ABB RELION 670 Series Technical Manual page 411

1MRK 502 066-UUS B
The healthy impedance measured at non-faulted conditions during the calibration is referred to as
the reference impedance in further text.
In IED the measured impedance is compared to the reference impedance in order to evaluate the
fault impedance.
1
Z
measured
1
=
Z Z
f
IECEQUATION16085 V1 EN-US
The real part gives the fault conductance.
1
=
Re
R
f
IECEQUATION16086 V1 EN-US
In case of an earth fault, the fault resistance is estimated and compared to the set values
RTrip . The values of RAlarm and RTrip are given in ohm.
and
An alarm signal ALARM is given after a set delay
A initiate signal BFI is given if
See figure
Technical manual
1 1
= +
Z Z
ref f
1 1
-
Z
measured ref
æ ö
1 1
-
ç ÷
ç ÷
Z Z
è ø
measured ref
R
<
f
220
tAlarm if R
RTrip
RAlarm
<
f
Section 7
Impedance protection
RAlarm
405