3.4 Primary Commissioning Tests with the Generator
375
7UM62 Manual
C53000-G1176-C149-3
3.4.6.1
Unit Connection
General
In the event of an external (high-voltage side) short-circuit, an interference voltage is
transmitted via the coupling capacitance C
C
(Figure 3-31) which induces a neutral
displacement voltage on the generator side. To ensure that this voltage is not
interpreted by the protection as an earth fault within the generator, it is reduced by a
suitable loading resistor to a value which corresponds to approximately one half the
pick-up voltage
(address
). On the other hand, the earth fault current
resulting from the loading resistor in the event of an earth fault at the generator
terminals should not exceed 10 A, if possible.
Figure 3-31
Unit Connection with Earthing Transformer
Calculation of
Protected Zone
Coupling capacitance C
C
and loading resistor R
B
represent a voltage divider
(equivalent circuit diagram Figure 3-32), whereby R
B
' is the resistance R
B
referred to
the machine terminal circuit.
Figure 3-32
Equivalent Diagram and Vector Diagram
U
E
R
B
7UM62
R
T
C
C
C
Tr
C
L
C
G
R
B
– Loading resistor
R
T
– Voltage divider
U
E
– Neutral displacement voltage at 7UM62
C
G
– Generator-earth capacitance
C
L
– Line–earth capacitance
C
Tr
– Winding–earth capacitance of block transformer
C
C
– Coupling capacitance of block transformer
U
R
’
R
B
’
U
C
ω
C
C
U
NU
√
3
~
U
NU
/
√
3
U
R
’
U
C
U
NU
Rated voltage on upper–voltage side of block transformer
U
C
Voltage at coupling capacitance C
K
C
C
Total coupling capacitance between upper–voltage and lower–voltage windings
U
R
’
Voltage across loading resistor
R
B
’
Loading resistor of earthing transformer, referred to machine circuit.
