ABB RELION REX640 Technical Manual page 525

1MRS759142 C
REX640
Technical Manual
However, the generator breaker normally exists between the protected generator and
its power transformer.
Stator winding
x E
3H
Neutral
side
x
R
3*C
g/2
C = 3*C /2
N g
GUID-118C9E20-43A9-4F78-95E5-049D8A48E66D V1 EN
Figure 284: Capacitance seen at generator terminal and neutral side
C Phase capacitance of generator stator winding
g
C Total external phase capacitance of the system as seen from the generator
t
C Phase-to-ground lumped capacitance of the generator stator winding, between the earth-fault
N
location and the generator neutral
C Phase-to-ground lumped capacitance of the generator stator winding, between the generator
T
terminal and earth-fault location
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 Beta setting with a set constant CB open
factor setting.
U U CB open factor Beta
+ ≥
3 H T _ 3 H N _
GUID-F3A88FC1-EA44-499E-B1D2-EEAEC72BDAA1 V1 EN
The CB open factor setting is obtained during commissioning.
(1-x) E Generator CB
3H
Terminal
side
(1-x)
f
3*C
g/2
Fault at distance 'x'
from generator neutral
U
⋅ ⋅
3 H N _
Section 4
Protection functions
Step-up unit
3*C
t
C = 3*C /2 + 3*C
T g t
or
C = 3*C /2
T g
Circuit breaker
(Equation 74)
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