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Electrical transmission line in Southwestern United States

Path 27, also called the Intermountain or the Southern Transmission System (STS), is a high voltage direct current (HVDC) electrical transmission line running from the coal fired 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 (LADWP), and 29 smaller Utah municipalities. Path 27 consists of an overhead power line 488 mile (785 km) long, and is capable of transferring up to 2,400 megawatts (MW) of power at ±500 kilovolts (kV), 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 (DC) is preferred to the more common alternating current (AC) 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 (in place of three conductors for AC lines). 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. Sections of Path 27 are paralleled by many other AC transmission lines, including some of 500 kV. The line is also viewable from Interstate 15 which it crosses several times on its trek. 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.

High voltages 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 (AC) to move electricity in a to and fro fashion, high voltage direct current (HVDC) lines, such as Path 27, carry power in only one direction. A direct current (DC) 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, 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 (WECC), which oversees the electrical transmission grid across the American West, to distinguish this line from the other critical transmission routes. though the system is able to handle such heavy usage. and Glendale (as shown in the table below). Altogether, these members acquire the biggest proportion of energy under the IPA. +End Users of Path 27 (based on net rating of 1,800 MW)LADWP48.617%Anaheim13.225%Riverside7.617%Pasadena4.409%Burbank3.371%Glendale1.704%Total78.943%Source: Participants & Services

The primary component of Path 27 is the transmission line itself, which travels a length of 488 mile (785 km) entirely aboveground through southwestern Utah, southern Nevada, and southern California. It exerts ±500 kV and is rated for 2,400 MW, giving it a maximum current rating of 4,800 amperes; by comparison, the Intermountain Power Plant (IPP) generates up to 1,900 MW, thus adding to the line's fault tolerance. As opposed to three phases, Path 27 has two poles; one positive (cathode) and another negative (anode); On most HVDC systems, power can be sent in either direction; Path 27 transports electricity from Utah to California under normal circumstances. 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.

Both sides of Path 27 feature a converter station, which bridges electrical power between AC and DC circuitry and is ubiquitous to HVDC projects worldwide. The converters are located at the IPP station north of Delta, UT, and the Adelanto Converter Station in Adelanto, north of San Bernardino, CA. The heart of the conversion process transpires in clusters of electronic valves that procedurally modify the flow of electricity in a manner similar to switches. The type of valves on Path 27 is thyristor, technology popular in HVDC since the late 1960s. The valve configuration is identical for both converters: 24 stacks about 50 feet (15 m) in height of 24 valves apiece (or three "quadruple valves"), The valves are affixed on tall insulators that separate them from the interior walls, floor, and ceiling; done to hinder induction and premature grounding, as well as aid cooling. Path 27's valve halls are also fortified against earthquakes. As the IPP converter is adjacent to the power plant, electricity is generally fed into the HVDC line from this terminus. Ingress for power to the system is preceded by a row of transformers and serve to cushion the facility from disturbances as a result of power surges. The electricity is then sent on its way down Path 27. The Adelanto Converter Station demarks the receiving end of the HVDC line and has relatively the same layout, but the conversion mechanism is reversed. Beyond another set of reactors and filters, the inpouring power enters the valve hall to be carried over to AC circuitry anew—this is inversion, and it blocks any power backflow on the AC conduit from reaching the valves to enforce unidirectional movement. They are reputedly the largest ever manufactured to the U. S. by ABB Group. although this seldom happens out of practicality.|group=lower alpha
Like any HVDC scheme, Path 27 needs to be grounded at both ends in order to operate with respect to the earth. Grounding provides the electric current a common path back into the earth, which serves as a neutral point for the circuit. Even with safeguards and a substantive fault tolerance, failures on the line can and do occur, thence grounding becomes a workaround for continued reliability of the system. If one pole develops a fault, its current is diverted via the ground return to complete the circuit. This de energizes the problem conductor, while the second pole remains active; These spots were chosen in part for high conductivity within the earthen minerals. The electrode line for the IPP converter has a total length of , and the Adelanto's extends .

Following the regular power flow, Path 27 begins at the Intermountain Power Plant in Utah and heads briefly west before curving southward, passing over the U. S. 50/U. S. 6 duplex west of Hinkley. The DC line ventures though legions of dry basins and high mountains in the state due southwest; all the while crossing Utah Routes 21, 56, and 18; before reaching Nevada about 5.5 mile (9 km) north of Arizona's northwestern corner, simultaneously entering the Mojave Desert. Inside Nevada, Path 27 traverses the Mormon Mesa, bypassing Glendale and crossing the Muddy River. It bisects the Moapa River Indigenous Reservation lands A 345 kV circuit runs beside the DC line interlinking the IPP and a wind farm near Milford.

The coal firing powerhouses at the IPP (Intermountain Power Plant) site are to be retired by 2027; this adheres to LADWP's wishes for reducing its dependence on fossil fuels in favor of greener energy. and paired with at least two solar farms on nearby parcels. The sources will beget 840 MW and 300 MW at peak, respectively, or 1,140 MW when combined, still lower than Path 27's maximal capacity. This "renewal project" also calls for both converter stations to be replaced and activated by June 2026, as well as improvements to Path 27 to lengthen the power line's service life.