Schweitzer Engineering Laboratories SEL-421-4 - Page 117

3.13
Date Code 20171021 Instruction Manual SEL-421 Relay
Testing
Checking Relay Operation
The result of Equation 3.1 is an impedance magnitude that varies with the magni-
tude and angle of the applied current. Normally, a forward fault results in a nega-
tive Z2c relay calculation.
Test Current
Solve Equation 3.1 to find the test current values that you need to apply to the
relay to test the element. For the negative-sequence current I
2
, the result is
Equation 3.2
when:
Equation 3.3
Multiply the quantities in Equation 3.2 by three to obtain 3I
2
, the negative-
sequence current that the relay processes. With a fixed applied negative-sequence
voltage V
A
, the relay negative-sequence voltage is 3V
2
. Set Z2c = Z2F to find the
test current magnitude at the point where the impedance calculation equals the
forward fault impedance threshold. Equation 3.2 becomes:
Equation 3.4
when:
Equation 3.5
For a reverse fault impedance threshold, where Z2c = Z2R, Equation 3.2
becomes:
Equation 3.6
when the angle calculation is the same as Equation 3.5.
For more information on the directional elements, see Ground Directional Ele-
ments on page 1.18 and Quadrilateral Ground-Distance Elements on page 1.18.
For settings and application information, see Section 6: Protection Applications
Examples.
Checking the Negative-Sequence Directional Element (Phase Faults)
This test confirms operation of the F32Q and the R32Q negative-sequence direc-
tional elements. This test procedure is for a 5 A relay; scale values appropriately
for a 1 A relay.
I
2
V
2
Z2c
----------=
I
2
V
2
Z1ANG–=
I
TEST
3I
2
3V
2
Z2c
-------------
3V
2
Z2F
-------------== =
I
TEST
3I
2
3V
2
Z1ANG–==
I
TEST
3I
2
3V
2
Z2c
-------------
3V
2
Z2R
-------------== =
NOTE: As you perform this test,
other protection elements can assert.
This causes the relay to assert other
targets and possibly close control
outputs. Be sure to isolate the relay
from the power system to avoid
unexpected system effects.