Sel 411L - Page 745

P.11.31
Date Code 20151029 Protection Manual SEL-411L Relay
Testing and Troubleshooting
Checking Relay Operation
The relay calculates the negative-sequence impedance Z2 from the
magnitudes and angles of the negative-sequence voltage and current.
Table 11.13 defines this function (the ‘c’ in Z2c indicates “calculated”).
Equation 11.13
The result of Equation 11.13 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 11.13 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 11.14
when:
Equation 11.15
Multiply the quantities in Equation 11.14 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 11.14
becomes:
Equation 11.16
when:
Equation 11.17
For a reverse fault impedance threshold, where Z2c = Z2R, Equation 11.14
becomes:
Equation 11.18
when the angle calculation is the same as Equation 11.17.
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)
Z2c
Re V
2
1 Z1ANG I
2
• 

• 
I
2
2
-------------------------------------------------------------------------=
V
2
I
2
---------
• cos V
2
Z1ANG I
2
––=
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
-------------===