Unbalanced output (single phase) from 3 phase transformer 3

[marquette]

We have a power transformer problem. We have lost a 1250 kva single phase (2 phases at 12,470 volts phase to phase) oil filled transformer with a 600 volt secondary. We want to temporarily replace it with a 3000 kva three phase 12,470 volt delta to 600/346 wye transformer and power it up using all three phases at 12,470, but only connecting the single phase (ie 600 volt) output (eg. X1- X2). We want to find the primary line currents; Will it handle 1250 kva temporarily (90 days)? Should we ground the neutral; What is good and bad with this idea?

 

[weh3]

What is your actual load?

William

 

[marquette]

The actual secondary load could be 1600 amps.

 

[weh3]

You are driving a 1600 A load at 600 V for 960 kVA.

You cannot exceed 1000 kVA because you are only connecting one winding on the primary.

You are using two windings of a 3000 kVA xf, amounting to 2000 kVA of capacity on the secondary.

Unless there is a problem with phase shift, between primary and secondary, which, it seems to me, should be 30 degrees, your scenario should not present a problem. I would not connect the neutral, but you may need to ground one of the secondary lines, depending on the load.

My esteemed colleagues out there in cyberspace, however, may have other considerations that I would be interested to know.

William

 

[jghrist]

The current in the two primary windings associated with the loaded secondary phases will be the turns ratio, 600/sqrt(3)/12470 = 0.02778 times the secondary current, or 44.45 A. There will be no current in the other primary winding. The current in two primary lines will be the same as the winding current, 44.45 A. Two winding currents will add (they are in phase) to give 88.9 A in the other primary line.

 

[stevenal]

You cannot exceed 1000 kVA because you are only connecting one winding on the primary.

You are using two windings of a 3000 kVA xf, amounting to 2000 kVA of capacity on the secondary.

More power coming out than going in? I'll take a dozen. Only one primary and one secondary winding are involved, so I'd keep the limit to 1000 kVA. You might be able to push a little more since you have extra oil and cooling capacity, but keep it to 1000 to be safe.

 

[rbulsara]

Little observation..

Both weh3 and jghrist are on right track.

You can either find your way thru the web (

http://www.sandc.com)

or look up S&Ccatalog if available to you. I looked up S &C's data bulletin 240-110. On page 11 it shows current relationships for all combination of delta-wye currents in case of a 'fault' on the secondary side. Fault or not, curents still follows the same physics. So the figure c there shows the curent relationship when there is a line- line fault on the seconday side and no current in the third leg. This holds true for your scenario. According to that for 0.87 pu current in each phase (and line) of the two wye secondary, while no current in the third leg, will cause .87pu current to flow in each related 'delta' primaries as jghrist righly calculated. But the line current on the delta side will be phase current/1.732 or 0.5pu. The two will add up on the third line as 1.0 pu current.

So the theoratical maximum capacity available if you use 600V line-line on the secondary side is 2*(0.87*1000)=1740 kVA,when one of the primary line side will be fully loaded.

For 1600A, 600V or 960kVA load, the current in the two primary 'lines' will be 44.45*0.58=25.8A and the third line will have 2*25.8=51A.

Stevenal: There are two sets of windings involved.

Feel free to correct me..

 

[weh3]

Stevenal,

I was referring to the capacity, which is different for primary and secondary, not to the actual input and output. You would obviously limit it to the lower value.

William

 

[stevenal]

Unless there is a capacity rated tertiary, the capacity of the primary winding will equal that of the secondary. On reread, Rbulsara is correct that two sets of these windings are being used. I think the one connected primary winding comment threw me off. Three primary windings are connected but only two are loaded.

 

[marquette]

Thanks to all that responded to our question. You have been very helpful. Marquette