500 kV Compact Structure Design Approach
By Buck Fife, POWER Engineers, and Paul M. Legrand II, P.E., Entergy
Entergy Corporation is a major utility with headquarters in New Orleans, LA and operations in four southern states and the city of New Orleans. Entergy’s five operating companies have 15,000 miles of transmission lines, 40 generating plants powered by a diverse fuel mix with 30,000 megawatts of power capacity serving 2.8 million customers. The company has annual revenues of $11.5 billion and 13,000 dedicated employees.
The Entergy service territory, particularly in south Louisiana, has been experiencing significant growth, mainly from industrial development. Planners at Entergy have determined that additional 500 kV transmission capacity is needed to service the area. The current Entergy standard structure at 500 kV is a horizontal configuration on lattice towers or tubular H-frames requiring a 225 foot right of way. Due to the short lead times associated with some of the industrial development projects and the difficulty in acquiring the standard right of way on a timely basis to facilitate the development projects, the Entergy line design team identified a need to explore ways to compact the design. The planners have suggested that the line design team explore ways of fitting a new 500 kV line with an ampacity rating of 3000 amps in an existing 230 kV corridor that is 125 feet wide. This would eliminate the need for the lengthy right-of-way acquisition process.
Entergy contracted POWER Engineers, Inc. to support the development of a 500 kV, single circuit, compact design to supplement their current 500 kV structure families. The design approach included evaluating the challenges specific to Entergy’s system, defining constraints and parameters important to those challenges, and preparing a cost-benefit analysis of the available options.
The design team of Entergy and POWER assigned priorities to common performance metrics and the design parameters that impact those metrics to highlight the important aspects of the structure design. The team then performed a parametric study of phase geometry, structure configuration and conductor configuration for the predetermined key performance metrics to quantify the importance of the parameters involved. The team studied the highest impact parameters in detail to understand the limits of compaction and related challenges.
Numerous structure configurations were developed based on the design efforts. The configurations included single and two-pole structure types of varying phase arrangements. Clearance and insulation requirements were varied, ranging from Entergy’s standard configurations to the limits determined through detailed analysis of those requirements. As phase spacing was decreased, the conductor configurations were adjusted to achieve audible noise requirements. The advantages and disadvantages of the various structure configurations were compared. This comparison included consideration for reliability performance, galloping, aesthetics, design flexibility, constructability, maintenance and costs.
The analysis led to the conclusion that one structure configuration did not necessarily satisfy the requirements of all of the possible situations and criteria that engineers may face on future projects. To best address the anticipated challenges, primary and secondary configurations were selected. These configurations will be used individually or in combinations to address a wide variety of likely design situations.
This paper was presented at the 2016 Transmission and Substation Design and Operation Symposium.
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