Handbook of Electrical Engineering

FAULT CALCULATIONS AND STABILITY STUDIES 289

Hence

Pload=Phm1+Phm2+Plm+Phs+Pls

=90.0 MW

Qhm1=19.84 MVAr,Qhm2=14.17 MVAr

Qlm=3.10 MVAr,Qhs=13.65 MVAr

Qls=0.5 MVAr

Qload=(Qhm1+Qhm2+Qlm+Qhs+Qls) 1. 015

= 52. 03

Sload=

√

(Pload^2 +Qload^2 )= 103 .96 MVA

The operating power factor PFloadis,

PFload=

Pload

Sload

=

90. 0

103. 96

=0.8657 lagging

The generator MVA isSgenwhich equals 37.5.

Choose the base MVA to beSbase=100.

Assume all five generators are operating when the three-phase zero impedance fault occurs.

Calculate the rms symmetrical fault currents for the generators and each type of load.

a) The generators and unit transformers

Ig′′=

5 × 1. 1 × 34. 0

j

(

0. 13 +

0. 08

1. 25

)

100. 0

= 0. 0 −j 9 .639 pu

• The high voltage motors and unit transformers.
  These consist of two groupsShm1andShm2, let their total beShm.

Ihm′′ =

1. 0 ×( 40. 23 + 28. 74 )

(

0. 033 +j 0. 164 +

j 0. 06

1. 10

)

100. 0

=

0. 6897

0. 033 +j 0. 164 +j 0. 0545

= 14. 1184 ( 0. 033 −j 0. 2185 )

= 0. 4659 −j 3 .0849 pu

• The high voltage variable speed drive motors.
  These can be ignored as sources of sub-transient current.