Instruction Manual

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Date Code 20210405 Instruction Manual SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System
Application Examples
The following two example applications show how a single protective zone using
the SEL-FT50/SEL-FR12 system allows both fuse-saving and fuse-blowing
schemes in service.
Example 1: Switchover Without Interruption From a Fuse-Blowing
Scheme to a Fuse-Saving Scheme
In this switchover scheme, utilities have the option to tailor protection for specific
line segments with different characteristics. If the SEL-FT50 declares that a fault
is present on a candidate line section, the scheme enables fuse-saving while the
fault is in progress. For other faulted line segments, the fuse-blowing scheme
works as intended.
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 clut-
ter 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-cur-
rent fault causes an instantaneous trip with no reclosing permitted.
While this approach protects equipment, it also often causes an unnecessary per-
manent outage when the fault is on the portion of the overhead line where avail-
able 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 out-
age 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 11. When an overhead fault occurs, the relay instantaneous element trips
the recloser or feeder breaker, but reclosing is allowed when the SEL-FT50 sig-
nals that the fault is on the overhead portion of the feeder. This simple modifica-
tion 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.