TIME GRADED OVER CURRENT PROTECTION IN RING MAIN SYSTEM
Ring main system – In this system, various power stations or sub-stations are interconnected alternate routes, thus forming a closed ring. In case of damage to any section of the ring, that section may be disconnected for repairs and power will be supplied from both ends of the ring, thereby maintaining continuity of supply
Fig 5 shows the single line diagram of a typical ring main system consisting of one generator G supplying four sub-stations S1, S2, S3 and S4. In this arrangement power can flow in both directions under fault conditions. So, it is necessary to grade in both directions round the ring and also to use directional relays. In order that only faulty section of the ring is isolated under fault conditions, the types of relays and their time settings should be as follows:
(i) The two lines leaving the generating station should be equipped with non directional over current relays ( relays at A and J in this case).
(ii) At each sub-station, reverse power or directional relays should be placed in both incoming and outgoing lines (relays at B, C, D, E, F, G, H and I in this case).
(iii) there should be proper relative time-setting of the relays. As an example, going round the loop G S1 S2 S3 S4; the outgoing relays (via at A, C, E, G and I) are set with decreasing time limits
Similarly, going around the loop in opposite direction (i.e. along G S4 S3 S2 S1 G), the outgoing re1ays (J, H, F, D and B) are also set with decreasing time limits
Suppose a short circuit occurs at the point as shown in Fig 5. In order to ensure selectivity, it is desired that only circuit breakers at E and F should open to clear the fault whereas other sections of the ring should be intact to maintain continuity of supply. In fact the above arrangement accomplishes this job. The power will be fed to the fault via two routes, via (i) from G around S1 and S2 and (ii) from G around S 4 and S3. It is clear that relays at A, B, C and D as well as J, I, H and G will not trip. Therefore only relays at E and F will operate before any other relay operates because of their lower time setting.