High-Impedance Differential Protection - Siemens SIPROTEC 4 7UT6 Series Manual

Functions
2.7 Single-Phase Time Overcurrent Protection
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Figure 2-87

High-impedance Differential Protection

2.7.2
Application Example
With the high-impedance scheme all current transformers at the limits of the protection zone operate parallel
to a common relatively high-ohmic resistance R whose voltage is measured. With 7UT6x the voltage is regis-
tered by measuring the current through the external resistor R at the high-sensitivity single-phase current
measuring input.
The current transformers have to be of equal design and provide at least a separate core for high-impedance
differential protection. They also must have the same transformation ratio and approximately the same knee-
point voltage.
With 7UT6x, the high-impedance principle is very well suited for detection of earth faults in transformers,
generators, motors and shunt reactors in earthed systems. High-impedance differential protection can be used
instead of or in addition to the restricted earth fault protection (refer also to Section
Protection).
Figure 2-88 shows an application example for an earthed transformer winding or an earthed motor/generator.
The example on the right side shows a non-earthed transformer winding or an non-earthed motor/generator
where the earthing of the system is assumed to be somewhere else.
178
Logic diagram of the single-phase overcurrent protection — example for detection of the
current at input Ι
8
2.3 Restricted Earth Fault
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016