Base Z

[jdd18vm]

Am I missing something here or is this a typo in 2.11? They show the Zbase as 13.4=1pu.

This is a 3 phase wye so I line = I phase= 16.6 amps, Z is given per phase 8.4 ohms

Every formula I use (and I prefer Graingers kV L-L ^2 x 1000/ base kVA3 phase) I get 7.74

.22^2 x 1000/6.25=7.74

EERM Example 35.1 (6th edition) page 35-8 shows this same "error" Zbase=Vbase/Ibase), which after beating my head I saw the errata that corrected it to Zbase=Vbase/sqrt3xIbase which changes THAT answer from 22.4 ohms to 12.9 (wasted an hour on THAT one)

While typing I noticed they do the same thing in 2.12. ugh...What am I missing.....

JD

 

[Flyer_PE]

John,

You aren't missing anything. The correct answer for Zb is 7.74 ohms. I typically use the same method that you used to determine the base impedance. There's no reason to calculate the base current in this problem. The intermediate step is just one more opportunity to make a math error (which they did).

Where they went wrong was in using the line voltage instead of the phase voltage in calculating the base impedance. Instead of 220V they should have used 127V.

Jim

 

[jdd18vm]

John,
You aren't missing anything. The correct answer for Zb is 7.74 ohms. I typically use the same method that you used to determine the base impedance. There's no reason to calculate the base current in this problem. The intermediate step is just one more opportunity to make a math error (which they did).

Where they went wrong was in using the line voltage instead of the phase voltage in calculating the base impedance. Instead of 220V they should have used 127V.

Jim

Thanks Jim I thought so, but you continually doubt yourself when you see this in print.