Substation1
Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

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.
auxiliary relay
a relay employed in power system protection schemes that does not directly sense fault presence and location. Typical auxiliary relays include lockout relays, reclosing relays, and circuit breaker anti-pump relays.
auxiliary winding
a winding designed to be energized occasionally for a specific purpose, such as starting a single-phase motor. The power to the winding may be controlled by various means including a timer, centrifugal switch, current sensing relay, or voltage (counter EMF) sensing relay.
average power
the average value, taken over an interval in time, of the instantaneous power. The time interval is usually one period of the signal.
average-value model
a mathematical representation in which the average value of variables are used to model a system. In electric machines and drives, system variables are typically averaged over various switching intervals. This eliminates the high-frequency dynamics, but preserves the slower dynamics of the system.

AVR
See automatic voltage regulator
AWG
American Wire Gauge, a system of wire sizing used in the USA especially in smaller conductors used in residential and
commercial wiring.
B coefficient
See loss coefficient
B-ISDN
broadband integrated services digital network.

back EMF
See counter-EMF
backfeed
in power distribution work, power which flows from the secondary lines into the primary lines through the distribution transformer, e.g.,from an emergency generator connected to customer load.