utility x/r ratio 1

[tulum]

The utility provided the following;

Supply Voltage: 12.48KV,

Short Circuit Data:
L-G Amps: 1869A
P-P Amps: 1782A
3P Amps: 2058A
Z1 = 1.05 +j3.34
Z0 = 1.22 +j4.39

This is on the primary of a pole top transformer that feeds one of our buildings. It is unclear yet, but I think the transformer is a 375kVA 12.48kV - 600V transformer (not sure of configuration). The transformer impedance is around 2.5%.

What does the z1 and z0 represent? If it is the positive and zero sequence impedances, where is the negative sequence impedance? How do I determine the x/r ratio for the 3p and L-g SC currents? How do I translate this to the secondary.

Sorry for the elementary question but this is not what I do in my day to day operations.

Regards,
TULUM

 

[davidbeach]

Z1 and Z0 are the source impedances, Z2=Z1. Z=R+jX, so you ought to be able to do X/R. Add the transformer impedance to the source impedance (best do set it all in per unit) and now you have the source impedance on the secondary.

 

[tulum]

Thanks David,

So then the z1 is the positive sequence impedance, and the Z0 is the zero sequence impedance... it has been awhile that is all... I just did not rememeber that the negative sequence impedance is usually a replica of the positive sequence impedance.

Regards,
TULUM

 

[davidbeach]

Z2 is that same as Z1 for all non-rotating equipment, and your service is well away from the generation that the difference between Z1 and Z2 is far smaller than the accuracy of the measurements and calculations used to determine the Z1 value.

 

[tulum]

While I am at it...

How do you calculate the x value from the transformer nameplate impedance...

TULUM

 

[davidbeach]

Your library should include useful reference that will have all the necessary calculations. Your Power System Analysis text from university would be a good start. You should also have a copy of J. Lewis Blackburn's Protective Relaying: Principles and Applications and it also has those calculations.

 

[tulum]

Thanks..

 

[DanDel]

As far as the X/R ratio calculation, don't forget that the 3-phase fault X/R = X1/R1, and the L-G X/R = (X1+X2+X0)/(R1+R2+R0).

Calculating X from the transformer %Z will require you to get the actual or assumed X/R ratio for that transformer.