Path 27
Path 27, also called the Intermountain or the Southern Transmission System, is a high-voltage direct current electrical transmission line running from the Intermountain Power Plant near Delta, Utah, to the Adelanto Converter Station at Adelanto, California, in the Southwestern United States. It was installed by Asea, a company based in Sweden, and commercialized in July 1986. The system is designed to carry power generated at the power plant in Utah to areas throughout Southern California. It is owned and operated by the Intermountain Power Agency, a cooperative consisting of six Los Angeles-area cities, the largest member being the Los Angeles Department of Water and Power, and 29 smaller Utah municipalities.
Path 27 consists of an overhead power line long, and is capable of transferring up to 2,400 megawatts of power at ±500 kilovolts, higher than the power plant's operational output of 1,900 MW. The resulting maximum current is 4,800 amperes. Given its length, a direct current is preferred to the more common alternating current as it allows the electrical energy to travel farther with minimal loss to resistance and requires no intermediate stations. It is bipolar, meaning that it has two conductors of opposite polarity. Both conductors for the entire length are three cables bundled together; this is done to reduce the effects of EM interference and enhance the power line's performance. At each end of the line is a converter station that changes AC to DC on one side and back again on the other. Each terminus also features a dedicated ground which is connected by an electrode line to a grounding site away from the converters to provide ample earth return; this helps protect the main line and equipment from faults, and allows the system to operate at partial capacity when one conductor is out of service.
Sections of Path 27 are paralleled by other AC transmission lines, including some of 500 kV. The powerline is also visible from the Interstate 15 which it passes over multiple times. The HVDC line's converter stations will be replaced as part of a project to repurpose the Intermountain Power Plant as a hydrogen-burning facility. The stations are expected to go online by June 2026.
Overview
Background
s are typically needed to convey large quantities of electrical power over a vast distance, while also minimizing the amount of energy lost to resistance in the conductive cables as a byproduct from a high current; that is, raising the voltage lowers the amperage for the circuit, according to Ohm's law. Whereas high-voltage transmission routes commonly employ a three-phase alternating current to move electricity in a to-and-fro fashion, high-voltage direct current lines, such as Path 27, carry power in only one direction. A direct current also incurs less loss of energy than AC over the same distance; in other words, DC can sustain power much further than AC, which may require intermediate stations, or "taps", along the route. Another disadvantage for AC is that power tends to flow on the outer layer of the conductor, a phenomenon called the skin effect. DC eschews this problem; it allows power to penetrate the entire thickness of the conductor for optimal capacity. Finally, DC circuitry has been chosen for Path 27 because the cost of transmitting power over its distance is lower than with AC, which is best economically suited for shorter stretches. HVDC is also cheaper because it utilizes fewer than three conductors, translating to less materials and subsequently allaying the cost of infrastructure and equipment used.Asea, a multinational electrical supplier based in Sweden, managed the framework and supplies for Path 27, and was a chief contributor for the power line's installment. The number in "Path 27" is assigned by the Western Electricity Coordinating Council, which oversees the electrical transmission grid across the American West, to distinguish this line from the other critical transmission routes in the region. A 2010 report indicated that Path 27 was also the most congested electrical pathway within WECC's territory, though the system is able to handle such heavy usage.
Path 27 is owned and operated by an inter-municipal syndicate known as the Intermountain Power Agency, thus lending it the alternative name Intermountain. The IPA's roster includes 29 Utah municipalities and six Southern California cities. The cooperative was founded around this mission:Among its objectives is funneling excess energy to communities in Southern California; Path 27 was built to fulfill that purpose. The topmost SoCal client by population on the list is the Los Angeles Department of Water and Power, the preeminent utility provider for Los Angeles, which also receives the greatest share of power through the plan ahead of Anaheim, Riverside, Pasadena, Burbank, and Glendale. Altogether, these members acquire the biggest proportion of energy under the IPA.
History
The earliest concept of Path 27 likely originated in 1973, in a decade when the U.S., along with much of North America, was in the throes of a debilitating energy crisis. During that year, after the U.S. Bureau of Reclamation warned of a forthcoming energy shortage, representatives for the Utah-based Intermountain Consumer Power Association convened with Southern California localities in a quest for newer power sources and for investors. The IPA was established in June 1977.A crucial step for Path 27 came with the creation of the Intermountain Power Project in 1974, which emerged as an initiative for financing the construction of a coal-fired generator, though it was not until 1977 that Utah's legislatures formally endorsed IPA's implementation of the project. Caineville in Wayne County, UT, was one of several locations suggested for the power plant, prior to settling upon the site north of Delta in Millard County, or west of Lynndyl as stated in the sanction. After clearing environmental checks, the generator broke ground on October 9, 1981, with $300 million in initial funds. The original plan called for four thermal units, each unit producing 750 MW, but it had decreased to two due to concerns over precipitous power demand. The generator's first unit was erected in 1983. The first batch of coal was delivered by train on July 2, 1985, and the first unit came online later that year. The second unit was actuated on June 13, 1987, bringing the plant to its fullest capacity of 1,500 MW. It would be bolstered to its present-day 1,900-MW productivity in 1989, hence rendering the electrical generator the largest in Utah by yield.
However, Path 27 fully materialized when the transmission line's construction commenced on May 1, 1984, and took slightly more than sixteen months to complete. Various American contractors were commissioned for certain aspects and segments of the line. For instance, the Commonwealth Electric Company based in Marietta, Georgia, built of the line from Adelanto, California to Moapa, Nevada, while Irby Construction of Jacksonville, Mississippi, took on the remaining till Delta. But perhaps the most notable obligation for this turnkey project rested upon Asea, who imported the conductive cables and pieces for the galvanized steel-lattice support pylons to be assembled on the spot. One phase entailed the emplacement of the pylons' cement foundations, which are in average depth per pylon and vary in shape based on the tower's height and weight. Newspapers reported that rocky, jagged terrain often posed challenges for accessibility and had likely necessitated copious excavation and even dynamiting. In all, six hundred workers were deployed for the labor. The line achieved revenue service in July 1986 after the ignition of the power plant's first coal-burning unit, even though a document by LADWP attests that it might have been energized as early as December 1985. Depending on official tallies and news accounts, the total cost was $1.1 billion.
Finally yet equally integral for Path 27 are two converter stations, which were inaugurated along with the transmission main, and are vital for its functionality. IPP's converter was installed in conjunction with the power generator, while the second converter in Adelanto and a tangent switchyard began rising on May 26, 1985, and finished in June 1986 in time to receive freshly-borne energy from borders over. LADWP supervised the construction of the combined facility and tasked Asea with the responsibility of the accouterments.
Later history
Since its inception, Path 27 saw upgrades and refinements around the latter turn of the century—the IPP's latest proliferation included. At the behest of LADWP and Southern California Edison, ABB Group and Hitachi Energy modernized the transmission line in 2008 and 2011, respectively, with improved control and protection technology alongside supplemental filters and cooling systems at each terminal. This helped raise the power line's capacity to its modern-day level of 2,400 MW.Installation
Transmission
The primary component of Path 27 is the transmission line itself, which is constructed entirely aboveground on a length of through southwestern Utah, southern Nevada, and southern California. As opposed to three phases, Path 27 has two poles; one positive and another negative ; making this a bipolar configuration. It exerts 500 kV per pole, or 1,000 kV total, and is rated for 2,400 MW, giving it a maximum current rating of 4,800 amperes. By comparison, the Intermountain Power Plant generates up to 1,900 MW, thus adding to the line's fault tolerance. On most HVDC systems, power can be sent in either direction; Path 27 transports electricity from Utah to California under normal circumstances.Any conductive material engenders electromagnetic radiation whenever electricity flows through it. A single cable carrying such high voltage would produce particularly strong radiation in the form of a corona discharge that can deprive electrical energy and cause EM interference on radio and communication devices. For this reason, Path 27's conductors comprise three cables fastened together, or are triple-bundled. This not only reduces the detrimental effects of discharge, it also provides more surface area for power to flow, thereby improving transmission efficiency. Each cable is made of aluminium strands enveloping a steel core for strength and durability, and the composite measures in breadth.
Two shield wires are mounted above the main conductors and shared by the same support pylons. These wires guard the power line against lightning strikes.