ABB RELION REX640 Technical Manual page 904

Section 4
Protection functions
898
impedance element is frozen after a fixed 100 ms of fictive voltage use, but the
memory function enables impedance polarization to operate up to a maximum of 300
s. This time can be set with the Voltage Mem time setting. After set Voltage Mem time
elapses, fictive voltage (and frozen impedance direction) can no more be used without
memory refreshment by a healthy positive-sequence voltage and also the frozen
impedance direction is reset.
Switch onto fault function (CVPSOF) should be used together with
distance protection. CVPSOF accelerates and secures the operation of
the protection if a close-in three-phase fault is detected immediately
after circuit breaker closing.
Mho circular characteristic
When the zone characteristic is "Mho (circular)", the distance protection has a circular
tripping characteristic. The zone reach is defined independently by the line reach for
phase-to-earth and phase-to-phase or three-phase elements. The resistive reach of the
tripping characteristic depends on the selected polarization method defined with
setting Pol quantity zone = "Cross pol", "Pos. seq. volt." or "Self pol". The non-
directional mho characteristic is an exception as it is independent of the selected
polarization method. It is always explicitly defined by the reach settings and thus fixed
in the impedance plane.
Table 844: Polarization methods and corresponding polarization voltages
Fault loop Self pol
Z U
A
Z U
B
Z U
C
Z U
AB
Z U
BC
Z U
CA
Z
ABC
In case of cross-polarization "Cross pol", the polarization voltages are theoretically
quadrature to the fault loop voltage. The phase angle of the polarization voltage is
rotated 90° to match the original fault loop voltage. The cross-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 . In case of an earth fault, the circle expands as a function of the
1source
source impedance Z
1source
Z + Z )/3 + Z
1source 0source
Cross pol
j·U
L1 L23
j·U
L2 L31
j·U
L3 L23
j·(U
L12 L23
j·(U
L23 L31
j·(U
L31 L12
, Z and earthing impedance magnitudes Z
0source
.
E
1MRS759142 C
Pos. seq. volt.
/√3 U
1
/√3 U ·1∠-120°
1
/√3 U ·1∠120°
1
- U )/√3 U ·√3·1∠30°
L31 1
- U )/√3 U ·√3·1∠-90°
L12 1
- U )/√3 U ·√3·1∠150°
L23 1
U
1
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