10-22
Testing and Troubleshooting
Date Code 20020129
SEL-387E Instruction Manual
U87P Unrestrained Differential Element
Step 1.
Purpose: Verify the expected unrestrained differential element pickup setting.
Method: Execute
the
SHOWSET
command via the relay front panel or serial port
and verify the setting (i.e.,
SHO U87P<ENTER>
).
Note:
This value is
in per unit of tap.
Step 2.
Purpose: Calculate the required current to pick up the unrestrained differential
element.
Method: Calculate the expected pickup for the 87U element by multiplying the
U87P setting by the TAP1, TAP2, or TAP3 setting and the compensation
constant A shown in Table 10.3. The CT connection compensation
settings W1CTC, W2CTC, and W3CTC determine the A constant for the
calculations. Use the corresponding TAPn and WnCTC settings for the
winding under test.
&$87,21
The continuous rating of the current inputs is 3 • I
nom
. For this
test, you may want to choose low values of U87P and TAPn, in
order to limit the required test current to a safe value.
Step 3.
Purpose: Display the appropriate Relay Word bit on the front-panel LEDs.
Method: Execute
the
TARGET
command (i.e.,
TAR F 87U<ENTER>
). The
SEL-387E Relay now displays the state of several differential elements in
the second row of the front-panel LEDs. The 87U bit is the fourth from
the left.
Step 4.
Purpose: Connect and ramp a single-current test source until the appropriate LED
illuminates.
Method: Connect a single-current test source as shown in Figure 10.6. Turn on the
current test source for the winding under test, and slowly increase the
magnitude of current applied until the 87U element asserts. Note the
magnitude of the current applied. It should equal the value calculated in
step 2, ±5% ±0.02 I
nom
.
Step 5.
Purpose: Repeat the test for each phase for each winding if desired.
Method: Repeat Steps 1 through 4 for each phase. Remember to view the
appropriate TARGET and apply current to the appropriate winding. The
computer terminal will display the LED labels from left to right when the
TAR F
command is issued.