Date Code 20011026
Maintenance and Testing
7-21
SEL-321/321-1 Instruction Manual
You may wish to use the connections shown in Figure 7.3 or Figure 7.4 (modified to
use I
B
and I
C
). If you use either of these alternative connections, set the magnitude of
I
C
equal to the magnitude of I
B
and set the phase angle of I
C
equal to the phase angle
of (I
B
+ 180
°
).
Step 4.
The 50PP overcurrent elements operate based upon the magnitude of the vector
difference between any two-phase currents. Using the current connections shown in
Figure 7.7, the magnitude of I
BC
is twice the magnitude of the applied current. This
is illustrated by the following equations.
TEST
BC
B
BC
C
B
BC
C
B
TEST
I
•
2
I
I
•
2
I
I
I
I
I
I
I
=
=
−
=
−
=
=
Equation
7.1
With a 50PP2 setting of 2.22 amps, 50PP2 picks up when I
TEST
is greater than 1.11
amps.
Step 5.
Turn on the current test source and slowly increase the magnitude of current applied
until the 50PP2 element asserts, causing OUT8 to close. This should occur when
current applied is approximately 1.11 amps.
Note:
As you perform this test, other protection elements may assert, causing the relay to
close other output contacts and assert relay targets. This is normal and is not a cause
for concern.
Negative-Sequence Directional Element
The SEL-321 Relay includes a directional element that operates based upon the calculated
magnitude and angle of negative-sequence impedance applied to the relay. There are two
methods of testing the element. The first, using a single voltage and current, and the second
using three voltages and one current. Examples of both methods are provided below following
an explanation of the equations which define the element.
Negative-Sequence Directional Element Based Upon Negative-Sequence Impedance
The SEL-321 Relay calculates the magnitude and angle of negative-sequence voltage and current
applied to the relay. From that information, the relay calculates the magnitude of negative-
sequence impedance that lies collinear to the line positive-sequence impedance. The equation
defining that function is shown below:
(
)
2
2
*
2
2
I
]
I
•
ANG
1
Z
1
•
V
Re[
c
2
Z
°
∠
=
Equation
7.2
Where: Re indicates the real part of the term in brackets, for instance Re[A + jB] = A
*
indicates the complex conjugate of the expression in parentheses, (A + jB)* =
(A
-
jB).
The result of Equation 7.2 is an impedance magnitude value that varies with the magnitude and
angle of applied current. An example illustrates this operation.
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