ABB RELION REX640 Technical Manual page 550

Protection functions
conditions. However, the generator breaker normally exists between the protected
generator and its power transformer.
Neutral
side
C = 3*C
N
Figure 317: Capacitance seen at generator terminal and neutral side
C
g
C
t
C
N
C
T
When there is a generator breaker, the capacitive coupling to earth differs between
the operating conditions when the generator is running with the generator breaker
open (before synchronization) and with the circuit breaker closed.
With the generator breaker open, the total capacitance is smaller compared to
normal operating conditions. This means that the neutral side third harmonic
voltage is reduced compared to the normal operating condition. Therefore, there
is a need to reduce the sensitivity of the protection. When generator breaker is
open, H3EFPSEF desensitizes the protection by multiplying the
set constant
U
3 H T
The
550
Stator winding
x E
3H
x
R
f
3*C
g/2
Fault at distance 'x'
from generator neutral
/2
g
Phase capacitance of generator stator winding
Total external phase capacitance of the system as seen from the generator
Phase-to-ground lumped capacitance of the generator stator winding, be-
tween the earth-fault location and the generator neutral
Phase-to-ground lumped capacitance of the generator stator winding, be-
tween the generator terminal and earth-fault location
CB open factor setting.
U CB open factor
+ ≥
_ 3 H N _
CB Open factor setting is obtained during commissioning.
(1-x) E Generator CB
3H
Terminal
side
(1-x)
3*C
g/2
C = 3*C
T
C = 3*C
T
Beta U
× ×
3 H N _
1MRS759142 F
Step-up unit Circuit breaker
3*C
t
/2 + 3*C
g t
or
/2
g
Beta setting with a
(Equation 86)
Technical Manual
REX640