Functions
202
7UM62 Manual
C53000-G1176-C149-3
The rotor earth fault calculation calculates the complex earth impedance from the aux-
iliary AC voltage U
RE
and the current
I
RE
. The earth resistance R
E
of the excitation
circuit is then calculated from the earth impedance. The device also considers the cou-
pling capacitance of the coupling unit C
K
, the series (e.g. brush) resistance R
series
and
the capacitance to the earth excitation circuit C
E
. This method ensures that even rel-
atively high-ohmic earth faults (up to 30 k
Ω
under ideal conditions) can be detected.
In order to eliminate the influence of harmonics - such as occur in static excitation
equipment (thyristors or rotating rectifiers) - the measured quantities are filtered prior
to their evaluation.
The earth resistance supervision has two stages. Usually an alarm is initiated if the
earth resistance falls below an initial high-resistance stage (e.g. 5 k
Ω
to 10 k
Ω
). If the
value falls below the second low-resistance stage (e.g. 2 k
Ω
to 5 k
Ω
), tripping will be
initiated after a short time delay. The dropout threshold is defined for both stages as
125 % of the set value.
Note: The rotor earth fault protection uses for the detection of the voltage U
RE
the U
E
voltage input of the device. Therefore, the displacement voltage U
0
for the 90–% stator
earth fault protection (see Section 2.26) is in that case calculated from the phase-to-
earth voltages.
Measuring Circuit
Supervision
Since a current flows even during healthy operation, namely the capacitive charging
current of C
E
, the protection can recognize and alarm an interruption in the measure-
ment circuit, provided the capacitance to earth is at least 0.15
µ
F.
Stabilization of the
Resistance Mea-
surement
If the measuring current
I
RE
exceed an internal predetermined value (100 mA), a low-
ohmic earth fault (R
E
≈
0) is detected regardless of the calculated resistance. If this
current drops below the internal predetermined value of 0.3 mA, R
E
→ ∞
is detected
regardless of the calculated resistance.
Figure 2-96 shows the logic diagram of the rotor earth fault protection.
Figure 2-96
Logic Diagram of the Rotor Earth Fault Protection
10.00s
RE
FNr. 05383
FNr. 05400
OR
&
20V/25V (fixed)
FNr. 05398
FNr. 05397
FNr. 05399
I
RE
U
RE
&
FNr.05394
FNr. 05392
TMin
TRIP CMD
Tripping
matrix
