Using AC from inverter to run XW+ battery charger mode

NZRR · September 2017

I am looking for advice to determine if it is possible to charge a battery bank using the battery charger of an XW+ inverter, where the AC for the charger comes from a string inverter.

Details: I would like to install a tracking array with an ABB PVI-6000 inverter, which would be connected to a service panel powered by the AC output of an XW+ inverter, which would be connected to the battery bank. The XW+ would provide AC to the string inverter, and the string inverter would supply AC to the XW+ for battery charging as needed. The AC out from the XW+ would provide the "grid" for the ABB inverter, and the AC from the ABB inverter would power loads connected to the service panel, as well as AC for the battery charger of the XW+ to keep the batteries charged.

Sounds circular, and not entirely efficient, but would it work? What special considerations might there be?

Thanks.

BB. · September 2017

Yes, it can work well. Just need the proper XW+ inverter, firmware level, and configuration. It is a relatively complex setup so get your ducks in a row before buying the hardware (parts list, knowledgeable and experienced installer if used, etc.).

Is this a grid tied installation with battery back up, or a full of grid install? I.e., why this configuration vs typical panels-charge controller-battery bank-hybrid inverter?

-Bill

NZRR · September 2017

This would be a full off-grid installation. We want to use trackers as it gives the customer more power for the money, and they have a great location for it. If we went with a typical ground mount, we would need half again as many modules for the system. And we will need at least two battery inverters to make sure the peak loads are covered, as well as a giant battery bank (approaching 12,000 Ah if FLA).

Any advice on the hardware, or someone to talk to at Schneider would be appreciated. We have installed many off-grid systems using the XW series inverters, but nothing of this scale and complexity.

Lee

Dave Angelini · September 2017

I am working with them now so drop me an e-mail.
A comment, it does sound circular as you say. Why not just DC couple everything and not have the complexity with a reduced reliability?
Schneider builds HV mppts to make this a very reliable system.

Image: https://us.v-cdn.net/6024911/uploads/editor/p4/47qwzv2i7vak.png

NZRR · September 2017

The trackers run on 240VAC, thus the need for the complexity. While the company, AllEarth Renewables, claims about a 50% increase in production, PVWatts calculates 70% or more. Thus, instead of 60 to 70 modules on poles or a ground mount, I can get away with 48 on two trackers.

Ocean · September 2017

A certain thing just occurred to me - you want to use the XW to provide the "Grid" for the panel inverters - so the panel inverters can push energy back through the inverter - and I don't know the XW personally - but usually there is a separate input for the inverter so it can "sync" to the grid before connecting to via internal switches - and then using it to charge.

Most grid tie inverters I would think do not like being "backfed" through the main AC output.

so this would be like the chicken or the egg issue, because if you wire the panel inverters to the ACIN port of the inverter, they will not see the inverters output, and so will not have a grid to push against until the SW syncs and locks to them. They need to provide their own grid first. Unless you have some kind of one/way sensing for the panel inverters to see the XW signal without the XW thinking your are backfeeding it through it's ACOUT port which I think it would not like.

Others here agree / disagree? Am I wrong about the XW - will it take a backfeed directly through it's ACOUT? Is there a workaround for this?

So, it might not be a possibility for you to do this via a normal setup. Not sure what the workaround might be. But if your Trackers simply need 240 AC to run, why not just run some 14/3 out to them along side the solar wiring - and drive them off the Inverter? If the XW does not provide AC240, perhaps a small transformer would do it?
-Ocean

Dave Angelini · September 2017

There is not much an XW+ can't do. As long as the input and output are isolated, (not shorted externally). Do some reading Mr. Ocean.
Radians do this as well as many other Hybrid inverter chargers. Ac coupling is pretty common but just another level of complexity that flies into the wind with a less is more offgrid concept.

Pretty much marketing hype on getting any more output from an AC powered tracker than a DC powered tracker. Certainly not more than 1 or 2 percent. A single axis is about 30 percent more harvest than fixed and a dual axis is about 40%. Does not matter if they turn with DC or AC. I would look at Array technology for guidance on this. I have been involved with them for 12 years now. They are the leader for near 30 years.

Ocean · September 2017

I may read up on the XW+ although I've not much reason to right now. But you said "as long as the input and output are isolated" which is exactly what I was referring too. It seemed to me that the proposal was to directly backfeed the output of the panel "string" inverters into the output of the XW (at the XW's service panel). Does this not exactly contradict the idea of keeping the input and the output of the XW isolated? Why he wanted to do this? Because he needs a "grid" for the String inverters to work - and he wanted to use the XW to provide that grid. But this would mean backfeeding into the XW's output. Am I wrong on this? I just re-read the proposal at the beginning of this thread, and it seems pretty clear.

mike95490 · September 2017

Add up the maintenance and repairs for trackers, and then see if more panels might be better instead.

Dave Angelini · September 2017

My first 13 years offgrid I virtual tracked with the idea that someday I would add electronic tracking and spin the arrays. I even added a dual and single axis for the next 12 years. The bottom line was that while the tracking was great for automating the power, it really did not do much for the total harvest offgrid. That really is a grid tie concept where you are selling at max during sun hours.

The thing tracking did (any kind) offgrid was increase the hours that you were off the battery. Electronic tracking (or the gas system that is out there) also concentrated the harvest to enable that last hour when the sun peaked out of the clouds after a winter storm. It allows for cooling in the long days of summer. Pumping lots of water on a farm or ranch is the other reason. There is very little maintenance, but I can see how 2 squirts of grease every 6 months might be too much.

Ocean, it is basic AC coupling that he wants but to me is crazy offgrid. Look up, watch a u-tube on how AC coupling uses the XW to charge the battery and regulates by driving the XW charger on or off frequency. I would just design a DC coupled system and if he had to have that AC powered tracker, run AC out to it or build a small AC power system out at the array. They usually need 50 watt hours a day or so.

To be honest, I would use an array tek tracker system (I am a dealer and the store here is not anymore) that uses DC and a photo sensor built that way for over 30 years now. Dual axis off grid is mostly just for show as the real deal is the hours of sun and not that small peaking of the array. Follow the sun!

BB. · September 2017

OK--It sounds like the folks are recommending AC GT inverters with the trackers to harvest more energy from the Array... As stated above, that is not what happens with off grid power systems.

With utility power, you can pump 100% of the available array energy back out to the utility. And using GT inverters for the DC to AC conversion is both cheap and efficient.

For off grid power systems... On average, you will probably only use ~50-75% of the avialable energy from your array (whether tracking or fixed). The batteries only have so much storage, and you will be using whatever energy you need during the 24 hour period (which for a "reliable" off grid system that does not use a genset very much, is somewhere around 65-75% of the "average system capability"--If you use more than 100%, you will need a genset to keep up. And for the "average user", they are not going to modulate their energy usage based on hour by hour sun availability every day. For variable loads (say water pumping, you can do this--But a battery based system is expensive vs a no-battery system that pumps during the day to usage/storage tanks, and a small pressurization pump for in-home water usage is much cheaper).

As Dave also states--The advantages to a tracker (whether actual mechanical tracker or virtual tracker of 1/2 the array pointing south east and the other 1/2 pointing south west) is not the increase in WH per Watt of array (which is nice), but the fact that you get more than 3-5 hours of "high" charging current for your battery bank. Lead Acid batteries need many hours to fully charge (6-8+ hours of charging current per day). And it is "kinder" to the battery bank if you have lower peak current but more hours of lower current per day (i.e., 100 amps * 4 hours vs 50 amps * 8 hours per day).

At this point, I still do not see any advantages to a pure off grid system with GT Inverters coupled into an off grid AC inverter.

The only "nice" system that does something like this is the SMA Sunny Island family. The GT inverter are "throttled" by varying the AC island power grid frequency so that the system can modulate the available charging power to the battery bank based on state of charge and loads on the system.

Otherwise, a GT Inverter to an XW system (as far as I know) does not modulate the power from the GT inverters--It simple takes the AC line frequency +/- 1 Hz off of 60 Hz and "knocks" the GT inverters off line for 5 minutes--Then they reconnect (if the line frequency is back at 60Hz). This is "bang bang" type energy control. It does work, but is not very friendly to the battery bank (or electronics).

Of course, I could be wrong on the XW + ABB GT inverters--They could operate like the SMA Sunny Island--Which would be a good thing. And for a distributed power system (i.e., solar arrays, battery banks, inverters, etc.) distributed over a larger "village" type installation, could make sense...

-Bill

Dave Angelini · September 2017

I think almost all is true Bill except the XW has been able to AC couple quite easily with all of the SMA grid inverters 6 years old or newer.
Even I have a grid customer (hard to admit) doing this, and it is quite easy to implement. Also the CSW's from Schneider can do this AC coupling. There is another brand of grid inverter that works well with Outback Radian and Schneider but I can't remember.

It just is weird way to do this Offgrid as the OP originally surmised. Not even going there with reduced reliability, more things to go wrong, and most importantly the hassle of doing things complicated in a lifestyle that celebrates peacefulness!

Using AC from inverter to run XW+ battery charger mode

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