Siemens SIPROTEC 7UM62, Manual

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