P.11.28
SEL-411L Relay Protection Manual Date Code 20151029
Testing and Troubleshooting
Checking Relay Operation
Phase Overcurrent Elements
The phase overcurrent elements compare the phase current applied to the
secondary current inputs with the phase overcurrent element pickup setting.
The relay asserts the phase overcurrent elements when any of the three phase
currents exceeds the corresponding element pickup setting.
Negative-Sequence Overcurrent Elements
The negative-sequence overcurrent elements compare a negative-sequence
calculation of the three-phase secondary inputs with the corresponding
negative-sequence overcurrent element pickup setting. The relay makes this
negative-sequence calculation (assuming ABC rotation):
3I
2
= A-phase + B-phase (shifted by –120°) + C-phase (shifted by 120°)
The relay asserts negative-sequence overcurrent elements when the 3I
2
calculation exceeds the corresponding negative-sequence current pickup
setting. If balanced currents are applied to the relay, the relay reads 3I
2
0
(load conditions) and does not pick up the negative-sequence overcurrent
elements.
For testing, apply current to a single phase of the relay, causing the negative-
sequence overcurrent elements to operate. For example, assume 1 A of current
on A-phase and zero current input on the B-phase and C-phase:
3I
2
= 1 A + 0 (shifted –120°) + 0 (shifted 120°) = 1 A (a simulated ground
fault condition)
Ground Overcurrent Elements
The 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
The procedure in the following steps tests the 50Q1 negative-sequence
overcurrent element. Use a similar procedure to test other overcurrent
elements.
Step 1. Configure the relay.
a. Start
ACSELERATOR QuickSet and read the present
configuration in the relay.
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.
To Next Page
Loading...
