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System-Wide Automation Solution Prolongs Life of Existing Relays

Chattanooga Heo


Chattanooga Electric Power Board (EPB) serves a territory of more than 600 square miles. To ensure reliable power delivery to this service area, EPB was determined to engineer a dependable subtransmission system that would restore power more quickly than their existing system.

Adding motor-operated switches and automating the subtransmission system would help minimize outages and enable quicker recovery across their service territory, but there was a catch. EPB wanted to use existing electromechanical relays with the new automated subtransmission system. Designing a modern system that incorporates older technology required some creative thinking.

Customer Profile

  • Customer: Chattanooga Electric Power Board (EPB)
  • Location: Chattanooga, Tennessee
  • Industry: Utility
  • Application: Automation, Engineering Services

We knew we were going to have trouble developing that [technical strategy] in-house. SEL had the expertise that we didn’t have.

Don Nanny, Senior Manager of System Analysis and Control, Chattanooga EPB


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.


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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Innovation, Persistence, and a New Beginning

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Innovation, Persistence, and a New Beginning