2 Functions
128
7UM61 Manual
C53000-G1176-C127-3
2.21 90-%-Stator Earth Fault Protection (ANSI 59N, 64G, 67G)
The stator earth fault protection detects earth faults in the stator windings of three-
phase machines. The machine can be operated in busbar connection (directly con-
nected to the network) or in unit connection (via unit transformer). The criterion for the
occurrence of an earth fault is mainly the emergence of a displacement voltage, or ad-
ditionally with busbar connection, of an earth current. This principle makes possible a
protected zone of 90 % to 95 % of the stator winding.
2.21.1 Functional Description
Displacement
Voltage
The displacement voltage UE can be measured either at the machine starpoint via
voltage transformers or neutral earthing transformers (Figure2-41 2-41) or via the e-n
winding (broken delta winding) of a voltage transformer set or at the measurement
winding of an earthing transformer (Figure 2-41). Since the neutral or earthing trans-
formers usually supply a displacement voltage of 500 V (with full displacement), a 500
V/100 V voltage divider is to be connected in series in such cases.
If the displacement voltage can not be directly applied to the device as a measured
value, the device can calculate the displacement voltage from the phase-to-ground
voltages.
Address
223
UE CONNECTION
serves for notifying the device of the way the displace-
ment voltage is to be measured or calculated.
In all kinds of displacement voltage formation, the components of the third harmonic
in each phase are summed since they are in phase in the three-phase system. In order
to obtain reliable measured quantities, only the fundamental of the displacement
voltage is evaluated in the stator earth fault protection. Harmonics are filtered out by
numerical filter algorithms.
For machines in unit connection the evaluation of the displacement voltage is suffi-
cient. The possible sensitivity of the protection is only limited by power frequency in-
terference voltages during earth faults in the network. These interference voltages are
transferred to the machine side via the coupling capacitances of the unit transformer.
If necessary, a loading resistance can be provided to reduce these interference volt-
ages. The protection initiates disconnection of the machine when an earth fault in the
machine zone has been present for a set time.
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