Troy,
If "cost is not an issue" and you only have 50 amp load, sounds to me
like you don't need to shed loads at all. Just design the battery/
inverter system to handle this. Build a 20kW Sunny Island system or
perhaps a pair of Outback series/parallel Quad Stacks, 28.8kW (see: http://www.outbackpower.com/resources/documents/wiring_diagrams/)
. This will eliminate the extreme complexity of externally shedding
loads in stages. BTW, Outback inverters also have a loadshed relay
control function built in.
William,
(quote) "but I know of none that has a programmable relay to actuate
at a certain load level."
Outback Mate has a programable setting for load start. It has set
points from 1-50kW and delay time to start/stop the generator. I use
this setup in Mexico on beach front rental properties with very large
battery banks so when a customer starts up a mini-split AC, the
generator will start as needed, if needed.
Kindest Regards,
Larry Crutcher
Starlight Solar
(928) 941-1660
Renewable Energy Products, Service and Installation
Mailing Address (NO SHIPPING):
11881 S Fortuna Rd.
#210
Yuma, AZ 85367
Shipping and retail store (NO MAIL):
2998 Shari Ave
Yuma, AZ 85365
On Jun 23, 2009, at 11:30 PM, Troy Harvey wrote:
Load sheding is not a bad idea if the tech exists. Not particularly
complex either with networkable breakers and a smart controller in
the inverter. Just software. Cost isn't an issue, they are willing
to pay.
The issue on loads isn't battery cut-off (though that isn't a bad
idea), but shedding the less-important loads to make sure the house
does exceed the inverter power output. A 6kW inverter subpaneled
will only give you about four 15 AMP circuits. However a typical
house of this size will have 30 such circuits, yet good chance it
won't be drawing much more than 50 AMPs for the whole house at any
one time. How do you select what is important? Either the inverter
is complicated or the electricians wiring is complicated.
Troy Harvey
---------------------
Heliocentric
801-453-9434
tahar...@heliocentric.org
On Jun 23, 2009, at 10:01 PM, William Miller wrote:
Troy:
Let me guess... They want it right away and cheap, too. Generally
speaking, this is an impractical request. Electrically operated
circuit breakers or 20 A relays and controllers are expensive and
complicated. One thing to learn in this trade is when to try and
talk a client out of a bad idea, and when a client has such wacky
ideas that it is best to walk away.
I could, however, suggest two ways to do this:
1. Use two inverters and set the LBCO for one high. Connect one to
critical loads and one (with the high LBCO) to non-critical loads.
When the batteries start getting low, the non-critical-loads
inverter shuts down, leaving the critical-loads inverter running.
Realize that you now need four load centers: Grid, generator (you
have recommended a generator so they can use their wide screen TV
during a wind storm, correct?), non-critical loads and critical
loads. This type of design gets complicated fast. Will the AHJ be
able to track this? Set a clause that allows you to collect hourly
fees when they require three different meetings and three re-writes
of the permit application.
2. Use an Outback with external relays to shut off loads when the
battery voltage falls below a certain point. This is a crude
approach, the parameters are not flexible (hard coded delay values)
and it requires custom built relay panels, time consuming,
expensive and a potential service problem.
Either system is actuated on battery voltage rather than loads.
Inverters I am familiar with have relays and internal controls that
operate based a set-able battery voltages, but I know of none that
has a programmable relay to actuate at a certain load level. In
addition, loads change so rapidly that this type of switching would
be erratic. Loading is a component of battery voltage, anyway, so
you are including that indirectly.
Good Luck,
William Miller
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