High-Impedance Ground Fault Unit Protection - Siemens SIPROTEC 4 7SJ61 Manual

Functions
2.4 Single-Phase Overcurrent Protection
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Figure 2-22

High-impedance Ground Fault Unit Protection

2.4.2
Application Examples
The high impedance protection concept is based on measuring the voltage across the paralleled CT's to a
common high-resistive resistor.
The CTs must be of the same design and feature at least a separate core for high-impedance protection. In
particular, they must have the same transformer ratios and approximately identical knee-point voltage.
With 7SJ61, the high-impedance principle is particularly well suited for detecting ground faults in grounded
networks at transformers, generators, motors and shunt reactors.
Figure 2-23 shows an application example for a grounded transformer winding or a grounded motor/gener-
ator. The right-hand example depicts an ungrounded transformer winding or an ungrounded motor/generator
where the grounding of the system is assumed somewhere else.
78
Logic diagram of the single-phase time overcurrent protection
SIPROTEC 4, 7SJ61, Manual
C53000-G1140-C210-6, Edition 05.2016