电气英文论文
皇后的新装-
Page1 Generators
and Motors
From reference 1
1. Direct-current generators
impress on the line a direct or
continuous emf, one
that is always in
the same direction. Commercial dc generators have
commutators,
which distinguish them
from ac generators. The function of a commutator
and the
elementary ideas of generation
of emf and commutation are discussed in Div. 1.
Additional information about
commutation as applied to dc motors, which in
general
is true for dc generators, is
given below.
2. Excitation of generator
fields.
To generate an emf, conductors
must cut a
magnetic field which in
commercial machines must be relatively strong. A
permanent magnet can be used for
producing such a field in a generator of small
output, such as a telephone magneto or
the magneto of an insulation tester, but in
generators for light and power the
field is produced by electromagnets, which may
be excited by the machine itself or be
separately excited from another
-excited machines may be of the series,
shunt, or compound type,
depending upon
the manner of connecting the field winding to the
armature. In the
series type of
machine,the field winding (the winding which
produces the magnetic
field) is
connected in series with the armature winding. In
the shunt type, the field
winding is
connected in parallel,shunt, with the armature
winding. Compound
machines have two
field windings on each pole. One of these windings
is connected
in series with the
armature winding, and the other is connected in
parallel or shunt
with the armature
winding.
3. Armature winding
of dc machines may be of the
lap
or the
wave
type. The
difference in the
two types is in the manner of connecting the
armature coils to the
commutator.A coil
is the portion of the armature winding between
successive
connections to the the lap
type of winding (see Fig. 7.1) the two ends
of a coil are connected to adjacent
commutator segments. In the wave type of
winding (see Fig. 7.2) the two ends of
a coil are connected to commutator segments
that are displaced from each other by
approximately 360 electrical
degrees.
The type
of
armature winding employed affects the voltage and
current capacity of the
machine but has
no effect upon the power capacity. This is due to
the fact that the
number
of parallel paths between armature
terminals is affected by the type of
winding. For a wavewound
machine there are always two paths in
parallel in the
armature winding
between
armature terminals.
For a lap-wound machine there are
as
many parallel paths in the armature
winding as there are pairs of poles on
the
machine. For the same number and
size of
armature conductors,
a machine when
wave-connected would
generate a voltage that
would equal the voltage generated
when lap-connected times the number of
pairs of the current capacity
would be
decreased in the same proportion that the voltage
was
increased. The
current capacity of a machine when
wave-connected is therefore equal to
the
capacity when lap-
connected divided by the number of pairs of
poles.
4. The value of the
voltage generated by a dc machine
depends upon the
armature
winding, the speed, and the field current. For a
given machine, therefore,
the voltage
generated can be controlled by adjusting either
the speed or the field
current. Since
generators are usually operated at a constant
speed, the voltage must
be controlled
by adjusting the field current.
5.
Separately excited dc generators
are
used for electroplating and for other
electrolytic work for which the
polarity of a machine must not be reversed.
Self-excited machines may change their
polarities. The essential diagrams are shown
in Fig. 7.3. The fields can be excited
from any dc constant-potential source, such as a
storage battery, or from a rectifier
connected to an ac supply.
The field
magnets can
be wound for any voltage
because they have no electric
connection
with the
armature. With a constant field
excitation, the voltage will drop slightly fromno
load
to full load because of armature
drop and armature te excitation is
advantageous when the voltage generated
by the machine is not suitable for field
excitation. This is true for especially
low- or high-voltage machines.
6. Series-wound generators
have their armature winding, field
coils, and
external circuit connected
in series with each other so that the same current
flows
through all parts of the
circuit (see Fig. 7.4). If a series generator is
operated at no
load (external circuit
open), there will be no current through the field
coils, and the
only magnetic flux
present
in the machine will be that due
to the residual magnetism which has been
retained by the poles from previous
operation. Therefore, the no-load voltage of a
series generator will be only a few
volts produced by cutting the residual flux. If
the
external circuit is closed and the
current increased, the voltage will increase with
the
increase in current until the
magnetic circuit becomes saturated. With any
further
increases of load the voltage
will decrease. Series generators have been used
sometimes in street-railway service.
They have been connected in series with long
trolley feeders supplying sections of
the system distant from the supply point in
order to boost the voltage. However,
power rectifiers have replaced dc generators
for most installations of this type.
Keywords:
generator
From reference 2
Since triphased asynchronous generators
are mainly used in conversion systems
of a eolian energy into electric
energy, their functional stability represent is
of great importance. As a first step,
the factors that radically affect the functional
stability of these generators have been
established. Thus, it was decelat the
powerful influence of the capacitor
bank
–
that provides the
necessary reactive
power for the
magnetization of the ferromagnetic core
–
over the functional
stability of the
triphased asynchronous
generator with short circuit rotor. The functional
stability is
greatly influenced by the
charge character (type) as well. The experimental
work
emphasized
–
through the functional features
–
the way these parameters
influence
the stability area of the
asynchronous generators. As far as triphased
asynchronous
generators with coiled
rotor are concerned, the controllable blind power
was
analyzed the analogy being made
with the situation of the necessary controllable
generating capacity for of the
triphased asynchronous generator with short
circuit
rotor.
Keywords
: triphased asynchronous generator.
[1]
D.M. Eggleston, F.S. Stoddard
–
Wind turbine engineering
design, Van Nostrand
Reinhold Company
New York 1986;
[2] V. I
lie,
L. Almaşi, şa –
Utilizarea energiei
vântului,
Ed. Tehnică, Bucureşti,
1984;
[3] Kovacs Pal
–
Analiza
regimurilor tranzitorii ale maşinilor electrice,
Ed. Tehnică,
Bucureşti 1980
;
[4] R.J. Harrington, F.M.M. Bassiouny
–
New Approach to
Determinate the Critical
Capacitance
for Self - Excited Induction Generators, IEEE
Trans. On Energy
Conversion, vol. 13,
no.3, sept. 1998, pp.244 - 250;
[5]
Colliez, C., Tounzi, A., Piriou, F.
–
Vector Control of a
Autonomous Induction
Generator
connected to a PWMRectifier. EPE `97, Trondheim,
Norvegia, vol. 2, pp.
711-716;
[6] Alan, I., Lipo, A. T.
–
Control of a Polyphase
Induction-Generator/ Induction-
Motor
Power Conversion System Completely Isolated from
the Utility. IEEE Trans. On
Ind. App.,
vol.30, no.3, may/june 1994, pp. 636-647
[7] Florin Iov
–
Stadiul actual î
n conversia energiei
eoliene (Referat nr.1
–
î
n cadrul
pregătirii
tezei de
doctorat) martie 1998;
[8] Florin Iov
–
Studiul
ansamblului turbină eoliană –
generator
asincron autoexcitat
(Referat nr.2
–
î
n
cad
rul pregătirii tezei de doctorat)
iunie 1999;
Page2 Electrical Energy Transmission
From reference 1
Growing populations and industrializing
countries create huge needs for
electrical energy. Unfortunately,
electricity is not always used in the same place
that
it is produced, meaning long-
distance transmission lines and distribution
systems are
necessary. But transmitting
electricity over distance and via networks
involves
energy loss.
So,
with growing demand comes the need to minimize
this loss to achieve two
main goals:
reduce resource consumption while delivering more
power to users.
Reducing consumption
can be done in at least two ways: deliver
electrical energy
more efficiently and
change consumer habits.
Transmission
and distribution of electrical energy require
cables and power
transformers, which
create three types of energy loss:
the
Joule effect, where energy is lost as heat in the
conductor (a copper wire,
for example);
magnetic losses, where energy
dissipates into a magnetic field;
the
dielectric effect, where energy is absorbed in the
insulating material.
The Joule effect
in transmission cables accounts for losses of
about 2.5 % while
the losses in
transformers range between 1 % and 2 % (depending
on the type and
ratings of the
transformer). So, saving just 1 % on the
electrical energy produced by a
power
plant of 1 000 megawatts means transmitting 10 MW
more to consumers,
which is far from
negligible: with the same energy we can supply 1
000 - 2 000 more
homes.
Changing consumer habits involves
awareness-raising programmers, often
undertaken by governments or activist
groups. Simple things, such as turning off
lights in unoccupied rooms, or
switching off the television at night (not just
putting it
into standby mode), or
setting tasks such as laundry for non-peak hours
are but a
few examples among the myriad
of possibilities.
On the energy
production side, building more efficient
transmission and
distribution systems
is another way to go about it. High efficiency
transformers,
superconducting
transformers and high temperature superconductors
are new
technologies which promise much
in terms of electrical energy efficiency and at
the
same time, new techniques are being
studied. These include direct current and ultra
high voltage transmission in both
alternating current and direct current modes.
Keywords: electrical energy
transmission
From reference
2
Disturbing loads like arc furnaces
and thyristor rectifiers draw fluctuating and
harmonic currents from the utility
grid. These non sinusoidal currents cause a
voltage drop across the finite internal
grid impedance, and the voltage waveform in
the vicinity becomes distorted. Hence,
the normal operation of sensitive consumers
is jeopardized.
Active
filters are a means to improve the power quality
in distribution networks.
In order to
reduce the injection of non sinusoidal load
currents shunt active filters
are
connnected in parallel to disturbing loads (Fig.
1). The active filter investigated in
this project consists of a PWM
controlled three-level VSI with a DC link
VSI is connected to the point of common
coupling via a transformer. The
configuration is identical with an
advanced static var compensator.
The
purpose of the active filter is to compensate
transient and harmonic
components of
the load current so that only fundamental
frequency components
remain in the grid
current. Additionally, the active filter may
provide the reactive
power consumed by
the load. The control principle for the active
filter is rather
straightforward: The
load current ismeasured, the fundamental active
component is
removed from the
measurement, and the result is used as the
reference for the VSI
output current.
In the low voltage grid, active filters
may use inverters based on IGBTs with
switching frequencies of 10 kHz or
more. The harmonics produced by those inverters
are easily suppressed with small
passive filters. The VSI can be regarded nearly as
an
ideally controllable voltage source.
Inmedium voltage applications with power
ratings of several MVA, however, the
switching frequency of today’s VSIs is
limited to
some hundred Hertz. Modern
high power IGCTs can operate at around 1 kHz.
Therefore, large passive filters are
needed in order to remove the current ripple
generated by the VSI. Furthermore, in
fast control schemes the VSI no longer
represents an ideal voltage source
because the PWM modulator produces a
considerable dead-time.
In this project a fast dead-beat
algorithm for PWM operated
VSIs is
developed [1].This algorithm improves the load
current tracking performance
and the
stability of the active filter. Normally, for a
harmonics free current
measurement the
VSI current
would be sampled
synchronously with the tips of the triangular
carriers. Here, the
current acquisition
is shifted in order to minimize the delays in the
control loop. The
harmonics now
included in themeasurement can be calculated and
subtracted from
the VSI current. Thus,
an instantaneous current estimation free of
harmonics is
obtained.
Keywords: active filters
From reference 3
This report
provides background information on electric power
transmission
and related policy issues.
Proposals for changing federal transmission policy
before
the 111th Congress include S.
539, the Clean Renewable Energy and Economic
Development Act, introduced on March 5,
2009; and the March 9, 2009, majority
staff transmission siting draft of the
Senate Energy and Natural Resources
Committee. The policy issues identified
and discussed in this report include:
Federal Transmission
Planning
:
several current
proposals call for the federal
government to sponsor and supervise
large scale, on-going transmission planning
programs. Issues for Congress to
consider are the objectives of the planning
process
(e.g., a focus on supporting
the development of renewable power or on a broader
set of transmission goals), determining
how much authority new
interconnection-
wide planning entities should be granted, the
degree to which
transmission planning
needs to consider non-transmission solutions to
power
market needs, what resources the
executive agencies will need to oversee
the planning process, and whether the
benefits for projects included in the
transmission plans (e.g., a federal permitting
option) will motivate developers to add
unnecessary features and costs to qualify
proposals for the plan.
Permitting of Transmission
Lines
:
a
contentious issue is whether the federal
government should assume from the
states the primary role in permitting new
transmission lines. Related issues
include whether Congress should view
management and expansion of the grid as
primarily a state or national issue,
whether national authority over grid
reliability (which Congress established in the
Energy Policy Act of 2005) can be
effectively exercised without federal authority
over
permitting, if it is important to
accelerate the construction of new transmission
lines
(which is one of the assumed
benefits of federal permitting), and whether the
executive agencies are equipped to take
on the task of permitting transmission lines.
Transmission Line Funding and Cost
Allocation
:
the primary
issues are whether
the the federal
government should help pay for new transmission
lines, and if
Congress should establish
a national standard for allocating the costs of
interstate
transmission lines to
ratepayers.
Transmission Modernization
and the Smart Grid
:
issues
include the need for
Congressional
oversight of existing federal smart grid research,
development,
demonstration, and grant
programs; and oversight over whether the smart
grid is
actually proving to be a good
investment for taxpayers and ratepayers.
Transmission System
Reliability
:
it is not clear
whether Congress and the
executive
branch have the information needed to evaluate the
reliability of the
transmission system.
Congress may also want to review whether the power
industry
is striking the right balance
between modernization and new construction as a
means of enhancing transmission
reliability, and whether the reliability standards
being developed for the transmission
system are appropriate for a rapidly changing
power system.
Keywords:
electric power transmission
[1] D. A.
G. Pedder, A. D. Brown, and J. A. Skinner, “A
contactless electrical
energy transmission system,”
IEEE Trans. Ind. Electron.
,
vol. 46, pp. 23
–
30, Feb.
1999.
[2] A. Ghahary and B. H. Cho,
“Design of transcutaneous energy
transmission
system using a
series resonant converter,” in
Proc.
IEEE PESC’90
, 1990, pp.
1
–
8.
[3] E. Dahl,
“Induction charging system,” U.S. Patent 3 938
018, Feb. 10,
1976.
[4] N.
Ishi
et al.
, “Electric power
transmitting
device with
inductive coupling,”
U.S.
Patent 5 070 293, Dec. 3, 1991.