Utility X/R Ratio

[DMG_1]

Hello.

I just started doing Short Circuit Calulations and require available short circuit current and X/R Ratio from the Utility. Can anyone tell me how the X/R ratio is used in the short circuit calculation when it comes form the Utility and then is referred through a transformer?

Thanks.

 

[ron]

X/R is the impedance of the system. Z=R+jX

If you are using a software package, it will need it to calculate down stream current.

 

[rcwilson]

X/R ratio is used to estimate the asymmetry of a short circuit current, or the DC current offset and its duration. It is calculated at the point of a fault. A high X/R means a long DC offset. A very high X/R can result in the short circuit current sine wave being so offset that the current never reaches zero during the 3-8 cycles the breaker is trying to open. Some breakers' interrupting ratings have to be reduced for high X/R faults.

For a fault on the secondary of the transformer the X/R would be the sum of the utility X and transformer X divided by the sum of the utility R and transformer R, if we negelect fault contribution from downstream loads.

X/R fault = (Xutil + X txmr) / (Rutil + Rtxmr ). All X's and R's have to be on the same base.

If there are other sources of fault currents, like motors or generators, their impedances and the impedances of the interconnecting cables have to be combined in the proper series or parallel equations to come up with the equivalent X and R at the fault location. Most short circuit programs do this automatically.

 

[ron]

X/R ratio is used to estimate the asymmetry of a short circuit current, or the DC current offset and its duration. It is calculated at the point of a fault. A high X/R means a long DC offset. A very high X/R can result in the short circuit current sine wave being so offset that the current never reaches zero during the 3-8 cycles the breaker is trying to open. Some breakers' interrupting ratings have to be reduced for high X/R faults.

For a fault on the secondary of the transformer the X/R would be the sum of the utility X and transformer X divided by the sum of the utility R and transformer R, if we negelect fault contribution from downstream loads.

X/R fault = (Xutil + X txmr) / (Rutil + Rtxmr ). All X's and R's have to be on the same base.

If there are other sources of fault currents, like motors or generators, their impedances and the impedances of the interconnecting cables have to be combined in the proper series or parallel equations to come up with the equivalent X and R at the fault location. Most short circuit programs do this automatically.

That's what I meant to say