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Transformer Differential Relaying Upgrade

The site has five (5) substations at the main base. Substations 3, 4, and 5 have a total of eight (8) large power transformers that supply to critical operations at the installation. None of the transformers had dedicated differential protection when CLP took over the system in 2015.

The transformers were simply protected by overhead power fuses. The main issues with fuse protection to large power transformers at the substation are (1) the fuse operation is too slow for any internal faults to the transformer and (2) when a single fuse operates for a single-phase fault, the transformer will continue to supply the load with remaining two phases causing major stress to the transformer and poor power quality to the customer loads. CLP Engineering identified this problem as a system deficiency and implemented a differential relaying system. This required adding current transformers (CTs) to the existing transformer primary bushings, digital differential relay, and control wiring. The differential protection scheme utilizes existing primary side circuit switchers and secondary side main circuit breakers to create a differential zone. With the upgrade, the transformers are well protected and coordinated with the rest of the distribution system providing safer, more reliable, and quality power to end-users.


Project Overview

Location: Western United States
Market Sector: Government
Services Type: Design Engineering, Testing and Commissioning
Timeline: 2015-2016
Client: U.S. Air Force


Design Features

  • Advanced relaying and controls
  • Retrofit instrumentation and control wiring


Construction Features

  • Slip-over CT installation
  • Complex CT and control wiring
  • Relay and control panel adds



  • Improved reliability and safety
  • Dedicated and fast transformer protection
  • Advanced transformer monitoring

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