Siemens siprotec SJ62 User Manual page 128

High-impedance
Protection with
7SJ62/63/64
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
Figure 2-38 Principle of ground fault protection according to the high-impedance principle
When a ground fault occurs in the protected zone (Figure 2-38 right), there is always
a starpoint current I . The grounding conditions in the rest of the network determine
SP
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 transform-
ers 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 ground fault current
to be detected, it generates a secondary voltage which is equal to the half knee-point
voltage of current transformers (see also notes on dimensioning in Section 2.5.4).
With 7SJ62/63/64 the sensitive measuring input I
measuring input I is used for high-impedance protection. As this is a current input,
N
the protection detects current through the resistor instead of the voltage across the re-
sistor R.
Figure 2-39 shows the connections diagram. The protection relay 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 2-39 Connection diagram of the ground fault differential protection according to the
high-impedance principle
2.5 Single-Phase Overcurrent Protection
or alternatively the insensitive
NS
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