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

Tue Jun 23 22:29:05 PDT 2009

We've looked at a variety of approaches over the years. My current  
thinking, is that you want some non critical loads to shut off during  
an outage, so the customer knows they are on backup power. With whole  
house backup, they never know till the batteries are gone too. I've  
found that with creative picking of circuits, the customer can be  
signaled that the grid is out, without creating any panic or  
hardship. Most houses have so many circuits that its just not a big  
deal to have the washing machine not work for a few hours.
I'd back off to a proven design, otherwise your going to spend a lot  
of time with them making this work. Manually switchable double throw  
breakers are available and work well, but yes, you do have to exert  
some slight mental and physical effort to operate them. Automatic  
anything means the installer is always to blame. Good system design  
puts some responsibility on the operator as well.

R. Walters
Solarray.com
NABCEP # 04170442	

On Jun 23, 2009, at 10:30 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
> taharvey at 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
>>> taharvey at heliocentric.org
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