9.6.7 Operation principle
SEMOD155869-4 v6
The protection is a combination of the 95% fundamental frequency ground fault protection and
the 3
rd
harmonic based stator earth fault protection, (STEFPHIZ, 59THD). The 3
rd
harmonic based
100% stator ground fault protection is using the 3
rd
harmonic voltage generated by the generator
itself. To assure reliable function of the protection it is necessary that the 3
rd
harmonic voltage
generation is at least 0.8 V RMS on VT secondary side.
The 3
rd
harmonic voltage generated by the generator has the same phase angle in the three
phases. It has the characteristic of a zero sequence component. If the generator is connected to
the power system via a block transformer that cannot transform zero sequence voltages between
the voltage levels, the 3
rd
harmonic voltage, that is V
3N
and V
3T
in fig
355, in the generator system
is not influenced by the external power system. At normal operation the generator third harmonic
voltage characteristic can be described as in figure 355.
Note that angle between V
3N
and V
3T
is typically close to 180°.
en06000448_ansi.vsd
V
3N
V
3T
- DV
3
+
- V
3
+
- V
3N
+
+
V
3T,A
-
+
V
3T,B
-
+
V
3T,C
-
ANSI06000448 V1 EN-US
Figure 355:
Generator 3
rd
harmonic voltage characteristic at normal operation
The generator is modeled as parts of a winding where a 3
rd
harmonic voltage is induced along the
winding, represented by the end voltages V
3N
(voltage drop across resistor) and V
3T
in the figure.
Via the winding capacitances to ground and the neutral point resistor there will be a small 3
rd
harmonic current, giving the voltages V
3N
and V
3T
. The 3
rd
harmonic voltage in the generator
Section 9 1MRK 502 066-UUS B
Voltage protection
656
Technical manual
To Next Page
Loading...
