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Date Code 20011026
Maintenance and Testing
7-35
SEL-321/321-1 Instruction Manual
The 50QF negative-sequence overcurrent element operates based upon the
magnitude of 3I
2
applied. Using the current connections shown in Figure 7.7, we can
calculate the magnitude of 3I
2
applied based upon the magnitude of I
TEST
.
(
)
(
)
(
)
(
)
(
)
°
−
∠
=
−
=
−
=
−
=
=
+
+
=
+
+
=
90
732
.
1
•
I
I
•
3
a
a
•
I
I
•
3
I
•
a
I
•
a
I
•
3
:
so
,
I
I
and
0
I
I
•
a
I
•
a
I
I
•
3
I
•
a
I
•
a
I
•
I
B
2
2
B
2
B
B
2
2
B
C
A
C
B
2
A
2
C
B
2
A
3
1
2
Because the 50QF element operates based upon magnitude only, the equation above
is simplified.
732
.
1
•
I
I
•
3
TEST
2
=
With a 50QF setting of 0.5 amps, 50QF picks up when I
TEST
is greater than 0.288
amps.
In this example, select
│
I
TEST
│
= 2.5 amps, thus I
BC
= 5.0 amps and 3I
2
= 4.33 amps.
This selection fulfills the supervisory overcurrent conditions described above.
The reach of the distance element under test is defined by the element setting. In this
case Z2P = 9.36 secondary ohms.
Calculate the magnitude of V
BC
using Equation 7.8.
ondary
sec
volts
8
.
46
V
36
.
9
•
0
.
5
V
Z
•
I
V
BC
BC
BC
BC
BC
=
=
=
Equation
7.8
Refer to Figure 7.9 and use the equations below to calculate the magnitude and angle
of V
B
and V
C
based upon the magnitude of V
BC
calculated above.
Figure 7.9: Phase Distance Element Test Voltage Signals
When the magnitude of V
BC
calculated above lies between 67 and 35 volts, use
β
=
150° and
│
V
B
│
=
│
V
C
│
=
│
V
BC
│
.
Содержание SEL-321
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