Electrical Engineering

Electrical Utility Power Systems

McGill Associates’ electrical engineering team members have extensive experience designing electrical power distribution systems for institutional, commercial, industrial and power utility facilities for both government and private entities. These power systems encompass the full range of electrical power from small low-voltage (600 V class) through bulk power transmission facilities up to 500 kV. Our experience includes both underground and overhead medium voltage (2.4 through 34.5 kV) systems, including distribution, transmission and substation facilities. We understand the special issues of replacing power systems while minimizing the impact to the served buildings and facilities.

case study

CLIENT: City of Elizabethton, Tennessee
PROJECT DESCRIPTION: McGill Associates provided design, bidding and construction administration services to rebuild the 1940s-era Elizabethton District Substation. The existing substation contained a single 69-12.47 kV, 30 MVA OA transformer bank consisting of three single-phase units, plus a spare. The 69 kV space-frame switching structure connected three transmission lines and the primary of the transformer bank. The 12.47 kV substation provided eight – 12.47 kV overhead distribution circuits for the system. The majority of the equipment within substation was original equipment. The relays were antiquated and the structures had significant corrosion. One of the transformers would not pass testing and was therefore not available for service.

Due to the location and high visibility of the substation, the City not only wanted the new station to be its “flagship” state-of-the-art station, but also have aesthetic appeal. Complicating the project was a Tennessee Department of Transportation (TDOT) project replacing the Highway 400 Watauga River Bridge. TDOT had acquired a large portion of the substation property, resulting in significant substation rearrangement. Each of these objectives had to be met with minimal outages due to the difficulty in transferring load to other substations.

With these constraints in mind, McGill Associates designed a low-profile substation configuration that conformed to the available space while providing free-flowing access to all major equipment. The new substation incorporates 2-25 MVA ONAN rated transformers, each supplying a 12.47 kV bus via main and bank breakers. The two buses are configured with main and transfer buses, with the main buses connected with a tie-breaker. Each bus supplies four – 12.47 kV circuits via underground medium voltage cable exits to improve their appearance. The transformers are protected by high-side 69 kV circuit breakers. The station is outfitted with a control house containing modern microprocessor relaying and SCADA equipment. The protection scheme employs overcurrent relaying for the distribution circuits, high-impedance differential relaying for the buses, and differential relaying for the transformers. Backup relaying is incorporated throughout, including transfer trip via radio to the supplying transmission substation. Substation automation features main-tie-main switching upon transformer or line failure to automatically restore all distribution circuits.

at a glance
  • Utility Distribution Substation Design
  • Standby Generation Power System Design
  • Commercial and Industrial Facility System Design