Functions
2.4 Single-Phase Overcurrent Protection
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
86
Figure 2-22 Principle of ground fault protection according to the high-impedance principle
When a ground fault occurs in the protected zone (Fig. 2-22 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 2.4.4).
High-impedance Protection with 7SJ61
With 7SJ61 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 2-23 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 satura-
tion are cut by the varistor. At the same time, voltage is smoothed without reduction of the mean value.
Figure 2-23 Connection diagram of the ground fault differential protection according to the
high-impedance principle
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