Handbook of Electrical Engineering

290 HANDBOOK OF ELECTRICAL ENGINEERING

• The low voltage motors and distribution transformers.
  There is one group of low voltage motors connected to various switchboards. Their total MVA
  isSlm. The transformer ratings also require the value of the total static MVA which isSls.

Ktd 2 = 2 Ktd

(

1. 0 +

Sls

Slm

)

= 2 × 1. 3

(

1. 0 +

2. 06

5. 88

)

= 3. 511

Ilm′′ =

1. 0 × 5. 88

(

0. 054 +j 0. 143 +

j 0. 055

3. 511

)

100. 0

=

0. 0588

0. 054 +j 0. 143 +j 0. 0157

= 2. 0924 ( 0. 054 −j 0. 1587 )

= 0. 1130 −j 0 .3321 pu

• The total rms symmetrical sub-transient fault current.

The total rms fault currentIfrms′′ is,

Ifrms′′ =Ig′′+Ihm′′ +Ilm′′

= 0. 0 −j 9. 639

+ 0. 4659 −j 3. 0849

+ 0. 1130 −j 0. 3321

= 0. 5789 −j 13 .056 pu

ThebasecurrentIbaseis,

Ibase=

Sbase

√

3 Vbase

=

=

100 × 106

√

3 × 33 , 000

=1749.6 amps

Hence the total fault current in rms amps is,

Ifrms′′ = 1749. 6 ( 0. 5789 −j 13. 056 )

= 1012. 8 −j22842.7 amps

The magnitude is,
|Ifrms′′ |=22,865 amps

Find the peak sub-transient fault current.