Functions
2.5 Single-Phase Overcurrent Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
132
Figure 2-40 Principle of ground fault protection according to the high-impedance principle
When a ground fault occurs in the protected zone (Fig. 2-40 right), there is always a starpoint current I
SP
. The
grounding 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-resistive, 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 trans-
formers.
Resistance R is dimensioned such that, even with the very lowest ground fault current to be detected, it gen-
erates a secondary voltage, which is equal to half the saturation voltage of current transformers (see also notes
on "Dimensioning" in Subsection 2.5.4).
High-impedance Protection with 7SJ62/64
With 7SJ62/64 the sensitive measurement input I
Ns
or alternatively the insensitive measurement input I
N
is
used for high-impedance protection. As this is a current input, the protection detects current through the resistor
instead of the voltage across the resistor R.
Figure 2-41 illustrates the connection scheme. The protection device is connected in series to resistor R and
measures its current.
Varistor B 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 2-41 Connection diagram of the ground fault differential protection according to the
high-impedance principle
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