[RE-wrenches] Parts List For AC Couple Disconnect

[William Miller]

Mark:



I do not think line voltage is an appropriate criteria for controlling an
AC coupled system for these reasons:

1.       My understanding of how an AC coupled system works does not match
yours.  The concept is still black magic to me, but as I understand it,
when you use GT inverters to charge batteries through a battery inverter,
there is no charge control.  This is the essence of this problem.  The
batteries will over-charge if you do not intervene.  Remember, in an AC
coupled system you are charging batteries by running power *into* the
*output* of the battery inverter, or backwards to the normal power flow.  I
understand this is why the normal charge control system does not work.

2.       If the battery charging does not stop, there can be no cause and
effect that will cause line voltage to rise.

3.       Even if 2 above were not true, batteries, as they approach full
charge, taper their use of charging source, so you cannot expect any
significant change in charge current when charging stops.

4.       There are too many other factors that can affect line voltage.  If
you set some circuit to do something when live voltage changes beyond a
certain thresholds, this trigger will happen randomly as line voltage
changes for external reasons.



I hate to break the news, but you need to find some way to communicate the
battery voltage to the GT inverter output.  You don’t have to run the GT
inverter AC output all the way to the battery inverter panel, but you will
need to get a control signal connection between the two systems.



Caveat:  I don’t pretend to know thoroughly about how AC coupled systems
work, but this is how I understand them.  Maybe others can verify my
conclusions.



William





Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com

CA Lic. 773985



-----Original Message-----
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Mark Frye
Sent: Sunday, November 3, 2019 9:39 AM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] Parts List For AC Couple Disconnect



Hi William,



Thanks, you bring up issues that are important to me here.



In particular, my situation is that the GT inverter is inter-tied a

couple sub panels upstream of where I want to put the BB inverter. The

distance is long, so I am looking for a solution where I don't have to

run a cable between the two.



In general, I do wonder about using AC line voltage rise to take the Gt

inverters off line. The main goal is to prevent excess voltage at the

battery, so monitoring battery voltage is most direct, and there are

simple solutions for that.



Is AC line voltage a suitable metric for achieving the same goal?



Here is where I could use Wrench knowledge to confirm my thinking, that

being:



- With excess energy in the system, the charger moves it into the

battery, raising it's voltage until it reaches it high charging voltage

set point



- Once the battery reaches it's high voltage set point, the charger

stops putting energy into the battery



- With no other place to put the excess energy, the AC voltage rises



Am I getting this right, the reason to disconnect AC coupled inverters

when the battery if full is not to prevent the batteries from being

overcharged, but rather to prevent the AC line from becoming unstable?



I am hoping this is correct and that with $200 of industrial grade

devices from Digikey I can implement a robust control that will

disconnect the GT inverters before the AC line goes so high that the BB

inverter faults.



Mark







_______________________________________________

List sponsored by Redwood Alliance



List Address: RE-wrenches at lists.re-wrenches.org



Change listserver email address & settings:

http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org



List-Archive:
http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html



List rules & etiquette:

www.re-wrenches.org/etiquette.htm



Check out or update participant bios:

www.members.re-wrenches.org
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches_lists.re-wrenches.org/attachments/20191103/1ca588f9/attachment.htm>