Electrical Engineering Dictionary - part 167

of the state variables at any time

> t

can

be determined uniquely provided the system
input is known for. The vector whose com-
ponents are state variables is called the state
vector. The state space is the vector space
whose elements are state vectors.

state vector

a vector formed by the state

variables.

state-space averaging

a method of ob-

taining a state-model representation of a cir-
cuit containing switching elements by aver-
aging the state models of all the switched
topologies.

state-space averaging model

a small-

signal dynamic modeling method for PWM
switching circuits. The circuit is viewed as
two linear subcircuits, one with the switch
on and one with the switch off. A duty-ratio
weighted average of the state-space equa-
tions for the two subcircuits is then linearized
and used to obtain the small-signal transfer
function for the switching circuit.

static excitation system

an excitation sys-

tem derived from solid state devices such as
thyristors that convert the AC terminal volt-
age to DC before application to the rotor.

static induction thyristor (SITH)

a self-

controlled power device with high switch-
ing frequency. The structure is similar to
the static induction transistor (SIT) (hence,
not really a thyristor), but has an additional
p-layer added to the anode side.

It is a

normally-on device with the n-region satu-
rated with a minority carrier. The device does
not have reverse blocking capability.

static induction transistor (SIT)

a high-

power, high-frequency device that is essen-
tially a solid-state version of the triode vac-
uum tube. It is a normally-on device, and a
negative gate voltage holds it off. The cur-
rent ratings of the SIT can be up to 300 A,
1200 V, and the switching frequency can be
as high as 100 kHz.

static prediction

a method of branch pre-

diction that relies of the compiler selecting
one of the two alternative instructions for af-
ter the branch instruction (either the next in-
struction or that at the target location speci-
fied in the branch instruction). A bit is pro-
vided in the branch instruction, which is set
to a 0 for one alternative and 1 for the other
alternative. The processor then follows this
advice when it executes the branch instruc-
tion.

static random access memory (SRAM)
random access memory that, unlike dynamic
RAM, retains its data without the need to be
constantly refreshed.

static system

a system whose output does

not depend upon past or future input is a
static, or memoryless system. For example,
consider a voltage

(t) applied to an ampli-

fier with gain

K that yields the output

out

(t) = Kv

(t) .

The output voltage at a particular instant in
time depends only on the input applied at that
same instant, thus the amplifier is a static sys-
tem. If a system is not static, then it is a
system with memory, or a dynamic system.

static var compensator

a device for fast

reactive compensation, either inductive or
capacitive, brought about by thyristor-based
control of an effective shunt susceptance. It
is typically used to regulate voltage at a bus
on the high voltage transmission system.

static VAR regulator

also called a static

VAR compensator. A nonrotating electrical
device designed to adjust the reactive power
flow of an AC power system. It typically
consists of a reactive load (either inductive
or capacitive) and a series electronic switch
(thyristor) that controls the reactive power.

static-column DRAM

DRAM that is or-

ganized in the same manner as a page-mode
DRAM but in which it is not necessary to tog-

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gle the column access strobe on every change
in column address.

station battery

a battery used to provide

operating energy for the protective relay op-
erations and to initiate circuit breaker opera-
tions in a generating station. The battery is
necessary, as the equipment must work reli-
ably during severe voltage sags and outages
on the AC system.

station control error

in economic dis-

patch studies, the difference between the de-
sired generation of all plants in a control area
and the actual generation of those plants.

station insulator

refers to a large-sized

insulator used in substations.

stationarity interval

the interval of either

time (temporal stationarity interval) or space
(spatial stationarity interval) over which the
conditions required for a WSSUS approxi-
mation is valid. That is, the stationarity in-
terval is the period of time or spatial sepa-
ration over which the scattering function of
the channel, and consequently also the delay
and Doppler power spectra, stays fixed. This
requires that the significant scatterers should
remain the same.

stationary

a dynamic system described

by a first-order vector differential equation
that does not depend explicitly on time. In
other words, such a system is governed by an
equation of the form

˙x(t) = f (x(t), u(t))

stationary process

a stochastic process

x(t), for which the joint probability distri-
bution of

x(t

) and x(t

) depends only on

− t

stationary reference frame

a two-dimen-

sional space that is fixed (nonrotational). In
electric machines/power system analysis, an
orthogonal coordinate axis is established in
this space upon which fictitious windings are

placed. A linear transformation is derived in
which the physical variables of the system
(voltage, current, flux) are referred to vari-
ables of the fictitious windings.

statistical quality control

methods of

quality improvement based on statistical
techniques. The main idea is to use statistical
methods for identification of unusual varia-
tions of the controlled process and to pin-
point the causes of such variations. By col-
lecting data at every stage of the production
process and statistical analysis of those data
( See

control chart

), the process is main-

tained in a state of statistical control. The
main difference between the statistical qual-
ity control and quality inspection is that the
latter enables only quality control while the
former leads to quality improvement. This in
turn results in increased productivity.

statistical multiplexing

multiplexing of

a number of variable bit rate (VBR) sources.
A result of statistical multiplexing is that for
a sufficiently large number of VBR sources,
the aggregate bit rate is less than the sum of
the peak bit rates of the individual sources.

statistical pattern recognition

methods

for carrying out the recognition of patterns on
the basis of statistical analysis. These meth-
ods are typically based on the learning of un-
known pattern probability distributions from
examples.

statistical sensitivity

a statistic derived

from statistical interpretation of the variation
of the circuit function,

F (s, x) around the

average value

Let

F (s, x) be a circuit function that de-

pends on a random parameter

x. Then the

statistical sensitivity can be approximated as

F (s, x) ≈ F s, x

+ x − x

∂F(s, x)

∂x

x=x

The values

x, F (s, x), and ∂F (s, x)/∂x

calculated at

x = x are considered con-

stants (the last one is usually denoted as

∂F (s, x)/∂x). If instead of x and F (s, x)

2000 by CRC Press LLC

are used, their relative deviations from the
average values, namely

δx = (x − x)/x and

∂F (s) = (F − F )/F , one obtains that

δF (s) ≈

F s, x

∂F s, x

∂x

δx =

F (s,x)

δx

Hence, in the first-order approximation the
random variables

δF (s) and δx are propor-

tional. Then this equation is interpreted sta-
tistically. For example, on the

jω axis the

averages of

δF (jω) and δx are related by

δF

≈

F (jω,x)

δx

The relationships for other statistical param-
eters can be obtained as well.

statistical spectral compression

a com-

mon approach to compression used in picture
coding. In this approach, the statistical re-
dundancy of the image is exploited and the
compression is also obtained by coding the
spectral components of the transformed im-
age.

stator

the portion of a motor that includes

and supports the stationary active parts. The
stator includes the stationary portions of the
magnetic circuit and the associated windings
and leads.

status callback

a request made by a con-

sumer for the utility to give them a phone call
which indicates the change of status of their
service request. An example of this would be
calling the consumer once a crew has arrived
on the scene of an outage or has located the
root cause of an outage.

status register

a register in a processor

that holds the status of flags; individual bits
in the register represent flag status.

steady-state control

operation and mech-

anisms of the control system in which the
main objective is to keep the controlled pro-
cess in the condition where the state vari-
ables relevant to the controlled process per-
formance are constant — i.e., to keep the pro-

cess in a required operating point. Steady-
state control structure may be composed of
several control layers, including direct reg-
ulatory layer, optimization layer and, even-
tually, other layers; steady-state control is
widely used in chemical and power indus-
tries.

steady-state gain

the gain that a system

applies to DC (constant) input signals.

steady-state error

the difference between

the desired reference signal and the actual
signal in steady state, i.e., when time ap-
proaches infinity.

steady-state response

in network analy-

sis, a condition that the response reaches a
constant value with respect to the indepen-
dent variable. In control system studies, it is
more usual to define steady state as the fixed
response at infinity with respect to the fixed
input under the stable circumstances.

steady-state stability

a power system

is steady-state stable if it reaches another
steady-state operating point after a small dis-
turbance. See

dynamic stability

steepest descent algorithm

See

gradient

descent

steering vector

in an antenna array, the

complex weights associated with each an-
tenna element to form a specific radiation
pattern are called the steering vector, since
these weights steer the radiation pattern in a
specified direction.

Steinmetz constant

a constant

n that re-

lates the area of the hysteresis loop of a mag-
netic material to the maximum flux density
in the material.

= K

max

where

is the area of the hysteresis loop,

is a constant of the material,

max

is the

maximum flux density, and

n is the Steinmetz

constant.

2000 by CRC Press LLC

step and repeat camera

See

stepper

step edge

an idealized edge across which

the luminance profile takes the form of a
step function, i.e., a line separating two re-
gions having different average gray-levels.
See

edge

step index fiber

a type of optical fiber

where there is an abrupt transition from the
core to cladding region, each region having
a different refractive index; optical fiber in
which the a homogeneous core region has
a higher index of refraction than a homoge-
neous cladding region, in contrast to a graded
index fiber. This configuration is more typ-
ical of single mode fibers, than multimode
fibers, which suffer from modal dispersion
effects.

step index optical fiber

See

step index

fiber

step response

the output of a linear time-

invariant system when the inputs are varied
as a step signal.

step size

when solving for the transient be-

havior of an electrical circuit, the associated
differential equations are solved at specific
points in time. The difference between two
adjacent solution time points is known as the
step size.

step-and-scan

a type of projection print-

ing tool combining both the scanning motion
of a scanner and the stepping motion of a
stepper.

step voltage

in power system safety stud-

ies, the voltage measured across two points
on the ground which are separated by a dis-
tance equal to an average person’s step while
walking over the area in question.

step-down converter

See

buck converter

step-up converter

See

boost converter

stepped leader

in lightning, a discharge

following the preliminary breakdown that
propagates from a cloud toward the ground in
a series of intermittent luminous steps with
an average speed of 10

to 10

m/s. Neg-

atively charged leaders clearly step, while
positively charged leaders are more pulsat-
ing than stepped.

stepper

a type of projection printing tool

that exposes a small portion of a wafer at
one time, and then steps the wafer to a new
location to repeat the exposure. Also called
a step-and-repeat camera.

stepping correlation

a principle of opera-

tion of a correlation receiver in channel mea-
surement, where pseudo-random sequences
are utilized. The transmitted signal consists
of a carrier modulated (typically employing
phase-shift keying) by a pseudo-random se-
quence.

The received signal is correlated

(multiplied) by an exact replica of the trans-
mitted signal by stepping the chip position
of the reference signal with respect to the re-
ceived signal through all or part of the chip
positions. The output of a stepping correlator
is a time-scaled version of the autocorrelation
function of the pseudo-random sequence, or
a part of it. The time-scaling factor depends
on the rate with which all the chip positions
of interest can be stepped. The rate of pro-
ducing autocorrelation functions can be made
much higher than in sliding correlation. See
also

sliding correlation

stereo imaging

See

binocular imaging

stereo vision

a vision model in which im-

aged objects are projected onto two image
planes, to extract depth information from the
scene. See also

binocular vision

stereospecific

directional covalent bond-

ing between two atoms.

sticky bit

the least significant guard digit

in floating-point representations. It is an in-
dicator bit obtained through a logical “OR”

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