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

Larry Crutcher, Starlight Solar larry at starlightsolar.com
Wed Jun 24 08:35:00 PDT 2009


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):
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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
> 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



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