10-16 Testing and Troubleshooting Date Code 20080110
SEL-387E Instruction Manual
Negative-Sequence Overcurrent Elements
The SEL-387E Relay has 12 negative-sequence overcurrent elements. They all operate based on
a comparison between a negative-sequence calculation of the three-phase inputs and the
negative-sequence overcurrent setting. The negative-sequence calculation that is performed on
the three-phase inputs is as follows:
3I2 = (A-phase + B-phase (shifted by –120
°
) + C-phase (shifted by 120
°
))
This means that if balanced positive-sequence currents are applied to the relay, the relay reads:
3I2 = 0 (load conditions)
For testing purposes, apply a single-phase current to the relay and the negative-sequence
overcurrent elements will operate. For example, assume one ampere on A-phase and zero on B-
and C-phases:
3I2 = (1 + 0 (shifted –120
°
) + 0 (shifted 120
°
)) = 1 (simulated ground fault condition)
Test the instantaneous and time-delayed negative-sequence overcurrent elements by applying
current to the inputs and comparing relay operation to the negative-sequence overcurrent
settings. These tests were outlined earlier in this section.
Residual Overcurrent Elements
The SEL-387E Relay has many residual overcurrent elements. They all operate based on a
comparison between a residual calculation of the three-phase inputs and the residual overcurrent
setting. The residual calculation that is performed on the three-phase inputs is as follows:
IR = (A-phase + B-phase + C-phase) (all angles are considered as well)
This means that if balanced positive-sequence currents are applied to the relay, the relay reads
IR = 0 (load conditions) because the currents cancel one another.
For testing purposes, apply a single-phase current to the relay and the residual overcurrent
elements will operate. For example, assume one ampere on A-phase and zero on B- and
C-phases:
IR = (1 + 0 (shifted 120
°
) + 0 (shifted –120
°
)) = 1 (simulated ground fault condition)
Test the instantaneous and time-delayed residual overcurrent elements by applying current to the
inputs and comparing relay operation to the residual overcurrent settings. These tests were
outlined earlier in this section.
Torque-Control
SEL
OGIC
®
control equations are provided for various overcurrent elements (i.e., 51P1TC) that
provide a torque-control (required to be true for element operation). Test the torque-control
equations.
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