[RE-wrenches] AC Coupling

Sun Oct 29 14:19:24 PDT 2023

William,

My whole email is in regards to the point you made when you said
"This is not how battery inverters were originally designed to operate.  These systems require careful consideration to avoid battery overcharging."

I both agree and disagree with these statements... depending on what you mean. I don't think it's super helpful to say that careful considerations are required without mentioning what those considerations are, and I'll get to those below.

When you say that the battery inverters "weren't originally designed for this" I'd agree to an extent.. The "original" battery based inverters 25 years ago were certainly not designed for AC coupling. They weren't "originally" designed for it. But that was then and this is now.

Now William is correct that the Gen input becomes an AC output, and you can't wire the gen into that "input" if you are using it as a PV input... that being said, Jay didn't think otherwise, he was just critiquing the wiring diagram in the original post to say that it wires up differently, so I'd say that you are both right on this one. That said, I don't think anyone says you have to wire it into the gen port, I think that even with the hybrid inverters, the way in William's original post is still a fully acceptable way to do it.

I am not a huge fan of AC coupling, however, I have done it a fair amount and I have a fair amount of experience with it off-grid with Lithium and with Lead Acid. Most of my experience is with Outback and Victron, but I have some experience with Schneider as well. These systems have been working really well for a good while, but that being said, Schneider had some real problems at first (major headache).

Now when it comes to hybrid inverters, I'm still not a huge fan... but I'm starting to try them out. I have not tested AC coupling with any of them yet. I was at a conference less than a year ago and one of the major brands was saying that there is no hybrid inverter that supports both AC coupled PV and a generator at the same time for a grid tied system. They said "you can't have it all." There may be an exception to that, I've never looked into it, but with the Victron and Outback (and probably Scheider... IDK) it's easy to have it all. I've done it and it works well in every mode. The frequency changes super fast and assuming you have a properly programmed rule 21 compliant Grid tied inverter, it responds very quickly, and in the systems I've monitored, the frequency never had to go above 62 Hz, which is good enough for most sensitive loads. The inverters are totally designed for this these days, but if you have highly sensitive loads, then it may not work well.

Considerations...
First of all, I will be assuming that you are using quality rule 21 grid-direct inverters and a good battery inverter that works as well as the Outback Radian or the Victron. The early Schneiders were too slow in changing their frequency, and special considerations had to be accounted for because of that, but I heard that was fixed a while ago. You had to reduce the charging voltages to account for the delayed throttling, but I can attest that even in the early days, Outback never had this problem, and Victron works smoothly as well.

The primary problem for AC coupling is in off-grid scenarios or prolonged power outages, and that's the black start issue (or dark start). Simply put, if your battery gets low and the battery inverter turns off, then your PV can't charge the battery. This problem is lessened when you connect the PV Grid Direct inverters to a dedicated output on the inverter so that the loads shut down while the battery is still a few percent above the inverter shutdown level. That is a nice advantage of the Victron inverters. I don't know if the Hybrid inverters do this or not. In any case, even with this little safety net, the inverter can still discharge the battery to the point where the inverter turns off and you have a problem. My solution is that you should have at least a little bit of the PV DC coupled so that when the sun comes out, the voltage will rise and the inverter will turn on. This is also recommended in Victron's AC Coupling manual.

The second consideration is the minimum inverter size... Your battery based inverter needs to be capable of handling and controlling the full PV power. Victron recommends that the maximum PV power (DC or AC, whichever is lower) does not exceed 100% of the rating of the inverter (no more than 10kW of power from the PV on a 10kW inverter). Outback recommends no more than 6 kW of Grid-Tied inverter per 8 kW of Radian (so that's 75% of the battery inverter's power). In some circumstances, this could demand upsizing the battery inverter, which is another great reason to DC couple half of the PV and AC couple the other half... because then you could effectively have more PV with less battery based inverter, depending on your loads.

As was mentioned by William, monitoring is generally not as good with an AC coupled system, but here again, that's not always true. Victron has been really leading the way with integrating with other PV grid-tied inverters so that you can monitor them through Victron's own monitoring. This depends on the brand you are connecting to, but most of these brands have a way to get this information via TCP/IP, and if so, Victron will read that and report it on their own website, in their app, and on their touchscreen. With some brands, you can even see the error codes etc. remotely through Victron's monitoring. It almost makes it seamless as though it was all one brand.

So in case it's not clear, I'm never a fan of 100% AC coupled PV, but these days there are very few legitimate concerns or issues... Most if not all of which disappear with a 10% - 60% DC coupled system.

Thanks,
Kienan

Green-Go Solar Wholesale Distribution
maxfieldsolar at hotmail.com<mailto:maxfieldsolar at hotmail.com>
(801) 631-5584 (Cell)
www.distribution.solar
________________________________
From: RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> on behalf of Jerry Shafer via RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Saturday, October 28, 2023 11:14 PM
To: William Miller <william at millersolar.com>; RE-wrenches <re-wrenches at lists.re-wrenches.org>
Cc: Jerry Shafer <jerrysgarage01 at gmail.com>
Subject: Re: [RE-wrenches] AC Coupling

AC coupled systems used to work like this but with solark and sort of with outback they use the gen set input to the inverter, do the freq shift and charge via a preset voltage. This seams to work well but if you connect a genny it must be on the grid input via an ATS, alot more complicated the any DC side connection.
Funtimes

On Sat, Oct 28, 2023, 8:51 PM William Miller via RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>> wrote:

Jay:

I am not sure how it would be possible for a grid-tied inverter to operate with any battery inverter if it was not connected to the battery inverter AC output.  The grid-tied inverter needs to see correct voltage and frequency before it can start up.  How else could this work?

I looked up all of the inverters you cited.  In each case it may look to you like you are connecting to an input, it is really just reprogramming the inverter to turn the generator input into an extra output.  All of the manufacturers provide warnings about doing this.  Here is more information from each of the manufacturers:

Sol-arc:  From the manual for the Sol-arc 15K-2P-N: You convert the Gen input to an output and make that your connection point.  “A full AC coupled solar system is not recommended as power control and monitoring is limited.”

EG4:  I could not find an “ev4” inverter anywhere online.  There is an EG4 inverter.  I read the manual and it looks like the EG4 works just like the Sol-arc:  You program the generator input to actually be an output.  The EG4 abruptly disconnects that output when battery SOC reaches a programmed level-- There is apparently no frequency shift function.  The manual is pretty inadequate on this point but they do provide a video<https://www.youtube.com/watch?v=vRAq9g34VcI&ab_channel=EG4Electronics>.

Fortress:  The same as above is true about using the generator input for AC-coupling on the Fortress FP-Envy-12k.  From that manual:  It is forbidden to connect the Generator in Gen Port when AC Coupled.  That is because when AC-coupling, the generator input is no longer an input.

So while it appears one connects an AC-Coupled inverter to battery inverter inputs, it is just a reprogramming of the generator input to be an output.  In all cases that I can see there is still no voltage control of AC-coupled battery charging.

I am aware that to try and provide some modicum of control over the battery charging from AC-coupled sources, a scheme was invented to shift the output frequency and taper or shut down AC-coupled power.  This is a work-around, not a design.  In each and every one of these systems it is pretty clear there is little control over battery charging voltage.  These systems do not support three stage battery charging like a DC charge controller can.

When AC-coupled charging is connected to batteries with BMS in certain conditions battery voltage spikes can occur.  See section 2.2 of this article.<https://www.victronenergy.com/live/ac_coupling:start>

There are limited advantages to AC coupling:

•        AC coupling allows reduced wire gauge for long-haul feeders.  This is no longer as necessary with new 600 volt input charge controllers.

•        AC Coupling allows an easier upgrade path for a customer that has grid-tied inverters and wants to add batteries.  I am not sure most batteries can survive for long with the crude charging algorithm provided, so I would hesitate to recommend this.

In my opinion AC-coupled battery charging with even the newest inverters is a crude affair.  I am not a fan.

William

Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com<http://www.millersolar.com/>

CA Lic. 773985

From: Jay [mailto:jay.peltz at gmail.com<mailto:jay.peltz at gmail.com>]
Sent: Saturday, October 28, 2023 1:58 PM
To: william at millersolar.com<mailto:william at millersolar.com>; RE-wrenches
Subject: Re: [RE-wrenches] AC Coupling

I’m going to both agree and disagree

Agree The older inverters ( Schneider, Outback, etc) your drawing is correct.

Disagree For many new inverters ( sol ark, ev4, fortress) they bring ac coupling into the inverter directly.

Usually on the generator input and not in the backup loads panel.

And I will also disagree about the statement that these inverters are not designed for ac coupling.

Agreed that the earlier non rule 21 compliant versions didn’t have any internal control and required external relay control to prevent overcharge.

Newer versions of Schneider, OB, have  frequently shift which works with newer gt inverters to address the ac coupling overcharge concerns. While creating potential new issues for loads with up to 64 hz.

Jay

On Oct 28, 2023, at 2:15 PM, William Miller via RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>> wrote:

Friends:

I am starting to get the impression that there is a diversion on the understanding of the definition of AC coupling within the industry.  I think this definition needs to be clarified.

Below is a diagram of what I understand is a DC coupled system (I hope these diagrams come through):

<image003.png>

A battery inverter that receives AC power into an AC input port is not AC coupled.

Here is an AC-coupled system:

<image005.png>

What makes AC-Coupled systems unique are the problems presented by sending power backwards through a battery inverter.  This is not how battery inverters were originally designed to operate.  These systems require careful consideration to avoid battery overcharging.

Do we have consensus on this definition?

William Miller

Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com<http://www.millersolar.com/>

CA Lic. 773985

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