Schweitzer Engineering Laboratories SEL-251 - Section 5: Applications;Section 5: Applications

Date Code 20000421 Applications i
SEL-251, -2, -3 Instruction Manual
TABLE OF CONTENTS
SECTION 5: APPLICATIONS 5-1
Fuse-Saving Schemes................................................................................................................... 5-1
Fuse-Saving Scheme Using the 79SH Bit............................................................................ 5-2
Fuse-Saving Scheme Using the 52AT Bit............................................................................ 5-3
Trip-Saving Scheme..................................................................................................................... 5-4
Blocking Schemes for Cold Load Inrush ..................................................................................... 5-5
Blocking Scheme for Cold Load Inrush Using the 50C Bit................................................. 5-5
Blocking Scheme for Cold Load Inrush Using the 52AT Bit .............................................. 5-7
Disable Ground/Residual Overcurrent Elements......................................................................... 5-8
Disable Ground/Residual Overcurrent Elements with Permissive Trip Input ..................... 5-8
Disable Ground/Residual Overcurrent Elements with Block Trip Input ............................. 5-9
Disable Ground/Residual Overcurrent Elements with Torque Control Input.................... 5-10
Minimum Response Time for Time-Overcurrent Elements....................................................... 5-11
Negative-Sequence Elements Protect for Phase Faults.............................................................. 5-11
Negative-Sequence Overcurrent Elements and Delta-Wye Transformers......................... 5-13
Summary of Figure 5.8............................................................................................... 5-13
Summary of Figure 5.9............................................................................................... 5-16
Underfrequency Load Shedding Input ....................................................................................... 5-16
Undervoltage Load Shedding..................................................................................................... 5-17
Breaker Failure........................................................................................................................... 5-17
Breaker Failure Implementation Notes .............................................................................. 5-18
Qualify Trip Condition With Presence of Load Current............................................ 5-18
Limit Trip Condition Time Duration.......................................................................... 5-18
Residual Current Unbalance Alarm ........................................................................................... 5-19
Block Voltage Regulating Load-Tap-Changer Operation.......................................................... 5-19
Extra Alarm Output Contact ...................................................................................................... 5-20
Backup Feeder Relays With a Bus Relay................................................................................... 5-20
Feeder Relay Setting Changes.................................................................................................... 5-22
Bus-Tie Relay Setting Changes.................................................................................................. 5-22
Clock-Driven Setting Group Selection....................................................................................... 5-22
Demand-Driven Setting Group Selection................................................................................... 5-22
System Restoration After Underfrequency Load Shedding....................................................... 5-23
Settings Sheets............................................................................................................................ 5-25
TABLES
Table 5.1: System Conditions and Currents Generated ......................................................................... 5-12