Page 712 - Industrial Power Engineering and Applications Handbook

201672 Industrial Power Engineering and Applications Handbook

8.u
x: + xz
and ZSsc (phase-to-phase fault) = ~
- 43. ti (20.7)
-
0.117 + 0.088
= 8.45u and for the impedances assumed, when all the four
machines arc running in parallel
If there is more than one generator operating in parallel,
when the fault occurs, the reactances of all such machines = 12.24 x 4~
will fall in parallel and diminish the effective reactance or 48.96~ (as against 34.2~ on a symmetrical three-phase
of the faulty circuit and enhance the fault current. For fault).
instance, when all four machines are operating in parallel, The ground fault circulating current is now as many
the effective reactance will become times more, as many grounded machines will be operating
in parallel. Such high ground fault currents may cause
disturbance in the supply system and severely damage
the machines. This can be reduced by grounding just one
machine at a time, as illustrated in Figure 20.20, and
and the enhanced fault current
adopting the normal protective scheme. Equation (20.7)
will now be modified to

i.e. 4 x 8.5511 or 34.2~
Generalizing the above, the symmetrical fault current, x:
n
when n number of machines are operating in parallel, where x:~, = -
x?
2) x2,, = -
’2
Issc = 7.
xd and
This is irrespective of whether the generator neutral is xoc, =xg
isolated or solidly grounded, and if grounded, whether Reactances with the suffix ‘el’ denote the equivalent
operating singly or in parallel, whereas the ground fault, reactances when all the machines are running in parallel,
when the generators are grounded but only one machine but neutral if only one machine is grounded at a time.
is operating at a time,
(20.8)
3u 3.v
I =-= (Table 13.9)
E x x:+x2+xo n n ”
and for the values of reactances considered; For the reactances assumed, when all the four machines
are running in parallel
3ti
I, =
0.1 17 + 0.088 + 0.04 I” = 3v
= 12.24~ 0.117 ~ 0.088 + ,04
4 4
(which is much higher than 8.55~011 symmetrical three- = 32.88~
a
phase fault). When more than one machine are operating
in parallel at a time and all of them are individually which is much less than before and even less than the
grounded, the effective reactances for n number of three-phase symmetrical fault current. To restrict the
machines will be ground fault current, when more than one generator are
operating in parallel it is advisable to ground only one
machine at a time. Although provision must be made for
all the machines for grounding, so that when one or
more machines are out of service, one of those in service
may be grounded. To achieve this, all the neutrals may
be connected to a common neutral bus through individual
neutral grounding switches as shown in Figure 20.20,
x
0
x,, = - and the common neutral grounded solidly.
n
Reactances with the suffix ‘e’ denote the equivalent Impedance grounding
reactances when all the machines are grounded and
running in parallel The above problem can also be overcome by impedance
grounding rather than solid grounding. It can be a
1
and x, = - (x; + x2 + x,) ) resistance R or inductance L or both, as discussed above.
n In the present case, if we consider a p.u. resistance r of
and the new fault current will be just 9% in every neutral, the improved ground fault current