MV Feeder Protection Settings: 50/51, 67, 79 and IEC 60255 Evidence

A setting workflow for medium-voltage feeder IEDs covering overcurrent, earth fault, directionality, auto-reclose, CT burden, grading and minimum/maximum fault

1. Begin with Verified Inputs

IEC 60255-1:2022 states common requirements for measuring relays and protection equipment. IEC 60255-151:2009 specifies minimum requirements for overcurrent and undercurrent protection, including measurement and time-delay characteristics.

A setting file is only as good as the source data. Freeze CT ratio and class, VT ratio and earthing, feeder impedance, source minimum and maximum fault, cable thermal limit, transformer inrush, motor/inverter infeed and breaker clearing time before calculating settings.

2. Function Map

FunctionTypical purposeMain setting risk
50 / 51Phase instantaneous and time overcurrentLost selectivity or failure to detect remote faults
50N / 51NEarth-fault protectionWrong zero-sequence path, residual CT error or excessive neutral sensitivity
67 / 67NDirectional phase or earth faultIncorrect polarizing quantity, angle convention or network operating mode
79Automatic reclosingUnsafe reclosing onto a permanent fault, trapped generation or an out-of-sync source
50BFBreaker-failure backupMissing breaker status, wrong timer or inadequate upstream trip path

3. Grading Workflow

1. Calculate the highest load, cold-load pickup, motor start, transformer inrush and permissible emergency loading.

2. Determine minimum and maximum fault current at each protected bus and the remote end of the feeder.

3. Select pickup above the declared load envelope but below the minimum fault that must be detected.

4. Apply the selected time-current curve and grade with downstream relays, fuses, reclosers and upstream protection.

5. Recheck directional elements for every source and line configuration.

Do not claim selectivity from a single fault case. The final evidence must show operating time, breaker clearing time, CT saturation margin and grading margin for all required cases.

4. Commissioning and Change Control

  • Test injection, binary I/O, trip circuit supervision, breaker failure, interlocks, autoreclose block and event records.
  • Confirm the relay firmware, setting group, CT polarity, phase rotation, time source and communication map.
  • Store the approved setting file with the one-line revision and the source-data calculation.
  • Any feeder topology, transformer tap, source contribution or CT change requires a controlled protection review.

*Reference: IEC 60255-1:2022; IEC 60255-151:2009; IEC 60255-27:2023; IEC 61850 where digital interfaces are used.*