ABB REG670 - General Measurement of Earth Fault Impedance

The injection unit REX060 is connected to the generator and to IED as shown in figure
125.
7.8.9.3 General measurement of earth fault impedance
From the REX060 the injected voltage and current are delivered as AC voltages to
IED. The injected voltage and current is measured and analyzed in the protection
function software within IED. The measured injected voltage and current are first
processed by means of special filtering so that the signals are referred to the injected
frequency only.
The bare complex impedance is, see fig
128:
Inj
Inj
bare
U
Z
I
=
EQUATION2500 V1 EN
An equivalent circuit for the measured impedance is shown in figure 128.
+
U
inj
-
I
inj
Z
shunt
R
f
Zseries
IEC11000003-2-en.vsd
Z
Bare
Z
Measured
IEC11000003 V1 EN
Figure 128: 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
Section 7 1MRK502052-UEN B
Impedance protection
288
Technical manual