Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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6.127
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
Out-of-Step Logic Application Examples
Out-of-Step Unblocking During Three-Phase Faults
The trajectories of a three-phase fault and a power swing appear the same to
phase-distance elements because both a three-phase fault and a power swing con-
sist of positive-sequence quantities only (V
1
and I
1
). Therefore, if a power swing
evolves into an internal three-phase fault, typical OSB logic cannot detect the
occurrence of the balanced fault. The SEL-421 includes an additional set of inner
blinders to provide proper detection of the internal three-phase fault (see
Figure 6.32 ). If the positive-sequence impedance resides between these blinders
for a specific duration, OSB logic unblocks. The relay calculates this duration
(UBOSBD) each time the power swing enters Zone 7. A short timer setting is
adequate for fast swings, but the relay needs a longer timer setting for slow power
swings. For example, if the positive-sequence impedance passes gradually
between the two inner blinders during a slow swing, a short timer setting would
cause unwanted tripping.
The UBOSBD timer length is the expected duration of the power swing within
the inner blinders. The relay bases the calculation on the actual time required for
the swing to traverse from Zone 7 to Zone 6, before entering the inner blinders. If
the swing remains inside the inner blinders for a period greater than UBOSBD,
an unblock signal asserts.
Figure 6.32 Inner Blinders
S
R
B
A
Zone 2
R1
X1
Zone 6
Zone 7
Right
Blinder
Left
Blinder