Siemens SIPROTEC 7UT613 series Manual page 199

High-Impedance
Protection with
7UT613/63x
7UT613/63x Manual
C53000-G1176-C160-2
Figure 2-87 Earth fault protection using the high-impedance principle
In case of an earth fault in the protection zone (figure2-87 right) a starpoint current I
will certainly be present. The earthing conditions in the rest of the network determine
how strong a zero sequence current from the system is. A secondary current which is
equal to the total fault current tries to pass through the resistor R. Since the latter is
high-ohmic, a high voltage emerges immediately. Therefore, the current transformers
get saturated. The RMS voltage across the resistor approximately corresponds to the
knee-point voltage of the current transformers.
Resistance R is dimensioned such that, even with the very lowest earth fault current
to be detected, it generates a secondary voltage, which is equal to half the saturation
voltage of current transformers (see also notes on "Dimensioning" in subsection
2.7.4).
With 7UT613/63x a high-sensitivity single-phase measuring input is used for high-im-
pedance protection. As this is a current input, the protection detects current through
the resistor instead of the voltage across resistor R.
Figure 2-88 shows the connection example. The 7UT613/63x is connected in series
to resistor R and measures its current.
Varistor V limits the voltage when internal faults occur. High voltage peaks emerging
with transformer saturation are cut by the varistor. At the same time, voltage is
smoothed without reduction of the mean value.
Figure 2-88 Connection scheme for restricted earth fault protection according to the high-im-
pedance principle
2.7 Single-Phase Time Overcurrent Protection
St
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