MV design guide - Schneider Electric

Design rules
Short-circuit currents
G
Synchronous generators
(alternators and motors)
Calculating the short-circuit current across the terminals of a
synchronous generator is very complicated because the internal
impedance of the latter varies according to time.
c When the power gradually increases, the current reduces passing
through three characteristic periods:
v sub-transient (enabling determination of the closing capacity of circuit
breakers and electrodynamic contraints), average duration, 10 ms
v transient (sets the equipment's thermal contraints),
average duration 250 ms
v permanent (this is the value of the short-circuit current in steady state).
c The short-circuit current is calculated in the same way as for
transformers but the different states must be taken account of.
Example:
Calculation method for an alternator
or a synchronous motor
c Alternator 15 MVA
c Voltage U = 10 kV
c X'd = 20 %
Sr
Ir = =
15
= 870 A
e • U e • 10 000
Isc =
Ir 870
= = 4 350 A = 4.35 kA
Xcc trans. 20/100
fault
appears
Ir
healthy
state
courant
subtransient
state
transient
state
Isc
time
permanent
state
short-circuit
c The short-circuit current is given by the following equation:
Isc =
Ir
Xsc
Xsc : short-circuit reactance c/c
c The most common values for a synchronous generator are:
State Sub-transient X''d Transient X'd Permanent Xd
Xsc 10 - 20 % 15 - 25 % 200 - 350 %
M
Asynchronous motor
c For asynchronous motors
v the short-circuit current across the terminals equals the start-up current
Isc z 5 at 8 Ir
v the contribution of the motors (current feedback) to the short-circuit
current is equal to:
I z 3 ∑ Ir
The coefficient of 3, takes account of motors when stopped and the
impedance to go right through to the fault.
14 Merlin Gerin MV design guide Schneider Electric