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2.26
90–%–Stator Earth Fault Protection (ANSI 59N, 64G, 67G)
General
2.26.1 Functional Description
Displacement
Voltage
7UM62 Manual
C53000-G1176-C149-3
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
connected to the network) or in unit connection (via unit transformer). The criterion for
the occurrence of an earth fault is mainly the occurrence of a neutral displacement
voltage. This principle results in a protected zone of 90 % to 95 % of the stator winding.
The displacement voltage U
voltage transformers or neutral earthing transformers (Figure 2-82) or via the e-n
winding (broken delta winding) of a voltage transformer set or the measurement
winding of a line connected earthing transformer (Figure 2-83). Since the neutral
earthing transformer or the line connected earthing transformer usually supply a
displacement voltage of 500 V (with full displacement), a voltage divider 500 V/100 V
is to be connected in such cases.
If the displacement voltage can not be directly applied to the device, the device
calculates the displacement voltage from the phase-to-ground voltages.
Address 0223 UE CONNECTION serves for specifying the way the displacement
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
sufficient. The achieved sensitivity of the protection is only limited by power frequency
interference voltages during an earth fault in the network. These interference voltages
are transferred to the machine side via the coupling capacitances of the unit
transformer. If necessary, a loading resistor can be provided to reduce these
interference voltages. The protection initiates disconnection of the machine when an
earth fault in the machine zone has been present for a set time.
90–%–Stator Earth Fault Protection (ANSI 59N, 64G, 67G)
can be measured either at the machine starpoint via
E
177
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