ABB Relion REG670, Technical Manual

Page: 317 / 1298

+
U
inj
-
I
inj
Z
shunt
R
f
Z
series
IEC11000003-2-en.vsd
Z
Bare
Z
Measured
IEC11000003 V1 EN
Figure 140: Equivalent of the impedance measurement
In non-faulted operation R
f
is very large. A “healthy” impedance is calculated as:
1 2 Measured bare
Z
k Z k
= × +
EQUATION2501 V1 EN
For definition of k1 and k2 , see figure 129
The factors k1 and k2 [Ω] are derived from measurements during commissioning,
where calibration to known fault resistance will be used to convert the measurement
to true primary impedance. The factor
k1 will compensate for transformer ratio and
other factors to achieve impedance values related to the primary system. The factor k2
[Ω] will compensate for the series impedance Z
series
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
earth 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.
1MRK502052-UEN B
Section 7
Impedance protection
311
Technical manual