AC Coupled Systems

[Grouch]

Hi all,
I understand this diagram of an AC coupled system. This diagram matches the diagram shown for an AC coupled system in the 2017 NEC (Article 690).

What I don't understand is the diagram below from LG, which is also an AC coupled system. During a power outage, the PV array and batteries (the battery is the largest box in the diagram, in the middle, with its inverter to the left) can both still be providing power to the critical loads panel, via their respective inverters. However, in this LG diagram, won't the PV array still be providing power to the main / normal panel, while it's providing power to the critical loads panel? the main panel is in series with the critical loads panel; the power from the PV array has to go through the normal panel to reach the critical loads panel. The auto-tx switch box is a transfer switch, but I don't see how this is supposed to work.

 

[ramsy]

However, in this LG diagram, won't the PV array still be providing power to the main / normal panel..

Yes, looks like Energy Meter with RS 485 breaks off utility, and Auto TX breaks off Main panel, so solar runs Main during day & battery runs standby panel at night.

 

[jaggedben]

I seem to recall looking at this diagram a few years back and being told that the LG battery inverter doesn't actually support having PV on when the grid goes off. That is certainly what that diagram shows. But maybe I'm misremembering.

LG Chem's battery can be used with other inverters such as Solaredge and SMA that do support coupling off-grid. The battery itself doesn't have a great safety record, there was a large recall.

 

[jaggedben]

Yes, looks like Energy Meter with RS 485 breaks off utility, and Auto TX breaks off Main panel, so solar runs Main during day & battery runs standby panel at night.

Doesn't make much sense.

 

[ramsy]

With both designs no direct solar DC to DC battery charge is possible. Unnecessary inverter losses in absence of any direct connection switch between PV arrays & batteries

 

[ramsy]

Doesn't make much sense.

Minimizing standby power circuits exclusively for the battery makes sense.

 

[Grouch]

Yes, looks like Energy Meter with RS 485 breaks off utility, and Auto TX breaks off Main panel, so solar runs Main during day & battery runs standby panel at night.

Wouldn't the battery run the standby panel during day AND night? Assuming not that many loads are on so that the battery can last that long.

Also, if the main panel is kept running by the PV array, shouldn't that also be called a critical / standby loads panel? It looks as if the main panel should be off if the grid goes down, now that I'm looking at it again.

 

[Grouch]

I seem to recall looking at this diagram a few years back and being told that the LG battery inverter doesn't actually support having PV on when the grid goes off. That is certainly what that diagram shows. But maybe I'm misremembering.

If this is the case, why would LG design a system where only the battery inverter is providing power to the critical loads panel? What the 2017 NEC shows in their diagram seems to be more efficient, where both the PV inverter and battery inverter can provide power to the CL panel when the grid goes down.

 

[ramsy]

Wouldn't the battery run the standby panel during day AND night? Assuming not that many loads are on so that the battery can last that long.

Also, if the main panel is kept running by the PV array, shouldn't that also be called a critical / standby loads panel? It looks as if the main panel should be off if the grid goes down, now that I'm looking at it again.

How do you mix solar power with battery power at the same time, especially without utility available to harmonize frequency?

 

[Grouch]

How do you mix solar power with battery power at the same time, especially without utility available to harmonize frequency?

Good point. From the article where i got the first diagram from, Mayfield Renewables, it mentioned both the pv and battery inverters could power the CL panel when the grid goes down. So I'm not sure.

 

[ramsy]

If this is the case, why would LG design a system where only the battery inverter is providing power to the critical loads panel?

If they don't mix, they both might run critical loads.

However, If designed to last longer than a day on battery power, a critical loads panel may exclude small appliance branch circuits.

Residential power walls & battery systems don't have capacity for electric heat, or HVAC for more than 1 night, much less run more than a few large appliance loads for longer than a typical day.

Many regional power failures resulting from natural disasters, fires, floods, hurricanes, & tornadoes are sufficiently destructive to knock utility power down for several weeks or months.

 

[jaggedben]

With both designs no direct solar DC to DC battery charge is possible. Unnecessary inverter losses in absence of any direct connection switch between PV arrays & batteries

There are losses either way, just a bit more with AC. But AC coupling allows a lot more flexibility in system design because the PV and battery voltages don't need to match.

 

[jaggedben]

Minimizing standby power circuits exclusively for the battery makes sense.

There's no major reason the PV can't be on the standby panel. Most system designs do that.

 

[jaggedben]

Wouldn't the battery run the standby panel during day AND night? Assuming not that many loads are on so that the battery can last that long.

Also, if the main panel is kept running by the PV array, shouldn't that also be called a critical / standby loads panel? It looks as if the main panel should be off if the grid goes down, now that I'm looking at it again.

Correct. It makes no sense to have two panels if both can be powered off grid.

If this is the case, why would LG design a system where only the battery inverter is providing power to the critical loads panel?

Good question. If my memory of the system limitations is correct, it calls into question their competence. Maybe they have an updated diagram at this point.

 

[jaggedben]

How do you mix solar power with battery power at the same time, especially without utility available to harmonize frequency?

The battery inverter plays the role of the utility when the utility is disconnected. That's why it's called a 'Multi-Mode' inverter in the NEC diagram. Many systems do this.

The PV inverter in the diagram is 'Interactive' which means it can NOT run without syncing with another source. So if the Main Panel it's connected to gets disconnected from both grid and battery inverter, the PV will not power the panel at any time of day.

 

[Grouch]

The PV inverter in the diagram is 'Interactive' which means it can NOT run without syncing with another source.

Nice explanation.

 

[Carultch]

What I don't understand is the diagram below from LG, which is also an AC coupled system. During a power outage, the PV array and batteries (the battery is the largest box in the diagram, in the middle, with its inverter to the left) can both still be providing power to the critical loads panel, via their respective inverters. However, in this LG diagram, won't the PV array still be providing power to the main / normal panel, while it's providing power to the critical loads panel? the main panel is in series with the critical loads panel; the power from the PV array has to go through the normal panel to reach the critical loads panel. The auto-tx switch box is a transfer switch, but I don't see how this is supposed to work.

View attachment 2558734

This is a setup that doesn't permit the PV and battery backup to operate simultaneously. And if it does through a method not obvious to me, the design misses the entire point of anti-islanding requirements of grid-tied inverters. "Island" being a metaphorical term, for a power distribution system that operates on its own, instead of depending on the connection to the "mainland" of the utility.

It appears as if the Auto-TX switch box is an automatic transfer switch that isolates the protected subpanel from the main panel, in the event of a utility power outage. This means that it is a battery back-up system that is independent of the fact that you have a PV system. It would run until the batteries no longer have any energy they can deliver, and the energy would never be replenished by the PV source.

The fact that this is called an "Auto-TX switch box", leads me to believe that this might be both an auto-transformer and a transfer switch, based on typical abbreviations I've seen. An auto-transformer is typically needed when using an inverter that ordinarily works with 240V, to be able to grid-form the 120/240V grid. It is center-tapped to grid-form the neutral in off-grid mode. Unless the inverter has its own internal way of doing this.

If instead, the Auto-TX switch box doesn't isolate from the main panel during off-grid mode, the design would miss the entire point of anti-islanding requirements. The anti-islanding requirements are put in place, so that no customer-owned generation will energize the utility feeder, for the safety of the linesmen. Any grid-forming back-up sources are required to be isolated behind transfer switches, so that the backup power stays on the property, and doesn't energize the rest of the grid.

To get the PV to operate during backup mode, you'd either need a need a second transfer switch to redirect its power, or you'd need to interconnect on the backup loads panel instead of the main panel.

 

[ggunn]

Yes, looks like Energy Meter with RS 485 breaks off utility, and Auto TX breaks off Main panel, so solar runs Main during day & battery runs standby panel at night.

The way it is drawn it looks to me like the PV shuts off when the grid goes down and the protected loads run off batteries only. I don't think the Energy Meter has an isolation switch.

 

[ramsy]

The battery inverter plays the role of the utility when the utility is disconnected. That's why it's called a 'Multi-Mode' inverter in the NEC diagram. Many systems do this.

The PV inverter in the diagram is 'Interactive' which means it can NOT run without syncing with another source. So if the Main Panel it's connected to gets disconnected from both grid and battery inverter, the PV will not power the panel at any time of day.

Thanks for explaining how AC Coupling works. The pace of solar technology change is incredible.

I see AC Coupling could be designed to charge batteries, which exclusively run critical loads, if isolated from grid.

Solar charged batteries would also eliminate dependence on reliable weather or night time secondary generation.

 

[ramsy]

To get the PV to operate during backup mode, you'd either need a need a second transfer switch to redirect its power, or you'd need to interconnect on the backup loads panel instead of the main panel.

It also occurred to me this LG diagram may be intended to add battery backup to existing solar equipment that shuts down the PV during utility outages.

LG diagram may not charge the battery during power failures, much less is that possible unless that power meter with RS 485 can break off utiliy.