3.3 Commissioning
313
7SD610 Manual
C53000-G1176-C145-4
I
4
from Own Line To generate a displacement voltage, the e–n winding of one phase in the voltage
transformer set (e.g. L1) is bypassed (see Figure 3-27). If no connection on the e–n
windings of the voltage transformer is available, the corresponding phase is open cir-
cuited on the secondary side. Via the current path only the current from the current
transformer in the phase from which the voltage in the voltage path is missing is con-
nected; the other CTs are short-circuited. If the line carries resistive-inductive load, the
protection is basically subjected to the same conditions that exist during an earth fault
in the direction of the line.
The voltages can be read on the display at the front, or called up in the PC via the op-
erator or service interface, and compared with the actual measured quantities as
primary or secondary values. The absolute values as well as the phase differences of
the voltages are indicated so that the correct phase sequence and polarity of individual
transformers can also be seen. The voltages can also be read out with the Web-Mon-
itor.
The same manipulation is carried out with the current and voltage transformers at the
other end.
Figure 3-27 Polarity check for I
4
, example with current transformer configured in a
Holmgreen connection
If the current flows towards the protected object according to the circuit in Figure 3-27,
the currents I
L2
and I
L3
are virtually zero. An earth current 3I
0
of the approximately
same level as I
L1
occurs. Accordingly, the voltage U
L1E
is missing and a zero sequence
voltage 3U
0
appears.
In the event of a polarity fault, 3I
0
is in opposite phase withI
L1
or the zero voltage 3U
0
supplements the other two voltages to a voltage star. Open the circuit breakers, short-
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