Application; Function Description - Siemens 7XT3300-0 Serie Handbuch

7XT3300-0*A00

Application

The 20-Hz generator 7XT33 is used in combination with the bandpass 7XT34 in the machine
protection (7UM62/7UM85). In order to implement a 100 % stator earth fault protection, it
provides the source for injection into the generator starpoint. Injection can be performed by
connecting the 7XT33 and the 7XT34 to the load resistor of an earthing or neutral
transformer. The 100 % stator earth fault protection detects earth faults in the stator windings
of generators which are connected with the network via a unit transformer. The protection
device working with the 7XT33 detects earth faults in all windings including the machine
starpoint. The measuring principle used is not influenced at all by the generator's operation
and allows measurements even with the generator at standstill.

Function Description

Basic Principle
The 7XT33 injects into the generator starpoint a voltage of max. 1 % of the rated generator
voltage. If an earth fault occurs in the generator starpoint, the 20 Hz voltage drives a current
through the fault resistance. From the driving voltage and the fault current, the protective
relay determines the fault resistance. The protection principle described here also detects
earth faults at the generator terminals, including connected components such as voltage
transformers.
Figure 1 Basic principle of voltage injection into the generator starpoint
C53000-B1174-C129-8
NOTE
The 7XT33 is always used in combination with the 7XT34.
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