User's Manual

9
Date Code 20170317 Instruction Manual SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System
Application Examples
Example 2: Switchover Without Interruption From a Fuse-Saving
Scheme to a Fuse-Blowing Scheme
In this switchover scheme, the SEL-FT50/SEL-FR12 system is used to
indicate which line section contains a fault. However, the fuse-saving scheme
is the default operating mode. When the SEL-FT50 declares that a fault is
present on a candidate line section, the scheme enables fuse-blowing while the
fault is in progress. For other faulted line segments, the fuse-saving scheme
works as intended.
Improve Feeder Cable
First-Span Protection
The SEL-FT50/SEL-FR12 system improves first-span feeder cable protection.
Feeder cables are often used for substation egress, eliminating overhead line
clutter and improving working safety. These feeder cables radiate from a
substation, continuing for a few feet to one mile. These cables are usually
terminated on a riser pole and then connected to the overhead conductors.
To protect cable sections, some utilities use instantaneous overcurrent
elements with pickup levels set to cover the entire cable length, plus some
margin that overreaches onto a portion of the overhead line. In these
applications, a high-current fault causes an instantaneous trip with no
reclosing permitted.
While this approach protects equipment, it also often causes an unnecessary
permanent outage when the fault is on the portion of the overhead line where
available fault levels are still very high. The majority of overhead faults are
caused by temporary events and are far more likely to occur than underground
faults. By not reclosing for close-in overhead faults, the entire feeder suffers a
permanent outage that could have been avoided.
To improve the first-span feeder cable protection, use a set of three SEL-FT50
Fault Transmitters to monitor the first span of overhead line, as shown in
Figure 9. When an overhead fault occurs, the relay instantaneous element trips
the recloser or feeder breaker, but reclosing is allowed when the SEL-FT50
signals that the fault is on the overhead portion of the feeder. This simple
modification of an existing scheme improves system availability. This
application extends to any line that transitions between overhead and
underground lines. Knowing whether a fault is on an overhead or underground
section of a feeder helps when coordinating reclosing and protection schemes.
Figure 9 Feeder Cable Egress Protection With Enhancements
SEL-FR12
Relay
Breaker
Substation bus
Substation
Underground
feeder cable
Wireless
Riser pole Overhead conductor
SEL-FT50