9.5 100% Stator earth fault protection, 3rd harmonic
based STEFPHIZ
9.5.1 Identification
Function description IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
100% Stator earth fault protection, 3rd
harmonic based
STEFPHIZ - 59THD
9.5.2 Functionality
Stator earth fault is a fault type having relatively high fault rate. The generator systems
normally have high impedance earthing, that is, earthing via a neutral point resistor.
This resistor is normally dimensioned to give an earth fault current in the range 3 – 15
A at a solid earth-fault directly at the generator high voltage terminal. The relatively
small earth fault currents give much less thermal and mechanical stress on the
generator, compared to the short circuit case, which is between conductors of two
phases. Anyhow, the earth faults in the generator have to be detected and the generator
has to be tripped, even if longer fault time compared to internal short circuits, can be
allowed.
In normal non-faulted operation of the generating unit the neutral point voltage is
close to zero, and there is no zero sequence current flow in the generator. When a
phase-to-earth fault occurs the neutral point voltage will increase and there will be a
current flow through the neutral point resistor.
To detect an earth fault on the windings of a generating unit one may use a neutral
point overvoltage protection, a neutral point overcurrent protection, a zero sequence
overvoltage protection or a residual differential protection. These protections are
simple and have served well during many years. However, at best these simple
schemes protect only 95% of the stator winding. They leave 5% close to the neutral
end unprotected. Under unfavorable conditions the blind zone may extend up to 20%
from the neutral.
The 95% stator earth fault protection measures the fundamental frequency voltage
component in the generator star point and it operates when the fundamental frequency
voltage exceeds the preset value. By applying this principle approximately 95% of the
stator winding can be protected. In order to protect the last 5% of the stator winding
close to the neutral end the 3rd harmonic voltage measurement can be performed. In
100% Stator E/F 3rd harmonic protection either the 3rd harmonic voltage differential
principle, the neutral point 3rd harmonic undervoltage principle or the terminal side
3rd harmonic overvoltage principle can be applied. However, differential principle is
strongly recommended. Combination of these two measuring principles provides
coverage for entire stator winding against earth faults.
1MRK 502 048-UEN A Section 9
Voltage protection
285
Technical manual
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