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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
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 determines the
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
N
) in ohms is:
Z
V
S
N
N
N
=
2
GUID-FBC519FF-D196-40CE-BF5C-9826D44E2569 V1 EN
(Equation 131)
V
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
=
GUID-8F02750E-98A4-4AD9-82C6-0632CCE84F01 V1 EN
(Equation 132)
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.
Section 4
1MAC059074-MB A
Protection functions
702
615 series ANSI
Technical Manual
Содержание Relion 615 series
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