It is widely accepted that grid expansion is necessary. In July, the Department of Energy announced up to $371 million in projects to expedite the permitting of high-voltage interstate transmission projects as part of its $760 million Transmission Siting and Economic Development (TSED) Grant Program. In addition, MISO released its Tranche 2 plan for $17B to $23B to expand 765 kV transmission in early 2024. Why the push for extra high voltage (EHV) transmission and why now?
Renewable Integration and Increasing Power Demand:
Wind and solar farms are often found in remote areas, away from population centers. To realize visions of integrated renewable energy systems delivering reliable, efficient power over long distances, we need transmission lines with high-voltage capacity. EHV transmission lines, typically 345kV and above, can efficiently transport electricity over long distances with minimal transmission losses while enabling better load balancing and managing voltage drop to ensure grid stability. Secondly, the rise in digitalization and electrification is also driving the need for high-capacity transmission lines to manage the growing load. Due to AI, data centers are significantly contributing to the increasing demand for energy.
However, planning and engineering EHV transmission lines requires a new perspective. One that considers different design standards from typical high-voltage or medium-voltage systems with compact designs to transfer more power in smaller corridors. It’s important to understand and address the following distinguishing factors to ensure EHV transmission lines are designed and implemented safely and effectively, maximizing benefits while minimizing risks and costs for public stakeholders and the environment.
System Reliability and Stability:
EHV lines play a critical role in grid stability and resilience; proper planning is essential to balance supply and demand over large areas. They require advanced protection systems to detect and isolate faults and often use larger diameter or bundled conductors to reduce losses and manage higher current levels. EHV systems are susceptible to corona discharge, which can cause energy losses and noise. Mitigations, such as bundled conductor configurations and smooth conductor surfaces help reduce corona discharge to ensure an efficient and quiet operation. There are programs and guidelines to ensure requirements, such as minimum ampacity ratings, audible noise, electric field, magnetic field, and corona grading ring requirements, are met.
Managing reactive power and voltage stability is critical at higher voltages, often requiring the use of shunt reactors, capacitors, and other compensation devices. Calculations to determine the resistive losses, reactive losses and transposition requirements all play a role in managing power quality with EHV lines.
Technical and Engineering Considerations:
Electrical clearance. Higher voltage systems require larger clearances between conductors and the ground or nearby structures to prevent electrical arcing. When calculating clearance, Section 23 of the National Electrical Safety Code® (NESC®) provides minimum safety code requirements, and applying design buffers can help address a wide range of potential variables and uncertainties. These buffers may include setting depths, wire stringing, easement use, slip joint variances, and grade change potential.
Stronger towers. EHV systems need taller and more resilient transmission towers to maintain clearances and support heavier conductors. Structural analysis of existing towers and/or review of tower specifications can ensure towers will withstand expected loads including wind, ice, seismic events, and weight of conductors. For example, after it was determined existing towers needed to be modified to accommodate a new 6-bundle conductor assembly, Black & Veatch designed a new series of guyed-vee lattice steel towers for a utility’s 765kV transmission line project.
Insulation: EHV lines use robust insulation systems, such as ceramic, non-ceramic, or toughened glass insulators, to handle higher mechanical and electrical stresses and extreme environmental conditions. Electrical studies can help determine length, geometry, and electrical properties for certain voltages. An insulation evaluation should consider power frequency, switching surge impulses, and lightning impulses for the site of the transmission facility.
Vibration control. Higher voltages and larger spans can lead to increased aeolian vibration and oscillation of conductors requiring vibration dampers and spacers. Vibration can induce mechanical stresses and hardware damage over time. Controlling vibrations boosts system reliability, minimizes damages due to fatigue, and helps conserve operational and maintenance costs.
Right of Way
EHV lines require a wider right of way and vegetation widths to ensure safety and compliance with regulatory clearances. Determining the recommended transmission line right of way is typically based on considering such impacts as structure deflection, wire blowout, design buffers, and minimum vegetation clearance and approach distances. Considering the environmental impact of EHV lines is also important. Higher voltages produce stronger Electromagnetic Fields (EMF) requiring careful assessment and mitigation to minimize impacts on health and the environment. Studies to determine the impact of mutual coupling to parallel existing infrastructure should be performed, the results of this analysis will determine if existing relay settings require modification.
Implementing the Solution
Designing and engineering EHV transmission lines involves a multidisciplinary approach, integrating advanced electrical, mechanical, structural, and environmental engineering principles to ensure the safety, reliability, and sustainability of power infrastructure. By understanding and addressing these complexities, stakeholders can deploy robust, efficient, and future-proof infrastructure. This is not only crucial for meeting growing energy demands, but also enhancing the stability and resilience of our power grids and facilitating the seamless integration of renewable energy sources. Black & Veatch provides design-build solutions for overhead and underground transmission systems in the United States and globally. This experience includes solving the unique challenges associated with implementing EHV transmission systems. These are just some of the key considerations, Black & Veatch’s Transmission Engineering team is available to discuss more on design considerations and implementing EHV transmission systems. Contact us today to get started.
