severe magnetic imbalance and heavy rotor vibrations that soon lead to a severe
damage.
Therefore, it is essential that any occurrence of an insulation failure is detected
and that the machine is disconnected as soon as possible. Normally, the device is
tripped after a short time delay.
A 50/60 Hz voltage is injected via the injection device REK 510 to the rotor field
winding circuit of the synchronous machine as shown in
voltage is 100 V AC via the coupling capacitors. A coupling capacitor prevents a DC
current leakage through the injection device.
Figure 232: Principle of the rotor earth-fault protection with the current injection
device
The auxiliary AC voltage forms a small charging current I
1
to flow via the coupling
capacitors, resistances of the brushes and the leakage capacitance between the
field circuit and earth. The field-to-earth capacitance C
E
affects the level of the
resulting current to an extent which is a few milliamperes during normal no-fault
operating conditions.
If an earth fault arises in the rotor field circuit, this current increases and can
reach a level of 130 mA at a fully developed earth fault (fault resistance R
E
= 0,
one coupling capacitor C
1
= 2μF is used). The integrated current transformer of
the injection device REK 510 then amplifies this current with the ratio of 1:10 to a
measurable level. MREFPTOC is used to measure this current.
An example of the measured curves with various field-to-earth leakage capacitance
values is given in
.
It is recommended that the alarm and operation stages of MREFPTOC are both
used. The alarm stage for giving an indication for weakly developed earth faults
with a start value setting corresponds to a 10 kΩ fault resistance with a 10-second
delay. The operation stage for a protection against fully developed earth faults with
a start value setting corresponds to a 1...2 kΩ fault resistance with a 0.5-second
delay.
The current setting values corresponding to the required operating fault
resistances can be tested by connecting an adjustable faultsimulating
resistor between the excitation winding poles and the earth. Whether
only one of the coupling capacitors or both should be used in a parallel
connection should be determined on a case-by-case basis, taking into
consideration the consequences of a possibly excessive current at a
direct earth fault.
1MRS757644 H
Protection functions
620 series
Technical Manual
457
