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2.32
Sensitive Earth Fault Protection B (ANSI 51GN)
The IEE-B sensitive earth current protection feature of 7UM62 provides greater flexibility and can be used for
the following applications.
Applications
• Earth current monitoring to detect earth faults (generator stator, terminal lead, transformer).
• 3rd harmonics earth current measurement for detection of earth faults near the generator star point. The
connection is accomplished in the secondary circuit of the neutral transformer.
• Protection against load resistances by means of single-phase current monitoring.
• Shaft current protection in order to detect shaft currents of the generator shaft and prevent that bearings take
damage. The function is mainly used for hydro-electric generators.
2.32.1
Functional Description
General
The sensitive earth current protection IEE-B uses either the hardware input I
signed in a way that allows them to cut off currents greater than 1.6 A (thermal limit, see technical data). This
has to be considered for the applications or for the selection of the current transformers.
Application as Shaft Current Protection (Bearing Current Protection)
As most of the applications mentioned above are rather simple, we will focus on the function's application as
shaft current protection. This function is of particular interest in conjunction with hydro-electric generators. Due
to their construction, the hydro-electric generators have relatively long shafts. The shaft is connected to earth
at one point via the turbine wheel and water. In cylindrical-rotor generators the shaft is earthed at one point via
the earthing brush. A number of factors such as friction, magnetic fields of the generators and others can build
up a voltage across the shaft which then acts as voltage source (electro-motive force - emf). This voltage also
contains harmonics with the 3rd harmonic being stronger. This induced voltage is still dependent on the load,
the system and the machine. The induced voltage can range between 0.5 and 2 V in cylindrical-rotor genera-
tors and between 10 and 30 V in hydro-electric generators. Detection is only possible during operation.
If the oil film covering a bearing is too thin, breakdown can occur. Since the bearing housing is earthed, the
electric circuit is now closed. Due to the low resistance (shaft, bearing and earthing), high currents may flow
that destroy the bearing. Past experience has shown that currents greater than 1 A are critical for the bearings.
As different bearings can be affected, the current is not measured at each bearing, but the current entering the
shaft is detected by means of a special transformer. It is a folding transformer that is mounted around the shaft.
The basic connection of the shaft current protection is shown in figure 2-101. The shaft current transformer is
then connected to the selected sensitive earth current input (I
value, the generator should be shut down.
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
2.32 Sensitive Earth Fault Protection B (ANSI 51GN)
or I
. These inputs are de-
ee1
ee2
or I
). If the shaft current exceeds a certain
ee1
ee2
Functions
231
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