Areva MiCOM P125, Техническое руководство

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Application Guide P12y/EN AP/D32
MiCOM P125/P126 & P127 Page 15/74
2.2.5.2 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.
A
2,1s
51
51 2,1s
Load
F
E
D
C
B
Load
67
67
67
67
67
67 67
67
67
67
1,7s
0,1s
0,5s
1,3s
0,9s
0,9s
1,3s
0,5s
1,7s
0,1s
Load
Load
Load
Load
P0082ENa
TYPICAL RING MAIN WITH ASSOCIATED OVERCURRENT PROTECTION
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 counter-
clockwise. 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 co-
ordinated 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.