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 Post subject: Service Transformer X/R ?
PostPosted: Tue Nov 08, 2011 6:16 pm 

Joined: Tue Apr 12, 2011 6:11 am
Posts: 17
Location: Oregon, USA
Well I have been away from my Arc Flash project for awhile, but am finally getting back to it. Last spring I went through the data request process with the electric utility company. That was a learning experience, but seemed to turn out fairly satisfactory. I am left still wanting only one data parameter I was hoping they might have had. This would be the X/R ratio or values for the service transformer. This is a parameter the electric utility doesn’t have a record of.

Although I’m not sure just why it is so, the tutoring materials I have studied indicate the X/R ratio is often difficult to impossible to obtain for many transformers in general. Therefore it is often necessary to simply go with a good educated guess, or ‘approximation’ for a transformer’s X/R ratio. I must suppose many a short circuit analysis / arc flash study necessarily proceeds in that way. I can accept that, and probably even get used it, if necessary.

However, because I am a novice at this, and because this is the service transformer itself we are talking about, at the head of the entire power distribution system of the facility, I am still reluctant, so far as this primary component is concerned, to go with an approximation, as of yet, and am still questing for the actual value. I realize this may be somewhat like tilting at windmills, but there it is.

Accordingly, I am presently pursuing this quest with the transformer manufacturer, which is General Electric. The transformer product line is ‘Prolec’, and at least I have the unit’s exact serial number, and of course, the size, going for me. Nevertheless, I have thus far been trundled from one ‘product specialist’ to the next, until I am now anxiously awaiting a response from the 4th one.

While at a lull in the throes of this quest, I thought I might pose this question to the forum :

Will a good X/R ratio value for the service transformer be very significant to the results of the entire study of the system downstream ? I mean, if I end up needing to go with an approximation, but that approximation were for any reason significantly off, could that seriously compromise the entire study results ? Or, am I just henpecking this horse to death unnecessarily ?

However my present quest turns out, I thought this may be a good point for the scrutiny of this forum.

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PostPosted: Wed Nov 09, 2011 12:58 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
The X/R is good to have but very difficult to obtain. I would not considere it a show stopper. It is used to define the angle of the transformer impedance which could have a slight effect when adding the transformer impedance to the primary and / or secondary impedance. My experience is this is normally not a huge difference because if the X/R is 6 or 8 or 10 you are only talking a few degrees difference which doesn't dramatically affect the results.

You can look at the IEEE Red Book, or Buff Book (also ANSI C57 but it is more expensive) and they will have typical X/R ratios. Although using a typical X/R from one of these standards is generally acceptable (a lot of the computer programs use these values) you do NOT want to ever us a typical transformer impedance.

Sounds like you have taken a few more steps forward!

Jim Phillips, P.E.

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