Functions
2.31 100-%-Stator Earth Fault Protection with 20 Hz Voltage Injection (ANSI 64G - 100%)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
224
To prevent the secondary load resistance from becoming too small (it should be greater than 0.5 Ω where pos-
sible), a high secondary rated voltage should be chosen for the earthing or neutral transformer. 500 V has
proven to be a practical value.
Figure 2-98 Circuit Design of the 100-% Stator Earth Fault Protection with Earthing Transformer or Neutral Transformer
R Loading resistance
U
SEF
Displacement voltage at the protective relay
I
SEF
Current measured at the protective relay
The same measuring principle can also be used with a primary loading resistor. The 20 Hz voltage is connected
in this case via a voltage transformer, and the starpoint current is directly measured. The setting notes (section
2.31.2) contain the connection scheme and information on the circuit design.
Measurement method
From the two measured quantities U
SEF
and I
SEF
in the above picture, the 20 Hz current and voltage vectors
are calculated, and from the resulting complex impedance the ohmic fault resistance is determined. This
method eliminates disturbances caused by stator earth capacitance and ensures a high sensitivity. The mea-
suring accuracy is further increased by using mean current and voltage values obtained over several cycles for
calculating the resistance.
The model takes into account a transfer resistance R
PS
that may be present at the neutral, earthing or voltage
transformer. Other error factors are taken into account in the angle error.
In addition to determination of the earth resistance, an earth current stage is provided which processes the
current rms value and thus takes into account all frequency components. It is used as a backup stage and
covers approx. 80 to 90 % of the protection zone.
A monitoring circuit checks the coupled fed in 20 Hz voltage and the 20 Hz current and detects by evaluating
them any failure of the 20 Hz generator or of the 20 Hz connection. In such a case resistance determination is
blocked. The earth current stage remains active.
To Next Page
Loading...
Need help?
General
