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3 Installation and Commissioning
3.3.7.2
Rotor Earth Fault Protection (1 to 3 Hz)
354
After the completion of the test, check that all provisional measures for testing have
been reversed:
− Earthing bridge or resistor has been removed,
− Measurement circuit has been closed,
− Controller unit connected to its operational supply a.c. voltage (refer also
connection diagram in Figure 2-95 in Section 2.25.1).
Tests with the running machine are described later under section 3.4.9.
The rotor earth fault protection can be checked with the machine at stand-still. For this
purpose, the series device 7XT71 must be supplied with an external AC voltage of
100 V to 125 V AC (see also to connection diagram Figure A-36 in the Appendix).
Switch the rotor earth fault protection (address 6101 REF 1-3Hz) to Block relay.
In fault-free condition, the following operational measured values are read out from the
protection device and evaluated (see Table 3-24).The operational measured values
can be found in the earth fault measured values (in DIGSI see tab "earth fault mea-
sured values").
Table 3-24
Operational Measured Values of the Rotor Earth Fault Protection
Measured Value
Comments
fgen = xx.x Hz
Shows the frequency of the injected square-wave voltage. The fre-
quency can be set by a jumper in the 7XT71. The default setting is
approx. 1.5 Hz (tolerance approx. ± 10 %).
Ugen = xx.x V
This measured value indicates the current amplitude of the injected
square-wave voltage. It amounts to approx. 50 V (tolerance of the
7XT71 can be up to ± 4 V).
Igen = X.xx mA
This measured value is nearly zero in fault-free condition. If a fault
resistor is installed between rotor and earth, the current to be ex-
pected can be estimated as follows:
R
Rtotal: Coupling resistance (20 kΩ + 720 Ω = 20.720 kΩ)
Qc = x.xxx mAs
This measured value indicates the charge which is a function of the
rotor earth capacitance. Half of the measured value must be set at
address 6106 as Qc <. If the capacitance is very small, it may be
necessary to deactivate the measuring circuit monitoring (setting val-
ue 0).
Rearth = xxx.x kΩ
This measured value indicates the rotor earth resistance. In fault-
free condition, the upper limit value 999.9 kΩ is displayed. If it is not,
there must be some additional capacitance in the excitation system.
The square-wave voltage frequency in the 7XT71 must be reduced
by a jumper. No polarity reversal may occur in the measuring current
Igen for at least 3 cycles. To visualize what is going on, a test fault
record (instantaneous values record) should be started and track
TD2, which models Igen, should be checked (see Figure 3-20).
Ugen
≈
------------------------------ -
Igen
R
+
Rtotal
E
: Fault resistor
E
7UM62 Manual
C53000-G1176-C149-3
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