Mitsubishi NF-S Series - Page 76

75
Z
S
= = R
S
+ j X
S
Z
L
+ Z
T
+ Z
M
(Z
L
+ Z
T
) · Z
M
R
S
=
(R
L
+ R
T
+ R
M
)
2
+ (X
L
+ X
T
+ X
M
)
2
(R
L
+ R
T
+ R
M
) {R
M
(R
L
+ R
T
) – X
M
(X
L
+ X
T
)}
+ (X
L
+ X
T
+ X
M
) {X
M
(R
L
+ R
T
) + R
M
(X
L
+ X
T
)}
[
X
S
=
(R
L
+ R
T
+ R
M
)
2
+ (X
L
+ X
T
+ X
M
)
2
(R
L
+ R
T
+ R
M
) {X
M
(R
L
+ R
T
) + R
M
(X
L
+ X
T
)}
– (X
L
+ X
T
+ X
M
) {R
M
(R
L
+ R
T
) – X
M
(X
L
+ X
T
)}
[
]
]
Thus, when calculating the short-circuit current at
various points in a load system, if the value Z
S
is first
computed, it is a simple matter to add the various wire
or bus-duct impedances. Table 9.4 gives values of
total supply impedance (Z
S
), using transformer imped-
ance per Table 9.1, power-supply short-circuit capacity
of 1000MVA, and X/R of 25.
Z
M
Z
B
Z
W
Z
L
L
T
B
W
Short-
circuit
point
M
Z
T
Z
B
Z
M
Z
L
Z
W
Z
T
Z
B
Z
W
Z
Z
S
Fig. 9.2 3-Phase Equivalent Circuits
Table 9.4 Total Impedances for 3-Phase Power Supplies
50
75
100
150
200
300
500
750
1000
1500
2000
Impedance based on
1000kVA(%)
Ohmic value (mΩ)
Transformer capacity
(kA)
Notes: 1. Total power-supply impedance
Z
S
=
Z
L
+ Z
T
+ Z
M
(Z
L
+ Z
T
)Z
M
2. For line voltages (E') other than 230V, multiply the ohmic value by
()
230
2
E'
33.182 +j26.482
21.229 +j22.583
15.473 +j17.109
9.56 +j 12.389
6.977 +j12.15
4.306 +j 8.795
2.089 +j 7.27
1.427 +j 5.736
0.969 +j 4.336
0.671 +j 3.142
0.467 +j 2.544
230V
17.553 +j 14.009
11.230 +j 11.946
8.185 +j 9.051
5.057 +j 6.554
3.691 +j 6.427
2.278 +j 4.653
1.105 +j 3.846
0.755 +j 3.034
0.513 +j 2.294
0.355 +j 1.662
0.247 +j 1.346
440V
64.240 +j 51.269
41.099 +j 43.720
29.956 +j 33.123
18.508 +j 23.985
13.507 +j 23.522
8.336 +j 17.027
4.044 +j 14.074
2.763 +j 11.104
1.876 +j 8.394
1.299 +j 6.083
0.904 +j 4.925