Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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5.66
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Out-of-Step Logic (Zero Settings)
Detection of Ground Faults During a Pole-Open
Regarding the ground-distance elements supervision, if the pole-open OOS logic
(OSBA, OSBB, OSBC, see
Figure 5.41 ) is de-asserted, AND 4 turns off. When
AND 4 turns off, the ground-distance elements cannot cause the swing signature
detector to assert.
Figure 5.48 shows the pole-open logic that blocks the ground-
distance elements during a power swing condition.
If a power swing occurs during an open-pole condition, the power swing as seen
by the relay is no longer balanced. The open-pole OOS blocking logic determines
which phase is open so that the relay can correctly identify faults that may occur
on the closed phases during the power swing. To identify the open phase, the
relay calculates the angle of the ratio of the zero-sequence current and the nega-
tive-sequence currents. If the angular relationship indicates a fault, the logic
shown in
Figure 5.41 turns off AND 4, thus preventing the swing signature detec-
tion (SSD) from asserting. When SSD is deasserted, the distance elements can
clear the fault.
For example, if the A-Phase is open, the angle of the ratio normally lies between
–60 and +60 degrees. If a fault now occurs on B- or C-Phase (or both), this angu-
lar relationship is no longer true. In
Figure 5.48 , OSBA asserts if either B-Phase
or C-Phase is open, and no fault is present. If a fault occurs on B-Phase or
C-phase (or both), OSBA deasserts because the angular relationship indicates a
fault.
Figure 5.47 Logic Diagram of the Three-Phase Fault Detector
| d2SCV1_UF |
0.23
max{0.1, cos(Z1ANG)}
| SCV1 |
| dSCV1_S |
0.01
OSB_I
R1T
| dSCV1_S |
max{0.1, cos(Z1ANG)}
0
CYC
6
CYC
2
CYC
0
CYC
20
CYC
0
CYC
5
CYC
0
CYC
DTF
Figure 5.48 Pole-Open OOS Blocking Logic
SPO
SPOA
AND 3
AND 1
AND 2
OR 1
OR 2
AND 4
AND 5
AND 6
AND 7
AND 8
SPOB
SPOC
3IOLFA — 3IA2LFA
50QR
50QF
OSBB
OSBC
OSBA
ENABLE
50GR
50GF
OR 3
OR 4
OR 5
—60 to 60
60 to 180
—60 to —180