AC coupling SMA-Schneider

Tropical_Installer

Hello everyone,

I consult this forum regularly but it is the first time I’m writing a post.

I am a solar installer in Haiti. I am designing a 20kW PV system and would like to hear your opinion.

It is for a vocational school that will only work on the day from 8:00 AM to 4:00 PM.

The system includes:

- 72 solar panels 320W, Yingly

- 24 Trojan L16RE-2V batteries (2V, 1100Ah)

- 3 inverter Conext XW+6848 (three-phase current). They have three-phase equipment

- There is a backup generator too.

Solar conditions here in Haiti is + 300 days of sunshine/year

The maximum possible load is 16kW, if all the appliances are running at the same time! I don’t think this will happen.

At the beginning I thought using 4 charge controller Schneider MPPT 600-80 but I realized that the charging current would be much too high: 320A. The maximum charging current allowed by Trojan is 143A (13% of C20 capacity)!

So, I thought about an AC coupling system. I can use 3 Sunny Boy 7000 instead of the DC charge controller.

Sunny Boy # 1 L1 L2

Sunny Boy # 2 L1 L3

Sunny Boy # 3 L2 L3

I know that the XW + have the "frequency shift" function which will “disconnect” the Sunny Boys when the batteries are charged. So, I won’t have the over-current charging problem anymore. I have never used this function in the XW+

I have never done the SMA - Schneider coupling. I really don’t know if it will work.

What do you think?

JR

mcgivor

The question is why not use Schneider CL series string inverter which are available in 3 phase https://solar.schneider-electric.com/products/three-phase-string-inverters-commercial-and-residential-grid-tie-inverters/ these would work with the XW, documents available from Schneider, unless of course you already have the SMA equipment.

Tropical_Installer

Mcgivor,

Thanks you for your answer. I checked the CL36, it might be a good solution but it is not available here. There is only a 2 pages technical specifications on the Schneider website and I can't even find a website to buy it.

JR

mcgivor

When I needed a vendor, I filled out this https://solar.schneider-electric.com/purchase-products/ , they sent a list of distributors

Dave Angelini

The charge current on an XW can be set to any value you want by lowering the percentage from 100% down to the value of current you want for your battery. The Power system will still have the full available power for AC loads but limit max DC charge current. Same thing with Outback. Make sure you use surge protection in Haiti with all of that lightning.

Tropical_Installer

Dave,

The charge current is not the problem in an AC coupling system. It is limited by the XW charger. My concern is about the frequency shift from the XW to stop the current coming from the Sunny Boys when the batteries are full.

Dave Angelini

Well of course it is a concern and a major reason not to AC couple. That said it should work as modern SMA grid chargers do work with the XW. I would never do this for a customer because to me it is just the wrong way to go. The mppt-80-600 is just too easy to accomplish the same thing without risking the battery. I never add more risk to charging a battery. In Haiti I think you should feel the same.

For a large micro grid with a skilled on site operator it works as a way to easily join dozens+ of arrays.

Tropical_Installer

I would need 4 mppt-80-600 to handle the 72 panel 320W. Charging current will be too high for 1100Ah battery bank. I would need to triple the size of the battery bank. I will end up with a large amount of unused energy, since I only have day-time consumption

mcgivor

What is the 3 phase load count? If the number is small, there are ways of converting single phase to 3 phase using frequency drives. The management of single phase system is much easier than 3 phase in conjunction with single phase. Keeping everything within a single operating parameter would open doors not available with the current setup, just thinking outside the box for the time being.

Dave Angelini

https://forum.solar-electric.com/discussion/comment/396022#Comment_396022

Not correct! Charging current can be limited as I said before in post 5. As mcgivor also asked what is the AC load? Seems overly complex but if the load is large then you have to use 3 phase.

Tropical_Installer

Mcgivor, there are only 2 3-ph equipment, 2HP each. So maybe it possible to install in 120/240 and use a converter for those equipment.

The maximum load possible is 16kW. Since all the equipment are not going to run together, we assume a 10kW average. for 8h/day that gives an energy consumption of 80 kWh/day. PV array is 72x320 = 23 kW. Peak sun hour here is 5. So my arrays will produce 115 kWh/day. Fair enough to cover the daily load.

So Dave, If I limit the charging current coming from the MPPT -80-600, I will get less power from the sun. In that situation, it would be better to buy 2 MPPT and less panels, this will save money. but I will not meet the 80 kWh daily load.

The problem is the size of the battery bank which can support a maximum charging current of 143A and the size of the PV array 23kW.

In fact the question can be general : how do you design a system that runs only during the day, no load at night, with a large PV array and a "small" battery bank?

This is why I decide to explore the AC coupling solution. Question is, will the frequency shift work between the XW+ and the Sunny boy

mcgivor

The maximum possible load is obviously during the day, this can be subtracted from the charging current. Limiting the controllers would, in effect, neuter them in their sole purpose of production,, which as you say not a logical solution. There are two thoughts which come nind, if there are no loads overnight the battery would be at a high state of charge, this in itself would limit the charging current to much less than the PV could produce, the excess would be diverted to the loads. Worst case scenario would be if the batteries are depleted, thereby accepting maximum available current, this would be in excess of maximum allowable, the solution may be in blocking some of the controllers from actually charging with the use of diodes, just a thought in its infancy.

Dave Angelini

You guys are really not understanding a networked power system of the same make equipment.

Mcgivor, you have one I think so test it. Limit the charge current on the mppt and run loads on a good day.

You will see that the full power of your solar is used for your loads and the current can never go higher to you battery than the percentage you limit the battery to.

So, to the OP, (2) 1100 ah batteries at 10% is going to be 220 amps for max charge. (4) 80amp mppts can max out 320A.

You would set the networked mppt to 60% and then dial it up or down to get the 220A max to the battery. The full 320A is available to the loads as they will take priority over the battery.

This is how Outback and Schneider have run their systems for a very long time. Other make may also but I use these. The network knows what the inverter is doing and the solar works with it to achieve your goal. You give this and other features up when you mix equipment.

mcgivor

That's interesting Dave, will try it today, didn't find information on that in the manual, not to say it'not there somewhere, I've never had the need to limit the amperage.

mcgivor

So I conducted the test, set the maximum charge current to 5A, plugged in a 500W rice cooker, no change to current output of the controller, it remained at 5A, all current dischargeing from the battery to power the load, Available PV at the time 1040W when set at 60A maximum so it would appear not to work as decided.

Dave Angelini

I do not understand your response? It works or it does not? I only have done this with XW BTW.

The OP would lose most all of the data by using AC coupling. All the charts and being able to see a "picture" of what is going on. He would also lose the monitoring remotely and getting help with warranty.

mcgivor

With the equipment I have it dose not work, perhaps with a Schneider battery monitor it could work, unfortunately that is not part of my system, it may be the missing part of the puzzle. Additionally mixing components of various manufacturers would result in finger pointing as to which component is at fault, probably voiding all warranties.

Tropical_Installer

Dave, well I did not know that! Never heard about it

At home, I have a networked power system of the same make equipment Schneider SW4024 and a MPPT 60-150. I'l will do the test this week-end since I'm out all day long.

Thank you guys for your help, I will let you know.

Tropical_Installer

Dave, I have test what you said about the networked systems: XW+, MPPT 60-150 and 3kW PV

At 10h00 AM, PV output was more than 2000W. I reduce the charge rate of the charge controller to 20%, I got fixes values : 618W and 11.8A. I lunched a 1540 Watt load, the overall amperage was -26.7A, the XW+ was -38,5A and the MPPT +11.8A.

Voltage started to decrease from 52.2V to 52, 51, 50

So it doesn't work as you predicted . I was expecting to see a "pass through" from the MPPT but that was not the case. The energy above the 20% was lost. Maybe there are special settings to achieve this behavior from the MPPT.

Dave Angelini

Hi TI,

I got to thinking about it and there was a project that I did with Schneider and they wrote firmware for a controller to basically limit the percentage in bulk only. Another way is to to set one controller to come on after the aux output detects absorption. With (4) mppts I would think only one needs this. The other way is to just manually set the 4th controller to disabled until the others go to absorb.

This also could be done with modbus but you need to know that...

The conext Bridge I worked on does this and that is what I have here. It was a failed project that LG Chem decided they were not going to certify for Offgrid. I run just over 6KW of tracked solar and the bridge limits charge in Bulk only.

Still another way is to virtual track your solar and that way you would be in absorb before the current exceeded the battery max charge.

I could give you a sales Engineers email and you could beg. A project this size might get some ears. It might not also because warranty is always a force that stops creativity. Good Luck to you!

Estragon

Midnite Classics can use a "whizbang jr" to sense current into the battery as distinct from current to loads. In my case, this allows for better use of end-amps for transition to float, but it may also allow for charging current limits to be more useful in this application as well.

I'm not sure it works that way though. May be able to test on my classics in the next day or two.

Dave Angelini

Yes try it! The XW system does this but can't limit battery current and supply max solar to the loads without going over the 220 amp max battery charge with a 320 amp PV source in this case.

Even with the conext battery monitor and it's shunt the limiting of charge current will limit the same in bulk,absorb, and float..Really all that is needed is firmware to limit in bulk only.

One of the great things about the XW/CSW system is it does not need shunts to sense the battery current and loads. They are internal but can't determine SOC without the battery monitor.

Tropical_Installer

Dave, I'm not really aware of the electronic inside the MPPT. If I understand well, it is possible to use 4 MPPT without exceeding the battery maximum current. This can be done with a firmware update or manually. Right?

Since I can't change the firmware, I need to go with the second option. Having someone managing the breakers of the MPPT will not be a problem here. So, if I start with 2 MPPT, as soon I as get to absorption, I can turn ON the 2 others? What about the load? what will happen if the load drops to 1kW with the 4 MPPT ON?

It seems like you are saying the DC coupling can limit the charging current into the battery and power the load, which is what the AC coupling does. Can the MPPT split the PV current in % for the battery and % for the load?

mike95490

Lead acid batteries will regulate their charge amps, according to the recharge voltage & the battery state of charge.

You "could" possibly rely on that, and the slow ramp up of power as the sun rises, and it only falls apart if it;s a foggy day and clears at noon, then - wham 320 noontime amps hit. if you generally have clear mornings, I'd gamble on that factor, rather than someone not falling asleep at the switches.

Tropical_Installer

Mike,how can the lead acid batteries avoid getting 320Amp from the 4 MPPT at bulk stage?

mcgivor

https://forum.solar-electric.com/discussion/comment/396249#Comment_396249

It's a ballancing act that is probably something not worth getting into, it would require someone to know when to throw a switch. Humans are notoriously unreliable, for this reason I personally think it wouldn't be a good idea, just an opinion.

The best solution would still be AC coupling with like components, did you request a dealer contact? Personally I'd be surprised if there were not dealers suppling the required equipment in Haiti, or at very least somewhere close, then again it's the third world, something I have to deal with on a regular basis.

Dave Angelini

Maybe it might be best to use one grid inverter limiting the charge current to an XW and the rest of the system on mppts? I just do not like using curtailment in an offgrid system. I probably never will and that is my bias. I hate to lose the combox or gateway views of system components running and the data. Seeing the picture is very valuable in a large system.

As I said you could automate by using the AUX to control a mppt, but as Mike said you will probably be fine most of the time and you probably do not need that 4th mppt except on tropical cloud days. On those days you will not have the risk in bulk.

You can contact me privately as I see other things that might help you.Good Luck!

Tropical_Installer

Thank you guys for your help. I read a lot about AC coupling, I think this might be possible to mix SMA and the XW+. If the frequency shift does not work, I still have the "relay option". This is what Outback uses to shut off the PV inverter.

The solution I'm designing is based on a limited budget, otherwise I would add more batteries and go to DC coupling. But, remember storage is not really necessary in that case. I should review my calculations to see if I can run with 3 mppt and less solar panels.

Mixing PV inverter and MPPt might be a good solution for battery charging since the PV inverter will not follow the "normal" charging algorithm. What would be the impact on battery lifespan with only the PV inverter? adding an MPPT can ensure the battery bank is fully charge.

Tropical_Installer

Dave I understand what you were talking about regarding the charge controller limiting the current.

I saw that last week-end at home. Around noon, the load was 400W and the sun was very bright, I was probably producing 1300W from my 1635W PV array. I couldn't know exactly because the batteries were floating. On my combox, I saw around 300W coming form the PV and 200W coming from my battery bank!! So I decided to raise the load by turning ON some fans and TV to bring the load at 800W. The power coming from the batteries remain the same while the power coming from the PV rise up to 700W. So I guess the load was "connected directly" to the PV so the charge controller was regulating the current. In that situation I can't have the overcharging current problem.

To get back to my original design, one possible solution can be to start with 2 MPPT at 80% to bring the batteries to float and then use the AUX port of the XW+ to drive a relay to connect the 2 other MPPT. The AUX port can be trigger by the SOC since I will have a battery monitor. The 2 other MPPT will come ON when the batteries are floating and will power the load directly. No need for AC coupling in this case.

Dave Angelini

Your last sentence is not what I and others here are saying. You will waste all the time in absorb that you can be powering large loads. Once the battery finishes bulk, the current into the battery will self regulate below the batteries max charge rate.

jakobw

Hi all!

Tropical_Installer.? Were you able to AC-Coupled or did you decide to go DC-Coupled?

I'm going to be installing some SMA AC-Coupled systems soon and looking for some real-world stats on different configurations.

Jakob