High-impedance Protection with 7SJ80
With 7SJ80 the sensitive measuring input Ι
NS
or the measuring input Ι
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.
Das Figure 2-40 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.
[sj6x-ueb-einph-hochimpedanz3-141103, 1, en_US]
Figure 2-40 Connection diagram of the ground fault differential protection according to the high-impe-
dance principle
For protection against overvoltages it is also important that the device is directly connected to the grounded
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, the high-impedance differential protection can be used analo-
gously. All current transformers at the overvoltage side, the undervoltage side and the current transformer at
the neutral point have to be connected in parallel when using auto-transformers.
In principle, this procedure 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 ground leakage — even high-ohmic — between a phase
and the frame of a power transformer. The tank must be isolated from ground. A conductor links the tank to
ground, and the current through this conductor is fed to a current input of the relay. When tank leakage
occurs, a fault current (tank leakage current) will flow through the grounding conductor to ground. 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 input is used for tank leakage protection.
2.5.3
Functions
2.5 Single-Phase Overcurrent Protection
SIPROTEC 4, 7SJ80, Manual 115
E50417-G1140-C343-A8, Edition 12.2017
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