As the U.S. continues to use more power in more places, utilities are trying to keep up with increased demand and an aging transmission system, parts of which date back to the 1880s. Furthermore, the United States alone has approximately 360,000 miles of transmission lines, 70 percent of which are estimated by the U.S. Department of Energy (DOE) to be more than 25 years old. With the increased demand for system reliability and resiliency, utilities are now being asked to strengthen their systems.
In many cases, the option to build new transmission lines does not exist. The existing corridors are at capacity, and acquiring the land for a new transmission line route is often not feasible for a variety of reasons. Consequently, utilities are turning to other options to get more out of their existing transmission lines. This can mean several things: simple structure replacements and raises, dynamic line ratings or reconductoring and rebuilding existing lines. All of these will help to push more power through existing facilities, remediate issues like clearance concerns, add extra years to a lines life, and to improve the overall reliability of the line.
Where to start
In order to effectively address the issue, a clearly defined goal and approach must be set. What needs to be achieved? Is more ground clearance required to reach the current rating? Is there a higher line rating needed?
To start, determine the desired facility rating. Next, develop a design criteria by which to analyze the existing line. Typically, a utility has predefined design criteria. However, in some instances, it is worth reviewing to determine if there are areas to tweak: Maybe a large clearance buffer isnt necessary, or a reduction on allowable swing angles may be appropriate. Making these modifications can allow more flexibility in design and potential cost savings. Finally, construction constraints must be considered. Lead time on materials, difficult terrain, equipment required for construction, and land access all play a large role in the engineering and design. Identifying these key items helps to ensure a successful project from inception to energization.
Examining minimal construction alternatives
The first, and perhaps most simple, option to gain system reliability, increase ground clearance and to effectively extend the life of a line is to replace structures on an as-needed basis. This alternative works best if the majority of the line is in good condition. Additionally, replacing structures with taller ones can increase the line capacity and allow a higher line rating if not already at the conductors maximum capacity. Structure material selection depends on how quickly the structure needs to be installed, what is available, and the terrain. Wood is the most common material and lead times can be relatively fast if not already in stock. However, it is not always the ideal choice. For areas where land access is by foot traffic only, fiberglass structures are becoming more common. They are light and durable, sections can be carried in, and are installed quickly. Steel is a great option for helicopter work and for lines that require a longer lifespan, however lead times can be 24 weeks or more. If gaining additional ground clearance is the objective, lattice tower extensions and phase raisers essentially jack up existing structures if they are in good condition.
To determine a lines true capacity in the field and optimize an existing line, especially at times of peak load, dynamic line ratings are a great solution. These devices monitor the conductor over time, such characteristics as conductor temperature, clearance, load, and weather to provide a reliability-based rating. With this, the maximum capacity of the facility can be determined. There are several companies that manufacture the equipment, and it can minimize the amount of additional money put into a line based on a facility rating. They may be moved around to critical spans for monitoring and on several lines to develop a picture of the broader system. Because of this, dynamic line rating is a great option if the goal is to increase the efficient use of an existing line and provide higher asset utilization and reliability.
Looking at reconductoring and rebuilding
If a line in an existing corridor is relatively new, but a higher rating is needed, reconductoring the existing line is a viable option. Many different types of higher capacity cables such as 3M and ACCC (aluminum conductor, composite core) are manufactured. Generally, these are high capacity, high temperature, low sag conductors that can often provide the necessary rating while still maintaining the required ground clearance to operate at a higher temperature. In these cases, it is important to recognize that the existing structures must be reanalyzed for heavier cable and the sagging characteristics are different. It is likely that deadends will have to be replaced. However, it is still a cost-saving solution when compared to the alternatives.
Rebuilding a line, while not usually the least expensive option, can provide perhaps the most comprehensive solution ensuring reliability and increasing a facility rating to a line thats at the end of its life span. This provides more reliability and resiliency, the potential for a higher capacity conductor, and taller structures. It can also use existing right-of-way to mitigate the need for additional permitting, landowner involvement, and access issues that may be confronted when trying to build a new line in a new corridor. The scope of a line rebuild can be as simple as a structure-for-structure replacement with either a new, higher capacity conductor or taller structures, to upgrading to multi-circuit structures for additional capacity, if the easements allow. Ultimately, the line rebuild is the most comprehensive solution, cost permitting.
In order to keep up with the ever growing need for power in more places, a more reliable grid, and in an attempt to maximize existing facilities, utilities have several options: the life of a line may be extended and rating increased by replacing a handful of older, at-risk structures; using dynamic line rating to determine the true capacity of the existing line; reconductoring if it is in good condition and more capacity is needed; or the line may be rebuilt entirely to the most out of the existing utility corridor. In a time when we are trying to do more with what we have, these are all viable options to get more out of whats existing, saving money where possible, and providing the increased reliability that is required.
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12 December 2017