Fuji Electric BW32AAG - Page 105

103
6
Short-circuit current calculation
6-1 Calculating short-circuit current
s 3TEP#REATINGANIMPEDANCEMAP
Create an impedance map using the impedance calculated in
step 2. Sources of short-circuit current like a power supply and
motor constitute the same electric potential in an impedance
map. Connect these sources together with an infinite
busbar as shown in Fig. 6-2 and be very careful connecting
impedance in series or in parallel between the busbar and fault
point F when creating the impedance map.
s 3TEP5NIFYINGIMPEDANCE
Take the impedance map shown in Fig. 6-2 and use series-
parallel calculations to unify all impedance as shown in
Fig. 6-3.

%Z = %R + j%X
%Z = (%R)
2
+ (%X)
2
s 3TEP#ALCULATINGEFFECTIVEVALUESFORSYMMETRICAL
short-circuit current
IF = (3ø) = IF (rms) sym (3ø)
3VB%Z
PB u 10
3
= u 100
%Z
100
= IB u [A]
Fig. 6-2
Infinite busbar
j%X1
%RT
j%XT
j%X5
%R5
j%X5
j%X2
%R4
j%X4
F
j%X
n
j%X3
%Rn
j%Xn
Fig. 6-3
Infinite busbar
j%X
%R
F