Interesting question, Mick. It may depend on the point in the cycle when the
transformer lost power and  the duration of the blip.

Dick Ratico

--- You wrote:
Before I present a suggestion, here's a question:

If by chance William is able to energize the transformer without tripping
the breakers, what if there is a brief break in the power...such as a
half-second?

I don't suppose there's capacitance here...more like magnetic flux...but
would there be a brief window before the transformer bleeds down and
requires another equally strong inrush current?

Or...would a tiny blip in the input power result in another big surge
requirement?

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675


2010/5/13 Richard L Ratico <Richard.L.Ratico@valley.net>

> Sorry for the additional post here, but it includes a typo correction:
>
> Bill,
>
> Not a lot of personal experience with this issue. The one time I had to
> deal
> with it, we were able to leave the circuit on, once we got it energized.
> As you know, the crux of the problem is the nature of the voltage relative
> to
> it's cycle when connection is attempted.
>
> An internet search resulted in this explanation that was very helpful. See:
>
> http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm
>
> As I understand it now, counterintuitively, you don't want to switch at the
> zero
> crossing. SS relays aren't my strong suit either, but if they connect at
> that
> moment.......
>
> Is your step up transformer actually designed by the manufacturer to be a
> step
> up unit, or are you using a step down unit in reverse?
> Again, according to information found in my search, not personal
> experience, the
> latter can contribute to higher than expected inrush.
>
> Dick Ratico
>
>
> --- You wrote:
> Thanks for the reply.  To clarify, we have tried his with one transformer
> and with two transformers.  Our success rate is lower powering one
> transformer.  My hunch is that the capacitance of the feeder is
> buffering...
>
> I wonder if I use a large solid state relay with zero crossing switching I
> might have better luck.
>
>
> William Miller
>
>
> At 06:23 PM 5/12/2010, Richard L Ratico wrote:
> >Bill,
> >Your branch circuit breaker is pretty small.
> >Are you energizing both transformers simultaneously? If so, it might help
> to
> >energize them individually.
> >
> >The branch breaker can be up to 250% of the step up transformer's primary
> >rating. 10kVA/240V=41A.
> >41A x 2.5=100A. (round numbers). To do this you would also need
> overcurrent
> >protection on the transformer secondary.
> >
> >Of course, you've already got the 100A main tripping. That's why I asked
> if
> >you're
> >energizing them simultaneously.
> >
> >Transformer inrush can be a real problem.
> >
> >Hope this helps.
> >
> >Dick Ratico
> >Solarwind Electric
> >
> >--- You wrote:
> >Friends:
> >
> >I have on off-grid client that is trying to eliminate winter generator run
> >time.  He has a rental on another corner of the property that has grid
> >power.  When it was installed, he put in an extra meter, hoping to use it
> >to run a utility back up feeder to charge batteries.
> >
> >We are now installing this feeder.  We purchased two 10KVA 240/600 volt
> >transformers to bump the voltage to 600 and then back down to 120/240.
> >
> >Here's the problem:  when we energize the circuit breaker that feeds the
> >transformer, the circuit breaker AND the main breaker trip.  The
> >transformer is 10KVA, the main is 100 amps and the branch breaker is
> >60A.  The breakers hold sometimes but trip other times.  This is obviously
> >an inrush problem.  How do I correct this problem?
> >
> >Thanks in advance,
--- end of quote ---

