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1、Page1 Generators and MotorsFrom reference 11. 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 idea
2、s 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
3、 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 th
4、e machine itself or be separately excited from another source.Self-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
5、) 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 connect
6、ed in parallel or shunt with the armature winding.3. Armature winding of dc machines may be of thelap 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
7、the commutator.In 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 deg
8、reesThe. typeof 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 alwa
9、ys two paths in parallel in the armature winding between armature terminals. For a lap-wound machine there are as many parallel paths in the armaturewinding as there are pairs of poles on the machine. For the same number and size ofarmature conductors, a machine when wave-connected would generate a
10、voltage that would equal the voltage generated when lap-connected times the number of pairs of poles.But the current capacity would be decreased in the same proportion that the voltage wasincreased. The current capacity of a machine when wave-connected is therefore equal to the capacity when lap-con
11、nected 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
12、generators are usually operated at a constant speed, the voltage mustbe 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
13、 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 connectionwith the a
14、rmature. With a constant field excitation, the voltage will drop slightly fromno load to full load because of armature drop and armature reaction.Separate excitation is advantageous when the voltage generated by the machine is not suitable for field excitation. This is true for especially low- or hi
15、gh-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 flowsthrough all parts of the circuit (see Fig. 7.4). If a series generator is operated at no load (external circuit open), there
16、 will be no current through the field coils, and the only magnetic flux presentin 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
17、 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
18、 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: generatorFrom reference 2Since trip
19、hased asynchronous generators are mainly used in conversion systems of a eolian energy into electric energy, their functional stability represent isof great importance. As a first step, the factors that radically affect the functional stability of these generators have been established. Thus, it was
20、 decelat the powerful influence of the capacitor bank that provides the necessary reactive power for themagnetization 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
21、 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
22、 analogy being made with the situation of the necessary controllable generating capacity for of the triphased asynchronous generator with short circuitrotor.Keywords : triphased asynchronous generator.1 D.M. Eggleston, F.S. StoddardWind turbine engineering design, Van Nostrand Reinhold Company New Y
23、ork 1986;2 V. Ilie, L. Alma?i, ?a Utilizarea energiei vantului, Ed. Tehnic?, Bucure?ti, 1984;3 Kovacs PalAnaliza regimurilor tranzitorii ale ma?inilor electrice, Ed. Tehnic?, Bucure?ti 1980 ;4 New Approach to Determinate the Critical Capacitance for Self - Excited Induction Generators, IEEE Trans. O
24、n 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/ Induct
25、ion-Motor Power Conversion System Completely Isolated from the Utility. IEEE Trans. On Ind. App., vol.30, no.3, may/june 1994, pp. 636-6477 Florin Iov Stadiul actual ?n conversia energiei eoliene (Referat nr.1?n cadrul preg?tirii tezei de doctorat) martie 1998;8 Florin Iov Studiul ansamblului turbin
26、? eolian? generatorasincron autoexcitat (Referat nr.2 ?n cadrul preg?tirii tezei de doctorat) iunie 1999;Page2 Electrical Energy TransmissionFrom reference 1Growing populations and industrializing countries create huge needs for electrical energy. Unfortunately, electricity is not always used in the
27、 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 twomain goals: reduce resource co
28、nsumption 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 lo
29、ss: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 abo
30、ut 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 sam
31、e 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 stand
32、by 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 transformer
33、s 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.Keywor
34、ds: electrical energytransmissionFrom reference 2Disturbing loads like arc furnaces and thyristor rectifiers draw fluctuating andharmonic 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 vici
35、nity 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 l
36、oads (Fig. 1). The active filter investigated in this project consists of a PWM controlled three-level VSI with a DC link capacitor.The 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
37、 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 straight
38、forward: 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 harmon
39、ics 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 powerratings of several MVA, however, the switching frequency of today s VSIs is limited to some hundred Hert
40、z. 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 d
41、ead-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 currentmeasurement the VSI currentwould be sampled synchronously with the tip
42、s 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.Keyw
43、ords: active filtersFrom reference 3This 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 o
44、n 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
45、 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 interconnecti
46、on-wide planning entities should be granted, the degree to which transmission planning needs to consider non-transmission solutions to power market needs, what resources theexecutive agencies will need to oversee the planning process, and whether the benefits for projects included in the transmissio
47、n 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 transm
48、ission lines. Related issues include whether Congress should viewmanagement 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 transm