Schweitzer Engineering Laboratories SEL-421-4 - Page 460

6.124
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
Out-of-Step Logic Application Examples
Reactance Lines
Zone 6 inner reactance lines X1T6 and X1B6 should completely encompass the
outermost zone of phase-distance protection that you want to block from tripping
during a power swing. Include a safety margin (20 percent).
Equation 6.74
X1T6 := 11.52. Zone 6 Reactance—Top (0.05–140  secondary)
The distance between Zones 6 and 7 top reactance lines should equal the distance
between Zones 6 and 7 right-hand resistance blinders.
NOTE: The value for X1T7 must be at
least 0.1  greater than that for X1T6.
Equation 6.75
X1T7 := 21.29. Zone 7 Reactance—Top (0.05–140  secondary)
Out-of-Step Block Time Delay
When the Z
1
impedance locus initially moves inside Zone 7, the relay starts the
OSBD (out-of-step block time delay) timer. The Z
1
impedance trajectory is
shown in Figure 6.31 for the case of |E
A
| = |E
B
| (E
A
is the voltage at Node A and
E
B
is the voltage at Node B). The OSBD timer detects slow swings. If the OSBD
timer expires before the Z
1
trajectory enters Zone 6, the relay detects a power
swing blocking condition.
where:
Z2MP = Zone 2 mho phase-distance element reach
X1T6 1.2 Z2MP• =
1.2 9.60 • =
11.52 =
X1T7 X1T6 R1R7 R1R6–+=
11.52  15.53  5.76 –+=
21.29 =