Date Code 20000421
Applications
5-1
SEL-251, -2, -3 Instruction Manual
SECTION 5: APPLICATIONS
This section is included as a tool for applying the SEL-251 Relay. The information does not
represent suggested settings or standards. Please refer to
Section 2: Specifications
and
Section 3: Communications
and be sure the settings you choose result in relay performance
appropriate to your application.
F
USE
-S
AVING
S
CHEMES
In a fuse-saving scheme, the SEL-251 Relay should trip instantaneously for faults on the load
side of tap fuses near the substation, attempting to save the fuse if the fault is transient. If the
fault is permanent, the fuse will eventually operate on a subsequent shot where relay tripping is
delayed. The
50NL
ground/residual instantaneous overcurrent element in Figure 5.1 is incor-
porated in fuse-saving schemes in the following examples.
Figure 5.1: Fuse Saving Using Ground/Residual Elements
At least two fuse-saving scheme implementations are possible. One scheme uses the
79SH
Relay
Word bit while the other scheme uses the
52AT
Relay Word bit. In the following examples, the
goal is to enable element
50NL
for the first one or two shots and disable it for the remaining
shots. With
50NL
disabled, the tap fuses operate if the fault is permanent. More information
about these two schemes appears below.