2 Functions
140
7UM61 Manual
C53000-G1176-C127-3
2.23 100-%-Stator Earth Fault Protection with 3rd Harmonics (ANSI
27/59TN 3rd Harm.)
As described in Section 2.21, the measuring procedure based on the fundamental
wave of the displacement voltage serves to protect maximally 90 % to 95 % of the
stator winding. A non-line frequency voltage must be used to implement a 100 % pro-
tection range. With the 7UM61 device, the 3rd harmonic is used for this purpose.
2.23.1 Functional Description
Mode of Operation
The 3rd harmonic emerges in each machine in a more or less significant way. It is
caused by the shape of the poles. If an earth fault occurs in the generator stator wind-
ing, the division ratio of the parasitic capacitances changes, since one of the capaci-
tances is short-circuited by the earth fault. During this procedure, the 3rd harmonic
measured in the starpoint decreases, whereas the 3rd harmonic measured at the gen-
erator terminals increases (see the following figure). The 3rd harmonic forms a zero
phase-sequence system and can thus also be determined by means of the voltage
transformer switched in wye/delta or by calculating the zero phase-sequence system
from the phase-earth-voltages.
Figure 2-49
Profile of the 3rd Harmonic along the Stator Winding
Moreover, the extent of the 3rd harmonic depends on the operating point of the gen-
erator, i.e. a function of active power P and reactive power Q. For this reason, the
working range of the stator earth fault protection is restricted in order to increase se-
curity.
With busbar connection all machines contribute to the 3rd harmonic, which impedes
separation of the individual machines.
Measuring Princi-
ple
The content of the 3rd harmonic in the measurement value is the pickup criterion. The
3rd harmonic is determined from the displacement voltage measured over two cycles
by means of digital filtering.
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