U.6.28
SEL-421/SEL-421-1 Relay
User’s Guide
Date Code 20020501
Testing and Troubleshooting
Checking Relay Operation
Equation 6.3
Where:
V
2
is the negative-sequence voltage
I
2
is the negative-sequence current
Z1ANG is the positive-sequence line impedance angle
Re indicates the real part of the term in brackets, for example,
(Re[A + jB] = A)
* indicates the complex conjugate of the expression in parentheses,
(A + jB)* = (A – jB)
The result of
is an impedance magnitude that varies with the
magnitude and angle of the applied current. Normally, a forward fault results
in a negative Z2c relay calculation.
Test Current
Solve
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 6.4
when:
Equation 6.5
Multiply the quantities in
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.
becomes:
Equation 6.6
when:
Equation 6.7
For a reverse fault impedance threshold, where Z2c = Z2R,
becomes:
Equation 6.8
when the angle calculation is the same as
Z2c
Re V
2
1 Z1ANG
I
2
•
Ð
(
)*
•
[
]
I
2
2
-------------------------------------------------------------------------
=
V
2
I
2
--------- • cos
V
2
Z1ANG
I
2
Ð
–
Ð
–
Ð
(
)
=
I
2
V
2
Z2c
----------
=
I
2
Ð
V
2
Ð
Z1ANG
Ð
–
=
I
TEST
3I
2
3V
2
Z2c
-------------
3V
2
Z2F
-------------
=
=
=
I
TEST
Ð
3I
2
Ð
3V
2
Ð
Z1ANG
Ð
–
=
=
I
TEST
3I
2
3V
2
Z2c
-------------
3V
2
Z2R
-------------
=
=
=
Summary of Contents for SEL-421
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