[RE-wrenches] AC Coupling

Mon Oct 30 07:32:39 PDT 2023

I read through this thread, and I don't think one important aspect was
covered with respect to "overcharging" batteries. There may be risk with
lead acid batteries connected to a hybrid inverter that is AC coupled. But
LiPO batteries have a BMS that is there, in part, to protect the battery
from an overcharge scenario. The BMS is going to disconnect the battery
before catastrophe strikes. In my opinion, AC Coupling is better suited to
modern equipment that talks to each other in a closed loop. Crude
algorithms for throttling PV don't seem particularly confidence-inspiring.
This is one argument for sticking with a single manufacturer system
architecture (i.e. Enphase). While there are obvious downsides and
limitations to that, the system should work safely and flawlessly in an AC
Coupled scenario.

With respect to mixed systems with DC and AC Coupling, while I see the
advantages, it really complicates the design and installation. It also
greatly confuses the monitoring aspect for PV production. I think it
depends on the scenario, but going one way or the other makes the most
sense to me in most situations. With that said, there are exceptions. For
example, I have a Sol-Ark 15K client right now where all of the MPPT inputs
are taken, and reconfiguring the DC coupled strings is not easily achieved.
So, to add more PV, the obvious choice is to AC couple. But this system has
a generator, so the AC Coupled PV needs to be on the AC load output,
meaning it will not be monitored by the Sol-Ark. That's not ideal, but it's
nice to have that flexibility.

Jason Szumlanski
Principal Solar Designer | Florida Solar Design Group
NABCEP Certified Solar Professional (PVIP)
Florida State Certified Solar Contractor CVC56956

On Sun, Oct 29, 2023 at 4:19 PM Kienan Maxfield via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> 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 <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> 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
>
> CA Lic. 773985
>
>
>
>
>
> *From:* Jay [mailto:jay.peltz at gmail.com]
> *Sent:* Saturday, October 28, 2023 1:58 PM
> *To:* 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> 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
>
> CA Lic. 773985
>
>
>
>
>
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