ABB 615 series Technical Manual page 708

Section 4
Protection functions
702
Table 605: 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
where x
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
size of the impedance circle.
Displacement Displacement of the center of the circle from the reactance axis or the R-coordinate 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.
The setting parameters of the off-set mho circle are to be given in pu values. The base
impedance (Z ) in ohms is:
N
2
V
N
Z
=
N
S
N
GUID-FBC519FF-D196-40CE-BF5C-9826D44E2569 V1 EN
V rated (phase-to-phase) voltage in kV
N
S rated power of the protected machine in MVA
N
The corresponding calculation to convert ohms to pu values is:
X
ohm
X
=
pu
Z
N
GUID-8F02750E-98A4-4AD9-82C6-0632CCE84F01 V1 EN
X pu value
pu
X reactance in ohms
ohm
Z base impedance
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.
' is the transient reactance of the machine. The sign of the setting value
d
1MAC059074-MB A
'/2,
d
, which determines the
d
(Equation 131)
(Equation 132)
615 series ANSI
Technical Manual