Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

direct-axis subtransient open-circuit time constant
a constant that characterizes the initial decay of transients in the d-axis variables of the synchronous machine with the stator windings open-circuited. The interval characterized is that immediately following a disturbance, during which the effects of all amortisseur windings are considered. A detailed (derived) closed-form expression for the subtransient open-circuit time constant of a machine with a single d-axis amortisseur windings is obtained by taking the reciprocal of the smallest root of the denominator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the field winding resistance is very small and the detailed expression simplified.
direct-axis subtransient reactance
the high-frequency asymptote of the d-axis operational impedance of a synchronous machine. This value characterizes the equivalent reactance of the d axis of the machine during the initial time following a system disturbance. In models in which the rotor windings are represented as lumped parameter circuits, the d-axis subtransient reactance is expressed in closed form as the sum of the stator winding leakage reactance, and the parallel combination of the d-axis magnetizing reactance and the d-axis rotor leakage reactances.
direct-axis subtransient short-circuit time constant
a constant that characterizes the initial decay of transients in the d-axis variables of the synchronous machine with the stator windings short-circuited. The interval characterized is that immediately following a disturbance, during which the effects of amortisseur windings are considered. A detailed (derived) closed-form expression for the subtransient short-circuit time constant of a machine with a single d-axis amortisseur winding is obtained by taking the reciprocal of the largest root of the numerator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the field winding resistance is small and the detailed expression simplified.
direct-axis transient open-circuit time constant
a constant that characterizes the decay of transients in the d-axis variables of the synchronous machine with the stator windings open-circuited. The interval characterized is that following the subtransient interval, but prior to steady-state, during which the effects of the amortisseur windings are small (possibly negligible). A detailed (derived) closed-form expression for the transient open-circuit time constant of a machine with a single d-axis amortisseur winding is obtained by taking the reciprocal of the smallest root of the denominator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the amortisseur winding resistance is infinite and the detailed expression simplified.
direct-axis transient short-circuit time constant
a constant that characterizes the decay of transients in the d-axis variables of the synchronous machine with the stator windings short-circuited. The interval characterized is that following the subtransient interval, but prior to steady-state, in which the effects of the amortisseur windings are small (possibly negligible). A detailed (derived) closed-form expression for the short-circuit transient time constant is obtained by taking the reciprocal of the smallest root of the numerator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the amortisseur winding resistance is infinite and the detailed expression simplified.
directional overcurrent relay
an over-current relay that operates only for overcurrents flowing in the tripping direction. Direction sensing is typically done with respect to a voltage or current signal, which is not affected by fault location.
directional power relay
a protective relay that operates for power flow in a given direction. Applications are in cases where normal power flow is in one direction, including anti-motoring protection on a turbine-generator and fault backfeed protection on parallel step-down transformers.
disconnect switch
a manually operated switching device used to disconnect circuit conductors and their associated load(s) their source of electrical power.
discontinuity effect
an appropriate equivalent circuit model for discontinuities that have a tendency to disturb the electric and magnetic fields in their vicinity.
dispatch
the determination of the power output of each plant in an electric power system.
dissipation factor (DF)
the ratio of the effective series resistance of a capacitor to its reactance at a specified frequency measured in percentage.
See loss tangent
dissipation power
(1) ratio of real power (in phase power) to reaction power (shifted 90. out of phase).
(2) the ratio of the imaginary to real parts of the complex permittivity, expressed as a dimensionless ratio.
distributed generation
small power plants at or near loads and scattered throughout the service area.
distribution
(1) the possibility to execute different parts of a system on different processors.
(2) that class of electric power system work which is concerned with the distribution of electric power within a load area such as a residential or commercial area, or within an industrial installation. The distribution circuit extends from the local substation and terminates at the customer's meter.
distribution switchboard
a switchboard used in the distribution system, typically within a building.
distribution transformer
a transformer designed for use on a power distribution system (typically 2.4 kV to 34.5 kV) to supply electrical power to a load at the proper utilization voltage.
disturbance
a sudden change or a sequence of changes in the components or the formation of a power system. Also called fault.
diversity combining
a communication technique that combines the signals received through different, possibly independent channels. The most common methods of combining are maximum-ratio combining, equal gain combining, selection combining, and switched combining.
diversity frequency
a method for increasing the reliability of digital communications in which multiple copies of the signal, or other types of redundant information, are transmitted. Frequency diversity implies that the received signal occupies a much wider bandwidth than the minimum bandwidth needed to carry the information.
diversity path
a form of diversity in which multiple copies of the signal are created via different paths from the transmitter to receiver.