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Distance Relay Course |
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The most
common form of feeder protection on high voltage
transmission systems is distance relay protection.
Power lines have set impedance per kilometre and
using this value and comparing voltage and current
the distance to a fault can be determined. The main
types of distance relay protection schemes are
* Three step distance protection
* Switched distance protection
* Accelerated or permissive intertrip protection
* Blocked distance protection
In three step distance protection, the relays are
separated into three separate zones of impedance
measurement to accommodate for over reach and under
reach conditions. Zone 1 is instantaneous in
operation and has a purposely set under reach of 80%
of the total line length to avoid operation for the
next line. This is due to measurements of impedance
of lines not being entirely accurate, errors in
voltage and current transformers and relay
tolerances. These errors can be up to ±20% of the
line impedance, hence the zones 80% reach. Zone 2
covers the last 20% of the feeder line length and
provides backup to the next line by having a slight
over reach. To prevent mal-operation the zone has a
0.5 second time delay. Zone 3 provides backup for
the next line and has a time delay of 1 second to
grade with zone 2 protection of the next line. |
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