9.6 100% Stator ground fault protection, 3rd harmonic
based STEFPHIZ (59THD)
SEMOD156719-1 v3
9.6.1 Identification
SEMOD158987-2 v4
Function description IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
100% Stator ground fault
protection, 3rd harmonic based
STEFPHIZ - 59THD
9.6.2 Functionality
SEMOD143258-4 v9
Stator ground fault is a fault type having relatively high fault rate. The generator systems normally
have high impedance grounding, that is, grounding via a neutral point resistor. This resistor is
normally dimensioned to give an ground fault current in the range 3 – 15 A at a solid ground-fault
directly at the generator high voltage terminal. The relatively small ground 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 ground 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-ground fault occurs
the neutral point voltage will increase and there will be a current flow through the neutral point
resistor.
To detect a ground 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 ground 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 ground faults.
Section 9 1MRK 502 066-UUS B
Voltage protection
652
Technical manual
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