Page 214 - Practical Power System and Protective Relays Commissioning

214 Practical Power System and Protective Relays Commissioning

FIGURE 18.6.7 Distance relay protection zones.

To have good coordination in a distance relay application on high-voltage
transmission lines we use three zones or more, as shown in Fig. 18.6.7.
Zone 1 is set to 80% of the line of Mho characteristic and is used to trip
the line instantaneously in approximately one cycle based on a 50-Hz system
(20 ms) this operating time is different from one manufacturer to the other
and can be less than this depend on the relay speed of operation.
Zone 2 is set to about 120% of the line of Mho characteristic to cover the
remaining part of the line (this means the remaining 20%) and a trip in a
time delay set to about 500 ms.
Zone 3 is set about 225% of the line as offset-Mho characteristic to cover
the other line sections but to trip in about 1000 ms this zone is also used as a
starting element in the distance relay and this offset-Mho characteristic has
the following advantages:
1. Working as backup protection for busbar faults in the reverse direction of
the line;
2. Providing a local backup protection for close-up faults near the local end
busbar of transmission line for faults which have a voltage near zero and
the forward direction zone 1 cannot see;
3. Used in a permissive overreach blocking scheme in distance protection,
which is explained in later sections of this chapter.