ABB RET670 Applications Manual page 248

Section 3
IED application
242
When the calculation of XFw gives a negative value the zone 1 must be
permanently blocked.
For protection on non compensated lines facing series capacitor on next line. The
setting is thus:
• X1 is set to (XLine-XC · K) · p/100.
• K equals side infeed factor at next busbar.
When the calculation of XFw gives a negative value the zone 1 must be
permanently blocked.
Fault resistance
The resistive reach is, for all affected applications, restricted by the set reactive reach
and the load impedance and same conditions apply as for a non-compensated network.
However, special notice has to be taken during settings calculations due to the ZnO
because 50% of capacitor reactance appears in series with resistance, which
corresponds to approximately 36% of capacitor reactance when the line current equals
two times the protective current level. This information has high importance for setting
of distance protection IED reach in resistive direction, for phase to ground- fault
measurement as well as, for phase-to-phase measurement.
Overreaching zone 2
In series compensated network where independent tripping zones will have reduced
reach due to the negative reactance in the capacitor and the sub-harmonic swinging the
tripping will to a high degree be achieved by the communication scheme.
With the reduced reach of the under-reaching zones not providing effective protection
for all faults along the length of the line, it becomes essential to provide over-reaching
schemes like permissive overreach transfer trip (POTT) or blocking scheme can be used.
Thus it is of great importance that the zone 2 can detect faults on the whole line both
with the series capacitor in operation and when the capacitor is bridged (short
circuited). It is supposed also in this case that the reactive reach for phase-to-phase and
for phase-to-ground faults is the same. The X1Fw, for all lines affected by the series
capacito, are set to:
• X1 >= 1,5 · XLine
The safety factor of 1.5 appears due to speed requirements and possible under reaching
caused by the sub harmonic oscillations.
1MRK504116-UUS C
Application manual