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First Post - Assistance needed on DC switching.


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Hi to all. My first post, and I hope it is of the standard required on this forum. I have been lurking for a while to get a feel for the personality's on the forum.

 

We are in Prince Albert in the Karoo.

Our main house has gables on either side of the roof we are putting our solar panels on. This leads to shadows been thrown over the solar array in the morning till about 9 and starting in the arvie from about 4.

This shortens our solar day by about 6 hours in the summer and 3 hours in the winter. A lot of generation time.

Option 1 is of course to bash the house down and rebuild. 
Option 2 is to add more panels, lets call them East and West and the main panels we call Center.

We can only have maximum 20 panels on line at a time in 2 banks of 10 panels each.

Our Center panels are 2 x 10 - total 20 and this pushes in the max amount allowed of DC power to the inverter.

But in the morning we want the 10 panels on the East roof to come into play, so we want to turn off one of the Center panels and turn on the East panels - total of 20 panels on line. This will be from 05h00 to 09h00.

After 09h00, we want to turn off the 10 East panels and turn on the second bank of Center panels till about 15h00 when we want to turn off one bank of center panels and turn on the 10 West panels. So all day we have a max of 20 panels on line at any one time.

So now we come the crux of the matter.

High voltage DC 3 way switches or any other suggestion to be able to allow us to configure any combination of 2 of the 4 banks online at a time - BUT NEVER BE ABLE TO HAVE 3 OR 4 BANKS ON AT ONCE.

Any legitimate advice and input will be welcomed.

I am posting this post on other sites as well as someone out there has had this problem and solved it, I am loathe to pay school fees on this. Need to know what worked on other sites.

Thank you in anticipation for assistance and advice. Cheers.

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3 hours ago, LLTHB said:

We are in Prince Albert in the Karoo.

I have family living up in Klipstraat :-)

3 hours ago, LLTHB said:

This shortens our solar day by about 6 hours in the summer and 3 hours in the winter. A lot of generation time.

That is rather a lot!

3 hours ago, LLTHB said:

Option 1 is of course to bash the house down and rebuild. 

You're going to have the historical committee (informally called the hysterical committee by the locals) on your case if you do that! I'd either make the panels free standing if I could, or if aesthetics is a concern, I'd just throw more panels at it, separate strings.

3 hours ago, LLTHB said:

High voltage DC 3 way switches or any other suggestion to be able to allow us to configure any combination of 2 of the 4 banks online at a time - BUT NEVER BE ABLE TO HAVE 3 OR 4 BANKS ON AT ONCE.

DC switchgear is often pretty expensive. Why not simplify things and just use more than one MPPT, even if you have to employ additional external ones? What kind of wattage are we talking about, what voltage will you operate at (I assume 48V)?

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Hi Plonkster, thanks for the replies, well informative.

The inverter has 2 MPPTs - the main panels are in 2 banks of 10, each bank uses up 1 MPPT on the inverter.

These panels bring us to just shy of absolute max input if the sun is shining and we at midday, all is running well.

We cannot now just flip on the West panels or the East panels as we will go over max input allowed into the inverter.

Parameters:

We have four sets of panels, only 2 can be online at a time. 2 equals the max the inverter can handle.

Four incoming wires from the 4 banks of panels come into the garage.

Our inverter has 2 MPPT  inputs.

Recommend the best way of being able to isolate the 4 panels and then be able to bring online only 2 of the 4 panels at a time.

 

Our intention:

At night, we will switch on East panel and 1 main roof panel - 2 online - West and 1 main roof panel off

This allows us to catch the early morning sun as it rises and supply power to the laundry geysers and washing machines.

By 10am, the sun will have moved past our East gable and the best power is now from the 2 main roof panels.

So we will isolate East panel, leaving only 1 panel on line.

Then we will turn on the 2nd main roof panel - 2 online, East and West offline

By 15h00, sun will be more to the west, so will isolate 1 main roof pane,, leaving only 1 panel on line.

Then we will turn on West panel - 2 online, East and 1 roof panel offline.

It is the isolating and switching off of the incoming lines from the PV panels that we need advice on.

 

Cheers.

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20 minutes ago, LLTHB said:

We cannot now just flip on the West panels or the East panels as we will go over max input allowed into the inverter.

You sure about this? MPPTs generally allow oversizing, in other words you are allowed to add more PV than the capacity of the controller. The controller simply limits the power to its maximum (sometime it will derate for temperature and limit a bit lower), and if there is an overcurrent event it shuts off and restart. Also, East/West arrays on the same controller do work, at a slight decrease in production.

Oversizing up to 120% is usually safe. I've done up to 150% at one point (stop gap measure with a charge controller that was too small), but then you get regular overcurrent events. What I am saying is look at the specs of the MPPT and maybe consult with the supplier/manufacturer. Chances are you can put the east/west on one controller and North on the other, especially if shading is such that you'll never get the full capacity in any case.

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26 minutes ago, LLTHB said:

It is the isolating and switching off of the incoming lines from the PV panels that we need advice on.

If you insist on doing this, then I'd say you need some DC-rated contactors with DC coils and an auxiliary contact to ensure they interlock. I'd expect around three of those, and some control circuitry. You can probably get a decent external MPPT for not much more.

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If I understand correctly, and what Plonskter is saying, after checking the controller specs:
East array and 1 main roof array on MPPT 1
West array and 1 main roof array on MPPT 2

Early morning and evenings is sorted.
Midday the main panels are predominantly providing power, with East and West producing less than optimal, or one side none at all.

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Thanks Plonkster and TTT.

Agreed 100%, except for the design and actual layout of our house.

There is a period from 10 to 2 that no sun is shining on the East or the West panels.

Steep pitched roofs, layout of house is in a "H" configuration. Our panels are at right angles to each other.

Looking at the house, the center faces exactly north, with East and West at 90 degrees to this. So with a high vertical sun in summer, it will only shine on the top edge of the East and West panels.

Our spread sheet calcs show 80% of the power comes from the main panels.

20% will come from East and West.

So the East panel will only help from sunrise to about 10h00. And, the West panel is still in shadow at this time.

And the West panel will only generate from about 15h00 onwards.

By doing it the suggested way, we only have one panel on line in the brightest part of the day. Seems to be that switching is going to have to be done.

 

cheers.

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1 hour ago, plonkster said:

... but simplicity is king. I'd rather spend 5k on an additional MPPT than tinker with contactors :-)

So, main array on the 2 x MPPT's, East and West on a 3rd new MPPT?

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6 minutes ago, The Terrible Triplett said:

So, main array on the 2 x MPPT's, East and West on a 3rd new MPPT?

Sounds good to me. Ideally of course you'd add two MPPTs to get full power from the east and west side, but this gets a bit expensive on a 48V system because the smallest controller (150/35) gets up there in price. I've never done an east/west, but I'm told it works well with only a slight penalty in production.

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10 minutes ago, plonkster said:

I've never done an east/west, but I'm told it works well with only a slight penalty in production.

May I ask why there is a penalty?

Mornings the east array is in full sun, west dead, main array on the other controllers. No penalties?
Midday makes no difference as the main array powers it all. Penalties but is is inconsequential.
Afternoon when main array starts being boycotted, the west gives the full input, as east array is dead. No penalties?

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2 minutes ago, The Terrible Triplett said:

May I ask why there is a penalty?

I don't know. I think the "dead" array shifts the voltage that the MPPT latches on a bit and causes a slightly less optimal harvest. I've never done it. Ek ploeg met ander man se osse. @Chris Hobson mentioned something about it in the past, for solar water pumps.

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23 minutes ago, plonkster said:

I don't know. I

Reason behind my interest, LLTHB's situation, your comments, made me think of my 100/30 with 2 x 200w panels on currently.

Max 880w on 24v right.

So I'm wondering, what if I put 4 x 200w panels facing west, 4 x 200w east, at an angle that midday, it has max 800w incoming. All to do with angles the panels are mounted facing west and east.

IF this can work, one controller at max power per array provided they are facing opposite sides.

EDIT: Which means LLTBH needs 1 x 150/35 or bigger controller.

Edited by Guest
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attached a graph for my small array facing exactly east west @ 23 degrees. This is today's graph

Penalty is that during winter ( May to end July) it actually generates less than half of that during dec to February.

Interesting fact is the time when the West panels get alive in the morning !!1401257782_East-West@23degrees.thumb.jpg.695de3380241ce0a02a04db880eee861.jpg

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