Sel SEL-421 - Page 273

U.6.33
Date Code 20020501 User’s Guide SEL-421/SEL-421-1 Relay
Testing and Troubleshooting
Checking Relay Operation
Distance Elements
Apply voltages and currents to the relay analog inputs that simulate fault and
load conditions to test distance elements. The relay supervises distance
elements so that these elements operate under the appropriate conditions. Be
sure to satisfy all the element supervisory conditions before testing a relay
element. For supervisory conditions for a particular element, see Mho Ground
Distance Elements on page R.1.46 in the Reference Manual.
Phase-to-Phase Distance Element MBC2
The SEL-421 Relay 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 Relay 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 W, 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=