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Answer Posted / joe 90
The rough-balance scheme shall be driven from three phase CTs fitted on the LV side of the transformer incomer circuit and on the bus section which shall be connected in parallel into rough-balance zone protection relays fitted on the bus section panel. These relays shall have a fast acting highset definite time overcurrent element and IDMT overcurrent and earth fault elements. The relays shall be set to be stable under normal and through fault conditions and to operate for an in-zone busbar fault where the fault is fed from each side (incomer and bus section) to summate at the relay. Note that for correct operation of this scheme the feeder circuits must be connected in-zone i.e. between the incomer and bus section.
A trip output from the rough-balance zone relay shall operate into the bus section trip coil and a ‘trip repeat’ output fed into the protection relays for the incomer and all feeders connected on to the faulted busbar zone via a busbar zone trip buswire. On receipt of the busbar zone trip input the incomer and feeder relays will initiate a trip output to trip their respective circuit breakers.
For a feeder fault both the feeder and bus zone protection may pickup. A means must
therefore be provided to discriminate between a feeder and busbar fault so that the bus zone protection does not have to wait to give the feeder protection time to operate in instances where the fault is on the feeder. This is done by a block signal from the feeder protection start which holds off the high set definite time bus zone protection giving the feeder protection time to operate to clear the fault. If the fault is on the busbar the feeder protection will not pick up, no block is present and the bus zone protection can operate almost instantaneously.
Where the feeder protection fails to clear a fault (e.g. circuit stuck breaker), the bus zone
protection IDMT elements will operate to trip the bus section and set the bus zone trip output high. The feeder protection start must be set low on completion of the feeder protection operation to remove any block to the bus zone protection to allow for stuck breaker situations.
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