Instruction Manual

12
SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System Instruction Manual Date Code 20210405
Application Examples
Implement a Low-Cost Fast Bus-Tripping Scheme
A fast bus-tripping scheme uses a short-time delayed overcurrent element in the
bus relay to provide a quick response to bus faults. To maintain coordination for
feeder faults, the scheme must block this fast element operation whenever a
feeder relay or recloser control is picked up and timing. Traditional fast bus-trip
schemes use a hardwired control circuit or communications link to allow each
feeder relay to drive a bus relay block signal when a fault occurs on a distribution
line outside the substation.
In stations without bus differential or a fast bus-tripping scheme, a bus fault has a
long duration because the bus relay only uses a time-overcurrent element coordi-
nated with the feeder protection.
In some substations, installing a fast bus-trip scheme is not feasible because the
feeder pickup-based block signal cannot be created or transmitted through nor-
mal means. For example, some feeder protection devices (reclosers or relays)
may not be able to provide a block signal, or the device may be located far across
a substation yard, requiring a costly cable or fiber run to bring a block signal back
to the bus relay panel. Figure 12 shows a substation one-line diagram with a
recloser on each feeder. Retrofitting a traditional fast bus-tripping scheme to this
type of substation might be expensive.
Instead of installing wiring or upgrading equipment, use the SEL-FT50/
SEL-FR12 Fault Transmitter and Receiver System to bring in the required feeder
pickup signal without making changes to the medium voltage system.
In Figure 12, each feeder (A through D) is equipped with SEL-FT50 transmitters.
An example fault F1 on Feeder D triggers one or more of the SEL-FT50 transmit-
ters to transmit. The SEL-FR12 receives the transmission and immediately sends
the fault status to the bus relay. At the same time, the bus relay is also picked-up
and timing the respective definite-time overcurrent element. As soon as the
SEL-FR12 fault signal arrives, the bus relay blocks the respective definite-time
overcurrent element and maintains the block state until the overcurrent element
has completely reset. The Feeder Recloser D operates as needed to clear the fault
or lockout the line.
For a bus fault F2, none of the SEL-FT50 transmitters trigger for fault current,
and the bus relay receives no fault signal from the SEL-FR12. In this situation,
the bus relay fast-acting definite-time overcurrent element times out and trips the
bus breaker after a brief coordination delay.
Figure 11 Feeder Cable Egress Protection With Enhancements
SEL-FR12
Relay
Breaker
Substation bus
Substation
Underground
feeder cable
Wireless
Riser pole Overhead conductor
SEL-FT50