Sel 421 - Page 291

U.6.29
Date Code 20090715 User’s Guide SEL-421 Relay
Testing and Troubleshooting
Checking Relay Operation
The result of Equation 6.3 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 Equation 6.3 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 Equation 6.4 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 6.4
becomes:
Equation 6.6
when:
Equation 6.7
For a reverse fault impedance threshold, where Z2c = Z2R, Equation 6.4
becomes:
Equation 6.8
when the angle calculation is the same as Equation 6.7.
For more information on the directional elements, see Ground Directional
Element on page R.1.27 and Quadrilateral Ground Distance Elements on
page R.1.54. For settings and application information, see Section 1:
Protection Application Examples in the Applications Handbook.
where:
V
2
= the negative-sequence voltage
I
2
= the negative-sequence current
Z1ANG = the positive-sequence line impedance angle
Re = the real part of the term in brackets, for example,
(Re[A + jB] = A)
* = the complex conjugate of the expression in parentheses,
(A + jB)* = (A – jB)
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
-------------===
Courtesy of NationalSwitchgear.com