[RE-wrenches] BASE Power added a battery to my solar customer's home and I'm concerned this not a safe design

[Mick Abraham]

scot.arey at solarcentex.com asked about AC coupling between Growatt battery
based inverters and his prior SolarEdge grid tie installation.

For PV to "AC couple" with the storage, it's correct for the SolarEdge AC
to land in the protected load center ("backed up side", as Scot wrote).
Here are the two big questions that both need to be "yes":

(1) Since Scot would have entered the SolarEdge "grid code" per power
company requirements, does that grid code allow the SE to curtail its
wattage via frequency shift? I think this answer depends on the SolarEdge
vintage as well as the particular grid code. The power company folks may
not know this answer but below are suggested steps for testing.

(2) When the grid is disconnected or blacked out, do the GroWatt battery
based inverters shift their frequency at high battery voltage (or high
state of charge)? There may be some setting in the menus to enable
this--which we hope are already done.

If both inverter types are A-OK for frequency/watt curtailment, data
communication between the two inverter types may be nice to have but that's
not mandatory. With frequency shift, the "comms" happen by way of the AC
wiring which is shared by both inverter types (primitive though that may
seem).

+++++++++++

Side note: In my own region, the utility & the code inspector would want a
permit, a code inspection & power company approval when adding storage. If
the work at the home of Scot's client was inspected & approved, all the UL
listings & functions should have already been vetted.

++++++++++

One could test an AC coupled installation as follows:

* Choose a sunny midday and a day with little risk of a power company
blackout.
* Make sure the protected loads are smaller than the likely PV wattage
during the test.
* Set up a way to watch the battery voltage if it's not already there...(or
state of charge or both).
* Clamp around one of the SolarEdge AC hot wires so one could monitor the
AC amperage with minimal lag time.
* (Optional) Put on a t-shirt that says: "Let's light this fuse and see
what happens."
* Shut down the SolarEdge inverter, then disconnect the grid power upstream
from the Growatt inverters. This fakes a blackout so the grid no longer
defines the AC frequency. AC power in the protected load center would now
be synthesized by the Growatt inverters.
* Run some of the protected loads to discharge the battery a bit. This will
give the observer some time in which to observe.
* Switch on the SolarEdge, which will now be fooled into waking up after
the 5 minute time-out. SE thinks there's a grid but I think Rule 21
algorithms also incorporate a frequency/watt curtailment function.

The AC frequency may shift more rapidly or more finely than what one could
easily measure. No problem there b/c the battery voltage & SOC should tell
the tale.

++++++++++++++++++++++++

I saw a setup with one Outback Radian plus a 2020-ish SolarEdge. I don't
know the grid code but I'm sure that includes Rule 21 compliance. Instead
of a smooth, gradual curtailment as the battery voltage rose, this one does
more of a "bang-bang". That may be because of high PV wattage relative to
battery size. The Outback frequency shifts so fast that it tosses the SE
into shutdown before meaningful curtailment can be observed. That's the
first bang, resulting in dropping the PV wattage to zero.

The second bang comes five minutes later when the SE again tries to connect
to the "microgrid". The PV wattage ramps up until there's another SE
shutdown. Rinse & repeat until the grid comes back on. This is literally
clunky but it does prevent overcharge & the lithium battery seems not to
mind. If this home was off-grid (or in prolonged grid blackouts), the bangs
might cause premature wear on the SolarEdge relay. I think Outback settings
don't allow any changes in how fast the frequency is changed relative to
the rate of increase on the battery voltage. SolarEdge may or may not allow
for nuance in its own freq/watt behavior.

I trust this might be helpful, Scot. Be sure to report back on-list with
further info. As always, "The Wrench List is the Bomb!"

Mick Abraham, Proprietor
www.abrahamsolar.com

Landline: 970-731-4675
Cell phone or for text messaging: 970-946-6584
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