Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

subtransient open-circuit time constant
See quadrature-axis transient open-circuit time constant and direct-axis subtransient open-circuit time constant.
superfluorescence
usually refers to the enhanced spontaneous emission that occurs due to self-organization into a coherent state by a system of atoms or molecules.
superheater
a heat exchanger that increases the steam temperature to about 1000 degrees F. It is heated by the flue gases.
superparamagnetism
a form of magnetism in which the spins in small particles are exchange coupled by may be collectively switched by thermal energy.
surge impedance
the ratio of voltage to current on that line for a high speed wave propagating down the line. The surge impedance of a line is a constant which depends on the line geometry and conductor characteristics. On power transmission lines, these waves are typically generated by lightning strokes, circuit breaker switching, etc.
See characteristic impedance
surge impedance loading (SIL)
of a transmission line, the characteristic impedance with resistance set to zero (resistance is assumed small compared to reactance). The power that flows in a lossless transmission line terminated in a resistive load equal to the line's surge impedance is denoted as the surge impedance loading of the line.
surge response voltage
the voltage that appears at the output terminals of surge protection equipment and is seen by loads connected to that device both during and after a surge condition.
susceptibility
the part of the permittivity or permeability that is attributable to the electromagnetic behavior of the medium. In a linear, isotropic medium, the electric susceptibility is numerically equal to the relative permittivity minus one, and the magnetic susceptibility is equal to the relative permeability minus one. See also electric susceptibility.
sustained interruption
all interruptions that are not momentary. Generally used when referring to long duration voltage interruptions of greater than 1 minute.
symmetrical component
the method by which unbalanced three-phase power system operation (particularly unbalanced fault performance) can be efficiently analyzed. Symmetrical components convert unbalanced line currents and voltages to three sets of balanced sequence components: positive sequence, negative sequence, and zero sequence.

The transformed phasor variables fC-0s are obtained by applying the appropriate Fortescue transformation to any multi-phase set of phasor variables. Denoted, f+s;f-s;f0s, these are the positive sequence, negative sequence, and zero sequence components, respectively. The variables are so named because any unbalanced
set of currents can be expressed (in phasor form) in terms of

1. a balanced set of currents with magnitude iCs that has a phase sequence which produces counterclockwise rotor rotation in a machine (positive sequence).

2. a balanced set of phasor currents with magnitude i-s that has a phase sequence which produces clockwise rotor rotation in a machine (negative sequence), and

3. a set of three equal phasors with magnitude f0 (zero sequence) which does not produce rotor rotation in machine.
synchronous operation
an operation that is synchronized to a clocking signal.
synchronous speed
speed of the rotating magnetic flux produced by three-phase currents in stationary coils in three-phase AC machines. The synchronous speed is calculated by a knowledge of the number of poles of the machine and the frequency of the stator currents as

Ns = 120fs/P
tap
a connection (actually one of several) to a coil, allowing the number of turns in the coil to be varied.
tap changer
a device to change the tap setting on a transformer coil, allowing voltage control. See also tap, tap changing under load.
tap changing under load (TCUL)
a type of transformer in which the output voltage can be adjusted while the load is connected to the transformer. The voltage is adjusted by changing the turns ratio of the primary and secondary coils. That, in turn, is accomplished by bringing out connections (taps) at several points on the coil. Changing from one tap to another either adds or subtracts turns from the coil and raises or lowers the voltage, respectively.
tape-wound core
a ferromagnetic core constructed by winding ribbon-like steel instead of stacking thicker, punched lamination. Usually used for higher frequency devices, or where it is desired to reduce the eddy current losses.
temperature coefficient of resistance
the change in electrical resistance of a resistor per unit change in temperature.
temporary fault
a fault that will not reoccur if the equipment is deenergized and then reenergized. An example of a temporary fault is when a lightning stroke causes an uninsulated overhead line to arc over an insulator, with no equipment damage.
temporary interruption
a loss of voltage of less than 0.1 pu for a duration of 3 seconds to 1 minute.
thermal expansion mismatch
the absolute difference in thermal expansion of two components.
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