Fuji Electric BW32AAG - Calculating Short-Circuit Current; Chapter 6 Short-Circuit Current Calculation

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6
Short-circuit current calculation
6-1 Calculating short-circuit current
6-1 Calculating short-circuit current
6-1-1 Calculation objective
The damage from a short-circuit fault in a system circuit must
be kept to a minimum with power receiving equipment. This
is why the short-circuit current generated when a fault occurs
must be accurately calculated for all points in the system.
Purpose of short-circuit current calculations
1) To select the appropriate breaking capacity for overcurrent
trip devices like MCCBs or fuses installed in the system.
2) To select the protective relay setting value that will ensure
proper coordination at all points when a fault occurs.
3) To see how much short-circuit current the cables,
disconnectors, current transformers and other series-
connected devices can thermally and electrically withstand
before the overcurrent trip device finally cuts off current.
6-1-2 Calculation formula
Short-circuit current is calculated from reference capacity and
reference voltage impedance. When the various quantities
needed in the calculation are expressed in percentages, then
simply use Ohm’s Law.
6-1-3 Calculating short-circuit current for
three- phase circuits
s 3TEP3ETTINGREFERENCEVALUES
First set calculation references, and then select appropriate
values for the references. The rated capacity of the transformer
is generally used for the reference capacity.
Reference capacity P
B = PT (kVA)
P
T = Transformer capacity
Reference voltage V
B = VT (V)
V
T = Line voltage
Reference current
3VT
PT
IB = IT = u 10
3
(A)
Reference impedance
PB u 10
3
VB
2
ZB =
=
P
T u 10
3
VT
2
(:)
Bus duct
M
Short-circuit capacity or
breaking capacity
Q (MVA) o j%X
1
PT (kVA)
/V
T (V)
%Z
T = %RT + j%XT
jX3 (:) o j%X3
Cable
R
5n (:) + jX5n (:)
%R
5n + j%X5n
Total capacity PM (kVA)
P
M (kVA) = 1.5x (kW sum total)
Cable
R
4 (:) + jX4 (:)
%R
4 + j%X4
jX2 (:) o j%X2
F
j%Xm
Fig. 6-1
s 3TEP#ONVERTINGIMPEDANCEVALUESTOREFERENCE
values
(1) Reactance on the primary side of the
transformer: %X1
The reactance can usually be obtained from the power
company. If the reactance is not known, then it can be found
by calculating back from the rated breaking capacity of the
circuit breaker installed on the primary side of the transformer.
Q u 10
3
PB
%X1 = u 100 (%)
Q = Short-circuit capacity on the primary side (MVA)
(2) Transformer impedance: %Zt
Transformer impedance is usually expressed as a percentage.
If the transformer capacity is used for the reference capacity,
then simply use %Z
T. When the reference capacity and the
transformer capacity are different, however, then convert them
to a base reference capacity (%Z
t) using the following formula.
%ZT
PT
=
%Z
t
PB
(Converting percentages into another capacity)
When there is a single-phase transformer, treat it like a three-
phase transformer, and set the impedance percentage at
√
3
/2
times the obtained current value.
2
1
%Z (3ø) = %Z (1ø)
Calculate three-phase short-circuit current using the formula
above, and multiply the obtained current by
√
3
/2.
(3) Motor reactance: %Xm
Motor capacity is generally expressed in kW, so convert the
kW into kVA.
Conversion to kVA | 1.5 u motor output (kW)
Use %Xm’ = 25% to convert the motor capacity into a base
reference capacity.
%Xm'
P
M
=
%X
m
PB
(Formula for converting into another capacity)
(4) Bus duct and cable impedance
These are normally expressed by cross sections (rated current
for the bus duct) and length.
ZC = (Ohms per unit length) u (Length) (:)
Convert this value into a percentage.
ZB
ZC
%ZC = u 100%
(If the value is given in ohms, convert it to a
percentage.)