ABB Relion 620 Series - Page 725

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
S
N
N
N
=
2
(Equation 142)
U
N
rated (phase-to-phase) voltage in kV
S
N
rated power of the protected machine in MVA
he corresponding calculation to convert ohms to pu values is:
X
X
Z
pu
ohm
N
=
(Equation 143)
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.
a) Z locus in under excitation for heavily loaded machine
b) Z locus in under excitation for lightly loaded machine
c) Z locus for a fault in the network
X (Reactance)
R (Resistance)
-X
Relay operation
characteristics
-R
a
b
c
Figure 377: Typical impedance locus in underexcitation: a) heavy load b) light load c)
fault in the network
1MRS757644 H
Protection functions
620 series
Technical Manual
725