EC&M Short Circuit Calculation Guide wrong??? 2

[EITengineering]

Hi friends!

I recently tracked down an EC&M article discussing the calculation of short circuit calculations using the p.u.
They have the following for fault impedance calculation of:
ZFault=(Zutility+ZT1)||ZMotor1||ZMotor2=(0.0021+j0.083+0.005+j0.07)||(0.49+j13.8)||(29.8+j298)=0.166+j2.817 pu

I added the Zutility and ZT1 together in series and calculated the rest in parallel but get 0.007006 + j0.15125 pu. Am I missing a step here? Can someone point me in the right direction??

The article can be found here:

http://ecmweb.com/mag/electric_shortcircuit_calculation_methods/

Thank you in advance!
-EITengineering

 

[ijl]

It seems that the reason for the discrepancy is explained in the discussion following the paper. That is, the transformer impedance must be converted to the correct base first (100MVA instead of 2MVA).

But why use such manual methods, when software is available, even freeware. See

http://pp.kpnet.fi/ijl

for example.

 

[EITengineering]

Thanks ijl!

Great! now my calculation comes pretty close

The reason I want to learn the manual calculation is so i can better understand the results and data input required for the software.

Thanks again for the help!

 

[luffy2011]

Try to change the Rt and Xt to:

Xt = .07 * (100/2) = 3.5
Rt = 3.5/15 = .2333

With this, the resulting values will be almost the same. The EC&M article forgot to change the old pu to new pu values using new base MVA.

http://www.powerqualityworld.com