(ZMCPDIS,ZMCAPDIS, 21). This function is necessary in the protection on
compensated lines as well as all non-compensated lines connected to this busbar (adjacent
lines). All protections that can be exposed to voltage reversal must have the special
directional function, including the protections on busbar where the voltage can be
reversed by series compensated lines not terminated to this busbar.
The directional function is controlled by faulty phase criteria. These criteria must identify
all forward and reverse faults that can cause voltage reversal. Setting of the corresponding
reach of the impedance measuring elements is separate for reactive and resistive reach and
independent of each other for phase-to-ground and for phase-to-phase measurement.
It is also necessary to consider the minimum load impedance limiting conditions:
Setting of zone 1
A voltage reversal can cause an artificial internal fault (voltage zero) on faulty line as well
as on the adjacent lines. This artificial fault always have a resistive component, this is
however small and can mostly not be used to prevent tripping of a healthy adjacent line.
An independent tripping zone 1 facing a bus which can be exposed to voltage reversal
have to be set with reduced reach with respect to this false fault. When the fault can move
and pass the bus, the zone 1 in this station must be blocked. Protection further out in the
net must be set with respect to this apparent fault as the protection at the bus.
Different settings of the reach for the zone (ZMCPDIS, 21) characteristic in forward and
reverse direction makes it possible to optimize the settings in order to maximize
dependability and security for independent zone1.
Due to the sub-harmonic oscillation swinging caused by the series capacitor at fault
conditions the reach of the under-reaching zone 1 must be further reduced. Zone 1 can only
be set with a percentage reach to the artificial fault according to the curve in 103
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 205
Application manual
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