ABB RELION REX640 - Page 1022

Table 948: Parameters of the circle
Setting
values
Description
Offset Distance of the top of the circle from the R-axis. This is usually set equal to
- x
d
’/2, where x
d
’ is the transient reactance of the machine. The sign of the
setting value determines the top of the circle regarding the R-axis. If the sign
is negative, the circle lies below the R-axis.
Diameter Normally set equal to the machine's synchronous reactance x
d
, which deter-
mines the size of the impedance circle.
Displace-
ment
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.
Rst offset X 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 Rst offset X angle .
If the sign of the Rst offset X setting is positive, the restraint line passes
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.
Rst offset X
angle
Angle between the R-axis (or Rst offset X line) and the directional restraint
line. If the sign of the OffsetXAng setting is positive, the angle is taken in
anti-clockwise direction from the R-axis (or Rst offset X line). Respectively, if
the sign is negative, the angle is taken in clockwise direction from R-axis (or
Rst offset X line).
The setting parameters of the off-set mho circle are to be given in pu values. The
base impedance (Z
N
) in ohms is:
Z
U
N
N
N
=
2
S
(Equation 276)
U
N
rated (phase-to-phase) voltage in kV
S
N
rated power of the protected machine in MVA
The corresponding calculation to convert ohms to pu values is:
X
X
Z
pu
ohm
N
=
(Equation 277)
X
pu
pu value
X
ohm
reactance in ohms
Z
N
base impedance
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.
Protection functions
1MRS759142 F
1022 REX640
Technical Manual