Success Story

System-Wide Automation Solution Prolongs Life of Existing Relays



EPB added approximately 100 motor-operated switch controls to their existing 46 kV subtransmission system. The closed-loop configuration of the EPB subtransmission system required directional fault detection capability in a compact footprint. EPB selected SEL-700GT Generator Protection Relays and installed them in pairs, enclosed in custom pole-mounted cabinets. To support the switch controls in isolating faults and to leverage data to automatically restore power, they also installed:

  • SEL-2411 Programmable Automation Controllers.
  • SEL-351S Protection Systems.
  • SEL-3530 Real-Time Automation Controllers (RTACs), configured as distribution automation controllers (DACs).

The DACs are installed in a central location close to the SCADA operations center and are the “brains” of the automation system. They communicate with the motor-operated switch cabinets and feeder breaker relays in the substation to know when a fault occurs, isolate the fault, and restore power to as many customers as possible.

To enable the central DACs to communicate with the legacy electromechanical relays, SEL designed a system that uses a remote terminal unit (RTU) to interface with the electromechanical relays’ contacts. This allowed EPB to incorporate their old relays as part of the fully functional automation system and reduced the time and cost to implement the project.

We knew we were going to have trouble developing that [technical strategy] in-house. SEL had the expertise that we didn’t have.
Don NannySenior Manager of System Analysis and Control, Chattanooga EPB


With the new motor-operated switches and automation of their entire subtransmission system (including areas protected by electromechanical relays), EPB can restore power faster than ever before after an outage. If their service territory experiences a major event, EPB can now depend on automatic fault location, isolation, and service restoration to bring the majority of customers back online. EPB has visibility into their entire subtransmission system and can determine which areas need to be repaired before they send technicians into the field.

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