Schweitzer Engineering Laboratories SEL-421-4 - Figure 5.56 Out-Of-Step Blocking for Zone 1 through Zone 5; Mho Ground-Distance Elements

5.72
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Mho Ground-Distance Elements
Mho Ground-Distance Elements
The SEL-421 has five independent zones of mho ground-distance protection. The
mho ground-distance protection operates only for single phase-to-ground faults.
You can set the reach for each zone independently. Zone 1 and Zone 2 distance
elements are forward only; you can set Zone 3 through Zone 5 distance elements
either forward or reverse. The mho ground-distance elements use positive-
sequence voltage polarization for security and generate a dynamic expanding
mho characteristic.
NOTE: The SEL-421-4 provides fast
and secure tripping but does not have
high-speed distance elements. Typical
detection time for the SEL-421-4 is
1.5 cycles.
The SEL-421-5 has three independent zones of high-speed mho ground-distance
protection. The high-speed mho ground-distance protection operates for single
phase-to-ground faults. The first three zones of mho ground-distance protection
(Zone 1 through Zone 3) are for high-speed operation; typical detection time is
less than one cycle.
NOTE: If the relay is using a remote
data acquisition system, such as TiDL,
the operating times will be delayed by
1.5 ms. Use caution when setting the
relay coordination times to account
for this added delay.
The Zone 1 zero-sequence compensation factor (k01) is independent from the
forward and reverse compensation factors (k0 and k0R) the relay uses for the
other zones.
If you set k0M1 to AUTO, the relay automatically calculates the values k01, k0,
and k0R based on the following equation:
Equation 5.28
The SEL-421 has a settable Zone 1 overcurrent supervision setting for phase-
distance elements (Z50P1) and for ground-distance elements (Z50G1). These
advanced settings (EADVS = Y) apply to both mho and quadrilateral distance
Figure 5.56 Out-of-Step Blocking for Zone 1 Through Zone 5
OSB1
OSB2
OSB3
OSB4
OSB5
OODBI = Y
OOSB5 = Y
OOSB4 = Y
OOSB3 = Y
OOSB2 = Y
EOOS = Y1
OSB
EOOS = Y
OOSDET
where:
Z
1L
= positive-sequence transmission line impedance
Z
0L
= zero-sequence transmission line impedance
k0
Z
0L
Z
1L
–
3Z
1L
•
----------------------=