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High voltage transmission lines interconnect utilities, generating stations, major load centers, and transfer power between utilities. The U.S. electricity transmission system is an extensive, interconnected network of high-voltage power lines that transport electricity from generators to consumers. The transmission system must be flexible enough, every second of every day, to accommodate the nation’s growing demand for reliable and affordable electricity.

Institutional and technological changes in transmission system engineering and operation are forthcoming and hold the potential for increased capacity and efficiency of the transmission grid.  However, there is a constant tension between the cost of such transmission improvements and the associated benefits.

In addition, due to load diversity between utilities and geographic regions, improved transmission links can often reduce the need for total installed generation and spinning reserve requirements. This lessened need for generation and spinning reserves can lead to a reduction in greenhouse gas emissions. Higher capacity transmission systems, including HVDC, could also increase the availability and economic viability of renewable resources that are remote from load centers.

The historical reasons for transmission system expansion have generally been tied to:

These reasons for transmission development, expansion, and reinforcement must now be reexamined in the context of competitive electricity markets. These markets require transmission expansion not only to interconnect new generation capacity but also to provide flexibility for the delivery of that generation capacity to customers. Both the customer’s selection of supplier and the customer’s load variations with time must be considered.

The ability of the electric transmission systems to transfer electric power among their interconnected elements and deliver power may be limited by the physical and electrical characteristics of the systems including thermal, voltage, and stability limits. Transmission systems are being subjected to power flows in magnitude and directions that were not considered when the systems were planned. In many instances, these new flow patterns result in an increasing number of transmission facilities being identified as limits to electric power delivery or transfers. Increasingly, more electricity is being shipped longer distances over a transmission system that was initially designed only to provide limited power and reserve sharing among neighboring utilities.

The text below discusses several specific areas for transmission improvements:

High Voltage Direct Current (HVDC) Transmission

High Voltage Direct Current (HVDC) systems transmit power using direct current (DC), which flows in one direction only. The vast majority of transmission lines in existence today use alternating current (AC), where the current reverses direction 60 times per second.

As AC power is delivered on a transmission line, the electricity tends to travel through the outer portion of the conductor, not evenly over the cross section of the conductor. This is called the "skin effect" and effectively increases the electrical resistance of the conductor to AC current. The increased resistance, in turn, slightly increases electrical losses on AC transmission lines (up to 0.5 percent).

DC power delivery produces just the opposite effect. Electricity travels more evenly over the entire cross section of the conductor. DC operation is bi-polar, requiring only two conductors, whereas three-phase AC transmission systems require at least three conductors. A typical HVDC line design can have less than 50 percent of the losses associated with an AC line of the same power transfer capability. The per mile construction costs of the HVDC line are also considerably less. However, the fixed terminal costs for HVDC equipment preclude the use of HVDC except for very long lines or other special situations. Applications of HVDC can be for new lines or conversion of existing AC lines.

Power system losses in the United States amount to billions of kWh annually. HVDC transmission lines could potentially reduce a portion of those losses and the electric generation necessary to replace them. To the extent that this generation was fossil-fired, greenhouse gas emissions would also be reduced.

Controlling Transmission Line Flows

Improved power electronics incorporating the new technology of large silicon, solid-state switches, called thyristors, can help utilities increase transmission system capacity while reducing susceptibility to power disturbances, thus enhancing the control of power flow.

By increasing or decreasing the power flow on specific lines, utilities can tailor power delivery strategies to best utilize their systems and reduce problems associated with loop flow. System optimization will allow more effective integration and use of renewable energy, energy storage, and demand-side management resources in the electric system, leading to possible further greenhouse gas reductions.

Savings may also be realized from reduced spinning reserve requirements in the generating capacity needed to serve as backup, rather than to meet actual demand for electricity. Reduced spinning reserve requirements could reduce emissions of greenhouse gases and air pollutants. Additional savings can result by balancing phase currents, thereby reducing the amount of losses associated with residual currents.

Increasing the current flow on a transmission line, however, will increase line losses. These losses require the generation of additional electricity, which could result in additional greenhouse gas emissions. Any increased emissions must be considered in evaluating the net impact of the project.

Conductor Loss Optimization and Phase Current Optimization

The resistance a conductor offers to the flow of electricity is inversely proportional to its cross-sectional area, i.e., the larger the diameter of the conductor, the less resistance the current will encounter. Resistance is also a function of the type of material of which the conductor is made. Thus, by replacing a conductor with one of a larger diameter or changing to a material which offers less resistance, power loss can be reduced when the same current is flowing through the conductor. Segmenting shield wires can also eliminate losses associated with loop flows through this path.  

Increasing Transmission Line Voltage

Increasing the voltage of a transmission line increases the efficiency of transmission of electricity over the line. Using the highest transmission voltage that is operationally and economically justified can reduce line losses. Increasing the voltage of an existing transmission line in many instances is an effective way of increasing the utilization of the line, and, because of the increased efficiency, less electricity would be required to be generated to provide the same service to the end customer. Consequently, less fuel is consumed, which could result in reduced greenhouse gas emissions.

Building New Transmission Lines

The location of power lines which transmit electricity from generating facilities to points for distribution to customers often does not optimize power delivery efficiency. This may be due to changes in customer demand or location after transmission lines were installed or attempts to avoid installation of expensive new transmission facilities.  

Proper placement of new transmission lines, especially around metro areas, can significantly reduce transmission losses. Reduced losses result in reduced generation requirements, with subsequent reductions in greenhouse gas emissions. 

Also, there are areas in the U.S. which have more generating capacity than their own system requires, even when considering reserve requirements. In many cases, this spare capacity is more efficient or produces less greenhouse gases than power produced in other systems (e.g., renewable or nuclear energy). This capacity could be made available to other entities that have a capacity shortage or more carbon-intensive fueled generation. There are other instances where diversity in system loads would permit utilities to share generation. This would reduce energy consumption for spinning reserve and reduce overall generation requirements.

The primary obstacle to efficiently resolving these situations is the lack of transmission line capacity between the affected entities. New transmission lines can make a contribution to reducing greenhouse gas emissions through more efficient operation of interconnected systems.

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ABB Group, “HVDC transmission for controllability of power flow”
http://www.abb.co.uk/global/abbzh/abbzh251.nsf!OpenDatabase&db=/global/gad
/gad02181.nsf&v=17EA&e=us&m=100A&c=C1256D71001E0037C125683200661468

In the majority of HVDC projects, the main control is based on a constant power transfer. This property of HVDC has become more important in recent years as the margins in the networks have become smaller and as a result of deregulation in many countries.  In many cases the HVDC link can also be used to improve the AC system performance by means of additional control facilities. Normally these controls are activated automatically when certain criteria are fulfilled. Such automatic control functions could be constant frequency control, redistribution of the power flow in the AC network, damping of power swings in the AC networks etc. In many cases such additional control functions can make it possible to increase the safe power transmission capability of AC transmission lines where stability is a limitation.

Electric Power Research Institute, “Overhead Transmission Inspection and Assessment Guidelines – 2006" (4th Edition, 30-Nov-2006)
http://my.epri.com/portal/server.pt?Abstract_id=000000000001012310 

The emphasis within the transmission arena has shifted from designing and building new transmission facilities to optimizing the use, performance, and life of existing facilities. In the early 1990s, EPRI established an initiative to examine the capabilities and limitations of existing inspection and assessment methods and technology. One of the key needs identified was information on methods and technologies for inspecting/assessing the conditions and life expectancy of overhead transmission line components. This need has been recently accentuated as the industry makes adjustments to cope with the impacts of a streamlined workforce and the accompanying loss of institutional memory. These guidelines - an outgrowth of this need - are an evolving resource designed to become a single living repository of information on the inspection and assessment of overhead transmission lines. The objective of these guidelines is to provide a self-contained, state-of-the-art resource on inspection and assessment methods and technology that is sufficient for the day-to-day needs of both experienced and novice asset managers, inspection personnel, and other maintenance stakeholders

Federal Energy Regulatory Commission, “Regional Transmission Organizations (RTO)/Independent System Operators (ISO)”
http://www.ferc.gov/industries/electric/indus-act/rto.asp

Independent System Operators grew out of Orders Nos. 888/889 where the Commission suggested the concept of an Independent System Operator as one way for existing tight power pools to satisfy the requirement of providing non-discriminatory access to transmission. Subsequently, in Order No. 2000, the Commission encouraged the voluntary formation of Regional Transmission Organizations to administer the transmission grid on a regional basis throughout North America (including Canada). Order No. 2000 delineated twelve characteristics and functions that an entity must satisfy in order to become a Regional Transmission Organization.

GE Energy, “HVDC Transmission Systems”
http://www.gepower.com/prod_serv/serv/energy_consulting/en/equip_app/hvdc_trans.htm

HVDC transmission systems are increasing in popularity due to the controllability of power flow, absence of short-circuit current contribution and the inherent technical and economic advantages of HVDC where moderate to long lengths of underwater or underground cables are needed. GE Energy provides a wide range of expert consulting services of value to the operator or owner of a power system to which a merchant HVDC system may be attached.

North American Electric Reliability Council, Transmission Adequacy Issues Task Force. “Transmission Expansion: Issues and Recommendations”. February 20, 2002
http://www.nerc.com/docs/docs/pubs/TransmExpansion_BOTapprvd_022002.pdf

This report by the Transmission Adequacy Issues Task Force of the NERC Planning Committee assesses the issues and obstacles that are impacting the planning and expansion of the transmission systems, including recommendations to reduce or eliminate these obstacles.

Siemens AG, Germany, W. Breuer et al, “Role of HVDC and FACTS in Future Power Systems”
http://www.iea.org/Textbase/work/2004/distribution/presentations/Retzmann_bkgd_2.pdf

To avoid large cascading system outages, transmission systems and system interconnections have to be improved by new investments, including the use of Power Electronics like HVDC, FACTS and other advanced technologies. In this paper, highlights of innovative FACTS and HVDC solutions are depicted and their benefits for new applications in high voltage transmission systems and for system interconnections are demonstrated.

Transmission & Distribution World, “Reinforcing the T&D Infrastructure” (April 1, 2002 article)
http://tdworld.com/mag/power_reinforcing_td_infrastructure/

One of the most important building blocks for the immediate future is a technology that ensures reliable power delivery over existing lines. That technology, Flexible Alternating Current Transmission Systems (FACTS), increases current-carrying capacity, improves system stability and results in a robust system.

U.S.-Canada Power System Outage Task Force, “Final Report on the August 14th Blackout in the United States and Canada” (April 2004)
https://reports.energy.gov/BlackoutFinal-Web.pdf

This report identifies the causes of the August 14, 2003 power outage and why the outage was not contained. It also presents comprehensive technical and policy recommendations to prevent or minimize the likelihood of future blackouts, and reduce the scope of those that do occur.

U.S. Department of Energy, Energy Information Administration, “Electricity Transmission Fact Sheet”
http://www.eia.doe.gov/cneaf/electricity/page/fact_sheets/transmission.html

This is a brief fact sheet summarizing data and regulatory authority regarding electricity transmission in the U.S.

U.S. Department of Energy, Energy Information Administration, “Upgrading Transmission Capacity for Wholesale Electric Power Trade”
http://www.eia.doe.gov/cneaf/pubs_html/feat_trans_capacity/w_sale.html

Due to the problems associated with constructing new transmission lines, it is important to examine .the possible options for increasing the transmission capability on present sites and making maximum use of existing transmission systems through upgrades. This article describes to policy makers and regulators the bulk electric power system and identifies the thermal, voltage, and operating constraints on a system's capability to transmit power from one area to another. Some of the potential remedies for these constraints through upgrades are presented along with a comparison of the cost to upgrade compared to the costs for new transmission lines.

U.S. Department of Energy, “National Transmission Grid Study” (May 2002)
http://certs.lbl.gov/certs-rtinakey-ntgs-p.html

DOE conducted an independent assessment of the U.S. electricity transmission system and found that our U.S. transmission system facilitates wholesale electricity markets that lower consumers’ electricity bills by nearly $13 billion annually. Despite these overall savings, interregional transmission congestion costs consumers hundreds of millions of dollars annually.

U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability
http://www.oe.energy.gov/

The Office of Electricity Delivery and Energy Reliability (OE) is leading Federal efforts related to several sections of the bill and is responsible for completing a number of activities and studies. The mission of this office is to lead a national effort to modernize and expand America's electric delivery system.

U.S. Energy Association and U.S. AID, “Handbook of Climate Change Mitigation Options for Developing Country Utilities and Regulatory Agencies”, Chapter 5, "Transmission System Actions"
http://www.usea.org/Archive/Climatechange/PDF/chapter5.pdf

The U.S. Agency for International Development and the U.S. Energy Association authorized the compilation of this handbook to increase awareness of the climate mitigation benefits from each practice among utility personnel and regulators in developing countries. Chapter 5 discusses Transmission System actions.

World Bank, “High Voltage Direct Current (HVDC) Transmission Systems: Technology Review Paper”
http://www.worldbank.org/html/fpd/em/transmission/technology_abb.pdf

This paper presents an overview of the status of HVDC systems in the world today. It reviews the underlying technology of HVDC systems, and discusses the HVDC systems from a design, construction, operation and maintenance points of view. The paper then discusses the recent developments in HVDC technologies; presents an economic and financial comparison of HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s electricity system development.

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