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For a fault in the busbar zone, the GRD150 is programmed to trip the bus section and bus coupler
circuit breakers via its instantaneous elements OC2 and EF2 set with short definite time delay
settings (minimum 50ms). This first stage trip maintains operation of half the substation in the
event of a busbar fault or incomer fault in the other half.
If the first stage trip fails to clear the fault, a second stage trip is given to the local incomer circuit
breaker via instantaneous elements OC3 and EF3 after a longer delay, thus isolating a fault on the
local busbar.
GRD150 integrated circuit breaker fail protection can be used to provide additional back-trips
from the feeder protection to the incomer, and from the incomer to the HV side of the power
transformer, in the event of the main trip failing to clear the fault.
A further development of this scheme might see directional relays being applied directly to the bus
section and bus coupler circuit breakers, to speed up operation of the scheme.
This scheme assumes that a busbar fault cannot be fed from the outgoing feeder circuits. In the
case of an interconnected system, where a remote power source may provide a back-feed into the
substation, directional relays must also be applied to protect the feeders.
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