transformers and has one sensitive alarm step and one trip
step.
100% Stator earth fault protection, 3rd harmonic based
STEFPHIZ
SEMOD143258-4 v10
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.
x E3
Rf
T
CB 2
(1- x) E3
over- voltage protection 10%
– 100%
Differential
0% – 30%
CB 1 may not exist
RN
N
CB 1
stator winding
uT
uN
x E3
Rf
Transformer
T
CB 2
(1- x) E3
x
Neutral point fundamental frequency
over-voltage protection 5% - 100%
3rd harmonic
differential
0% - 30%
CB 1 may not exist
1 or 100 %
RN
N
N
CB 1
stator winding
uT
uN
1 - x
1 - x
Samples of the
neutral voltage
from which the
fundamental and
3rd harmonic
voltages are
filtered out
Samples of the
terminal voltage
from which the
3rd harmonic
voltage is filtered
out
IEC10000202-1-en.vsd
IEC10000202 V1 EN-US
Figure 6.
Protection principles for STEFPHIZ function
1MRK 502 074-BEN A
Generator protection REG670 2.2 and Injection equipment REX060,
REX061, REX062
Product version: 2.2.1
28
ABB
