ABB RELION REX640 Technical Manual page 976

Protection functions
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.
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
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 922: Polarization methods and corresponding polarization voltages
Fault loop
Z
A
Z
B
Z
C
Z
AB
Z
BC
Z
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
source impedance Z
Z
1source
976
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.
Pol quantity zone = "Cross pol", "Pos. seq. volt." or "Self pol".
Self pol
U
L1
U
L2
U
L3
U
L12
U
L23
U
L31
. In case of an earth fault, the circle expands as a function of the
1source
, Z
1source 0source
+ Z )/3 + Z .
0source E
Cross pol
j·U /√3
L23
j·U /√3
L31
j·U /√3
L23
j·(U -U )/√3
L23 L31
j·(U -U )/√3
L31 L12
j·(U -U )/√3
L12 L23
U
1
and earthing impedance magnitudes Z
1MRS759142 F
Pos. seq. volt.
U
1
U ·1∠-120°
1
U ·1∠120°
1
U ·√3·1∠30°
1
U ·√3·1∠-90°
1
U ·√3·1∠150°
1
as (2 ⋅
E
REX640
Technical Manual