Page 304 - 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
is S lm . The transformer ratings also require the value of the total static MVA which is S ls .

S ls
K td2 = 2K td 1.0 +
S lm

2.06
= 2 × 1.3 1.0 +
5.88
= 3.511
1.0 × 5.88
I
lm =
j0.055
0.054 + j0.143 + 100.0
3.511
0.0588
=
0.054 + j0.143 + j0.0157
= 2.0924(0.054 − j0.1587)
= 0.1130 − j0.3321 pu

• The total rms symmetrical sub-transient fault current.
The total rms fault current I is,
frms

I = I + I + I
frms g hm lm
= 0.0 − j9.639
+ 0.4659 − j3.0849
+ 0.1130 − j0.3321
= 0.5789 − j13.056 pu

Thebasecurrent I base is,

= 100 × 10 6
S base
I base = √ √
3V base = 3 × 33,000
= 1749.6 amps

Hence the total fault current in rms amps is,

I = 1749.6(0.5789 − j13.056)
frms
= 1012.8 − j22842.7 amps

The magnitude is,

|I frms |= 22,865 amps
Find the peak sub-transient fault current.