Remove Congestion, Boost Capacity by Going Underground

By Roger Rosenqvist, ABB Inc.

Reliance on the transmission grid to meet load deliverability requirements in population centers continues to grow. Addition of new transmission capacity normally can meet increasing demands in densely populated urban areas. New overhead transmission lines, however, often face opposition.

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A new overhead transmission line initially might appear to be the preferred option for increasing capacity. Public and landowner opposition, environmental-impact restrictions and potential legal action by opponents, however, might make project construction so uncertain—or delay the estimated in-service date so much—that the project might not be a viable, practical option.

Utilities and transmission owners may elect less controversial and, therefore, more predicable and practical ways of increasing transmission capacity. For example, utilities may propose construction for new underground transmission circuits within existing rights-of-way of overhead transmission lines, public roadways, railroads and gas lines.

Boosting Transmission Capacity—Underground

In many areas where transmission capacity is already constrained by thermal limits of transmission lines or their associated equipment, construction of new transmission circuits may be the only feasible option to alleviate grid constraints and looming reliability problems. With fierce public opposition and rising construction costs for overhead transmission lines, however, utilities and transmission owners in densely populated areas increasingly find that underground cable circuits are a practical compromise solution that reasonably balances the construction costs of new transmission facilities with system reliability risks and transmission congestion costs from not completing needed transmission capacity expansion on time.

Polymeric insulated cables with pre-molded joints are a proven technology today for high-voltage (HV) and extra high-voltage (EHV) transmission systems. Polymeric insulated cables are safe for the environment and, therefore, appear a natural technology choice for construction of new underground EHV transmission circuits.

AC Cables for Short- and Medium-distance Underground Transmission

Polymeric insulated cable systems for AC are commercially available for conductor sizes up to 2,500 mm2. Reliability performance of EHV class polymeric cable has been proven in numerous projects around the world since the early 1980s. More recently, high-capacity 400 kV AC cross-linked polyethylene (XLPE) insulated underground cables have been installed in large underground transmission projects in the United Kingdom, Germany, Denmark and Spain, and the performance of 500 kV AC XLPE cables have been proven in projects in China and Japan.

An XLPE insulated underground cable circuit can be buried directly in a 3- to 4-foot-wide and 4- to 6-foot-deep trench at the inside perimeter of an existing transmission line right-of-way or along the shoulder of a roadway or railroad. As illustrated in Figure 1, an XLPE insulated underground cable circuit also can be installed in a traditional duct bank system inside a public roadway.

1 Incremental Development of HVDC Underground Cable Ratings
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Advantages of XLPE cable technology include the following:

XLPE insulated cables do not contain any insulating fluids and, therefore, there is no risk of accidental release of oil or other hazardous materials and substances into the environment;

Reduced visual impact—an underground cable circuit offers significant advantages in aesthetics and rights-of-way requirements:

  • XLPE insulated cables are suitable for direct burial in open trenches at a significantly lower overall installation cost than using concrete-encased duct bank systems.
  • Capacitance per mile and phase for XLPE insulated cable is significantly less than for fluid-filled cables and, therefore, XLPE insulated cable technology increases the practical distance for under-grounding of AC HV transmission circuits.
  • Once installed, XLPE insulated cables are virtually maintenance-free.
  • XLPE insulated underground cable circuits do not require any auxiliary systems.

At a relatively small additional cost, an XLPE cable system can be equipped with real-time temperature monitoring using a fiber-optic cable in a small, metallic tube inside the jacket of the power cable. The temperature monitoring system collects data along the entire cable route and displays the temperature for the length of the cable or vs. time.

Pre-molded joints for XLPE cables enable splicing in discontinuous shifts, a characteristic of special importance for the feasibility of installing underground cable circuits along congested public roadways. For example, splicing of the cable can be temporarily suspended during morning and afternoon rush hour traffic to avoid increased traffic congestion during construction. A typical design is of a modern, pre-molded joint with screen separation. The screen separation allows for cross-bonding or single-point bonding of the metallic sheaths in the power cables, which in turn increases the circuit capacity for a given cable size by eliminating the flow of induced continuous currents in the metallic sheaths.

Technical feasibility and construction costs will continue to be significant factors for evaluation and consideration of underground transmission systems. For traditional AC transmission, the charging current in underground cables consumes capacity cumulatively with distance. For example, a 25-mile-long 345 kV underground XLPE cable circuit requires approximately 600 Amperes of charging current. As a result, the transmission capacity of an underground AC cable circuit diminishes with distance, limiting the practical application of EHV AC cables to short- and medium-distance underground transmission.

DC Cables for Long-distance Underground Transmission

In response to industry and public demands for cost-efficient, medium- and long-distance underground transmission systems, ABB introduced in the late 1990s XLPE insulated cable systems for HV direct current transmission (HVDC). Unlike AC, there is no length limit for underground HVDC transmission using XLPE insulated cables. The first commercial underground transmission circuit using XLPE insulated HVDC cable was commissioned in 1999 at a circuit rating of 160 kV (±80 kV), 50 MW. The HVDC circuit, which is some 43 miles long, connects a large wind farm on the southern side of the island of Gotland in the Baltic Sea to the island’s principal population center on the northern side.

Since the 1990s, XLPE insulated HVDC cables have been introduced for commercial use at incrementally higher circuit ratings (see figure below).

2 Examples of Installation of XLPE Insulated Cable
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The incremental development of HVDC underground cable mirrors the introduction of XLPE insulated cable for increasingly higher AC voltage ratings since the 1970s and reflects the utility industry’s conservative approach and preference for proven designs.

For example, XLPE insulated HVDC underground cables for 300 kV (±150 kV) circuit ratings were commissioned in 2002, and since 2007, XLPE insulated HVDC underground cables have been available for commercial use at circuit ratings of up to 640 kV (±320 kV), 1100 MW. Development will continue and HVDC circuit ratings higher than 1,100 MW are likely to become available over the next two to three years as cable voltage ratings continue to increase.

HVDC underground transmission cables use pre-molded joints and terminations similar to those used for many years with XLPE insulated AC cables. Unlike AC joints, however, there is no requirement for screen separation and special sheath bonding for HVDC joints and, therefore, the HVDC joints are simpler and typically faster to install. Also, an HVDC underground circuit requires only two cables whereas an EHV AC underground circuit typically needs three cables and a continuous ground cable for the sheath bonding at the splice locations.

HVDC underground transmission circuit ratings of up to 1,100 MW, 640 kV, are feasible today using a pair of XLPE insulated cables, each with a voltage rating to ground of 320 kV. (For comparison, in a three-phase AC circuit rated 345 kV, each phase cable has a voltage rating to ground of 345 kV / √3 or approximately 199 kV.) Thus, an underground transmission line with a rating of up to 1,100 MW can be directly buried in a 1½- to 2-foot-wide and 4- to 6-foot-deep trench at the inside perimeter of an existing overhead transmission line right-of-way or along the shoulder of a roadway or railroad. Like XLPE insulated AC cable circuits, an HVDC underground cable circuit also can be installed in a traditional duct bank system: for example; inside a public roadway.

Like XLPE insulated AC cables, reliability performance of XLPE insulated HVDC cable has been proven over several years in large projects, including in Australia, United States, Finland, Estonia, Norway and Sweden. At this time, more than 1,000 linear miles of XLPE insulated HVDC cable supplied by ABB (HVDC Light) is installed in underground and submarine projects around the world.

Installation Methods for XLPE Insulated Underground Transmission Cable

XLPE insulated AC and DC cables do not contain insulating fluids. Because there is no risk of accidental release of oil or other hazardous materials and substances from the cable into the environment, XLPE insulated cables for AC and DC are well-suited for direct burial in open trenches. In a rural environment or along the perimeter of an existing overhead line right-of-way, direct burial of an XLPE insulated cable circuit typically offers significantly lower civil construction and installation costs than a traditional concrete-encased duct bank system.

The pictures above show examples of installation equipment and methods used for construction of XLPE cable-based underground transmission systems. The combined trencher and cable-laying machine was used during the installation of an approximately 110-mile-long underground HVDC transmission system in Australia. That cable project (Murray Link) was commissioned in 2002 and is the longest underground transmission system in the world.

They also show the inside work area in a mobile splicing unit that was used during the installation of an approximately 47-mile-long underground segment of a combined underground-submarine cable circuit in northern Germany. When completed in 2009, the project will include a continuous, 127-mile-long HVDC cable circuit from an offshore wind farm in the North Sea to a major substation in northern Germany.

Transmission grid capacity demand is expected to continue rapid growth in coming years, especially with an increased emphasis on renewable energies and retirement of aging, green-house gas-emitting, inefficient generation. This demand for increased grid capacity often clashes with increasingly fierce opposition from various stakeholders and communities whenever new overhead transmission lines are being proposed.

There are new technological advancements that utilities and transmission owners can tap into to overcome these transmission challenges. New, environmentally friendly underground transmission circuits within existing rights-of-way of overhead transmission lines, public roadways, railroads and gas lines, as described above, represent some of the most promising solutions in coming years.

Roger Rosenqvist is an executive consultant for ABB’s Grid Systems North America business unit. Rosenqvist is based at the ABB Power Systems division headquarters in Raleigh, N.C. Rosenqvist has been with ABB since 1980.

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