Ethical dilemma [RE-wrenches]

Bill Hoffer Billhoffer at hotmail.com
Tue Aug 14 09:52:20 PDT 2001


William Miller wrote:
> Fiends:
> 
> Here's a dilemma and how I dealt with it.  I am still wondering if there 
> is
> a better solution.  Your thoughts are appreciated:
> 
> I was called by the operations director for an off-grid elementary 
> school
> to replace fans on an SW inverter.  While on site, I took some time to
> learn how the system was built.  There are 4 SW inverters with outputs
> summed through 2 levels of transformers.   They run their generator 
> during
> the school day and ran on batteries and inverters during the afternoon 
> and
> evening for extra curricular, janitoring and etc.  The system runs great
> while fed by the generator, but when the generator shuts down, inverters 
> 1
> and 2 are not synced to inverters 3 and 4.  You can measure the voltage
> from phase to phase and over a half hour or so it goes from 240 to 0 and
> then back up.  The output of the equipment room is two hots and a shared
> neutral.  When the inverters are out of phase, the neutral is properly
> shared.  When the inverters drift in phase, the neutral current will
> increase, possibly until the current is double the rated current.  It is 
> my
> understanding that power is distributed throughout the school in this
> manner.  At any point where a neutral is shared--on a feeder or 
> multi-wire
> branch circuit--this condition is possible.
Sounds like a really bad install by someone who doesn't have a clue!  I 
am assuming that this is a series/parallel set up supplying 240.  Trace 
experimented (there are several functioning systems out there) with 
series/paralleling 4 SW inverters.  For awhile they actualy supplied a 
connection to keep all 4 inverters in sync, 2 parallel and 2 in series.  
I am guessing that this set up does not have the 4 inverters 
interconnected properly.  There are also some special programming tricks 
that need to be done to the inverters to insure that the #1 inverter 
always takes the lead and the other inverters follow when the inverters 
connect to outside sources (Gen and Grid) and when they disconnect.  And 
also timed relays were needed to insure that the #1 inverter saw and 
reacted to the inputs first.  It really is not so bad once you 
understand what is happening, but I would not recommend it unless you 
are very experienced and have time to play with it (and a few spare 
inverters in case you blow one!)  Unfortunatly the only guy who knows 
how to make it work, no longer works at Trace/Xantrex!  > William
> 
> __________________________________________________________________
> William Miller
> SLO Communications: Communications and Power Systems Consulting
> PO Box 50, Santa Margarita, CA 93453
> Voice :805-438-5600		Fax: 805-438-4607	VMail: 805-546-4875
> email: wrmiller at slonet.org
> License No. C-10-773985
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