[RE-wrenches] Grid-Connect Inverter with battery, AND auto backoff?

[William Miller]

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
>taharvey at heliocentric.org
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