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Project: T143700 #3398

The scope of this project includes a critical review of available technical information and recent developments paired with the rationale for these best practices and finally, the development of this guide. To address both the electrical and mechanical issues effectively, the present study was conducted in two phases. This report addresses the first phase of the study and includes the electrical aspects of EHV AC line design. The second phase of this study will address the mechanical aspects of EHV AC line design.

The first phase of this project dealt with the development of the following seven chapters: (1) Introduction, (2) A Basic Understanding of a Power System, (3) Corona Effects, (4) EMF Issues, (5) Conductor Selection Process, (6) Insulation Coordination, and (7) Lightning.

Chapter 2 – A Basic Understanding of a Power System. This chapter provides load-flow characteristics of a line, determination of an economic voltage level, definition of surge-impedance loading (SIL), description of AC versus DC transmission and the criteria when is DC preferred and the determination of line capacity based on electromechanic stability and thermal and voltage constraints. 

Chapter 3 - Corona Effects. This chapter describes the basic mechanism of audible noise generation by an EHV transmission line due to the presence of corona. EHV lines can also produce significant electromagnetic noise that can interfere with AM/FM radio and TV, even under fair weather conditions. Limits of these disturbances at the edge of the ROW are specified by national codes and standards. The design and selection of line conductors, accessories, hardware, insulators and clearances are dictated by these limits; hence the expected increase in cost of a transmission line with more strict limits.

Chapter 4 – EMF Issues. This chapter provides a basic description of the EMF at operating frequency and the impact of known health issues related to humans, animals, and plants. Also, a brief review of the international and national standards is presented. The computation of fields near the edge of the ROW of EHV lines is presented. This section also presents “best practices” on EMF effects associated with EHV line design and its impact on choice of conductor sizes, bundle configuration, attachment heights, and ROW.

Chapter 5 - Conductor Selection Process. This chapter provides a basic description on conductor selection process based on economics, such as the balance of initial capital cost against future cost of losses (due to corona and resistive losses, for example). The process of determinating the thermal limit (capacity) of an EHV line is also discussed.

Chapter 6 - Insulation Coordination. This chapter focuses on the design of EHV line insulation. This is an important component of EHV line design because it has a significant impact on line cost and operational reliability. This section presents best practices for dimensioning of the line insulators based on statistical methods described in international standards and guidelines. Both external and internal overvoltages should be considered in the design process.

Chapter 7 - Lightning. This chapter provides an overview of lightning effects on transmission facilities. Lightning flashes are a major source of outages and can last from a few seconds to several days due to damage of the line. A basic description of overvoltage due to a lightning strike is presented and the dissipation mechanism and mitigation are discussed.