ABB RELION 620 Series Technical Manual page 502

Section 4
Protection functions
496
Defining mho (circular) zone characteristics
The directionality of a particular zone is defined with the Directional mode Znx (x = 1...
5) setting. When selected "Forward", the zone trips on faults in forward direction and
respectively "Reverse" means the zone trips on faults in reverse direction.
The reach of the zone is defined by the settings Z1 zone x (x = 1...5) and Z1 angle zone x
(x = 1...5) respectively.
Directional mode Znx (x = 1...5) = Forward
X
Z
1
reach zone x
Z
1
angle zone x
R
GUID-4CD15EF5-0493-4445-B174-1CBA166917F4 V1 EN
Figure 235: Line reach for phase-to-phase or three-phase impedance measuring
elements
The actual resistive reach of the mho characteristic depends on the selected polarization
method defined by the Pol quantity setting, which can be set to "Cross Pol" or "Pos. seq.
volt.". The "Cross Pol" or "Pos. seq. volt." polarizations provide a variable mho feature
insuring protection near the origin of the RX-diagram.
Table 363: Polarization methods and corresponding polarization voltages
Fault loop
ZAB
ZBC
ZCA
ZABC
1) Regardless of the selected polarization method, the three-phase fault measuring element always uses
positive-sequence voltage for polarization. Memory voltage is included in the polarization voltage to secure
directionality if the positive-sequence voltage becomes too small.
In the case of cross-polarization, the polarization voltage is in the quadrature to the fault
loop voltage. The phase angle of polarization voltage is rotated 90 degrees to match the
original fault loop voltage. The cross-polarization expands the tripping characteristic
according to the prevailing system conditions, that is, in case of short circuit faults, the
circle expands as a function of positive-sequence source impedance magnitude Z
This provides a variable mho feature insuring protection near the origin of the RX-
diagram.
Directional mode Znx (x = 1...5) = Reverse
Z
1
R
angle zone x
Z
1
reach zone x
Cross Pol
j·(VBC - VCA)/√3
j·(VCA - VAB)/√3
j·(VAB - VBC)/√3
1MAC504801-IB E
X
Pos. seq. volt.
V1·√3·1∠30°
V1·√3·1∠-90°
V1·√3·1∠150°
1)
V1
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