Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-17
AP
Zone 3 may also be programmed with a slight reverse (“rev”) offset, in which case its reach
in the reverse direction is set as a percentage of the protected line impedance too. This
would typically provide back-up protection for the local busbar, where the offset reach is set
to 20% for short lines (<30 km) or 10% for longer lines.
Zone P
is a reversible directional zone. The setting chosen for Zone P, if used at all, will
depend upon its application. Typical applications include its use as an additional time
delayed zone or as a reverse back-up protection zone for busbars and transformers. Use of
zone P as an additional forward zone of protection may be required by some users to line up
with any existing practice of using more than three forward zones of distance protection.
The Zone 4
elements may also provide back-up protection for the local busbar. Where zone
4 is used to provide reverse directional decisions for Blocking or Permissive Overreach
schemes, zone 4 must reach further behind the relay than zone 2 for the remote end relay.
In such cases the reverse reach should be as below (depends on characteristic used):
Mho: Z4 ((Remote zone 2 reach) x 120%)
Quadrilateral: Z4 ((Remote zone 2 reach) x 120%) minus the protected line impedance
Note: In the case of the mho, the line impedance is not subtracted. This
ensures that whatever the amount of dynamic expansion of the circle,
the reverse looking zone will always detect all solid and resistive faults
capable of detection by zone 2 at the remote line end.
2.3.7 Quadrilateral phase resistive reaches (Distance option only)
Two
setting modes are possible for resistive reach coverage:
Common
- In this mode, all zones share one common fault resistive reach setting
Proportional
- With this mode, the aspect ratio of (zone reach): (resistive reach) is
the same for all zones. The “Fault Resistance” defines a reference
fault at the remote end of the line, and depending on the zone reach
percentage setting, the resistive reach will be at that same
percentage of the Fault Resistance set. For example, if the zone 1
reach is 80% of the protected line, its resistive reach will be 80% of
the reference “Fault Resistance”.
Proportional setting is used to mimic Germanic protection practice, and to avoid zones being
excessively broad (large resistive reach width compared to zone reach length). In general,
for easiest injection testing, the aspect ratio of any zone is best within the 1 : 15 range:
1/15th Z reach / R reach setting 15
The resistive reach settings (RPh and RG) should be selected according to the utility
practice. If no such guidance exists, a starting point for Zone 1 is:
Cables
- Choose Resistive Reach = 3 x Zone 1 reach
Overhead lines
- Choose Resistive Reach according to the following formula…
Resistive reach = [2.3 - 0.0045 x Line length (km)] x Zone 1
reach
Lines longer than 400 km
- Choose: 0.5 x Zone 1 reach
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