What is SEL in electrical engineering?

SEL (Schweitzer Engineering Laboratories) manufactures protective relays and automation equipment that detect abnormal power system conditions — faults, overloads, equipment failures — and initiate protective actions like opening circuit breakers.

What software configures SEL relays?

ACSELERATOR Quickset (SEL-5030) is the primary tool for SEL relay settings, commissioning, and event retrieval. It is available as a free download from selinc.com.

What protection functions do SEL relays provide?

SEL relays implement overcurrent (50/51), distance (21), differential (87), directional (67), voltage (27/59), frequency (81), and many other ANSI standard protection elements.

What communication protocols do SEL relays support?

SEL protocol (proprietary), DNP3, Modbus RTU/TCP, IEC 61850 (GOOSE and MMS), and MIRRORED BITS for peer-to-peer protection communication.

Complete Guide to Schweitzer Engineering Laboratories (SEL) Protective Relays and Automation

Quick Answer

Schweitzer Engineering Laboratories (SEL) manufactures protective relays, automation controllers, and power management systems used in electric utility transmission, distribution, and industrial power applications. SEL devices are configured using ACSELERATOR Quickset software and support protection functions including overcurrent, distance, differential, and directional elements. They communicate via SEL proprietary protocol, DNP3, Modbus, and IEC 61850, and integrate with SCADA systems through the SEL Real Time Automation Controller (RTAC).

What Is SEL?

Schweitzer Engineering Laboratories was founded in 1982 by Dr. Edmund O. Schweitzer III in Pullman, Washington. SEL is an employee-owned company that designs and manufactures protective relays, automation controllers, meters, and communication equipment for electric power systems. SEL relays are the most widely deployed protective relays in North American electric utilities and are extensively used in industrial power systems including refineries, data centers, pipelines, and manufacturing facilities.

SEL's product philosophy emphasizes open documentation (all instruction manuals are free), robust hardware designed for harsh substation environments, and continuous firmware improvement throughout the product lifecycle. This approach has earned SEL a dominant position in the power protection market.

SEL Product Families

Protective Relays — SEL-300 series (transmission), SEL-400 series (advanced transmission), SEL-700 series (generator/motor), SEL-351 (feeder), SEL-387 (transformer differential), SEL-651R (recloser)
Automation Controllers — SEL RTAC (Real Time Automation Controller) for substation automation, data concentration, and protocol translation
Power Quality Meters — SEL-734 advanced power quality and revenue meter
Network Infrastructure — SEL-2740 series industrial Ethernet switches, SEL-2407 GPS time servers

ACSELERATOR Software Suite

ACSELERATOR Quickset (SEL-5030) — Relay settings, commissioning, event retrieval, and monitoring. Free download from selinc.com.
ACSELERATOR Architect (SEL-5035) — IEC 61850 substation configuration and SCL file management
ACSELERATOR RTAC (SEL-5033) — RTAC programming using IEC 61131-3 languages
ACSELERATOR Team (SEL-5042) — Enterprise settings management and version control

Protection Functions

SEL relays implement ANSI standard protection elements:

50/51 — Overcurrent — Phase and ground overcurrent with IEEE and IEC time-current curves
21 — Distance — Impedance-based line protection with multiple zones and pilot schemes
87 — Differential — Current differential for transformers, generators, and lines
67 — Directional Overcurrent
27/59 — Under/Overvoltage
81 — Frequency

Communication Protocols

SEL Protocol — Proprietary ASCII serial protocol for relay configuration and data extraction
MIRRORED BITS — Peer-to-peer Boolean transfer for pilot protection schemes
DNP3 — SCADA integration with utility masters. See DNP3 protocol guide.
IEC 61850 — GOOSE messaging and MMS for modern substation automation
Modbus — Legacy SCADA and energy management integration. See Modbus protocol guide.

SEL in Power Systems

Transmission substations — Line protection (distance, differential), bus protection, breaker failure
Distribution feeders — Overcurrent protection, recloser control, sectionalizer logic
Generators — Generator protection (40 loss of field, 78 out of step, 87G differential, 32 reverse power)
Industrial facilities — Motor protection, transformer protection, arc flash detection
Data centers — Utility tie protection, generator paralleling, automatic bus transfer

SEL and NERC CIP Cybersecurity

SEL devices at BES (Bulk Electric System) substations are subject to NERC CIP requirements including CIP-005 (electronic security perimeters), CIP-007 (systems security management), and CIP-010 (configuration change management). SEL relays support password management, port hardening, audit logging, and DNP3 Secure Authentication for compliance.