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
is not influenced by the external power system. At normal operation the generator third harmonic
voltage characteristic can be described as in figure
Note that angle between V
3N
and V
3T
is typically close to 180°.
en06000448_ansi.vsd
V
3N
V
3T
-
D
V
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
Содержание Relion 670 series
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