Substation1
Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

asynchronous operation
a term to indicate that a circuit can operate or a communication system can transmit information when ready without having to wait for a synchronizing clock pulse.
asynchronous system
a (computer, circuit, device) system in which events are not executed in a regular time relationship, that is, they are timing-independent. Each event or operation is performed upon receipt of a signal generated by the completion of a previous event or operation, or upon availability of the system resources required by the event or operation.
asynchronous transfer mode (ATM)
method of multiplexing messages onto a channel in which channel time is divided into small, fixed-length slots or cells. In ATM systems the binding of messages to slots is done dynamically, allowing dynamic bandwidth allocation. ATM is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic.
ATM
See asynchronous transfer mode
atmosphere
a convenient measure of pressure. 1 std atm = 14.696 psia (pounds per square inch absolute).

atmospheric attenuation
decrease in the amplitude of a signal propagating through the atmosphere, due primarily to absorption and scatter.

atom
a particle of matter indivisible by chemical means, which is chemically neutral. It is the fundamental building block of the chemical elements.
attenuation
the exponential decrease with distance, in the amplitude of an electric signal traveling along a very long transmission line due to losses in the supporting medium. In electromagnetic systems attenuation is due to conductor and dielectric losses. In fiber optic systems attenuation arises from intrinsic material properties (absorption and Rayleigh scattering) and from waveguide properties such as bending, microbending, splices, and connectors.
attenuation constant
the real part of the complex propagation constant for an electromagnetic wave.

attenuator
a device or network that absorbs part of a signal while passing the remainder with minimal distortion.

automatic
(1) property pertaining to a process or a device that functions without intervention by a human operator under specified conditions.
(2) a spring-loaded tension sleeve into which a conductor or other wire is inserted for tensioning and attachment to a pole or other fixture.

automatic circuit recloser
See recloser
automatic frequency control (AFC)
(1) an automatic feedback control system that is used to maintain active power balance by means of the speed governor system. In an interconnected system, scheduled power interchanges are maintained by means of controlling area generations.
(2) electronic circuitry used to keep the received signal properly placed within the desired IF frequency range. In televisions, the AFC circuitry is also called the AFT or "automatic fine tuning" section. The AFC circuit will generate an error signal if the input frequency to the IF drifts above or below the IF frequency. The error signal is fed back to vary the local oscillator frequency in the tuner section. See also automatic fine tuning (AFT).
automatic gain control (AGC)
a method to control the power of the received signal in order to be able to use the full dynamic range of the receiver and to prevent receiver saturation.

automatic generation control (AGC)
phrase describing the computer-based process by which electric utilities control individual generating stations to maintain system frequency and net interchange of power on a highly interconnected transmission grid. Automatic generation control (AGC) systems monitor grid frequency, actual and scheduled power flows, and individual plant output to maintain balance between actual and scheduled power production, both within transmission control areas and at individual generating stations. Control is generally accomplished by adjusting the speed control (or droop) characteristics of individual generating units. Control actions are determined by planned production schedules and power exchange agreements among participating utilities.
automatic transfer switch
a self-acting switch which transfers one or more load conductor connections from one power source to another.
automatic voltage regulator (AVR)
an automatic feedback control system that is responsible for maintaining a scheduled voltage either at the terminals of a synchronous generator or at the high-side bus of the generator step-up transformer. The control is brought about by changing the level of excitation.
automation
refers to the bringing together of machine tools, materials handling process, and controls with little worker intervention, including
1. a continuous flow production process that integrates various mechanisms to produce an item with relatively few or no worker operations, usually through electronic control;
2. self-regulating machines (feedback) that can perform highly precise operations in sequence; and
3. electronic computing machines. In common use, however, the term is often used in reference to any type of advanced mechanization or as a synonym for technological progress; more specifically, it is usually associated with cybernetics.

autotransformer
a power transformer that has a single continuous winding per phase, part of this winding being common to both the primary and the secondary sides. As a result, these voltages are not isolated but the transformer is reduced in weight and size. Autotransformers are most suited for relatively small changes in voltage. Three phase autotransformers are by necessity connected in a wye configuration.
autotransformer starter
a single three-phase autotransformer or three single phase transformer used to start induction motors at a reduced voltage.