ABB Relion, Техническое руководство

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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
Figure 144:
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 neutral point (V
3N
) will be close to zero in case of a
stator ground-fault close to the neutral. This fact alone can be used as an indication of
stator ground-fault. To enable better sensitivity and stability also measurement of the
generator's 3
rd
harmonic voltage V
3T
is also used. In addition to the decrease of V
3N
the
generator voltage V
3T
will increase under the stator ground-fault close to the generator
neutral point. Therefore the 3
rd
harmonic voltage V
3T
, (which is a zero sequence voltage)
is used by the protection. In the 3
rd
harmonic voltage differential protection algorithm
equation 94 is used:
Section 9
1MRK 502 048-UUS A
Voltage protection
314
Technical manual