100–%–Stator Earth Fault Protection with 3rd Harmonics (ANSI 27/59TN 3rd Harm.)
189
7UM62 Manual
C53000-G1176-C149-3
2.28
100–%–Stator Earth Fault Protection with 3rd Harmonics
(ANSI 27/59TN 3rd Harm.)
2.28.1 Functional Description
General
As described in section 2.26, 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-frequent voltage must be used to implement a 100 %
protection range. With the 7UM62 device, the 3rd harmonic is used for this purpose.
The 3rd harmonic is created in each machine in a more or less significant way. It is
provoked by the shape of the poles. If an earth fault occurs in the generator stator
winding, the division ratio of the parasitic capacitances changes, as one of the
capacitances was short-circuited by the earth fault. During this procedure, the 3rd
harmonic measured in the starpoint decreases, whereas the 3rd harmonic measured
at the generator terminals increases (see figure 2-90). The 3rd harmonic forms a zero
phase-sequence system and can thus also be determined by means of the voltage
transformer switched in star/delta or by calculating the zero phase-sequence system
from the phase-earth-voltages.
Figure 2-90
Profile of the 3rd Harmonic along the Stator Winding
Moreover, the level of the 3rd harmonic depends on the operating point of the
generator, i.e. a function of the P active power and the Q reactive power. For this
reason, the working area of the stator earth fault protection is restricted in order to
enhance security.
In case of a busbar connection, all machines contribute to the 3rd harmonic, which
impedes the separation of the individual machines.
Measuring
Principle
The level of the 3rd harmonic in the measuring quantity is the pickup criterion. The 3rd
harmonic is determined from the displacement voltage measured during two cycles by
means of digital filtering.
Different measuring procedures are applied, depending on whether the displacement
voltage is detected or not (project configuration parameter
Normal operation
Earth fault close to starpoint
3. Harmonics
Terminals
Starpoint
L
1
L
2
L
3
