[RE-wrenches] Large loads, small service

[William Miller]

Colleagues:



I have an interesting project.  I am working for a family that lives part
time in a planned community built some years ago.  The HOA owns the 12 KV
distribution and it is old and undersized.  This client shares a 25 kVA
transformer with a neighbor.  This calculates out to 50 amps per home at
240 VAC.



The client has more stuff to power than they have power. To make it
possible to power their loads, I am imagining a pseudo off-grid
installation. We have grid power, but precious little of it, so we have to
stand-alone, at least to some degree.



I am thinking that we can provide a battery inverter system that can be
programmed to limit the draw from the grid.  All the loads will be
connected to the AC output of the inverter system.  If the load increases
over that programmed limit, a smart inverter system should be able to
synchronize to the grid input and assist with inverted power (like gen
support on the grid input).  I believe the Radian inverters can do this.



If occasionally the batteries get low, a generator can run intermittently
and top off the batteries.



There is an 8kW grid-tied system on site.  This should be able to help
power on-site loads or charge the batteries during day-light hours.  I will
specify a contactor to kill the GT feed if the grid goes out and the
batteries are fully charged.



We can add another 11 kW of GT solar.  However, I am concerned that we
might be over-feeding the transformer on sunny days.  The neighbor has
about 8 kW of GT solar.  If you add all of the GT solar into this one 25
kVA transformer, you arrive at 27 kVA.  This is too much.  Our additional
GT solar might be with a Solar Edge HD-Wave SE10000-US.  This inverter has
an external box that can monitor net back-feed and limit the output of the
SE inverter to a programmed level.



Anyway, this is a lot of detail that maybe you don’t need to know.  My
question is this:  How would you design a system to take 12 kVA from a grid
source and turn it in to 60, or more?



Any input is appreciated.



William Miller
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