Battery Backup for a Grid-Tied system

[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.

[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.

[jflorey2]

In this situation the hybrid inverter modulates the grid tie inverter by shifting the frequency that fools the grid tie inverter. In the case of a Solaredge inverter, it is the optimizers that drop off individually.

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.

[Ampster]

I started watching the video and the diagram at the beginning is not correct because it does not show the operation of the automatic transfer switch that isolates the hybrid inverter output from the meter. However his manual disconnect insures that the grid is disconected before he can connect the the hybrid inverter. The hybrid inverter transfer switch may not be able to handle all the loads of that electrical panel. Therefore he came up with the manual work around which may not satisfy the UL or local code requirements.

He does use the term frequency curtailment which is another way to describe the AC coupling process how the hybrid inverter uses to control the grid tie inverter.

[bcroe]

Do you have another explanation of how AC coupling works to enable a grid tie inverter power the loads when the grid is down?

I do not have anything else to add for now. Bruce Roe

[Ampster]

You are completely out in left field. The issues here are the flow and control of energy. I know
little of optimizers, but the idea of trying to control a grid tie inverter from its output terminals is
quite contrary to the design intent. Even if somehow a design flaw allowed some of this to
happen, there is absolutely no assurance that this unintended mode will be a reliable operating
function. Soon as it is not, look for the smoke. Bruce Roe

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."

[bcroe]

In this situation the hybrid inverter modulates the grid tie inverter by shifting the frequency that fools the grid tie inverter. In the case of a Solaredge inverter, it is the optimizers drop off individually. Less panel providing power means less output from the grid tie inverter.My understanding of the concept comes from another forum where Tesla Powerwall users discuss how the Powerwall AC couples to other grid tie inverters. I could be mistaken. AC coupling is complex.

You are completely out in left field. The issues here are the flow and control of energy. I know
little of optimizers, but the idea of trying to control a grid tie inverter from its output terminals is
quite contrary to the design intent. Even if somehow a design flaw allowed some of this to
happen, there is absolutely no assurance that this unintended mode will be a reliable operating
function. Soon as it is not, look for the smoke. Bruce Roe

[Ampster]

...........
A grid tie inverter DOES NOT try to match the load, it matches the available solar power. Bruce Roe

In this situation the hybrid inverter modulates the grid tie inverter by shifting the frequency that fools the grid tie inverter. In the case of a Solaredge inverter, it is the optimizers that drop off individually. Less panels providing power means less output from the grid tie inverter. My understanding of the concept comes from another forum where Tesla Powerwall users discuss how the Powerwall AC couples to other grid tie inverters. I could be mistaken. AC coupling is complex.

[bcroe]

I am on the road and can't watch the entire video until later.
My high level understanding of the concept is the hybrid inverter modulates the output of the grid tie inverter so that the output of the grid tie inverter matches the combined load of the house and charging load. The balancing would have to be done by the grid tie inverter, if not then an auto transformer would have to do that.
My understanding of why he didnt feed solar to the batteries was that he would have needed to buy a 400v charge controller or rewire his panels for a lower voltage charge controller. I think ButchDeal explained some of that in an earlier post.

I didn't go back to the beginning. But rewiring the panels is a trivial thing (which has happened to all
of mine), certainly a way better idea than this. 400 volt charge controller, fine buy it or rewire to 200V.

A grid tie inverter DOES NOT try to match the load, it matches the available solar power. Bruce Roe

[Ampster]

.......
It is concerning that he has a manual disconnect so that bimodal inverter is not connected during normal grid usage so how are the batteries maintained? or am I missing something?

other than hacking this together it looks like a basic AC coupled system.

Yes, I was wondering about the manual disconnect also. My Outback Skybox has an automatic transfer switch. For maintenance, I did install a manual transfer switch. I have the same question about charging batteries.

[ButchDeal]

I am on the road and can't watch the entire video until later.
My high level understanding of the concept is the hybrid inverter modulates the output of the grid tie inverter so that the output of the grid tie inverter matches the combined load of the house and charging load. The balancing would have to be done by the grid tie inverter, if not then an auto transformer would have to do that.
My understanding of why he didnt feed solar to the batteries was that he would have needed to buy a 400v charge controller or rewire his panels for a lower voltage charge controller. I think ButchDeal explained some of that in an earlier post.

Grid tie inverters do not generally have any way to balance the legs. Typically an auto-transformer is added to do that.

It is concerning that he has a manual disconnect so that bimodal inverter is not connected during normal grid usage so how are the batteries maintained? or am I missing something?

other than hacking this together it looks like a basic AC coupled system.

[Ampster]

And where is the solar power going to go if it exceeds the home drain? What about balancing the 120V
circuits? Why does he not just feed solar to the batteries? Bruce Roe

I am on the road and can't watch the entire video until later.
My high level understanding of the concept is the hybrid inverter modulates the output of the grid tie inverter so that the output of the grid tie inverter matches the combined load of the house and charging load. The balancing would have to be done by the grid tie inverter, if not then an auto transformer would have to do that.
My understanding of why he didnt feed solar to the batteries was that he would have needed to buy a 400v charge controller or rewire his panels for a lower voltage charge controller. I think ButchDeal explained some of that in an earlier post.

[bcroe]

If you watch the video, he claims the Schneider raises frequency above 60Hz, which tells SolarEdge to send less power

I am not sure about balancing, isn't that a separate topic ?

My neighbor messed up his neutral and lack of balance burned out the door opener transformer.
I fixed it 15 feet in the air. Every 120V appliance could be smoked by this idea. Bruce Roe

[NetComrade]

If you watch the video, he claims the Schneider raises frequency above 60Hz, which tells SolarEdge to send less power

I am not sure about balancing, isn't that a separate topic ?

[bcroe]

Found this guy using Ac Coupled Schneider SW to "fool" his SolarEdge Grid Tie inverter. Says it works, but grid disconnect is manual. Figured i'd share.. haven't looked too much into it, but seems like a fairly cheap solution

And where is the solar power going to go if it exceeds the home drain? What about balancing the 120V
circuits? Why does he not just feed solar to the batteries? Bruce Roe