Date Code 970103
Introduction
1-1
SEL-251, -2, -3 Instruction Manual
SECTION 1: INTRODUCTION
G
ETTING
S
TARTED
If you are not familiar with this relay, we suggest that you read this introduction, then perform
the Initial Checkout Procedure in
Section 7: Maintenance & Testing
.
O
VERVIEW
Conventional Terminal Block Model
The SEL-251 Relay is designed to protect distribution lines for all fault types. The following list
outlines protective features, performance, and versatility gained when applying the SEL-251
Relay to your installations.
•
Develop traditional and advanced schemes using flexible SEL
OGIC
TM
Control Equations
•
Phase-overcurrent elements have voltage control for load security
•
Negative-sequence elements reject load for more sensitive phase fault protection
•
Ground and residual overcurrent elements cover ground faults
•
Choose fast or electromechanical reset characteristic for time-overcurrent elements
•
Overcurrent elements inhibit recloser reset to prevent nuisance "trip-reclose" cycling
•
Sequence coordination avoids unnecessary tripping for faults beyond line reclosers
•
Undervoltage logic detects high-side transformer fuse operations
•
Six selectable setting groups cover all feeder protection contingencies
•
Circuit breaker monitor sums interrupted current in each pole to aid maintenance
•
Fault locator reduces line patrol and outage time for increased service reliability
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Eleven-cycle event report simplifies fault and system analysis
•
Comprehensive voltage, current, power, unbalance, and demand metering
•
Connects to SEL-RD Relay Display for easy information access
•
Improved
Fast Meter
•
Improved
Fast Operate
The SEL-251 Relay improves every aspect of feeder protection.
•
Security:
Undervoltage supervision and negative-sequence avoid load encroachment
•
Reliability:
Field-proven hardware; new backup concepts
•
Sensitivity:
Negative-sequence overcurrent elements for better phase fault coverage
•
Flexibility:
SEL
OGIC
Control Equations handle virtually every conceivable scheme