ABB RELION REX640 Technical Manual page 944

Section 4
Protection functions
938
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
U
N
Z
=
N
S
N
GUID-C0AD6AFD-4D8B-4224-851B-CB2055CB1445 V1 EN
U 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-23686146-F889-453A-AF9B-360335720153 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.
1MRS759142 C
(Equation 232)
(Equation 233)
REX640
Technical Manual