Can I connect two strings with 3 wires

davidwillis

This is for an off grid system.  I know your first thought is no, but here is the situation.

I have 3 4/0 wires running underground 1000 feet, and I want to setup two strings, one facing west, and one facing east, and will be using two flex max 80 charge controllers (one for each string).  Can I connect all negatives together and run them through one of the wires, then run the positive from one string through the second and the other positive through the third?

Estragon

I can guess why you might want to avoid adding a 4th 1000ft run of 4/0. 

I *think* it  would work, but you would be limited for ampacity and voltage drop by the single negative.  Obviously both east and west arrays can't be producing full current at the same time, but...

Hopefully others will weigh in.

davidwillis

Thanks.  I am not worried about the amps or the voltage loss.  Even with both arrays at full output, my voltage loss will be under 5% and not even close to the amperage limit of the 4/0 wire.

I am the same as you.  I think it should work, and don't see why it would not.  But there may be something I am missing...

Photowhit

I think this might be another one for the Outback forums, I can't see why, but perhaps it would have problems with measuring the MPPT during sweeps. Just something I vaguely recall from my faulty mind...

Horsefly

The only thing I can think of that would be a problem is if something was using a shunt to measure the current coming from the PV arrays. I really doubt if the Flex Max would have a shunt on that side, but if it did it would be on the negative line. The current on that line will obviously be the sum of the current of the two positive lines, so neither charge controller would get a real current measurement reading.

Like @Photowhit said, it is probably worth checking with the Outback homies to make sure, but I would imagine there is no shunt on that side, so no issue from that perspective.

I gotta say, I'm pretty impressed by the idea of 1000 ft of #4/0 wire. Wow! 

davidwillis

Thanks guys.  I will check in with the outback just to make sure.

davidwillis

This is what outback says:

This is not recommended, the array side of your system must be completely isolated, one positive and negative wire per controller.
Your idea of sharing the negative conductor will seem to work in the beginning; the controllers will seem operational, however, you will affect your total production. The MPPT algorithm will not perform at its maximum efficiency

bill von novak

davidwillis said:

This is what outback says:

This is not recommended, the array side of your system must be completely isolated, one positive and negative wire per controller.
Your idea of sharing the negative conductor will seem to work in the beginning; the controllers will seem operational, however, you will affect your total production. The MPPT algorithm will not perform at its maximum efficiency

Sounds like they rely on the negative side to do something.  Current sense resistor perhaps.

Estragon

I wish they had explained a bit more about why the MPPT would be messed up.  How would the east controller see current or voltage from the west array, or vice versa, in normal operating conditions?  Negative is ground referenced, and positive will be +Vmp on each array.  Where is the circuit path to mess with the tracking?

Also, ran across an interesting article on using single MPPT on multiple arrays...
http://www.interpv.net/tech/tech_print.asp?idx=418&part_code=030140054

It's about grid-tied specifically, but I think the principles would mostly hold for battery charging.  The author apparently works for Fronius, and seemingly wouldn't have much incentive to propose something that sells fewer MPPT inverters.

Vic

Agree,   cannot see the big issue.

In the recent FM CC Manual,  the wiring diagram shows the PV Negative connected to the negative battery Busbar,  along with the CC's PV negative,  and battery negative terminal.   So cannot see the CC trying to measure the input current on the low side,  as is done in the Schneider 60 A 150 V XW CC,  and the MN KID  CC.

A further clarification from OB would be helpful.   Wonder if this a bit of confusion from OB,  about just how "separate"  must  each PV array be,  on  each of two (or more)  CC.  With two CCs,  each with its own PV array,   and each CC's PV negative connected to the SAME negative bus,  is NOT making both PV arrays functionally-common,   IMO.

There may be a wiring diagram for the FP system,   showing just how two FM CCs are connected ...

FWIW,   Vic

davidwillis

Thanks for the comments.  Yes I wish outback would give more detail.  I tried to get some more, and this is the reply:

"Thinking out loud,,,,,, you may need to get a data connection for temp sensing but possibly put the CCs the panel end ? with 4/0 you should not get that much drop even just using 2 of the three conductors, use the three & I presume you will get morning then afternoon Sun so the neutral will not be too loaded ?"

I will let you know if I find anything else out.  But there is no way I am going to put the cc on the panel end since it is charging a 24v battery, I would loose too much power with that.

If for some reason outback will not work, I will use another brand, but the whole system is outback, so I just wanted to stay with that.

mcgivor

Vic said:

Agree,   cannot see the big issue.

In the recent FM CC Manual,  the wiring diagram shows the PV Negative connected to the negative battery Busbar,  along with the CC's PV negative,  and battery negative terminal.   So cannot see the CC trying to measure the input current on the low side,  as is done in the Schneider 60 A 150 V XW CC,  and the MN KID  CC.

A further clarification from OB would be helpful.   Wonder if this a bit of confusion from OB,  about just how "separate"  must  each PV array be,  on  each of two (or more)  CC.  With two CCs,  each with its own PV array,   and each CC's PV negative connected to the SAME negative bus,  is NOT making both PV arrays functionally-common,   IMO.

There may be a wiring diagram for the FP system,   showing just how two FM CCs are connected ...

FWIW,   Vic

If the positives of each array are isolated but share the positive of the battery there, is a possibility that conflict could occur there. Isolating the positives of the controllers using a diode on each positive output may solve the problem, if in fact that is where the conflict allegedly occurs. Or is there no real issue and the reaction to an obscure  question was, it can't be done, without justification. 

Estragon

@davidwillis
That answer isn't making much sense to me. If the issue is voltage drop, wouldn't that be an issue with the size of the wire, not just the shared negative per se.

If you did put the CCs at the panel end, you would likely just add a second negative (and move batteries and inverter to the panel end too)? I guess that solves the "problem", but it doesn't answer the question of why the shared negative won't work.

I see you posted on the MN forum as well. Maybe they know of some reason it won't work or some difference between MN and Outback? If anyone would know it would be boB, but they must be pretty busy trying to get the new inverter happening.

davidwillis

Sorry, I probably was not to clear.  Outbacks solution was to put the charge controller next to the panels, and only use two wires going from the controllers to the battery.  However I don't want to do this because the charge controller will drop the voltage down from 100+ volts to 24v, and increase the amps accordingly.  with 100V and 20amps I will only get a 1.96% loss in the wire, but with 24v and 83 amps (the same power after the controller), I will get a loss of 34%.

I can't move the inverter and battery bank to the panels because I already have an array next to the batteries, and the battery bank and inverters are already wired into the home.  The east/west array is simply to give me me extended hours of charge time because the current array is facing south, with a mountain in the way which only gives me sun until about 2:00pm.  This east/west array will be on top of the mountain to provide more hours of sun.

I hope that makes more sense now..  Sorry for not giving enough detail.

Yes, I posted on MN forum.   They seem to think the MN controller would work just fine in that configuration.   But I just can't see any reason why it would not work with the outback controller.

@mcgivor Your idea of the diode got me thinking.... What if we used just one charge controller with both arrays run in parallel with a diode on each leg to prevent any reverse current through the array that is not producing power.  I think I will ask outback if they think that will work.

mcgivor

@davidwillisWhat if we used just one charge controller with both arrays run in parallel with a diode on each leg to prevent any reverse current through the array that is not producing power.  I think I will ask outback if they think that will work.

That should work, there will be a slight voltage drop over the diode but not significant to worry about, use Schottky diodes, for low forward volt drop. 

davidwillis

@mcgivor Thanks!  I think that is what I will do.

mcgivor

Please update and don't be a stranger, everyone benifits from experimental findings.

davidwillis

Sorry, I have not really got any more information, other than on the midnight forum they say the 3 wires will work fine, and on the outback forum they say it will not (no explanation why other than  the  MPPT algorithm will not perform at its maximum efficiency).

But everyone seems to agree that one charge controller with a diode will work fine, so this is what I plan on doing.

BB.

I do not believe you will need a Diode in each + of the separate arrays. In theory, you do not even need a separate circuit breaker (int he positive lines) for each string (breaker/combiner box would nice for servicing/debugging, but not needed for safety--Other than you would need a box to install surge supressors if you have a good chance for lightning).

Diodes do have drops (0.2 volts * 20 amps = 4 watts of wasted energy--You need heat sink(s) to keep cool and insulated against shorts). Also Diodes to fail (both shorted and open). Another thing that can fail.

The first question--Can you then have the east+west array paralleled together to a single MPPT controller. Yes, it will work, but will not be optimum. One array will be at a different temprature vs the other array--So they each have a slighting different Vmp-array... There is no "one optimum" point--The MPPT will simply do its best to figure out what the less than perfect match will be (within 10%?).

If you have two different MPPT controllers. Outback FM series (and apparently Midnite's?) does nothing with the negative wire--The can share the single negative bus connection without issue between two (or more) controllers). And, you can even run the negative solar connection directly to the battery bank.

If you have a DC GFI protection system--That makes things more complicated (I do not like DC GFI systems). How the system grounding/negative wiring runs can affect the DC GFI system.

There are charge controllers that measure current in the negative leg. Schneider (Xantrex) units (some/all of the MPPT?) do that, and you cannot use a single negative between two or more controllers/arrays. Each controller must have its own (isolated) negative lead so it can measure its array current to make the MPPT calculations.

I am a little confused--Just to be clear, are the arrays being connected to a single MPPT controller or two different MPPT controllers?

When paralleling the arrays--You have combiner boxes on each of the East/West arrays? It sounds like you need it (more than 3 paralleled strings when E+W arrays are "added together").

I do not see the need for any diodes in any configuration here...

-Bill

Dave Angelini

Me neither!  Try it all and see what happens. Depending what you find this is probably another reason to stick with one brand of equipment and another reason for using a 600vdc Mppt.

davidwillis

BB.  Thanks!
Originally I was thinking about using two different mppt controllers, but on the outback forum they said not to use a common negative wire for both.

The reason for the Diode would be to keep from feeding power through the shaded array (wasting power).  Maybe this does not happen though (which it sounds like from your comment it does not).  But isn't this the reason why shading causes an entire array to loose so much power when just one panel gets shaded?  I know this is an off grid system, but I see many sales people pushing grid tie micro inverters saying how much just a little shading will effect a string inverter, but only effect the shaded panel of a micro inverter system.

Yes, I will have a combiner box for the west array and another one for the east array, but no GFCI.

Thanks!

Estragon

My understanding is the reason shading drops output so much is because of the serial connections. The two arrays would be parallel.

Having one MPPT controller for different array orientations causes problems in calculating max power point.

AFAIK, microinverters aren't designed for off-grid. Presumably they need external reference AC.

Dave Angelini

Shading can cause the controller to pick the wrong power point but not always. The thing that is worse is clouds causing the controller to be off or spend time searching. You will have to get feel for what it is doing. It may not be a big deal for you depending on your loads. If you have large loads cooling/heating, you will find out! Nothing worse than several KW of solar running a 1KW load and the solar goes to zero while searching. It really depends on the controller also. As you said, "the first thought is No" Good Luck.

Last thought is Outback has the Ultra charge controller out soon and it will be hi voltage. You could network the 2 controllers and run an extra #8 or 10 wire and do this the best way if you have problems here.