Since the polarizing voltage is also used for the Mho distance characteristics, the magnitude of
the voltage is just as interesting as the phase. If there are symmetrical conditions and the
measured per phase positive sequence voltage magnitude is above 75% of the base voltage
before the fault, the pre-fault magnitude will be memorized and used as long as there is a fault.
The phase angle however will only be memorized (locked) for 75 ms at a time, not to lose
synchronism with the real system voltage.
Should the positive sequence voltage drop below 2% of the base voltage, it will be considered
invalid. In this situation, directional signals and pickups from Mho elements will be sealed-in and
kept static as long as there is a fault.
For ZMFCPDIS, when option SeriesComp is chosen for OperationSC, the voltages of faulty phases
will be discarded in order not to affect the polarizing voltage with voltage reversal.
7.9.7.5 Fuse failure
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The ZMFCPDIS function has to be blocked by an additional function like the Fuse failure
supervision (FUFSPVC) or an equivalent external device. Typically, the binary input VTSZ is used for
this purpose.
However, to guarantee that also very fast operation is blocked in a fuse failure situation, there is a
built-in supervision based on change in current that will delay operation before the FUFSPVC
blocking signal is received. The delay will be introduced if no (vector) magnitude change greater
than 5% of IBase has been detected in any of the phase currents.
7.9.7.6 Power swings
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There is need for external blocking of the ZMFCPDIS function during power swings, either from the
Power Swing Blocking function (ZMRPSB) or an external device.
7.9.7.7 Measurement principles
Quadrilateral characteristic
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ZMFCPDIS implements quadrilateral and mho characteristic in all the six zones separately. Set
OpModePEZx
or
OpModePPZx
to
Quadrilateral
, to choose particular measuring loop in a zone to
work as quadrilateral distance protection.
All ZMFCPDIS zones operate according to the non-directional impedance characteristics
presented in figure
173 and figure 174. The phase-to-ground characteristic is illustrated with the
full loop reach while the phase-to-phase characteristic presents the per-phase reach.
The voltage and current phasors after the half-cycle filter are used in fault loop equations.
For phase-to-phase faults (Figure 175, lower part), the calculated impedances from the relay to the
fault
follow Equation 97 (example is given for a phase L1 to phase L2 fault).
IECEQUATION18003 V1 EN-US
(Equation 97)
Where and represents the corresponding voltage and current phasors in the respective
phase
Ln
(
n
= 1, 2, 3).
Section 7 1MRK 502 066-UUS B
Impedance protection
344
Technical manual
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