U.6.36
SEL-421 Relay
User’s Guide
Date Code 20090715
Testing and Troubleshooting
Checking Relay Operation
Figure 6.28
Finding Phase-to-Phase Test Quantities
One way to create a V
BC
phasor is to equate |V
B
| and |V
C
| and determine the
appropriate angles to make an equilateral triangle as shown in
Subtract 30 degrees (angle
β
1
) from 180 degrees to obtain the angle for test
source V
C
phasor
;
V
C
= 46.8
∠
150
°
V.
Similarly, add 30 degrees (angle
β
2
) to –180 degrees to obtain test source V
B
phasor; V
B
= 46.8
∠
–150
°
V.
Test voltage V
A
can be the nominal value, V
A
= 67
∠
0
°
V.
Thus, the resulting phase-to-phase voltage is V
BC
= 46.8
∠
–90
°
V, referenced
to the V
A
phasor at 0 degrees.
The relay measures phase distance element maximum reach when the faulted
phase-to-phase current lags the faulted phase-to-phase voltage by the distance
element maximum torque angle. In the SEL-421, the phase distance element
maximum torque angle is setting Z1ANG. Current I
BC
should lag voltage V
BC
by Z1ANG.
In this example, Z1ANG is 84.0 degrees. From
, the angle of I
B
is
the angle of I
TEST
, and the angle of I
C
is 180 degrees from the angle of I
TEST
.
The test source current for I
B
is the following:
Equation 6.13
And the test source current for I
C
is the following:
Equation 6.14
Checking the MBC2 Portion of the M2P Phase Distance Element
NOTE:
As you perform this test,
other protection elements can assert.
This causes the relay to assert other
targets and possibly close control
outputs. Be sure to isolate the relay
from the power system to avoid
unexpected system effects.
The following procedure describes how to test the B-phase to C-phase
distance element MBC2. Although this test refers directly to the Zone 2 phase
distance element, you can apply this procedure to any other forward-reaching
phase-to-phase distance element zone.
β
2 = 30˚
φ
= –150˚
θ
= 150˚
–90˚
90˚
VA
0˚
±
180˚
β
1 = 30˚
VBC
VB
VC
I
B
2.5
–90
°
Z1ANG
–
(
)
A
∠
=
2.5
–90
°
84
°
–
(
)
A
∠
=
2.5 –174
°
A
∠
=
I
C
–I
B
– 2.5 –174
°
A
∠
(
)
2.5 6
°
A
∠
=
=
=
Содержание 421
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