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What Is a Flexible AC Transmission System?

By Paul Reed
Updated May 17, 2024
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A flexible AC transmission system (FACTS) is a combination of hardware and software installed in power transmission substations designed to improve capacity and reliability. AC refers to alternating current, the electrical power supplied by electrical utilities to homes and businesses. FACTS technology can be used to optimize existing electrical transmission systems and often prevent or delay construction of new transmission systems.

Electric utilities face continued demands for greater capacity as customers consume greater quantities of electricity. Conservation programs, higher efficiency appliances and lighting, and smarter consumer electronics can slightly reduce electrical use. Environmental concerns for overhead electrical lines, magnetic fields and damage from new construction can delay new equipment construction. A flexible AC transmission system can provide better performance of existing transmission lines, improve power quality and reduce electrical losses.

Installation of FACTS equipment in series, or in-line, with the transmission lines is used to control system voltage. Equipment designed in shunt, or parallel across transmission lines, controls current or electrical flow. FACTS includes software, capacitors, voltage regulators and other equipment that manages the electrical system.

A flexible AC transmission system has several advantages over standard transmission networks. Existing transmission lines have significant voltage losses over long distances. This system can boost line voltage and maintain proper supply voltage across a transmission network. Most transmission systems are interconnected networks of supply lines, where power imbalances can occasionally occur. FACTS shunt systems can balance power across parallel supply lines to optimize performance.

Increasing development and population density can make construction of new transmission equipment difficult. FACTS technology is compact and can be installed within the existing footprint of a substation or electric transmission facility. Existing transmission lines carry higher loads without overheating or exceeding their rated power limits. Customers typically see improved electric supply quality and reliability.

Electrical demands are not steady, but fluctuate widely with customer needs. Static transmission lines cannot quickly adapt to these changes, which can result in voltage losses to customers. Reduced voltages and brownouts can also damage equipment. FACTS technology includes software that responds to changing utility conditions, optimizing system performance.

Another advantage of flexible AC transmission system technology is the reduced cost to a utility for installing this system versus designing and building new transmission systems. FACTS equipment consists of modules that are combined to meet the performance needs of the substation or transmission line. Construction time can be reduced significantly. Many utilities are regulated, and are faced with pressure to reduce costs and defer price increases to consumers. Optimizing existing equipment can be a very cost-effective way to meet increasing demand.

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