AC Coupled Systems

[Carultch]

Unless that power meter with RS 485 can break off utility.

It would be called something else entirely, if it could break off a connection to the utility. All a meter can do, is measure what power, energy, and characteristics of the voltage and current waveforms, pass through a point in the circuit. RS485 just refers to the communication technology.

Even if it did use the meter as feedback to prevent export during an outage, it still would not comply with anti-islanding requirements. The service drop needs to remain completely de-energized, as opposed to just having zero current.

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.

Agreed. That's probably the intent. The fact that there is existing solar would most likely be irrelevant to the functionality of the battery backup system.

The battery backup system would have a limited runtime on only what the batteries could provide. In concept, it could also perform load profile management tasks such as time-of-use energy arbitrage, demand mitigation, and zero-export recapture. However, these applications a battery are mutually exclusive to the application of backup power. Because you could have a power outage at a time when your batteries are intentionally discharged for the cycle of load profile management, unless the control system designates a portion of the battery capacity to each task.

 

[jaggedben]

It would be called something else entirely, if it could break off a connection to the utility. All a meter can do, is measure what power, energy, and characteristics of the voltage and current waveforms, pass through a point in the circuit. RS485 just refers to the communication technology.

...


Agreed. That's probably the intent. The fact that there is existing solar would most likely be irrelevant to the functionality of the battery backup system.

The meter is probably there to determine on-grid battery cycling for time-of-use arbitration. It wouldn't do anything when the system is off grid. This is generally true of such setups even when the PV does charge the battery off-grid.

 

[Grouch]

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.

Understood, thanks. The part that I don't follow, is where you explained the auto-transformer. What voltage is it converting from primary side to secondary? I understand it's a single winding set up, with no isolation.

 

[Carultch]

Understood, thanks. The part that I don't follow, is where you explained the auto-transformer. What voltage is it converting from primary side to secondary? I understand it's a single winding set up, with no isolation.

The prefix "auto-" means "self". An autotransformer has only one winding, unlike a standard transformer that has two windings coupled by a shared magnetic field. It still is a series of conductive windings, wrapped around an iron core, that magnetize the core, and have voltage proportional to the number of turns in a section of the winding.

Suppose the autotransformer contains 100 turn in total within the winding. The first turn is connected to the L1 of the split phase grid, and the final winding is connected to L2 of the split phase grid. The 50th turn is connected to the neutral. This establishes the fact that L1 and L2 are equal and opposite in voltage relative to neutral. This enables a 240V inverter, to feed 120V loads, whether the load currents on L1 and L2 are balanced or not. Essentially, it is converting 240V that would otherwise be ungrounded, into a grounded grid of 120/240V, and is converting the voltage from 240V into two 120V lines for the portion of the loads that need it.

 

[ggunn]

Understood, thanks. The part that I don't follow, is where you explained the auto-transformer. What voltage is it converting from primary side to secondary? I understand it's a single winding set up, with no isolation.

What an autotransformer does in this application is to convert unbalanced 120V loads to a balanced 240V load.

 

[GoldDigger]

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.

We have gotten used to POCO smart meters with contactors to allow remote disconnect and reconnect of service. But not all energy meters have that capability. In addition, I do not know whether the approvals for the contactor in the POCO meters allow them to be used in an NEC rather than NESC context. They are certainly very small for their nominal capacity.

 

[PWDickerson]

The LG diagram the OP provided is not an AC coupled system. It is a diagram of a simple grid-tied PV system and an independently installed battery backup system. The two systems do not interact. The energy meter is not an isolating device. The PV will shut down during a grid outage with this arrangement.

I prefer to use the term backup loads panel, since the NEC uses the term "critical" to describe specific circuits in patient care facilities.

 

[tallgirl]

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.

AC coupling is fairly old. I don't know if Magnum inverters supported it 15+ years ago, but the Outback Power inverters did.

What's new is the way the power electronics handle things like uncontrolled charging from non-battery-based inverters, such as string or microinverters. SMA inverters use a proprietary frequency shifting signal which increases the line frequency when running disconnected from the grid with the batteries reaching a fuller state of charge.

If I were designing an AC coupled system I'd have a second not-so-critical-loads panel which could be powered on when running disconnected from the grid with the battery state of charge over 80% of so.

 

[jaggedben]

If I were designing an AC coupled system I'd have a second not-so-critical-loads panel which could be powered on when running disconnected from the grid with the battery state of charge over 80% of so.

Enphase supports this with auxilliary contacts that can be used to control a relay for such a panel. Or you can use it for specific loads. They have four contacts. I've done it for specific not-so-critical loads but not for a panel yet.

You could also use a Span Smart Panel as the backup panel, and I'm sure you're aware of other options for home circuit automation.

 

[PWDickerson]

There are quite a few inverter manufacturers that are using frequency shifting to reduce the output of a grid-interactive PV inverter. Nice to see it catching on.

With Enphase, the IQ inverters support this, but the older M inverters do not. I once asked Enphase tech support if I could set each individual micro-inverter in an older installation to a different frequency disconnect set point, which would essentially accomplish the same thing, but they said I would have to have each inverter on its own Envoy to make it work.

 

[jaggedben]

...

With Enphase, the IQ inverters support this, but the older M inverters do not. ...

I think that may be out of date now. As of earlier this year, Enphase supports M-Series inverters in an off-grid islanded mode with Encharge batteries. So they must have some way of controlling them, although it's possible it's still the more bang-bang approach wherein they all just turn off instead of throttling. They might have reduced reconnect times now.

 

[ramsy]

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.

Just read this Feb-2020 article regarding 1 & 2kw batteries under solar modules, integrated with micro inverters.

https://www.pv-magazine.com/2020/02/18/batteries-under-solar-modules-just-like-microinverters-and-optimizers/

One start up claims 96% battery-charge efficiency, powered directly by the solar module, complete UL testing, & battery warranty extendable to 20yrs.

https://www.yottaenergy.com/

Don't know if they are designed for grid-storage solutions, building back-up power, or both.

 

[jaggedben]

Just read this Feb-2020 article regarding 1 & 2kw batteries under solar modules, integrated with micro inverters.

https://www.pv-magazine.com/2020/02/18/batteries-under-solar-modules-just-like-microinverters-and-optimizers/

One start up claims 96% battery-charge efficiency, powered directly by the solar module, complete UL testing, & battery warranty extendable to 20yrs.

https://www.yottaenergy.com/

Don't know if they are designed for grid-storage solutions, building back-up power, or both.

The roof is the worst place to put a battery, in my opinion. We'll see if that idea pans out, I'm not holding my breath.

 

[ramsy]

The roof is the worst place to put a battery, in my opinion. We'll see if that idea pans out, I'm not holding my breath.

One company has already deployed thru industry partnerships in several states, on flat-roof commercial and industrial markets.

Another company sells batteries compatible with most solar panels and racking systems, and partners with residential installers.

Regardless of how this market emerges my question is, if a typical residential Solar array fits <=6 panel modules on the roof, what can <=6kW of backup storage do. Will that run an HVAC appliance over night?

 

[tallgirl]

You could also use a Span Smart Panel as the backup panel, and I'm sure you're aware of other options for home circuit automation.

I'll have to look at smart panels one of these days.

The truth is that working for manufacturers, which I've been doing for going on 10 years, I just know what I make or use for work. We had problems with some badge readers at work and the technicians have the turnstiles open and are fidgeting with the things. Buried inside a turnstile is something I work on.

Basically, I can run your building, I can't run my house. I mean, I can. I just can't do it in a UL-listed and code-compliant manner -- I made a PLC once (circuit layout, PCB manufacture, hand-stuffed parts and soldered, firmware, ladder logic) so I could make an ATS.

 

[tallgirl]

The roof is the worst place to put a battery, in my opinion. We'll see if that idea pans out, I'm not holding my breath.

The only place worse than the roof is on the roof and under a solar panel.

I've got a photo of a thermometer on a white rubber roof, under a solar panel, reading 115F. When I was learning how irradiance and -dP / dT were related, I started understanding how much heat is transferred THROUGH the panels.

 

[ramsy]

I've got a photo of a thermometer on a white rubber roof, under a solar panel, reading 115F. When I was learning how irradiance and -dP / dT were related, I started understanding how much heat is transferred THROUGH the panels.

Don't know how these panel batteries got engineered for -4 to 150°F operating temperatures, NRTL tested for UL9540, UL1973, with a standard 10 year warranty, but apparently they work fine right next to the micro-inverters.

 

[tallgirl]

Don't know how these panel batteries got engineered for -4 to 150°F operating temperatures, NRTL tested for UL9540, UL1973, with a standard 10 year warranty, but apparently they work fine right next to the micro-inverters.

They have an integrated passive thermal management system. I'd have to read more, but I doubt it actually works in more extreme environments. Like, Austin TX where they are headquartered.

US10424821B2 - Thermally regulated modular energy storage device and methods - Google Patents

In some embodiments, a device may include one or more thermal insulation panels defining an enclosure and a phase change material (PCM) within the enclosure. The device may further include at least one unidirectional heat pipe including a proximal portion extending into the enclosure, a distal...

patents.google.com

 

[jaggedben]

Don't know how these panel batteries got engineered for -4 to 150°F operating temperatures, NRTL tested for UL9540, UL1973, with a standard 10 year warranty, but apparently they work fine right next to the micro-inverters.

I'll much more readily believe they can be made safe in that location than that they will have a long enough life to justify that choice economically compared to other options.

 

[ramsy]

They have an integrated passive thermal management system. I'd have to read more, but I doubt it actually works in more extreme environments. Like, Austin TX where they are headquartered.

US10424821B2 - Thermally regulated modular energy storage device and methods - Google Patents

In some embodiments, a device may include one or more thermal insulation panels defining an enclosure and a phase change material (PCM) within the enclosure. The device may further include at least one unidirectional heat pipe including a proximal portion extending into the enclosure, a distal...

patents.google.com

They choose not to use active-cooling electronics, fans, or liquid convection, as their rivals do, but could always ad it later.

Apparently, with passive thermal, lithium-phosphate chemistry, and satellite-aerospace engineering bluster, their 96% efficiency don't need no stinking cooling.