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Hypothesis: Can a double conversion UPS energize a grid-tied micro inverter based solar array during power outage if the UPS input and output are both connected to main panel?

Assumptions below diagram

Questions:

  • Can the UPS provide reference voltage to the solar micro inverters to operate?
  • Can the the UPS get damaged due to excess AC current backfeeding into UPS's inverter?
  • Can we limit AC current backfeed into the UPS by dumping excess solar power to run a 5 ton HVAC?
  • Can the UPS keep a fixed 60Hz output (as the utility grid does) if the micro inverters adjust their output frequency?
  • Can the UPS batteries charge safely from micro inverters output?
  • What other safety and functional concerns should be addressed?

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Assumptions:

  • Both the UPS and solar micro inverters are split phase 208V 60Hz
  • 7.8KW solar array based on Enphase IQ6+ grid tie micro inverters.
  • 3000VA APC double converion UPS model SURTD3000XLT with sine wave output
  • UPS output frequency tracks input frequency
  • input of UPS powered from breaker panel with 30A 2-pole breaker
  • output of UPS is plugged safely to breaker panel bus bar with 30A 2-pole breaker
  • grid is disconnected.
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  • \$\begingroup\$ One GTI is more cost effective than two \$\endgroup\$ Sep 2, 2020 at 5:47
  • \$\begingroup\$ Thanks to ThreePhaseEel, Brian and Simon for guiding me to further understand the topic of AC-Coupled PV systems with focus on Microgrid. I found this Enphase resourse to be a good place to start: enphase.com/sites/default/files/downloads/support/… \$\endgroup\$
    – Frank
    Sep 3, 2020 at 14:45

3 Answers 3

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I think the odds of that working are very slim.

The UPS is much smaller than the inverters, so it will struggle to maintain a stable 208V 60Hz. That's especially true if it's not intended to be back-fed.

It will be very difficult to get the load to exactly match the generation. Too much load, and the UPS will shut down, followed immediately after by the inverters. To little load, and the inverters will attempt to push out current into no load.

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  • \$\begingroup\$ Thanks Simon. So hypothetically speaking then, if the UPS were rated the same as the solar array then this scenario is plausible - of course equipment and people safety not withstanding that is. \$\endgroup\$
    – Frank
    Sep 2, 2020 at 17:24
  • \$\begingroup\$ @Frank plausible, but difficult. If the load is to low for the solar generation, it may go wrong as the inverters try to backfeed the UPS. If a cloud goes across the sun, then the UPS will need to be big enough to make up the shortfall. \$\endgroup\$
    – Simon B
    Sep 2, 2020 at 19:31
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This has a name

The general topology you are describing with a battery-powered inverter (represented by the double-conversion UPS in your diagram) forming a grid for one or more grid-tied solar inverters is called AC coupling in the solar business, and is used in some multimode (off-grid/grid-tied hybrid) and even perhaps a few off-grid systems. While theoretically feasible, and practical with the correct hardware (vs. what you have), there are some drawbacks to this setup:

  • It requires a rather hefty battery-inverter or premade Energy Storage System (ESS) so that the battery/ESS inverter can maintain control of your tiny "grid" when the utility supply is down and your solar generation is peaking. In particular, you need at least as many VA of battery/ESS inverter output as you have of peak solar generation capacity; as it stands right now, you're nowhere close, and without it, everything will collapse if insufficient load is present as the AC 'grid' voltage then soars, causing all the solar inverters to trip out on overvoltage and then potentially overloading the battery inverter, or causing it to shut off due to the overvoltage condition as well.

  • Charge control requires more care and attention in AC-coupled setups too, since the battery/ESS inverter has to be able to tell the solar inverters "whoa dude, that's too much generation!" when neither loads nor battery capacity can absorb the solar output. Most setups use what's called frequency-watt control for this purpose, where the battery/ESS inverter raises the mains frequency to tell the solar inverters to back off on the juice; you may also see references to volt-watt control as well.

  • Finally, as with any multimode-type solar setup, you need something smarter than your average transfer switch, or even your double-conversion UPS, to isolate your 'grid' from the utility grid when the utility grid goes dark while still permitting your solar system to export power when the grid is available. Note that this may be built into a battery inverter (along with auxiliary transfer functionality), or a separate "add-on" module such as the Backup Gateway used with the Gen2 and newer Tesla Powerwalls.

However, even with these drawbacks, it can work well enough with existing grid-tied hardware that's new enough to support niceties like frequency-watt control, and also has the advantage that there's no nasty high voltage DC roaming around the system, which eliminates the need for PV-specific arc and ground fault protection. It also permits the use of microinverters, with their attendant module-level array-optimization and rapid-shutdown advantages.

What you'd need if you wanted to do this for real

If you actually wanted an AC-coupled backup setup, you'd need to first decide if you're going to go with an integrated ESS, or an "open architecture" setup using a field-wired battery-bank connected to a multimode inverter. Current generation Powerwalls are the prime example of an integrated ESS; with their compact configuration, high degree of integration, and high power and energy densities, they are an attractive option if equipment space is at a premium or in an environment where field-integrated equipment is frowned upon. However, while the Tesla setups are capable of multimode operation using the Backup Gateway, and some other manufacturers (such as Sonnen) now support similar features, some integrated ESS systems do not provide for this configuration (look for a "backup" or similar mode in the user documentation).

The alternative option is to use what's basically an open architecture system, with a separate battery-inverter rated to UL 1741 in North America or the equivalent local grid code for where you're at in the world, DC-rated switchgear (which while non-trivial to find, is not completely unavailable), individual battery packs/blocks, and a cabinet or rack for said batteries. These systems take up much more space and require more integration work from the installer, as well as a much larger balance of system with fat cabling and high amperage, DC-rated, yet mains-circuit-suitable switching and fusing as they run at 48V DC, but provide a higher degree of customizability, and can work with inexpensive (and highly recyclable!) deep-cycle lead-acid batteries.

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  • \$\begingroup\$ Thanks @ThreePhaseEel for your robust answer; I conclude that a system as I described is not viable.I will redirect my attention to fully understand AC-coupled setups. \$\endgroup\$
    – Frank
    Sep 3, 2020 at 14:15
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Tesla Powerwall essentially does this as explored here.

The crucial takeaway is that a safety approved Gateway component is required, so that when the grid fails, and your UPS takes over as a source of AC to keep the grid tie inverter (GTI) running, the gateway safely isolates your island from the rest of the grid thus (a) protecting the linemen, and (b) not supplying your neighbours with your precious power. The link referred to UK installation, your region will have its own legal requirements.

The second takeaway is the role of frequency in your local island to control charging from the GTI.

Whether this will work with any UPS other than the Powerwall I can't comment.

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  • \$\begingroup\$ Thanks @Brian 1- Agreed; this hypo is only to explore electrical behavior of the system IFF a) grid is physically disconnected b) UPS is manually introduced via 30A power inlet and 30A breaker and connected to common equipment ground 2- The UPS output freq could govern solar array behavior if it can be modulated by battery charge level; solar array would produce when battery is below full charge level and stop production when battery is full. UPS would support demand when solar is off. I will deep dive into Enphase + APC to understand how they might interact if connected. \$\endgroup\$
    – Frank
    Sep 2, 2020 at 22:12

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