Grid-Tie Inverters with Batteries [RE-wrenches]

Christopher Freitas --- OutBack Power cfreitas at
Mon Sep 13 14:47:39 PDT 2004


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Mark Edmunds wrote:
> The transfer time for the SW/SW+ depends a lot on what it is doing at 
> the time the grid fails. 

The transfer time for any inverter / UPS is dependant on three different 
things:  (1) the detection time, (2) the speed of the AC transfer switch 
and (3) the ability of the inverter to regulate its output voltage.  

Often the detection time of the inverter is the greatest portion of the 
total transfer time period.  Some older designs can take an entire AC 
cycle (16 mS) to detect an outage - newer designs do it in a 1/4 cycle 
(4 mS) or less.  

In most utility outages the grid's AC voltage doesn't just "go away"  - 
the inverter actually sees a huge load (the entire local neighborhood) 
and it overcurrent trips and shuts off.   

The speed of the AC transfer switch is primarily a mechanical issue of 
the relay itself - and most of the AE grid-tie with back-up inverters 
use the same relay so this tends to be about the same for most designs.  

The issue of the inverter's ability to voltage regulate can be very 
significant - it doesn't help much if the inverter drops the grid 
because of a low voltage grid condition and then the inverter just 
provides low AC voltage once it picks up the load itself...

So - for some designs - stating a time period (like the 32 mSec / 2 AC 
cycles) doesn't fully describe what might actually happen in the real 
world under certain conditions.  

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @
Arlington WA USA
Tel 360 435 6030  

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