9
Date Code 20210405
Instruction Manual
SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System
Application Examples
Figure 8
gives a representation of a typical time-overcurrent coordination graph.
Fuse-Blowing Scheme Shortcomings
For a radial distribution system, the goal of the fuse-blowing scheme is to mini-
mize the number of customers exposed to an interruption. The scheme accom-
plishes this by allowing a fuse to clear a given fault. The recloser only trips for
faults that are not protected by a fuse. This scheme is sometimes called a trip-sav-
ing or fuse-blowing scheme because the recloser only trips when absolutely nec-
essary.
Refer to
Figure 8
for the recloser control time-overcurrent element slow curve
(shown in red). This curve must coordinate with the highest-rated fuse size pres-
ent on the system, which is shown in green. An intentional coordination margin
allows for prefault load and variances in fuse construction.
For faults on sections of the feeder that are not fuse-protected, the recloser must
still implement this intentional coordination margin. The recloser control cannot
determine which downstream branch the fault is on and assumes that the fault
will be cleared by a fuse.
Figure 9
shows an example of a main line feeder and a
fused lateral without a fuse. For a fault on the main line, the recloser control will
wait before using the slow curve to clear the fault (see
Figure 8
). For a fault on
the main line, the recloser control delays tripping unnecessarily because there is
no fuse present.
Figure 8
Example Time-Overcurrent Element Coordination
Recloser Control
Fast Curve
Recloser Control
Slow Curve
Fuse-Melting Curve
Fuse-Clearing Curve
0.01
0.1
1
10
100
100
Current (A)
1,000
1,000
Time (s)
