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Figure 2-15
Logic of the dropout delay for 67-1
2.3.4
Interaction with Fuse Failure Monitor (FFM)
Spurious tripping might be caused by a measuring voltage failure due to a short circuit, a phase failure in the
voltage transformer secondary system, or a pickup of the voltage transformer mcb (fuse). In the event of a
single-phase or two-phase failure of the measuring voltage, it is possible to recognize this condition and block
the directional overcurrent protection elements (phase and ground) (see logic diagrams).
For additional information on the operation of the fuse failure monitor, see Section 2.8.1 Measured Value Su-
pervision.
2.3.5
Inrush Restraint
The 7SC80 features an integrated inrush restraint function. It prevents the "normal" pickup of all directional and
non-directional overcurrent relay elements in the phases and ground path, but not the high-set elements. The
same is true for the alternative pickup thresholds of the dynamic cold load pickup function. After detection of
inrush currents above a pickup value, special inrush signals are generated. These signals also initiate fault an-
nunciations and start the associated trip delay time. If inrush conditions are still present after the tripping time
delay has elapsed, a corresponding message ("....TimeOut ") is output, but the overcurrent tripping is blocked
(for further information see "Inrush Restraint" in Section 2.2).
2.3.6
Determination of Direction
The determination of the fault direction for the phase directional element and the ground directional element is
performed independently.
Basically, the direction determination is performed by determining the phase angle between the fault current
and a reference voltage.
Method of Directional Measurement
For the phase directional element the fault current of the corresponding phase and the unfaulted phase-to-
phase voltage are used as reference voltage. The unfaulted voltage also allows for a correct direction determi-
nation even if the fault voltage has collapsed entirely (short-line fault). In phase-to-ground voltage connections,
the phase-to-phase voltages are calculated. In a connection of two phase-to-phase voltages and V
phase-to-phase voltage is also calculated.
SIPROTEC, 7SC80, Manual
E50417-G1140-C486-A1, Release date 12.2011
2.3 Directional Overcurrent Protection 67, 67N
Functions
, the third
N
77
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