ABB Relion 670 series - Page 433

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 RAlarm and RTrip .
If the measured impedance is larger than the setting openCircuitLimit , the output OPCIRC is set
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 -
OpenCircLim ) to reset. The hysteresis is hidden and set to a default value of 10%
of open Circuit limit for the stator.
openCircuitLimit
Open Circuit
Hysteresis
0
no open- circuit open- circuit
Open- circuit
characteristics
IEC11000073-1-en.vsd
Z
Measured
{ }
re
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
GUID-CFBC35D7-F434-47DB-9832-532CDF7578B5 v3
The healthy impedance is equal to the parallel connection of the neutral point resistor ( R
N
), the
capacitive reactance between the stator windings and ground (1/ω C
stat
) , the transformer
magnetization impedance (
Z
mT
) and the ground fault resistance ( R
f
). The series resistance in the
injection circuit is eliminated by k2 . R
f
is very large in the non-faulted case and the measured
impedance is equal to the stator reference impedance:
1 1 1
stat
ref N mT
j C
Z R Z
w = + +
EQUATION2502 V1 EN-US
Where
2
inj
f
w p = ×
EQUATION2503 V1 EN-US
1MRK 502 066-UUS B Section 7
Impedance protection
427
Technical manual