Doesn't the BB inverter have a relay output that can be linked to 'batteries
are full' and energize a relay to drop out the AC line to the GT inverter, thus
causing it to go offline?-GlennSent from my 'smart'phone, so please excuse
typos and spelling errors.------ Original message------From: Mark FryeDate:
Sun, Nov 3, 2019 1:58 PMTo: re-wrenches@lists.re-wrenches.org;Cc: Subject:Re:
[RE-wrenches] Parts List For AC Couple Disconnect
OK, the posts are coming in on the "no" side, but with
uncertainty about how any given BB inverter might work.
I still think that the BB charger is in charge (no pun) of
sending power to the battery and if it the battery is at it's set
point then the duty cycle on the transistors in the rectifier go
down - zero. At that point the impedance of the AC line would
appear to go up. In response the GT inverters would increase there
output voltage in an attempt to over come the higher impedance.
The voltage monitoring switch that I spec'd has a response delay
setting. If I am running in offgrid mode, the main worry is
voltage sag due to load. So generally, the inverter would be
running at or below AC voltage set point. There may be spikes
above voltage set point when a load is disconnected, but I could
filter that out with the delay on the sensor.
Still, this is why I am asking.
Anyone on the list have more confidence in their understand of BB
inverters in off-grid mode with AC coupled GT inverters?
Mark
On 11/3/2019 10:44 AM, Kienan Maxfield
wrote:
P {margin-top:0;margin-bottom:0;}
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
use a BB inverter that has frequency shifting control for AC
coupling
run a wire from the battery room to the GT inverters that
you can use for controlling them based on the battery
voltage
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?
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
maxfieldso...@hotmail.com
(801)
631-5584 (Cell)
From:
RE-wrenches <re-wrenches-boun...@lists.re-wrenches.org>
on behalf of Mark Frye <ma...@berkeleysolar.com>
Sent: Sunday, November 3, 2019 11:00 AM
To: re-wrenches@lists.re-wrenches.org
<re-wrenches@lists.re-wrenches.org>
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
<ma...@berkeleysolar.com> 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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