Function
Range or value
Accuracy
Curve type
IEEE
2
(0.18
)
:
(
1)
×
=
-
TD
IEEE t
M
EQUATION1645 V1 EN
(Equation 93)
where M = (E/f)/(Vn/fn)
± 5% + 40 ms
Minimum time delay for inverse
function
(0.000–60.000) s
± 0.5% ± 25 ms
Alarm time delay
(0.00–9000.00)
± 0.5% ± 25 ms
9.5
100% Stator ground fault protection, 3rd harmonic
based STEFPHIZ (59THD)
9.5.1
Identification
Function description
IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
100% Stator ground fault protection, 3rd
harmonic based
STEFPHIZ
-
59THD
9.5.2
Functionality
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
1MRK 502 048-UUS A
Section 9
Voltage protection
309
Technical manual
