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
At 07:41 PM 6/23/2009, you wrote:
Hi folks,
I need an inverter/charger/controller solution for a grid connect
house that:
1. grid-connects (net-meters)
2. Islands off of the battery in outages (whole house UPS)
3. Feeds into the whole house breaker, so the whole house is backed
up
4. Shuts down less important breakers as needed, if the load for the
whole house is over the inverter limit
Instead of guessing which breakers are important to put on a battery
backed up sub-panel, my client would like the whole house backed up.
But of course, can't guarantee that the house won't be drawing too
much for a 6000 Watt inverter at any given time. So would like to
have
the system intelligently remove less important breakers until the
system is below the inverter operation wattage.
Anyone know of a inverter system that is smart like that?
Troy Harvey
---------------------
Heliocentric
801-453-9434
tahar...@heliocentric.org
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