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 the ABB Group, a company based in Switzerland, 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 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.
Overview
Background
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, 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.
ABB Group, a multinational electrical supplier based in Switzerland, manages 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. 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.