8.14
SEL-551 Relay Instruction Manual Date Code 20110408
Testing and Troubleshooting
Commissioning Testing
Residual Ground
Overcurrent Elements
The SEL-551 has four residual ground overcurrent elements. They all operate
based on a comparison between a residual calculation of the three-phase
inputs and the residual ground overcurrent setting. The residual calculation
that is performed on the three-phase inputs is as follows:
Equation 8.4
This means that if balanced positive-sequence currents are applied to the relay,
the relay reads IG = 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:
Equation 8.5
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 previously outlined in this section.
Commissioning Testing
When: When installing a new protection system.
Goals:
➤ Ensure that all system ac and dc connections are correct.
➤ Ensure that the relay functions as intended using your settings.
➤ Ensure that all auxiliary equipment operates as intended.
What to test:
➤ all connected or monitored inputs and outputs
➤ polarity and phase rotation of ac current connections
➤ simple check of protection elements
SEL performs a complete functional check and calibration of each relay
before it is shipped. This helps ensure that you receive a relay that operates
correctly and accurately. Commissioning tests should:
1. Verify that the relay is properly connected to the power system
and all auxiliary equipment.
2. Verify control signal inputs and outputs.
3. Check breaker auxiliary inputs, SCADA control inputs, and
monitoring outputs.
4. Use an ac connection check to verify that the relay current
inputs are of the proper magnitude and phase rotation.
Brief fault tests ensure that the relay settings are correct. It is not necessary to
test every relay element, timer, and function in these tests.
At commissioning time, use the relay METER command to record load
currents.
IG A-phase B-phase C-phase++=
IG 1 0 0++=
1 (simulated ground fault condition)=
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