Mark,
Sorry, my post must not have been perfectly clear... I have total
confidence in how the BB inverters work, and no, the GT inverter there
will be no measurable deduction in impedance, even if you have a very
sensitive sensor and there are no other loads on the system. It just
doesn't work that way. It is impossible for the inverter to reduce the
current into the batteries when it's being fed into the inverter
through its output. And that's true no matter what inverter we're
talking about. It is impossible for it to increase it's impedance that
way.
The only thing I would say is that if you were an engineer at an
inverter manufacturer, it would be possible for you to design an
inverter that could change it's output voltage based on the battery
voltage, but nobody does that. That's why they change the output
frequency. I think my last email layed out the 4 possible options, and
I am confident that there is no other option that would work at all.
These were my four options...
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)
1. 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.
Sorry for the bad news, but unless you have frequency shifting, it's
totally impossible to control the GT inverter using the AC line.
Kienan
*Maxfield Solar*
*maxfieldso...@hotmail.com* <mailto: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 11:58 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, 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:
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)
1. 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*
*maxfieldso...@hotmail.com* <mailto:maxfieldso...@hotmail.com>
*
**(801) 631-5584 (Cell)**
*
------------------------------------------------------------------------
*From:* RE-wrenches <re-wrenches-boun...@lists.re-wrenches.org>
<mailto:re-wrenches-boun...@lists.re-wrenches.org> on behalf of Mark
Frye <ma...@berkeleysolar.com> <mailto:ma...@berkeleysolar.com>
*Sent:* Sunday, November 3, 2019 11:00 AM
*To:* re-wrenches@lists.re-wrenches.org
<mailto:re-wrenches@lists.re-wrenches.org>
<re-wrenches@lists.re-wrenches.org>
<mailto: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>
<mailto: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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