifting way OT sorry!

air

air wrote: »

Covered I said, not shaded.
Now (if you dare risk it) connect the two strings in parallel. The output from the unshaded string will flow backwards through the two with the shaded panel.

oh I dare. in the name of science. could I be bothered is another more poignant issue.
so if there' no parallels with partial pitch blackouts then it's not an issue?

I'm gonna document pwm kicking mppt bottom with panel to battery voltage suitablity first...or not...

air wrote: »

Have you an Irish supplier for these by any chance? I have 1 but need a few more.

nah mine are oooold. 90 cells iirc.
250w rated 275w performing [until i derated them with my install]
i'm happy to trade for 30 cells all black whatevers.

air wrote: »

Nobody is talking ELV here though and at the low current levels typical in Irish conditions the forward volt drop is going to be <0.5V nearly all the time in practice.

i'm doing 80amp solar installs these days. no diodes.

air wrote: »

You should really be going at least 24V and preferably 48V for your vehicles at this stage mind you.

i go whatever is a good deal. 12v, 24v, 48v and 60v. it's hard get away from 12v even when it's bonkers not to. the clients don't like having to change alternators.


i'm grid tying a truck; 4kva hybrid with 2.5kva synchronised backup and 1.5kw solar rough spec atm...trying to convince myself it doesn't need 3.5kva 3 phase.

air wrote: »

Running 32V and 48V in lithium myself, 12V/24V down converters for loads that need it, I would never use lead in anything mobile.

easy lose a quarter ton in a truck [or a barge]. i've 15kwh of freecycled lead li-ion can't compete with that.
meh, i use both. li-ion is good at being tenacious and light. i use it to torque fill the marathon running lead.

air wrote: »

Drifting way OT sorry!

i'm sure they'll get over it...someone will striaighten us out before long.