[RE-wrenches] AC Coupling

Sat Oct 28 20:49:17 PDT 2023

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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