[RE-wrenches] AC Coupling
Dave Angelini Offgrid Solar
offgridsolar at sti.net
Mon Oct 30 09:41:57 PDT 2023
From Schneider, who leads in it all the way back to Trace Engineering.
The interlock is to protect the genset from backfeed. Sometimes a block
diagram can help.
A link to their guide which explains the history back to Legacy grid tie
inverters.
https://www.solar-electric.com/lib/wind-sun/Conext-XW-Pro-SW-AC-Coupling-Guide.pdf
Dave Angelini Offgrid Solar
"we go where powerlines don't"
[9]https://offgridsolar1.com/ [10] [9]
e-mail offgridsolar at sti.net
text 209 813 0060
On 2023-10-30 9:14 am, Steve Higgins via RE-wrenches wrote:
> Good Morning Wrenches,
>
> AC-coupled systems are generally not pleasant to batteries of any type.
> I would not AC Couple any Valve Regulated Battery system as you are,
> in essence, "overcharging" them at times, and this can and has led to
> battery failures (usually overcharging and swelling) that all battery
> manufacturers will not cover under warranty. Flooded batteries can
> take this abuse a bit more, but this can still result in drying the
> batteries out and corrosion or degradation of the negative plate due to
> heat.
>
> Lithium batteries do not like this, but the BMS will shut off the
> charge to the battery bank. It doesn't power off the battery but stops
> the excessive charge to the battery, but these unloaded voltages on the
> input of the battery can stress the BMS depending on how much voltage
> is seen.
>
> Ideally, if you are AC coupling, it's better to use a system that
> tapers charging and voltage depending on SOC, to do this you'll need
> closed loop communication, with the inverter/charger so the battery can
> shut down or slow charging depending on SOC.
>
> [1]
> Steve Higgins ⋅ Technical Services Manager
> t +1.902.597.4020 m +1.206.790.5840
> f +1.902.597.8447 e steve at surrette.com
> [2] [3] [4] [5]
>
> -------------------------
>
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>
> On Mon, Oct 30, 2023 at 8:53 AM William Miller via RE-wrenches
> <re-wrenches at lists.re-wrenches.org> wrote:
>
> Jason:
>
> Thanks for contributing to this thread. I did not realize it would
> create so much interest. My main point is to establish a definition
> for AC-coupling, and I believe we do have consensus on that. The next
> question to ask is AC-coupling a good thing?
>
> You make the point that BMS systems protect lithium batteries from
> damage. They do so by shutting down, not correcting the problem. But
> what good is a system that shuts down? When that happens someone has
> to roll a truck to reset the system. We should not design outages into
> our systems.
>
> I spoke at length this weekend with a very knowledgeable member of this
> forum (thank you, sir) about closed loop. Their experience is that
> there were packet losses in the communications and when that occurred
> the system shut down. I am going to set up my first closed loop today
> and I hope I do not have the same experience.
>
> Traditional charge controllers may seem "crude" but they operate on
> real, measured values of voltage and current, values that are at the
> core of battery care. Modern charging systems seem to rely at least
> partially on sate of charge. State of charge is a calculated value
> that is notorious for being inaccurate and drifting more so over time.
> Every SOC scheme I know needs to be recalibrated frequently. The
> recalibration requires hitting certain charge parameters and if you
> can't hit those you don't recalibrate.
>
> I assumed that a BMS system provided within a lithium battery assembly
> by the manufacturer would be accurate. I spoke with a tech support
> person from a prominent LFP manufacturer at length recently and was
> told their BMS is not foolproof on calculating SOC. I am not confident
> that any of them are.
>
> I am new to lithium battery systems. It may be my inexperience
> talking, but it seems they incorporate a lot of complexity that may
> make them unreliable unless charged and discharged very carefully. I
> do not believe AC-coupled charging is a good way to charge lithium
> batteries, or any battery.
>
> William Miller
>
> Miller Solar
>
> 17395 Oak Road, Atascadero, CA 93422
>
> 805-438-5600
>
> www.millersolar.com [6]
>
> CA Lic. 773985
>
> From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
> Behalf Of Jason Szumlanski via RE-wrenches
> Sent: Monday, October 30, 2023 7:33 AM
> To: RE-wrenches
> Cc: Jason Szumlanski
> Subject: Re: [RE-wrenches] AC Coupling
>
> 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
>
> (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 [7].
>
> 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. [8]
>
> 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 [6]
>
> 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 [6]
>
> CA Lic. 773985
>
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Links:
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[6] http://www.millersolar.com/
[7]
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[8] https://www.victronenergy.com/live/ac_coupling:start
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