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2.31
100-%-Stator Earth Fault Protection with 20 Hz Voltage Injection
(ANSI 64G - 100%)
The 100 % stator earth fault protection detects earth faults in the stator windings of generators which are con-
nected with the network via a unit transformer. This protection function, which works with a 20 Hz injected volt-
age, is independent of the network frequency displacement voltage appearing in earth faults, and detects earth
faults in all windings including the machine starpoint. The measuring principle used is not influenced at all by
the generator operating mode and allows measurements even with the generator at standstill. The two mea-
suring principles – measurement of the displacement voltage and evaluation of the measured quantities at an
injected 20 Hz voltage – allow to implement reliable protection concepts that complement one another.
If an earth fault in the generator starpoint or close to the starpoint is not detected, the generator is operated as
"earthed". A subsequent fault (e.g. a second earth fault) causes a single-pole short-circuit that may have an
extremely high fault current because the generator zero impedance is very small.
The 100 % stator earth fault protection is for this reason a basic function for large generators.
2.31.1
Functional Description
Basic Principle
The basic principle is shown in the following figure. An external low frequency alternating voltage source (20
Hz) 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 princi-
ple described here also detects earth faults at the generator terminals, including connected components such
as voltage transformers.
Figure 2-97
Circuit Design
To implement the above concept, some additional equipment is required. The following picture shows a 20 Hz
generator generates a square-wave voltage with an amplitude of approx. 25 V. This square-wave voltage is fed
via a bandpass into the loading resistor of the earthing or neutral transformer. The bandpass serves for round-
ing the square-wave voltage and for storing energy. The 20 Hz resistance of the bandpass is approx. 8 Ω. The
bandbass assumes also a protection function. If the load resistor carries the full displacement voltage during a
terminal-to-earth fault, the higher series resistance of the bandpass protects the 20 Hz generator from exces-
sive feedback currents.
The driving 20 Hz voltage is taken directly at the loading resistor using a voltage divider. In addition the flowing
20 Hz current is measured using a miniature CT. Both values (U
The voltage to be injected into the generator starpoint depends on the driving 20 Hz voltage (voltage divider:
load resistor and bandpass), and on the transformation ratio of the neutral or earthing transformer.
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
2.31 100-%-Stator Earth Fault Protection with 20 Hz Voltage Injection (ANSI 64G - 100%)
Basic Principle of Voltage Injection into the Generator Starpoint
and I
) are fed to the protection device.
SEF
SEF
Functions
223
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