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Wrapping Around Transformer Inrush Point

Wrapping Around Transformer Inrush Point

Wrapping Around Transformer Inrush Point

(OP)
I have a 3000A breaker upstream of a 2800kVA, 600:375VAC delta-wye phase shifting transformer for a 3000A 12-pulse DC drive.  This SCR breaker is one of four coming off a switchboard bus fed only by three paralleled gensets.

I want to tighten the SCR breakers settings due to past SCR shorts.  I'm assuming a transformer inrush point of ~32,000A (ie 12X of 2,694A) @ 0.1sec.  See attached.

http://files.engineering.com/getfile.aspx?folder=95571284-2254-4717-b152-a2e36636c24b&file=tcc44.pdf

I show the breaker's instantaneous at 15X.  I want to set the short time setting about 3X as shown with 0.2sec delay and I^2t off.  The drive current limits to 3000A and the 3X setting is a drop from 6X currently with I^2t on.  I believe if the drive gets up to 9000A for even 200ms there must be something terribly wrong and want to trip the breaker.  The drive controls a pump motor with even the top end 3000A range rarely seen.

Since there is no "cold load" to be worried about here, does the short time delay of 0.2 still seem too low?  I can increase to 0.3 or 0.4 or of course leave I^2t on.  But, I have not been able to run across any other information on typical magnetization inrush multiples other than the 12X point (and 25X at 0.01sec used only when fuses are involved?).  Since I've read that the 12X is kind of the outside of a 8X-12X envelope, wondering if the 100ms isn't also an upper limit and I'll probably be fine with the 200ms-9000A "corner" sticking out there?  For example, Pg. 22 of below link suggests as much:

http://epowerengineering.com/guides-part-5.pdf

Thanks for any help.

RE: Wrapping Around Transformer Inrush Point

I don't see the fuses that should be protecting the SCRs.  You aren't going to get good SCR protection from a circuit breaker and relay/trip unit.

RE: Wrapping Around Transformer Inrush Point

(OP)
Thanks davidbeach.  Unfortunately, what's done is done with these old drives.  It may be a few years before I'd have the opportunity to convince management to lay out funds to retrofit in new fusing.

Sorry, didn't mention, I'm looking to aid system stability of the online generators, not protection of the SCR's.  We've had at least one case in the last few years where drive shorts have led to tripping out all three on-line gensets (black-out).  

On the fuses, though, I looked at Bussman's KRP-C-3000 or KTU-3000.  SKM gives me a line-side 3ph bolted fault of about 30kA.  These fuses would finish pre-arc at either 200 or 100ms respectively.  That's where it seemed a short-time trip in 200ms at 9000A, if possible, could be in some sense equivalent?

 

RE: Wrapping Around Transformer Inrush Point

(OP)
Well, just to put this thread out of its misery, I researched this enough to see that the roll-off of inrush typically would not allow me to set the short-time where I'd hoped.  I found the first page of this Siemens Power Engineering Guide helpful:
http://www.ee.kntu.ac.ir/pow/Articles/6tafreshi_6.3.4%20Protection%20coordination.pdf

Also, J. E. Holcomb's paper in AIEE Transactions, Dec. 1961, "Distribution Transformer Magnetizing Inrush Current" had some helpful data.  I put it into a spreadsheet to give some representative roll-off curves that relate.

http://files.engineering.com/getfile.aspx?folder=5afbbb48-53a6-4404-8a97-f05d13dddb26&file=Xfmr_Inrush_Holcomb_AIEE1961_Table1.xls

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