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3 Installation and Commissioning
376
Since the reactance of the coupling capacitance is much larger than the referred
resistance of the loading resistor R
(compare also vector diagram Figure 3-32), whereby U
displacement voltage with a full displacement of the network (upper-voltage) neutral.
The following applies:
----------------------- -
1
U
Inserting the voltage transformation ratio TR of the earthing transformer:
TR
,
U
------- - U
=
R
3
we obtain:
U
Together with the voltage divider 500 V/100 V this corresponds to a displacement
voltage of:
U
E
The pick-up value for the neutral displacement voltage U0> should amount to at least
twice the value of this interference voltage.
Example:
Network:
Voltage transformer
Earthing transformer
Loading resistor
1
36
⋅
----- - 10 Ω 314 s
-- -
U
=
E
5
3
10 V has been chosen as the setting value for U0> in address 5002 which
corresponds to a protective zone of 90% (Figure 3-33).
Note: When using a neutral earthing transformer, TR must be inserted as the voltage
transformation ratio instead of TR/3. The result is the same since the neutral earthing
transformer has only one winding.
', U
can be assumed to be U
B
C
,
,
R
U
B
R
---------------------------
=
⁄
(
ωC
)
⁄
(
)
U
3
C
NU
,
,
⋅
ωC
⋅
⁄
(
=
R
U
R
B
C
NU
⋅
and
R
TR
⋅
⋅
ωC
⋅
------- - R
=
U
R
B
C
3
1
TR
⋅
⋅
⋅
ωC
⋅
-- -
------- - R
U
=
B
C
5
3
U
=
110
kV
NU
f
=
50 Hz
N
0.01 µF
C
=
C
10 kV/0.1 kV
TR
=
36
Ω
R
=
10
B
1
–
⋅
⋅
⋅
⋅
0.01 10
≈ U
C
/√3 is the neutral
NU
)
3
2
æ
TR
ö
,
⋅
R
------- -
R
=
è
ø
B
B
3
⁄
(
)
3
NU
⁄
(
)
3
NU
110
6
3
–
⋅
⋅
--------- - 10
F
V
=
4.8 V
3
C53000-G1176-C149-3
/√3
NU
7UM62 Manual
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