ABB Relion 670 series, Technical Manual

Page: 411 / 1432

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 1 1
1 1 1
measured ref f
f measured ref
Z Z Z
Z Z Z
= +
= −
IECEQUATION16085 V1 EN-US
The real part gives the fault conductance.
1 1 1
f measured ref
Re
R Z Z
 
= −
 
 
 
IECEQUATION16086 V1 EN-US
In case of an earth fault, the fault resistance is estimated and compared to the set values
RAlarm
and
RTrip. The values of RAlarm and RTrip are given in ohm.
An alarm signal ALARM is given after a set delay tAlarm if R
f
< RAlarm
A initiate signal BFI is given if R
f
< RTrip
See figure 220
1MRK 502 066-UUS B Section 7
Impedance protection
405
Technical manual