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| Fig. 1: Overview of U.S. Electric Supply Chain. [1,3] (Image Source: A. Klingberg) |
The electric supply chain consists of three main components: electrical generation, transmission, and distribution. [1] In electricity generation, power can be produced from a multitude of sources such as fossil fuels, nuclear energy, and renewable sources (i.e., solar, wind, hydro). At generating stations, mechanical energy is converted into electrical energy using generators, which involves rotating conductors within magnetic fields to induce an electric current. [1] In the transmission of electricity, high-voltage power lines connect different regions with one another. The United States' electric system includes over 360,000 miles of transmission lines, with roughly 180,000 miles being high-voltage lines, which link to around 7,000 power plants. [1] Electricity is delivered to consumers by power distribution systems. Subtransmission lines operating at lower voltages carry the electricity closer to the end users. Distribution substations then reduce the voltage further to levels suitable for local distribution networks. Fig. 1 provides an overview of the U.S. electric supply chain, illustrating the flow of electricity from power generation to end users.
Unfortunately, transmitting electrical power over long distances results in energy losses. These losses occur primarily from resistive heating within conductors. Technical losses naturally occur during the transmission and distribution of electricity through power lines, transformers, and other equipment. Non-technical losses, such as energy theft or meterting errors, also contribute to the total power loss. [2]
Technical power losses can be categorized into two main types: power line losses and transformer losses. [2] Power line losses are influenced by factors like the conductivity of the line material, the cross-sectional area, the length of the line, ambient temperature, and the current density within the conductor. [2] These factors determine how efficiently electricity is transmitted over distances. Transformer losses are further divided into no-load losses and load losses. No-load losses occur continuously due to magnetization currents in transformers and reactors. These losses are independent of the current flowing through the transformer but may increase with higher input voltages. [2] Load losses arise from the flow of current through transformer windings and depend on the amount of current and its temporal fluctuations. Magnetic flux leakage also contributes to these losses. [2]
Typically, transmission and distribution grid losses account for anywhere between 4-15% of all generated power, and sometimes more. [2] Losses exceeding this range are often attributed to non-technical factors. [2] Since almost four trillion kWh of electricity energy is consumed every year in the United States, it is estimated that technical transmission and distribution losses account for at least 160-600 billion kWh of electric energy lost annually. [3]
Traditionally, total energy losses are calculated by finding the difference between the total energy supplied to the system and the total energy consumed (metered and billed) by customers. [4] This method captures both technical losses and non-technical losses. To differentiate between these losses, further analysis is required. A common method for estimating technical losses involves calculating active power losses during peak load conditions through computational models and recorded load data. A loss factor can then be applied to estimate average power losses over time. [4] By subtracting the estimated technical losses from the total losses, non-technical losses can be determined.
As the amount of losses in the United States seems high, it may appear that more effort needs to be made to prevent these losses. However, transmission is both an economic and engineering concern. The voltage level in the grid is crucial for determining power losses as higher voltage levels reduce the current needed to transmit the same amount of power. Hence, losses are decreased since they are proportional to the square of the current. Yet, higher voltage levels also entail increased infrastructure costs. [2] Additionally, losses may be prevented by upgrading power lines with materials that have higher conductivity and larger cross-sectional areas. Implementing technologies like High-Voltage Direct Current (HVDC) transmission for long distances, reactive power compensation, and smart grid systems optimizes energy flow and further reduces losses. [2] However, these advancements must be economically justified to ensure that the cost of infrastructure improvements does not exceed the value of the power savings achieved.
Although the electric supply chain is an intricate network involving generation, transmission, and distribution, significant losses in transmission and distribution persist. These losses are further compounded by the aging infrastructure in the United States. As of 2015, 70% of power transformers are 25 years or older, 60% of circuit breakers are 30 years or older, and 70% of transmission lines are 25 years or older. [3] As modernization becomes essential to maintain reliability and reduce losses, upgrading conductors and transformers is crucial. Innovating the electric supply chain with new materials and advanced technologies (such as high-temperature low-sag conductors and enhanced transformers) can boost efficiency and help prevent losses when economically viable. [3,5] While the technology currently exists to reduce losses, technological advancements may be hindered by prohibitive costs, requiring a careful balance between efficiency and affordability.
© Andrew Klingberg. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "United States Electricity Industry Primer," U.S. Department of Energy, DOE/OE-0017, July 2015.
[2] K. Sadovskaia et al., "Power Transmission and Distribution Losses - A Model Based on Available Empirical Data and Future Trends For All Countries Globally." Int. J. Electr. Power Energy Syst. 107, 98 (2019)
[3] "Quadrennial Technology Review 2015," U.S. Department of Energy, September 2015, p. 53.
[4] J. R. Agüero, "Improving the Efficiency of Power Distribution Systems Through Technical and Non-Technical Losses Reduction," IEEE PES Transmission and Distribution Conference and Exposition, IEEE 6281652, 7 May 12.
[5] M. Amin and J. Stringer, "The Electric Power Grid: Today and Tomorrow," MRS Bull. 33, 399 (2008).