[RE-wrenches] Parts List For AC Couple Disconnect
Mark Frye
markf at berkeleysolar.com
Sun Nov 3 14:51:38 EST 2019
Folks,
I do not want to be obdurate here, perhaps just a slow learner.
I have to ask the question though:
In the case of a BB inverter connected to the grid only, with no GT
inverters, you don't have to shutdown the whole inverter when the
battery is full because you are not required to endlessly push available
energy into the batteries. You can turn off the switch that connects the
charger to the battery.
Mark
On 11/3/2019 11:13 AM, Kienan Maxfield wrote:
> 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*
> *maxfieldsolar at hotmail.com* <mailto:maxfieldsolar at hotmail.com>
> *
> **(801) 631-5584 (Cell)**
> *
> ------------------------------------------------------------------------
> *From:* RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> on
> behalf of Mark Frye <markf at berkeleysolar.com>
> *Sent:* Sunday, November 3, 2019 11:58 AM
> *To:* re-wrenches at lists.re-wrenches.org
> <re-wrenches at 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*
>> *maxfieldsolar at hotmail.com* <mailto:maxfieldsolar at hotmail.com>
>> *
>> **(801) 631-5584 (Cell)**
>> *
>> ------------------------------------------------------------------------
>> *From:* RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org>
>> <mailto:re-wrenches-bounces at lists.re-wrenches.org> on behalf of Mark
>> Frye <markf at berkeleysolar.com> <mailto:markf at berkeleysolar.com>
>> *Sent:* Sunday, November 3, 2019 11:00 AM
>> *To:* re-wrenches at lists.re-wrenches.org
>> <mailto:re-wrenches at lists.re-wrenches.org>
>> <re-wrenches at lists.re-wrenches.org>
>> <mailto:re-wrenches at 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 <markf at berkeleysolar.com>
>> <mailto:markf at 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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