Time Graded Over Current Protection for Ring Main System of Bus Bar:
The ring main consists of various generating stations and substations interconnected by alternate routes. When the fault occurs in any section of the ring, that section can be disconnected for repairs and power is supplied from both ends of ring, maintaining the continuity of supply.
Figure represents the single line diagram of a typical ring main system, which consists of a generator and four sub-stations S1, S2, S3, S. As the power can flow in both the directions under fault conditions, it is necessary to grade feeder protection in both directions round the ring with directional relays. In order to ensure selectivity, the circuit breakers at E & F should open to clear the fault, there by maintaining other section intact. Actually the power is fed to the fault via two routes i.e. (i) From G around S1, S2 and (ii) from G around S. The relays at A, B, C and D as well as J, I, H and G do not trip. Therefore, only relays at E and F operate before any other relay operates.
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