Battery Backup for a Grid-Tied system

[ButchDeal]

You may be thinking of the SMA Sunny Island system. The Sunny Island system does that (changes freq) and the microinverters throttle back their output. In a Solaredge system, all the optimizers see is DC so they don't "know" they have to throttle back; that's all under control of the master inverter, and AFAIK the SolarEdge inverter will NOT reduce output as frequency climbs.

However, after a certain point, any grid tie inverter will decide that the frequency is too far out of range and drop off completely. That, of course, is a little hard on the hybrid inverter, since it has to go from absorbing thousands of watts to sourcing thousands of watts within one AC cycle.

A lot of systems use relay(s) to disconnect the grid tie solar. If you have a microinverter system with a lot of strings of inverters, you can do this one string at a time, which is less stressful to the inverter. But it takes logic that most charge controllers/inverters do not have.

SolarEdge can and does drop off production with frequency modulation But it does not drop off individual optimizers.

The bimodal system will need a disconnect method from the grid though to insure no back feeding or shorting from the grid.
The bimodal inverter will also need a way to disconnect the grid tie inverter as mentioned to shut it completely off when not needed.

Outback radian has three AC connections and frequency modulation in this way dedicating one of the AC connections to the grid, one to the grid tie inverter and one to the load.

[ButchDeal]

I have been trying to answer your questions as best as I can. Perhaps my explanation did not make sense. You clearly have more background and technical knowledge than I do.

I have spent a great deal of time studying the issue as it is done with a Tesla Powerwall. What I can tell you about that interface is that it uses a separate gateway that acts as the transfer switch. It also uses CT clamps and a Neurio to sense the loads and outputs. It communicates with the Powerwall to shift the frequency of the sync signal. I am not advocating that anyone do that on their own. Various amateurs have tried to "fool" a grid tie inverter with a cheap inverter and the result is the inevitable smoke as you predict.

Perhaps the term "fool" is what is misleading? I know that there are tens of thousands of Tesla Powerwalls that have been installed and they allow a grid tie inverter to function when the grid is down. There are other hybrid inverters that can be AC coupled to do the same thing. I haven't watched the complete video that started this dialogue and need to understand what was meant by manual transfer switch. That raised a red flag because if that in any way defeats the safety features built into a grid tie inverter that would be very dangerous.

As I mentioned earlier, the high level concept is that the control of the grid tie inverter by the hybrid inverter is done by changing the frequency of the sync signal that the grid tie inverter uses. I dont know any other place on my grid tie inverter except the output terminals where the grid tie inverter would get the signal that it syncs with. Does that make sense?

Do you have another explanation of how AC coupling works to enable a grid tie inverter power the loads when the grid is down? ButchDeal described the system in the video as a "basic AC coupled system."

You seem to be confusing two different function of the Powerwall. The CT clamps and modulation are used to prevent back feeding when the grid is operational. When the grid is down the CTs monitoring grid are NOT used, just the battery state. As long as the battery state is not full the grid tie inverter is allowed to generate as much as it can, when the battery starts to get full it throttles back the grid tie, or shuts it down (cold with a disconnect).

This is why the battery and bimodal system needs to be larger than the grid tie system, to handle the full charge and load.

All backup systems have to have a transfer switch to disconnect the grid and an auto-transformer to balance the legs. Not all battery systems have backup capabilities (no transfer switch and no auto-transformer); these would be used just for load shifting.

In the video the owner did not get an automatic transfer switch to save money. It is not clear that he has a way to shut off the grid tie inverter completely either. Thus he has hacked together an AC coupled system but it likely wouldn't pass code and would definitely not be automatic, and I am not sure that it can manage the batteries since it is disconnected most of the time.

[J.P.M.]

You seem to be confusing two different function of the Powerwall. The CT clamps and modulation are used to prevent back feeding when the grid is operational. When the grid is down the CTs monitoring grid are NOT used, just the battery state. As long as the battery state is not full the grid tie inverter is allowed to generate as much as it can, when the battery starts to get full it throttles back the grid tie, or shuts it down (cold with a disconnect).

This is why the battery and bimodal system needs to be larger than the grid tie system, to handle the full charge and load.

All backup systems have to have a transfer switch to disconnect the grid and an auto-transformer to balance the legs. Not all battery systems have backup capabilities (no transfer switch and no auto-transformer); these would be used just for load shifting.

In the video the owner did not get an automatic transfer switch to save money. It is not clear that he has a way to shut off the grid tie inverter completely either. Thus he has hacked together an AC coupled system but it likely wouldn't pass code and would definitely not be automatic, and I am not sure that it can manage the batteries since it is disconnected most of the time.

On the video: Looks like this may be another example of more non vetted and possibly dangerous nonsense from the idiot's bible.

At least such idiocy might have a side bennie of improving the gene pool a bit.

[jflorey2]

SolarEdge can and does drop off production with frequency modulation

Thanks! Good to know. It will certainly make AC coupled systems easier to implement.

[ButchDeal]

On the video: Looks like this may be another example of more non vetted and possibly dangerous nonsense from the idiot's bible.

At least such idiocy might have a side bennie of improving the gene pool a bit.

one would hope but the problem is that people see it and think he is an expert and tested something out. They don't know that he is just an idiot that has been lucky so far.

[ButchDeal]

Thanks! Good to know. It will certainly make AC coupled systems easier to implement.

not sure why it would make AC coupling any easier to implement? you still need all the other equipment to disconnect and sizing.

[Ampster]

..........
Outback radian has three AC connections and frequency modulation in this way dedicating one of the AC connections to the grid, one to the grid tie inverter and one to the load.

I am waiting for the firmware update on the Outback Skybox so I can try to AC couple it to my SolarEdge grid tie inverter. The Skybox solar is enough to keep my batteries charged and run my critical loads. In a major emergency the additional 4kW capacity of the grid tie inverter would be useful to charge my EVs.

[ButchDeal]

I am waiting for the firmware update on the Outback Skybox so I can try to AC couple it to my SolarEdge grid tie inverter. The Skybox solar is enough to keep my batteries charged and run my critical loads. In a major emergency the additional 4kW capacity of the grid tie inverter would be useful to charge my EVs.

What extra capacity? If you are AC coupling you have the solar capacity of the grid tie inverter and that is it.
You don't DC couple AND AC couple.

[jflorey2]

not sure why it would make AC coupling any easier to implement? you still need all the other equipment to disconnect and sizing.

Because it provides a way to reduce solar generation, rather than cut it off completely. Agreed that you still need all that other stuff.

[ButchDeal]

Because it provides a way to reduce solar generation, rather than cut it off completely. Agreed that you still need all that other stuff.

that doesn't make AC coupling any simpler or easier to implement since you need all the same equipment. It makes it a little less harsh on the equipment since it isn't going to flip the grid tie on and off but will instead ramp the grid tie down.
implementation is actually a bit more complex in that you have to do everything you would without frequency shifting as well as a few tasks to configure frequency shifting on bimodal and grid tie.

[Ampster]

What extra capacity? If you are AC coupling you have the solar capacity of the grid tie inverter and that is it.
You don't DC couple AND AC couple.

No problem for me when the grid is down. My Skybox solar is 3k west facing with shadows late afternoon. My girid tie solar is 4k south facing, no shadows. If I have to choose I will take the grid tie output.

[Ampster]

............
This is why the battery and bimodal system needs to be larger than the grid tie system, to handle the full charge and load.
...........

I am just getting into a video of an Outback Webinar that seems to suggest that at least with Rule 21 compliant Gridtie inverters in California they can modulate down using a Frequency/Watt algorithm. I am hoping that is the case for my installation if I choose to go AC coupled.

[SunEagle]

I am just getting into a video of an Outback Webinar that seems to suggest that at least with Rule 21 compliant Gridtie inverters in California they can modulate down using a Frequency/Watt algorithm. I am hoping that is the case for my installation if I choose to go AC coupled.

It may make sense for the POCO to be able to "adjust" your energy storage system to meet the needs of a stable grid but to some people it may seem like just more of "the man" taking control of my stuff. Hopefully there is a happy medium for consumers and the POCO.

[Ampster]

It may make sense for the POCO to be able to "adjust" your energy storage system to meet the needs of a stable grid but to some people it may seem like just more of "the man" taking control of my stuff. Hopefully there is a happy medium for consumers and the POCO.

Rule 21 has nothing to do with storage. It is the smart inverter algorithm that prevents a shock to the grid if all the grid tied inverters were to shut down at the same moment in time. At least that is my understanding. I am sure it is a result of the high rate of solar PV adoption. Hawaii and Puerto Rico have similar regulations. One of the side benefits is it makes it easier for a hybrid inverter to leverage grid tie inverter generation when the grid goes down.

[NetComrade]

Why do you have so many small inverters on a single site?


A bimodal inverter has batteries and can work on the grid and off the grid.
It will not "fool" the solarEdge inverters but will control them to keep the battery charged and provide power to an emergency panel.
take a look at the OutBack Radian inverters:
http://outbackpower.com/outback-prod...ategory_id=529

I think I finally have an answer to this question. After reading Outback's marketing material, I think this was sized so that we could connect it to Outback Radian, which has a maximum recommended wattage of 6000 (on the 8K model).

BTW, MTV Solar has since been recommending the Schneider XW+.. I had them come out last week to assess and see what they say.


Thanks for your patience and input.