Medium Voltage Application Guide

Current
13850.A
SWITCHGEAR
Synchronous generator
= U 2
syn
X syn
Where:
Z sc-syn = synchronous machine short-circuit impedance ()
X syn = synchronous reactance (%)
S syn 100
U syn = synchronous machine output voltage (kV)
S syn = synchronous machine output power (MVA)
A synchronous machine has three stages of reactance during a short-circuit fault. The reactance is lowest at the
beginning of a fault, causing the highest level of short circuit current. From this level, the short circuit current decays
to a steady state.
Subtransient stage
This is usually the first few cycles of a fault occurrence. The peak short circuit current at this stage determines the
fault “make rating” of a circuit breaker, and the mechanical withstand.
Transient stage
This stage typically lasts for 10 to 20 power cycles and determines the thermal withstand and “break rating” of a
circuit breaker.
Permanent stage
This is the short circuit current level until the fault is interrupted by protection and clearing of the fault. In reality, this
stage never occurs as the fault is cleared beforehand.
Stages of a short circuit
Ir
A
B
C
D
I sc
1
A
B
C
D
Point at which fault occurs
Healthy
Subtransient stage
Transient stage
Permanent stage
Time
Transient levels for a synchronous generator
Type Subtransient Xd´´ Transient Xd´ Permanent Xd
Turbo 10%~20% 15%~25% 200%~350%
Exposed poles 15%~25% 25%~35% 70%~120%
Transient levels for a synchronous motor
Type Subtransient Xd´´ Transient Xd´ Permanent Xd
High speed > 1500 rpm 15% 25% 80%
Low speed < 1500 rpm 35% 50% 100%
Asynchronous motors
Where:
Z sc-mtr = motor output short circuit impedance ()
U mtr = motor input voltage (kV)
P mtr = motor rated power (kW)
An asynchronous motor will contribute approximately 4 to 6 times its rated current into a short circuit fault.
Cables
Busbars
Z sc = 0.1 /km
Z sc = 0.15 /km
Page 158 Medium Voltage Application Guide 710-12280-00A