Functions
2.5 Single-Phase Overcurrent Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
133
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 high-impedance protection can be used analogously. All current
transformers at the overvoltage side, the undervoltage side and the current transformer at the starpoint 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.
2.5.3
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.
Figure 2-42
Principle of tank-leakage protection
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