Sel SEL-421 - Mho Ground Distance Elements; Open-Pole OSB Unblock Logic

R.1.49
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Mho Ground Distance Elements
Figure 1.34 Open-Pole OSB Unblock Logic
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-421-1 and the
SEL-421-2 provide fast and secure
tripping but do not have high-speed
distance elements. Typical detection
time for the SEL-421-1 and for the
SEL-421-2 is 1.5 cycles.
The SEL-421 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 (Zones 1 through 3) are for high-speed operation; typical detection
time is less than one cycle.
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 1.14
I0LFA–IA2LFA
SPO
50QR
50QF
50GR
OSBB
OSBC
OSBA
SPOA
SPOB
SPOC
50GF
–60 to 60
ENABLE
60 to 180
–60 to –180
where:
Z
1L
= positive-sequence transmission line impedance
Z
0L
= zero-sequence transmission line impedance
k0
Z
0L
Z
1L
–
3Z
1L
•
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