Functions
180
7UM62 Manual
C53000-G1176-C149-3
On the occurrence of earth fault in the machine zone, the disconnection of the
machine is initiated after a set delay time.
When the earth current is not decisive to detect an earth fault, e.g. because the circuit
breaker is open, the earth current detection can be switched off by a control signal via
a binary input of the relay. During this time, the displacement voltage stage is fully
operative as the only earth fault protection (e.g. during run-up of the machine).
Figure 2-87 shows the logic diagram of the stator earth fault protection.
If the stator earth fault protection is used as directional busbar connection protection,
this feature is assigned to the sensitive current measuring input of the 7UM62 device.
The user must be aware that the sensitive earth fault detection can use the same
measuring input (if I
EE2
has been configured) and thus the same measuring quantity.
For this reason, two additional, independent pickup thresholds Iee> and Iee>> could
be formed for this measuring quantity by means of the sensitive earth fault detection
(see section 2.27). If the user does not desire this, he should remove the sensitive
earth fault configuration at address
, or use it with I
EE1
.
If the rotor earth fault protection (see Section 2.30) is used, it occupies the additional
voltage input; the displacement voltage U
0
for the stator earth fault protection is
therefore calculated from the phase-earth voltages in that case.
Earth Fault
Detection
(Earth Differential
Protection with
Tripping via
Displacement
Voltage)
In the industrial sector, busbar systems are implemented with high- or low-resistance,
switchable starpoint resistances. For earth-fault detection, the starpoint current and
the total current are detected via toroidal current transformers and transmitted to the
protective relay as current difference. In this way, both the earth current portion
derived from the starpoint resistance and the earth current portion derived from the
power system contribute to the total earth current. In order to exclude an unwanted
operation due to transformer faults, the displacement voltage is used for tripping (see
figure 2-86). The protection feature detects a machine earth fault if the following two
criteria are fulfilled:
−
Displacement voltage larger than set value
−
Earth fault current difference
∆
I
E
higher than setting value
Figure 2-86
Earth Current Differential Protection with Busbar Connection
Appendix A.4 (Figure A-27) illustrates a connection example with a low-resistance
starpoint (earth current limiting to about 100 A). In this example as well, 3I0> and U0>
are the tripping criteria.
G
7UM62
I
EE
U
E
&
∆
IE
