Sel 421 - Page 288

U.6.26
SEL-421 Relay User’s Guide Date Code 20090715
Testing and Troubleshooting
Checking Relay Operation
Ground Overcurrent Elements
The SEL-421 ground overcurrent elements compare a residual ground
calculation of the three-phase inputs with the residual overcurrent setting. The
relay makes this residual current calculation:
3I
0
= A-phase + B-phase + C-phase
The relay asserts ground overcurrent elements when the 3I
0
calculation
exceeds the ground current element pickup setting. If balanced currents are
applied to the relay, the relay reads 3I
0
= 0 (load conditions) because the
currents cancel in the calculation; the relay does not pick up the ground
overcurrent elements.
For testing, apply current to a single phase of the relay, causing the residual
overcurrent elements to operate. For example, assume 1 A of current on
A-phase and zero current input on B-phase and C-phase:
3I
0
= 1 A + 0 + 0 = 1 A (a simulated ground fault condition)
Checking the Negative-Sequence Instantaneous Overcurrent Element,
50Q1
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 procedure in the following steps tests the 50Q1 negative-sequence
overcurrent element. Use a similar procedure to test other overcurrent
elements.
This example assumes that you have successfully established communication
with the relay (see Making an EIA-232 Serial Port Connection on page U.4.5).
In addition, you must be familiar with relay access levels and passwords (see
Changing the Default Passwords on page U.4.6 to change the default access
level passwords and enter higher relay access levels). You should be familiar
with
ACSELERATOR QuickSet (see Section 3: PC Software).
Step 1. Configure the relay.
a. Start
ACSELERATOR QuickSet and read the present
configuration in the SEL-421.
b. Click Settings > Read.
The relay sends all settings and configuration data to
ACSELERATOR QuickSet.
c. Expand the Group 1 settings and click the
Negative-Seq Inst O/C button of the Settings tree view
as shown in Figure 6.18.
You will see the Negative Sequence Instantaneous
Overcurrent dialog box similar to Figure 6.18.
d. Click the arrow in the Instantaneous and Definite
Time Overcurrent Element Levels E50Q dialog box
and select 1.
e. For this test, set the 50Q1P level to 1.00 and 67Q1TC
to 1.
Courtesy of NationalSwitchgear.com