Handbook of Electrical Engineering

FAULT CALCULATIONS AND STABILITY STUDIES 281

The contribution from the lower voltage motors is found as follows.

The average ratio of starting current to rated current (Is/In) for the lower voltage motors may be
assumedtobe,
Is
In

= 6 .5 pu for lower voltage motors

Their sub-transient impedanceZlm′′ is

Z′′lm=

1. 0

6. 5

= 0 .153 pu(atSlm)

Typical lower voltage motors have a starting power factor of between 0.25 and 0.35 lagging,
hence assume 0.35.

The sub-transient impedanceZlm′′ of the motors becomes,

Z′′lm= 0. 054 +j 0 .143 pu

The equivalent impedanceZtdof the distribution transformers can be found by the method in
sub-section 11.5.2.5 or taken as,

Ztd= 0. 0 +j 0 .055 pu(atStd)

Again assume that the air-gap emfElm′′ is 1.0 pu.

The contributionIlm′′ from the lower voltage motors is,

Ilm′′ =

Elm′′

(

Z′′lm

Slm

+

Ztd

2 Ktd(Slm+Sls)

)

Sbase

=

1. 0 Slm

(

0. 054 +j 0. 143 +

j 0. 06

Ktd2

)

Sbase

(11.3)

Where

Ktd2= 2 Ktd

(

1. 0 +

Sls

Slm

)

The total sub-transient symmetrical rms fault currentIfrms′′ is,

Ifrms′′ =Ig′′+Ihm′′ +Ilm′′ ( 11. 4 )

11.6.1 Calculate the sub-transient peak fault current contributions

Many power system networks can be reduced to a simple series-connected circuit containing a resis-
tanceRand an inductanceL, for the purpose of calculating the transient fault current. Furthermore a