ABB Relion 670 series, Applications Manual

Page: 191 / 760

When the calculation of XFw gives a negative value the zone 1 must be
permanently blocked.
Fault resistance
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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 earth- fault measurement as well as,
for phase-to-phase measurement.
Overreaching zone 2
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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-earth
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.
The increased reach related to the one used in non compensated system is recommended for
all protections in the vicinity of series capacitors to compensate for delay in the operation
caused by the sub harmonic swinging.
Settings of the resistive reaches are limited according to the minimum load impedance.
Reverse zone
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The reverse zone that is normally used in the communication schemes for functions like fault
current reversal logic, weak-in-feed logic or issuing carrier send in blocking scheme must
detect all faults in the reverse direction which is detected in the opposite IED by the
overreaching zone 2. The maximum reach for the protection in the opposite IED can be
achieved with the series capacitor in operation.
The reactive reach can be set according to the following formula: X1 = 1.3 · (X1
2Rem
- 0.5 · (X1
L
-
X
C
))
Settings of the resistive reaches are according to the minimum load impedance:
Optional higher distance protection zones
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When some additional distance protection zones (zone 4, for example) are used they must be
set according to the influence of the series capacitor.
7.1.3.6 Setting of zones for parallel line application
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1MRK 505 343-UEN B Section 7
Impedance protection
185
Application manual