Page 221 - Electrical Engineering Dictionary

edge of the structure of the inhomogeneous windings is obtained by taking the reciprocal
medium. of the smallest root of the denominator of the
d-axis operational impedance. An approxi-
direct semiconductor a semiconductor mate (standard) value is often used, in which
whose band gaps are between electronic it is assumed the field winding resistance is
states with the same momentum, thus allow- very small and the detailed expression sim-
ing optical transitions to occur between them. plified.

direct sequence refers to a technique for direct-axis subtransient reactance the
digital spread spectrum modulation. The high-frequency asymptote of the d-axis op-
data symbol is modulated by a higher-rate erational impedance of a synchronous ma-
pseudo-random sequence. The resulting sig- chine. This value characterizes the equiva-
nal is thus at a higher rate, or equivalently, lent reactance of the d axis of the machine
occupies a larger bandwidth. This is denoted during the initial time following a system
direct sequence spreading. disturbance. In models in which the rotor
windings are represented as lumped parame-
direct stroke (1) a lightning strike which ter circuits, the d-axis subtransient reactance
terminates on a piece of equipment is expressed in closed form as the sum of the
(2) an approach to the design of over- statorwindingleakagereactance, andthepar-
head electric transmission lines which as- allel combination of the d-axis magnetizing
sumes that only direct strokes to the line will reactance and the d-axis rotor leakage reac-
be disruptive to a power system. tances.

direct write lithography a lithography direct-axis subtransient short-circuit time
method whereby the pattern is written di- constant a constant that characterizes the
rectly on the wafer without the use of a mask. initial decay of transients in the d-axis vari-
ables of the synchronous machine with the
direct-access storage storage in which stator windings short-circuited. The inter-
an item can be accessed without having to val characterized is that immediately follow-
first access all other items that precede it in ing a disturbance, during which the effects
the storage; however, sequential access may of amortisseur windings are considered. A
be required to a few preceding systems. An detailed (derived) closed-form expression for
example is a disk, in which blocks may be the subtransient short-circuit time constant of
accessed independently, but access to a lo- a machine with a single d-axis amortisseur
cation within a block is preceded by access winding is obtained by taking the reciprocal
to earlier locations in the block. See also of the largest root of the numerator of the d-
sequential-access storage. axis operational impedance. An approximate
(standard) value is often used, in which it is
direct-axis subtransient open-circuit time assumed the field winding resistance is small
constant a constant that characterizes the and the detailed expression simplified.
initial decay of transients in the d-axis vari-
ables of the synchronous machine with the direct-axis transient open-circuit time
stator windings open-circuited. The interval constant a constant that characterizes the
characterized is that immediately following decay of transients in the d-axis variables
a disturbance, during which the effects of all of the synchronous machine with the stator
amortisseur windings are considered. A de- windings open-circuited. The interval char-
tailed (derived) closed-form expression for acterizedisthatfollowingthesubtransientin-
the subtransient open-circuit time constant of terval, but prior to steady-state, during which
a machine with a single d-axis amortisseur the effects of the amortisseur windings are

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