with various generator system earthing methods. It is therefore recommended to make
the injection via the open delta VT on the terminal side in most applications.
Accuracy for STTIPHIZ is installation dependent and it mainly depends on the
characteristic of grounding or voltage transformer used to inject signal into the stator.
Note that large variation of the ambient temperature and variation of stator
capacitance and conductance to ground between standstill and fully loaded machine
will also limit the possible setting level for the alarm stage. As a consequence 10 kΩ
sensitivity can be typically reached without problem. Depending on particular
installation alarm sensitivity of up to 50 kΩ may be reached at steady state operating
condition of the machine.
Note that it is possible to connect two REG670 in parallel to the REX060 injection unit
in order to obtain redundant measurement in two separate IEDs. However, at
commissioning both REG670 IEDs must be connected during calibration procedure.
7.9.9.4
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
Inj
Inj
bare
U
Z
I
=
EQUATION2500 V1 EN
An equivalent circuit for the measured impedance is shown in figure
Section 7
1MRK502052-UEN B
Impedance protection
310
Technical manual
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