Need Xantrex help (again) [RE-wrenches]

[Christopher Freitas --- OutBack Power]

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Allan Sindelar wrote:
> OK, so the above confuses me. If one of two inverter bypass switches is 
> in bypass mode, that inverter's input and output are disconnected from 
> both load and external source. While the neutral remains connected and 
> common with the other inverter, there is no current supplied to that 
> neutral conductor by the bypassed inverter. So if the inverter that is 
> in normal mode references to an external AC source, and the bypassed AC 
> leg uses that source to power loads (180 out of phase), where's the 
> problem, at least in reference to overloading the neutral?

The scenario goes like this - the inverter's won't stay sync'd to a 
generator - so the customer throws just one of the bypasses - resulting 
in that leg of the 120/240 vac split phase system operating at the 
generator's phasing.  The other inverter does not sync to the generator 
and operates on its own - unaware that the other inverter is not 
following it anymore.  The result is that the two AC legs can go in and 
out of phase with each other - resulting in anything from zero to 240 
vac applied to the L1-L2 and up to twice the current in the neutral 
conductor.  Usually the phasing goes in and out quickly - but I have 
seen them stay phased in / out of phased for several minutes at a time, 
slowly alternating between the two conditions.  This also occurs when 
the stacking cable of some inverters gets unplugged between the units - 
and they don't shut down or show and error - they just keep operating on 
their own... 

We all have lived with these kinds of problems for years - but I think 
this is one of those real-world, actually-does-happen type of problems 
which should be addressed (as opposed to the theoretically-possible, 
but-so-extremely-unlikely issues like islanding with a magically 
balanced load condition that UL tends to focus on...)
  
Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  

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