ABB REG650 Technical Manual page 256

Section 8
Voltage protection
250
- U +
3N
U
3N
IEC06000448 V2 EN
Figure 127:
Generator 3
The generator is modeled as parts of a winding where a 3
induced along the winding, represented by the end voltages U
figure. Via the winding capacitances to earth and the neutral point resistor there
rd
will be a small 3 harmonic current flow, giving the voltages U
easily be seen that the 3
will be close to zero in case of a stator earth-fault close to the neutral. This fact
alone can be used as an indication of stator earth-fault. To enable better sensitivity
and stability also measurement of the generator's 3
used. In addition to the decrease of U
under the stator earth-fault close to the generator neutral point. Therefore the 3
harmonic voltage U , (which is a zero sequence voltage) is used by the
3T
rd
protection. In the 3 harmonic voltage differential protection algorithm
equation 85 is used:
- U
3
- DU
+
3
rd
harmonic voltage characteristic at normal operation
rd
harmonic voltage in the generator neutral point (U
the generator voltage U
3N
1MRK 502 034-UEN -
+
en06000448.vsd
rd
harmonic voltage is
and U in the
3N 3T
and U
3N 3T
rd
harmonic voltage U
3T
will increase
3T
Technical Manual
+
U
3T,L1
-
+
U
3T,L2
-
+
U
3T,L3
-
U
3T
. It can
)
3N
is also
rd