Sel 421 - Page 297

U.6.35
Date Code 20090715 User’s Guide SEL-421 Relay
Testing and Troubleshooting
Checking Relay Operation
Phase-to-Phase Distance Element MBC2
The SEL-421 contains mho phase distance elements among the many
protection elements in the relay. The relay has phase distance elements to
detect phase-to-phase faults, phase-to-phase-to-ground faults, and three-phase
faults. The SEL-421 has five independent zones of mho phase distance
protection; each zone consists of phase-to-phase elements that the relay
combines to produce a particular zone output.
For example, the OR combination of MAB2, MBC2, and MCA2 produces the
M2P Zone 2 mho phase element. For more information on the mho phase
elements and other distance elements, see Section 1: Protection Functions in
the Reference Manual and Section 1: Protection Application Examples in the
Applications Handbook.
Test Current and Voltage for a Phase-to-Phase Fault
To find the test current for a phase-to-phase fault, consider Equation 6.9 for a
B-phase to C-phase fault:
Equation 6.9
The B-phase to C-phase current vector, I
BC
, is:
Equation 6.10
Choose a convenient test source current magnitude, |I
TEST
| = 2.5 A; then
|I
BC
|= 2 • |I
TEST
| = 5 A.
Find the magnitude of the test source voltage |V
TEST
|:
Equation 6.11
where relay setting Z2P (Zone 2 Reach) substitutes for the B-phase to C-phase
impedance Z
BC
. For setting Z2P of 9.36 Ω, the test voltage magnitude |V
BC
|
is:
Equation 6.12
I
TEST
I
B
–I
C
==
I
BC
I
B
I
C
– I
B
I
B
() +2 • I
B
2 • I
TEST
====
V
TEST
V
BC
I
BC
• Z
BC
I
BC
• Z2P ===
2 • I
TEST
• Z2P=
V
TEST
2 • I
TEST
• Z2P=
2 • 2.5 • 9.36 = 46.8 V=
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