ABB RELION REX640 - Page 977

X
R
Phase-to-Earth Phase-to-phase
X
R
(2·Z
1source
+ Z
0source
)/3 + Z
E
Z
1source
Z
1Line
(2·Z
1Line
+ Z
0Line
)/3
Figure 550: Tripping characteristic in case the zone characteristic is "Mho (circular)"
and Pol quantity zone is "Cross pol"
In case of positive-sequence polarization "Pos. seq. volt.", the polarization voltage
is the positive-sequence voltage. The voltage phase angle is rotated to match
the original fault loop voltage. The positive-sequence polarization expands the
tripping characteristic according to the prevailing system conditions. In case of
short circuit faults, the circle expands as a function of the source impedance
magnitude Z
1source
/2. In case of an earth fault, the circle expands as a function of
source impedance and earthing impedance magnitudes: (Z
1source
+ Z
0source
)/3 + Z
E
.
Phase-to-phase/three-phase
X
R
Z
1source
/2
Z
1Line
X
R
Phase-to-Earth
(Z
1source
+ Z
0source
)/3 + Z
E
(2·Z
1Line
+ Z
0Line
)/3
Figure 551: Tripping characteristic in case the zone characteristic is "Mho (circular)"
and Pol quantity zone is "Pos. seq. volt."
One alternative for polarization is also "Self pol" despite the drawbacks when using
it as a polarization method. The mho circle does not expand when using this
polarization method.
In case of cross and positive-sequence polarization, the expansion of the mho circle
is a positive feature as the fault resistance coverage is increased. The reach of the
zone is not affected by the expansion. The greatest expansion is obtained with
cross-polarization.
1MRS759142 F
Protection functions
REX640
Technical Manual
977