5.62
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Out-of-Step Logic (Zero Settings)
3. The ultra-fast derivative dSCV1_UF will be greater than 0.55
(pu V/cyc) for more than 4 cycles.
4. Either all three poles are open (3PO) or an internal loss-of-potential
(ILOP) occurred.
The Dependable Power-Swing Blocking Function
The dependable PSB detector function asserts the DOSB signal for power-swing
conditions where neither the slope detector nor the swing signature detector can
detect a power swing fast enough. An example of this type of situation might
occur after a slow-clearing fault right behind or at the remote end of a transmis-
sion line on a marginally stable network.
As shown in Figure 5.43, if a close reverse or forward fault clears with a signifi-
cant delay, there is a possibility that the network has entered a power swing. In
this case, the Z1 trajectory at the relay may cross into the Zone 2 or Zone 1
phase-mho characteristic right after the fault clears, but before the slope detector
has detected the power swing. In this case, the phase mho elements of the relay
may issue a trip signal as a result of the power swing and not because of a real
fault. To overcome this problem, the dependable power-swing detector asserts the
DOSB (see Figure 5.37) signal to block the distance elements until the slope
detector has had time to detect a power swing.
Figure 5.42 Reset Conditions Logic
AND 2
AND 3
C1
OR 4
OR 4
OR 3
OR 6
AND 4
EOOST = I, O, C
Z1 Outside
Zone 7
| SCV1 |
0.85
Z1 Outside
Starter Zone
C2
0.0026
| dSCV1_S |
Z2P
Z3P
Z4P
Z5P
C4
| dSCV1_UF |
0.55
3PO
ILOP
OR 5
T1
0
CYC
30
CYC
T2
0
CYC
10
CYC
T3
0
CYC
4
CYC
RST
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