ABB 615 series Technical Manual page 440

Section 4
Protection functions
434
Stator winding
x E
3H
Neutral
side
x
R
f
3*C
g/2
Fault at distance 'x'
from generator neutral
C = 3*C /2
N g
GUID-5A88B023-8F01-46B5-8A6F-CD9AD652E2B4 V1 EN
Figure 209: 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 ground-fault location
N
and the generator neutral
C Phase-to-ground lumped capacitance of the generator stator winding, between the generator terminal
T
and ground-fault location
When there is a generator breaker, the capacitive coupling to ground 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, 27/59THN desensitizes the
protection by multiplying the Beta setting with a set constant CB open factor setting.
V V CB open factor
+ ≥
3 H T _ 3 H N _
GUID-DE97C93B-50F3-48EA-A558-9FF3ED57FC62 V1 EN
The 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 /2 + 3*C
T g
or
C = 3*C
T
Beta V
⋅ ⋅
3 H N _
1MAC059074-MB A
Step-up unit Circuit breaker
3*C
t
t
/2
g
(Equation 53)
615 series ANSI
Technical Manual