Page 222 - Electrical Engineering Dictionary

small (possibly negligible). A detailed (de- nal, or both (dual directional coupler) in an
rived) closed-form expression for the tran- RF, microwave, or mmW circuit. Directional
sient open-circuit time constant of a machine couplers are usually described in terms of
with a single d-axis amortisseur winding is coupling factor and directivity. The coupling
obtainedbytakingthereciprocalofthesmall- factor describes what fraction of the incident
est root of the denominator of the d-axis op- (or reflected) power appears at the desired
erational impedance. An approximate (stan- coupled port. Directivity describes the frac-
dard) value is often used, in which it is as- tion of power coupled to the same port due to
sumed the amortisseur winding resistance is reverse power in the main arm of the coupler.
infinite and the detailed expression simpli-
fied. directional overcurrent relay an over-
current relay that operates only for overcur-
direct-axis transient short-circuit time rents flowing in the tripping direction. Di-
constant a constant that characterizes the rection sensing is typically done with respect
decay of transients in the d-axis variables to a voltage or current signal, which is not
of the synchronous machine with the sta- affected by fault location.
tor windings short-circuited. The interval
characterized is that following the subtran- directional power relay a protective re-
sient interval, but prior to steady-state, in lay that operates for power flow in a given
which the effects of the amortisseur wind- direction. Applications are in cases where
ings are small (possibly negligible). A de- normal power flow is in one direction, includ-
tailed (derived) closed-form expression for ing anti-motoring protection on a turbine-
the short-circuit transient time constant is ob- generator and fault backfeed protection on
tained by taking the reciprocal of the small- parallel step-down transformers.
est root of the numerator of the d-axis op-
erational impedance. An approximate (stan- directivity the maximum ratio of an an-
dard) value is often used, in which it is as- tenna’s ability to focus or receive power in
sumed the amortisseur winding resistance is a given direction relative to a standard; the
infinite and the detailed expression simpli- standard is usually an isotropic radiator or a
fied. dipole. Only depends on the radiation pattern
shape and does not include the efficiency of
direction cosine similarity between two the antenna.
variables x = (x 0 , ..., x n ) and y = (y 0 , ...,
y n ), it is defined as directory a table used in the directory
method to maintain cache coherence in mul-
(x, y)
cos θ = tiprocessors. Contains entries identifying the
k x kk y k
caches that hold copies of memory locations.
where (x, y) is the inner product of x and y
and k x k is the Euclidean norm of x. directory look-aside table (DLT) See
translation lookaside buffer.
direction line a curve to which the given
field is tangential at every point on the curve. Dirichlet conditions a set of conditions
Also called stream line or flux line. guaranteeing that a signal x(t) will be equal
to the N-term Fourier approximation of x(t)
direction of arrival See angle of arrival. as N →∞ except at isolated values of t for
which x(t) is discontinuous. The conditions
directional coupler a passive, 3 or 4 port are that
deviceusedtosampleaportionoftheforward (1) the signal x(t) must be absolutely in-
(incident) signal or the reverse (reflected) sig- tegrable,

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