Mark,
I think that you're not understanding what Brian is saying. In any case, your
proposal will not work with most equipment. There may be some equipment that
would work that way, but not anything I'm aware of. Let me tell you what will
happen in a normal system, assuming that the manufacturer hasn't built in an AC
coupling control mechanism (like frequency shifting)
The BB inverter will maintain a stable AC voltage by allowing the GT inverters
to back feed into the batteries. If the batteries start overcharging, the BB
inverter will not raise the AC voltage in response. What will happen instead is
that there will be no measurable difference in the AC voltage, but the DC
voltage will continue to rise until it hits the multimode BB inverter's high
battery cut out voltage, and the BB inverter will shut off, and your entire AC
line will drop to 0 volts. This will turn off the GT inverter, and as the
battery voltage falls back into an acceptable range, the BB inverter may or may
not automatically turn back on. If it does automatically turn back on, then the
GT inverter will wait for 5 minutes and then they'll start back-feeding until
the battery voltage goes above the overvoltage setpoint, and the whole system
shuts off again. So this is what is normally happening if you hear about an AC
coupling system that shuts off once every 6 minutes or so during the day... And
yes, I have seen a system where the homeowner was told that this was normal,
and it's just how the system was supposed to work..
So if the inverter has an AC Coupling control mechanism, and that AC coupling
mode is turned on, then it will change the characteristics of the AC line in
order to notify the GT inverter to turn off, or to decrease its output. The
only way that I've ever heard about any manufacturers implementing this is
through frequency shifting. Now it would be perfectly possible to do it through
voltage shifting (or voltage raising) and I've talked to some engineers who
said that on a technological level, it wouldn't be any harder for a
manufacturer to do voltage shifting than frequency shifting, but no
manufacturer has actually pursued that. So basically, your only options are
1. use a BB inverter that has frequency shifting control for AC coupling
2. run a wire from the battery room to the GT inverters that you can use for
controlling them based on the battery voltage
3. experiment with something custom and just expect a high probability of
eventual failure (but let us know how it goes)
* You might be able to find a device that would read the battery
voltage, and then transmit that information wirelessly, then have another
device at the GT inverters that decodes that information and then drives a
relay accordingly. (I have no idea how to do this, for me, this is in the
hypothetical realm.) Or maybe it could transmit the info via PLC?
4. or the fourth option is to set the BB inverter's high battery cut out
voltage to just above the absorb voltage and just plan on the power going out
multiple times per sunny day. I wouldn't go for this one unless it's your own
home and you're feeling very adventuresome.
Essentially the bottom line is that monitoring the AC line will never work
unless the BB inverter has a built-in AC coupling control mechanism, then
you'll be monitoring for the frequency rather than the voltage.
Sorry for not having anything more helpful to say.
Kienan
Maxfield Solar
[email protected]<mailto:[email protected]>
(801) 631-5584 (Cell)
________________________________
From: RE-wrenches <[email protected]> on behalf of Mark
Frye <[email protected]>
Sent: Sunday, November 3, 2019 11:00 AM
To: [email protected] <[email protected]>
Subject: Re: [RE-wrenches] Parts List For AC Couple Disconnect
OK Brian,
So I think you are confirming that monitoring AC side voltage is an
acceptable way of deciding when to disconnect the GT inverters.
With my parts list, I have a delay time that would hold the GT inverters
off-line for some period of time (I would probably set to 1 hour) before
coming back on line after tipping off.
Mark
On 11/3/2019 9:53 AM, Brian Mehalic wrote:
> With frequency-on/off, frequency power control, or other “inverter
> integrated” control based on battery voltage/state of charge there is also
> control in regards to when the ac coupled inverters come back on. And yes,
> this is definitely needed to prevent overcharge (or get three-stage charging
> from the AC coupled system). Of course if it doesn’t work, yes the bus
> voltage will rise and trip the BB offline.
>
> AC bus voltage will go down as soon as the ac couple inverters are kicked
> off, so if that’s your measured value unless your control system has a delay
> or other component to control reconnection and charging i think it could
> essentially chatter off and on and off and...
>
> Brian
>
>> On Nov 3, 2019, at 9:38 AM, Mark Frye <[email protected]> wrote:
>>
>> 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
>>
>>
>>
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