[RE-wrenches] AC Coupling

Sat Oct 28 22:14:46 PDT 2023

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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