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2.2 ELECTRIC TRANSMISSION
Regardless of the generation source, generated energy must be transmitted from the generation site to the consumer. During the latter part of the 19th century, this mainly consisted of distributed generation, where the power was generated at or near the location it was consumed. However, around the turn-of-the-century, George Westinghouse introduced the world to the first transmission system, which leveraged alternating current as opposed to the direct current systems primarily used previously. The benefits of this high voltage transmission system quickly became evident as these new systems provided the ability to transmit and distribute electricity over very long distances. The dream of providing electricity to every home in America was becoming a reality, and the electric market was changed forever. Over the last century, electric transmission has grown to a type of superhighway for electricity, with the capacity of sending hundreds of kilovolts across entire continents.
At the start of the 21st century, the electric transmission system in the United States had truly become an interconnected network with more than 150,000 miles of high-voltage transmission lines. These lines differ significantly from the distribution lines you commonly see outside your home or office. Their sole purpose is to transmit electricity from the generation site to the distribution site. While higher voltage is the primary difference, transmission systems also include step-up and step-down transformers to smoothly connect the generation source to the distribution substation. Outside of a generation site, a switchyard is where station transformers increase voltage to the level required by the transmission system. Also at this switchyard, breakers, busbars, and other protective equipment are used to safely and efficiently manage the flow of power away from the generation site while protecting the integrity of the transmission grid. At the tail end of the transmission system is the substation, which is most commonly associated with the distribution system. Like the switchyard, substations also include breakers, switches, and transformers. However, at the substation, transformers are used to decrease voltage to levels acceptable by the distribution system. It is common for utilities to have monitors and metering devices at a substation to ensure reliability and safety by using real-time data to better manage power levels flowing into and out of the substation.
In between the generation unit and the distribution substation are miles of high-voltage transmission lines. Transmission lines are generally twice as tall as distribution lines, ranging anywhere from 60 to 140 feet tall. These lines have multiple forms, as you can see from the figure below. While they are primarily made of metal, some can also be constructed of wood. However, wood is more commonly used for distribution poles. For above-ground transmission systems, these lines must have ample space around them free from trees, buildings, or other tall structures hazardous to the integrity of the lines.
Voltage flowing through transmission lines can range anywhere from 40 to 765 kilovolts. This differs greatly in comparison to distribution voltages which range from about 7 to 35 kilovolts. This is due to the unique purposes transmission and distribution systems serve. Distribution systems are used to deliver energy from a local substation to the consumer. Transmission systems are used to transmit generated energy over long distances to the distribution substation.
Due to the long distances transmission lines cover and the higher voltage levels they must sustain, system operators face unique challenges operating a transmission system at an optimal capacity. To best optimize the flow of power, transmission lines use copper or aluminum because these metals have low resistance. To prevent drops in voltage over long distances, series capacitors are used to periodically balance voltage, but unfortunately at the cost of lost energy. This is why transmission systems must enforce physical constraints despite attempts to connect supply with the ever-growing demand of today’s electric grid. For example, a line is overheating. Resistance within a transmission line can cause overheating when high-frequency currents flow through a transmission system. External factors, such as temperature and wind speed can increase the chances of overheating. This can be detrimental, because it damages lines and causes line sagging. The latter can in return cause inefficiency and potential short circuits if transmission lines come in contact with external resistors, such as trees or buildings. Also, when overheating occurs, electrical energy is lost as it is changed into heat energy. This is commonly referred to as line loss and is one of the leading causes of transmission system inefficiency.
Similarly, system operators set limits on voltage levels for transmission lines in hopes of limiting transformer overloads, short circuits, and radio interference. While the purpose of transmission systems is to indeed transmit electricity at high voltage levels, these sorts of limits are critical for maintaining a reliable grid that is both safe and environmentally friendly. Reliability is a priority for all utilities, and excess voltage can in fact impede that when voltage levels exceed the capacity of the transmission system. To limit these sorts of risks that lead to damaged assets and interrupting overloads, operators try to closely monitor the flow of power through transmission lines. This reveals a growing need for transmission smart grid technologies to ensure transmission systems expand in productivity and not just size and cost.
Key similarities exist among all utility transmission systems, including the need for vast amounts of land, large high-cost assets, complex operational procedures, and an emphasis on safety. When a utility or transmission company decides to expand its transmission network, they must first plan for additional infrastructure. This includes cost analysis, efficiency analysis, and determining possible routes for transmission lines. The next step is to get permission from federal, state, and local governments and agencies including ISOs, RTOs, and FERC. The final step is to actually finance the project. This process requires a large amount of resources, especially time and money. Due to the complexities in geographical conditions, careful planning is required to ensure new transmission lines are as efficient as possible in their use of materials and their ability to provide energy. What is required to transmit electricity through Oregon’s terrain is not the same as in Texas.
This process is further complicated by the required approval from many different governmental agencies, especially due to the multiple jurisdictions interstate lines can cross. Additionally, the price for establishing transmission lines and keeping them operational is very high. The table below shows the typical capital costs for electric transmission.
Today, the electric transmission system is the backbone of the electric market. The hundreds of miles of transmission lines interconnecting generation systems to distribution systems all across the globe is what gives the sense of a widespread electric “grid.” However, this sort of interconnectivity does have setbacks. Aging infrastructure and spikes in population growth are constant struggles utilities face every day. While fighting to keep up with demand, electric transmission systems struggle with electrical resistance, line sag, transformer and capacity limits, and line loss. The United States as a whole loses nearly ten percent of all the power it generates to these technical limitations. These same technical limitations are even hindering the large scale use of renewable energy sources like wind and solar. While advances in generation technologies grow, the current AC transmission system is failing to keep up with transferring that energy to the necessary service territories hundreds to thousands of miles away.
For the electric market, encouraging electricity consumers to load shift their demand from on-peak to off-peak hours can help improve transmission efficiency. However, successful load shifting alone is not enough to help ease the tension of such a vast, interconnected system. The introduction of computerized flexible AC transmission systems or FACTS has helped by providing effective ways of controlling power flow to allow for better utilization of power lines. This increased control can also reduce harmful high-voltage transmission side effects such as overheating and line loss. By transitioning from an estimation-based model of predicting capacity based off weather, historical trends, and other system flow factors to taking advantage of smart energy management systems (EMS), transmission operators can use real-time monitoring and visualization to prevent underutilized lines and potential overloads. Some of the resulting benefits of the electric transmission system are diversified generation and an interconnectivity of the grid across multiple service territories. With higher control of power flow through the strategic, coordinated actions of FACTS devices, utilities can limit outages, by redirecting power away from overloaded lines and areas. Similarly, these systems can redirect lines to reserve power sources when blackouts occur.
Further development of these types of systems along with an emphasis on replacing current transmission lines with newer, more efficient systems will help the grid as a whole utilize current generation methods more efficiently and also open the door for newer ones. However, better control of the system does not necessarily prevent the grid from all possible threats, such as storms or other natural events. That is why powerline technicians, also known as linemen, are a pivotal part in the operation of an electric transmission system. Linemen are responsible for constructing and maintaining electric power transmission systems by installing or fixing capacitors, insulators, line transformers, and fuses. The job of a lineman is one of the most dangerous jobs in the world, thus an emphasis on personal and operational safety are pivotal for successful daily operation of a transmission system. Linemen are entrusted with physically maintaining and expanding the grid, thus they are highly skilled and trained individuals.
Energy transmission is one of the most pivotal and yet overlooked components of the 21st century grid. Hundreds of kilovolts of electricity are transmitted cross-country every day in the United States alone. Utilities, transmission companies, and regulatory agencies attempt to support an efficient system capable of supplying reliable amounts of energy throughout the country through constant monitoring and auditing of their operational procedures. Continued innovation in design and materials are necessary for a complete, efficient system capable of keeping up with today’s “energy-literate” consumers. With companies like Tesla paving the way with new technologies in energy storage, it begs the question of what can we do to ensure our renewable energy sources are successfully transmitting clean energy to the new energy uses of tomorrow?
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