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2.3.2 Transmission Line Design

Transmission lines can be designed and constructed in a variety of ways to accommodate site-specific conditions, such as topography, soil types, and proximity to existing infrastructure, sensitive resources, and urban areas. While traditional overhead alternating current (AC) transmission lines are the most common, alternative transmission line types, such as direct current (DC), underground, and submarine, are becoming more prevalent. These types of technologies are discussed in the following sections.

DC Transmission Lines

According to DOE, several thousand miles of high-voltage DC transmission lines are installed in the U.S., which is relatively small compared to the over 200,000 miles of total installed high-voltage transmission lines (including AC and DC) in the U.S. However, the implementation of DC technology into project design is becoming increasingly more common. Direct current systems are most often implemented for large-scale bulk power transfers over long distances, such as undersea cables, or to connect different transmission networks between countries. In some applications, high-voltage DC (HVDC) systems can be more cost effective at long transport distances compared to high-voltage AC (HVAC) systems. DC technology allows for the use of fewer conductors or cables (two versus three for AC), allowing for typically more compact installations than a comparable AC system. However, DC systems require large conversion stations at each interconnection with the traditional AC grid. Precise, fast, and flexible control of energy flows at any level within the capacity limit of the line is another significant advantage of a DC system. While this technology is becoming more widely used, there are no current projects within Maryland proposing the use of this technology. This technology may be implemented in the 300-mile Atlantic Wind Connection project that is contemplated for support of offshore wind projects from New Jersey to Virginia (see Section 5.5.1).

Underground Transmission Cables

In September 2009, the PSC granted a CPCN to the Southern Maryland Electric Cooperative (SMECO) for the construction of a new 230 kV transmission line from Holland Cliff in Calvert County to the Hewitt Road Switching Station in St. Mary’s County. The project includes an underground construction component, for a short segment of the project under the Naval Recreation Facility (see below for submarine construction component of this project). Underground transmission lines are typically implemented in locations where overhead lines are difficult to place or would create aesthetic or environmental issues.

In this type of construction, underground transmission cables are typically placed four to five feet below ground surface in conduits or reinforced duct banks, or are directly buried in specially prepared soil, as shown in Figure 2-15. Instead of wide spacing between conductors, as is required for overhead transmission lines, underground cables are typically placed close together and insulated to protect the cables from one another. Often times, the individual cables required to make up a circuit are placed in polyethylene, PVC, or fiberglass conduits and are installed as a group.

Figure 2-15 Direct-burial Underground Transmission Line Installation

Figure 2-15

Modern underground cables, such as cross-linked polyethylene (XLPE), do not require pressurized liquid or gas insulating and cooling systems that were predominant in earlier cable types, and therefore, no longer have the environmental contamination risk associated with coolant releases. The cables can be designed for AC or DC systems and are manufactured in finite lengths that need to be spliced together, on the order of every 1,000 to 2,000 feet.

The advantages of underground transmission include reduced visual impacts and narrower right-of-way width requirements, due to the close spacing of the cables. For short distances, right-of-way widths of approximately 20 feet are possible, whereas in open country, a 30- to 50-foot width is preferred. Most of this width is to permit access for construction and maintenance equipment, since the duct bank itself is usually less than 10 feet wide. In some instances, these improvements may also coincide with reduced environmental impacts; however, in sensitive areas the installation of an underground transmission cable can be more disruptive than an overhead line.

Disadvantages of underground cables include thermal impacts during operation, significantly higher project costs versus comparable overhead installations, and longer cable repair times due to difficulties locating and accessing the cables and re-installation. Despite the longer repair times, underground cables generally have a longer useful life, are not damaged as often, and can be more secure.

Submarine Transmission Cables

Submarine cables are installed beneath a river bottom or seabed, via trenching or (for shorter lengths) horizontal directional drilling, or are laid on top of the river bottom or seabed. These cables have been used sparingly historically, but are becoming more common for higher voltage transmission lines, as the reliability of the technology is being proven. The above mentioned SMECO 230 kV transmission line from Holland Cliff in Calvert County to the Hewitt Road Switching Station in St. Mary’s County includes an approximately one-mile submarine crossing of the Patuxent River near Solomons, to be achieved with horizontal directional drilling. The construction of this project was completed in 2014 and was monitored by PPRP.

Submarine cables are typically manufactured and installed as one continuous line to provide the greatest reliability and can stretch up to 10 miles in one segment for AC cables, or several times longer for DC cables. Submarine cables are similar in design to underground cables with additional shielding layers. Like underground cables, submarine cables can be designed for both AC and DC systems and can be bundled and installed together in the same trench or conduit. Trenching techniques typically involve fluidizing the seabed using a jet plow pulled along the seabed in order to allow the cable to sink down to the desired installation depth of approximately 6 to 15 feet, depending on specific site conditions.

The benefits of implementing a submarine system are limited disruption to navigation and minimized visual impacts once the cables are installed, compared to the use of an overhead waterway crossing. Impacts from submarine cables are typically associated with disruption of the seabed, sedimentation, and release of nutrients sequestered in the sediments, as well as heat dissipation during operation.