Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

differential protection unit
a protective unit based on the difference of currents flowing in and out of a protected zone.
diode detector
a device that by use of rectification and the use of inherent nonlinearity separates a modulating signal from its carrier.
diode rectifier
a circuit in which the output voltage is fixed by the circuit parameters and the load. The direction of power flow is not reversible. An example of a single-phase diode-bridge rectifier with a capacitor filter is shown. Note that the diodes are on only for a short duration, while the rectified line voltage is greater than the capacitor voltage.
direct axis (d axis)
the magnetic axis of the rotor field winding of a synchronous machine. The axis between the poles of a DC machine.
direct axis magnetizing (armature) reactance
a reactance that represents all the inductive effects of the d-axis stator current of a synchronous machine, except for that due to the stator winding leakage reactance. In Park's d-axis equivalent circuit of the synchronous machine, this reactance is the only element through which both the stator and rotor currents flow. Its value may be determined by subtracting the stator winding leakage reactance from the steady-state value of the d-axis operational impedance or from the geometric and material data of the machine.
direct axis synchronous reactance
the sum of the stator winding leakage reactance and the direct-axis magnetizing (armature) reactance of a synchronous machine. This represents the balanced steady-state value of the direct-axis operational impedance of the synchronous machine, and thus characterizes the equivalent reactance of the machine during steady-state operation.
direct axis transient
reactance a value that characterizes the equivalent reactance of the d-axis windings of the synchronous machine between the initial time following a system disturbance (subtransient interval) and the steady state. This reactance cannot be directly mathematically related to the d-axis operational impedance. However, in models in which the rotor windings are represented as lumped parameter circuits, the d-axis transient 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 field winding leakage reactance.
direct burial
(1) the practice of burying a specially-armored power or communications cable in a ditch without the use of a surrounding conduit.
(2) a term applied to any cable which is meant for direct burial. direct control bottom, first, control layer
of a multilayer controller, directly responsible for adjusting the manipulated inputs to the controlled process; typical example of direct control is the regulation layer of an industrial control system, where the manipulated inputs are used to make the controlled variables follow the desired set point values.
direct converter
a frequency converter that converts an RF signal to a baseband signal directly in receivers. It converts a baseband signal to an RF signal directly in transmitters.
direct current machine
a DC machine is an electromechanical dynamo that either converts direct current electrical power into mechanical power (DC motor), or converts mechanical power into direct current electrical power (DC generator). Some DC machines are designed to perform either of these functions, depending on the energy source to the dynamo.
direct current motor
a rotation machine energized by DC electrical energy and used to convert electrical energy to mechanical energy.
direct drive
a drive in which no gear reducer is used.
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.
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