The protection element also provides a non-sensitive area to avoid instability due to small asymmetries and unbalances that can generally be present in the network system. This condition is represented by the characteristic reproduced below, where the hatched area is the tripping zone.
Ring Main Arrangement
A particularly common arrangement within distribution networks is the ring main circuit. The
primary reason for its use is to maintain supplies to consumers in the event of fault
conditions occurring on the interconnecting feeders.
A typical ring main with associated overcurrent protection is shown in the following figure.
As with the previously described parallel feeder arrangement, it can be seen here that
current may flow in either direction through the various relay locations.
Therefore, directional overcurrent relays are again required in order to provide a
discriminative protection system.
The normal grading procedure for overcurrent relays protecting a ring main circuit is to open
the ring at the supply point and to grade the relays first clockwise and then counterclockwise. The arrows shown at the various relay locations in above figure depict the
direction for forward operation of the respective relays, i.e. in the same way as for parallel
feeders, the directional relays are set to 'look into' the feeder that they are protecting. The
above figure shows typical relay time settings (if definite delay time co-ordination was set),
from which it can be seen that faults on the interconnections between stations are cleared
without any discrimination by the relays at each end of the feeder.
Again, any of the three overcurrent stages may be configured to be directional and coordinated as per the previously outlined grading procedure, noting that IDMT characteristics
are selectable on the first and second stage.
Note that the above requirements outlined for the parallel transformer arrangements are
equally applicable for plain feeders, which are operating in parallel.
