High-Impedance Protection with 7UT6x
With 7UT6x a high-sensitivity single-phase measuring input is used for high-impedance protection. As this is a
current input, the protection detects current through the resistor instead of the voltage across resistor R.
Das
shows the connection example. The 7UT6x is connected in series to resistor R and measures
its current.
Varistor V 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.
[ueb-einph-hochimpedanz3-020926-st, 1, en_GB]
Figure 2-90
Connection scheme for restricted earth fault protection according to the high-impedance prin-
ciple
For protection against overvoltages it is also important that the device is directly connected to the earthed side
of the current transformers so that the high voltage at the resistor can be kept away from the device.
For generators, motors and shunt reactors high-impedance differential protection can be used analogously. All
current transformers at the overvoltage side, the undervoltage side and the current transformer at the star-
point have to be connected in parallel when using auto-transformers.
In principle, this scheme can be applied to every protected object. When applied as busbar protection, for
example, the device is connected to the parallel connection of all feeder current transformers via the resistor.
Tank Leakage Protection
Application Example
The tank leakage protection has the task to detect earth leakage — even high-ohmic — between a phase and
the frame of a power transformer. The tank must be isolated from earth. A conductor links the tank to earth,
and the current through this conductor is fed to a current input of the relay. When a tank leakage occurs, a
fault current (tank leakage current) will flow through the earthing conductor to earth. This tank leakage
current is detected by the single-phase overcurrent protection as an overcurrent; an instantaneous or delayed
trip command is issued in order to disconnect all sides of the transformer.
A high-sensitivity single-phase current measuring input is normally used for tank leakage protection.
2.7.3
Functions
2.7 Single-Phase Time Overcurrent Protection
180
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
Summary of Contents for SIPROTEC 4 7UT6 Series
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