ABB RELION REX640 Technical Manual page 1022

Protection functions
Table 948: Parameters of the circle
Setting
values
Offset
Diameter
Displace-
ment
Rst offset X
Rst offset X
angle
The setting parameters of the off-set mho circle are to be given in pu values. The
base impedance (Z
Z
N
U
N
S
N
The corresponding calculation to convert ohms to pu values is:
X
pu
X
pu
X
ohm
Z
N
Example of impedance locus in underexcitation
In an example of a typical impedance locus, once the impedance locus enters the
relay operation characteristics, the relay operates after a settable definite time.
1022
Description
Distance of the top of the circle from the R-axis. This is usually set equal to
- x '/2, where x
d
setting value determines the top of the circle regarding the R-axis. If the sign
is negative, the circle lies below the R-axis.
Normally set equal to the machine's synchronous reactance x
mines the size of the impedance circle.
Displacement of the center of the circle from the reactance axis or the R-coor-
dinate of the center. The setting can be used to adjust the sensitivity of the
underexcitation protection. If the sign of the setting is positive, the circle is
shifted to the right, that is, closer to the normal operating point. Respectively,
if the sign is negative, the circle is shifted to the left and thus moves away
from the normal operating point.
Displacement from origin to a point on reactance axis (X-axis) through which
the directional restraint line passes. The setting can be used to adjust the
restrain area of the underexcitation protection along with
If the sign of the
through a point on X-axis which is above the R-axis and if it is negative, it
passes through a point on X-axis which is below the R-axis.
Angle between the R-axis (or
line. If the sign of the
anti-clockwise direction from the R-axis (or
the sign is negative, the angle is taken in clockwise direction from R-axis (or
Rst offset X line).
) in ohms is:
N
2
U
N
S
N
rated (phase-to-phase) voltage in kV
rated power of the protected machine in MVA
X
ohm
=
Z
N
pu value
reactance in ohms
base impedance
' is the transient reactance of the machine. The sign of the
d
Rst offset X setting is positive, the restraint line passes
Rst offset X line) and the directional restraint
OffsetXAng setting is positive, the angle is taken in
1MRS759142 F
, which deter-
d
Rst offset X angle .
Rst offset X line). Respectively, if
(Equation 276)
(Equation 277)
Technical Manual
REX640