SEL Communications Protocols — SEL Protocol, MIRRORED BITS, and Fast Message

Quick Answer

SEL devices use several proprietary communications methods that serve different jobs: SEL ASCII (human/engineering access for settings, metering, and events), MIRRORED BITS® (peer-to-peer Boolean exchange for protection and teleprotection), and SEL Fast Messaging (binary Fast Meter, Fast Operate, Fast SER, and related messages used by SEL RTACs and communications processors). Open protocols—DNP3, Modbus, and IEC 61850—are the usual choices for SCADA and multi-vendor integration. Do not confuse Fast Messaging with MIRRORED BITS or IEC 61850 GOOSE: Fast Messaging is not sub-cycle protection messaging.

SEL ASCII Protocol

Plain-language ASCII commands over EIA-232/EIA-485 serial ports and, on many Ethernet-capable relays, over Telnet or other engineering access paths. acSELerator QuickSet uses this access path for settings files, metering, and event retrieval. Common commands include ACC (request Access Level 1; password follows), 2AC (Access Level 2), SHO (show settings), TAR (targets / Relay Word), EVE (event reports), and MET (metering). Exact command sets and default passwords are model- and firmware-specific—confirm in the instruction manual.

MIRRORED BITS

Peer-to-peer exchange of a small set of Boolean bits (commonly eight transmit/receive bits mapped to Relay Word bits) between SEL IEDs—or between a relay and devices such as the SEL-2507 High-Speed Remote I/O Module. Channels are typically dedicated serial/fiber links or multiplexed IEEE C37.94 optical paths, not a general SCADA LAN poll.

End-to-end timing depends on baud rate, device processing interval, and channel equipment. Classic MIRRORED BITS links are often on the order of a few milliseconds. Selected high-speed configurations (for example SEL-2507 at 115.2 kbps) advertise about 1 ms end-to-end operate time—not a universal “<1 ms on any fiber” guarantee. Common protection uses include pilot schemes such as POTT and DCB, where relays exchange directional or scheme bits rather than only a single fixed “directional element” signal.

SEL Fast Messaging

SEL Fast Messaging is a binary SEL protocol family used so an RTAC or communications processor can collect and control data more efficiently than ASCII command/response. Typical message types include Fast Meter, Fast Operate, Fast Message Read, and Fast SER, plus unsolicited write support on some communications-processor links. On the SEL RTAC protocol list it appears as a client/server protocol alongside SEL ASCII—separate from peer-to-peer MIRRORED BITS and IEC 61850 GOOSE.

Not “sub-cycle Boolean messaging.” For protection-speed peer Boolean transfer use MIRRORED BITS or IEC 61850 GOOSE; use Fast Messaging for concentrator-style metering, status, SER, and control between SEL devices and an RTAC/communications processor.

Protocol Selection

• SEL ASCII → engineering access, settings, event retrieval, interactive metering
• MIRRORED BITS → protection / teleprotection peer Boolean exchange
• SEL Fast Messaging → RTAC or communications-processor Fast Meter/Operate/SER collection
• DNP3 → SCADA (events, timestamps, controls)
• IEC 61850 → MMS SCADA and GOOSE peer messaging in IEC 61850 substations
• Modbus → simple metering / legacy SCADA and energy dashboards