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-boun...@lists.re-wrenches.org] On Behalf Of Mark Frye Sent: Sunday, November 3, 2019 9:39 AM To: re-wrenches@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@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
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