ABB RELION 670 Series Technical Manual page 433

1MRK 502 066-UUS B
The healthy impedance measured at non-faulted conditions is referred to as the reference
impedance in further text.
In IED the measured impedance is compared to the reference impedance. In case of an ground
fault, the fault impedance is estimated and compared to the set values
If the measured impedance is larger than the setting
TRUE. If OPCIRC is set, it means there is a strong likelihood that the generator neutral resistor is
not anymore connected to ground, since only the capacitive part in the circuit is left. The open
circuit is only applied on the stator winding protection. To make the open circuit limit more stable
a hysteresis is added. If OPCIRC is TRUE, the measured fault impedance must drop below open
circuit limit * (1 -
of open Circuit limit for the stator.
IEC11000073 V1 EN-US
Figure 230: Open circuit characteristics
Blocking: The output OPCIRC is blocked during an error occurring and during initialization of
function.
Detailed Set: If the total measured real part of the impedance is greater than the setting
OpenCircLim the output OPCIRC is TRUE, see figure 230.
7.14.8.5 Measuring reference impedance
The healthy impedance is equal to the parallel connection of the neutral point resistor (
capacitive reactance between the stator windings and ground (1/ω
magnetization impedance (
injection circuit is eliminated by
impedance is equal to the stator reference impedance:
1
=
Z
ref
EQUATION2502 V1 EN-US
Where
w p
=
2
EQUATION2503 V1 EN-US
Technical manual
OpenCircLim ) to reset. The hysteresis is hidden and set to a default value of 10%
Open- circuit
characteristics
0
Z
) and the ground fault resistance (
mT
1 1
w
+ +
j C
stat
R Z
N mT
×
f
inj
openCircuitLimit , the output OPCIRC is set
no open - circuit
k2 . R
is very large in the non-faulted case and the measured
f
RAlarm and RTrip .
open- circuit
Open Circuit
Hysteresis
{ } re
Z
Measured
openCircuitLimit
IEC11000073-1-en.vsd
GUID-CFBC35D7-F434-47DB-9832-532CDF7578B5 v3
C ) , the transformer
stat
R
). The series resistance in the
f
Section 7
Impedance protection
R ), the
N
427