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On the occurrence of an 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 for detecting an earth fault when the circuit
breaker is open, the earth current detection can be switched off for a certain time via
a binary input. By this means it is possible to switch to sole evaluation of the displace-
ment voltage e.g. during run-up of the generator.
Figure 2-46 shows the logic diagram of the stator earth fault protection.
If the stator earth fault protection is used as directional or non-directional busbar con-
nection protection, this activates the sensitive current measuring input of the 7UM61
device. Note that the sensitive earth fault protection uses the same measuring input
and thus accesses the same measured value. Thus two additional, independent
pickup thresholds Iee> and Iee>> could be formed for this measured value by means
of the sensitive earth fault detection (see Section 2.22). If this is not desired, cancel
sensitive earth fault protection configuration at address 151.
Earth Current De-
In the industrial sector, busbar systems are designed with high or low resistance, swit-
tection (Earth
chable starpoint resistances. For earth-fault detection, the starpoint current and the
Current Differential
total current are detected via toroidal current transformers and transmitted to the pro-
Protection with
tective device as current difference. In this way, the earth current portions derived both
Tripping via Dis-
from the starpoint resistance and from the power system contribute to the total earth
placement Voltage)
current. In order to exclude an unwanted operation due to transformer faults, the dis-
placement voltage is used for tripping (see following figure).
The protection feature detects a machine earth fault if the following two criteria are ful-
filled:
• Displacement voltage larger than set value U0>,
• Earth fault current difference ∆I
Figure 2-45
Determination of
In addition to this, a supplementary function serves to determine the faulty phase. As
the Faulty Phase
the phase-earth-voltage in the faulty phase is less than in the two remaining phases
and as the voltage even increases in the latter ones, the faulty phase can be deter-
mined by determining the smallest phase-earth voltage in order to generate a corre-
sponding result as fault message.
7UM61 Manual
C53000-G1176-C127-3
2.21 90-%-Stator Earth Fault Protection (ANSI 59N, 64G, 67G)
larger than setting value 3I0>,
E
Earth Current Differential Protection with Busbar Connection
131
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