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1、 1 Section 2 Components of Power Systems Modem power systems are usually large-scale,geographically distributed,and with hundreds to thousands of generators operating in parallel and synchronously.They may vary in size and structure from one to another,but they all have the same basic characteristic
2、s:(1)Are comprised of three-phase AC systems operating essentially at constant voltage.Generation and transmission facilities use three-phase equipment.Industrial loads are invariably three-phase;single-phase residential and commercial loads are distributed equally among the phases 50 as to effectiv
3、ely from a balanced three-phase system.(2)Use synchronous machines for generation of electricity.Prime movers convert the primary energy(fossil,nuclear,and hydraulic)to mechanical energy that is,in turn,converted to electrical energy by synchronous generators.(3)Transmit power over significant dista
4、nces to consumers spread over a wide area.This requires a transmission system comprising subsystems operating at different voltage levels.The basic elements of a modem power system in U.S.A.are shown in Fig.6-1.Electric power is produced at generating stations(GS)and transmitted to consumers through
5、 a complex network of individual components,including transmission lines,transformers，and switching devices.It is common practice to classify the transmission network into the following subsystems：Transmission system;subtransmission system;Distribution system.The transmission system interconnects al
6、l major generating stations and main load centers in the system.It forms the backbone of the integrated power system and operates at the highest voltage levels(typically,230 kV and above in U.S.A.).The generator voltages are usually in the range of 11 to 35 kV.These are stepped up to the transmissio
7、n voltage level,and power is transmitted to transmission substations where the voltages are stepped down to the subtransmission level(typically,69 to 138kV).The generation and transmission subsystems are often referred to as the bulk power system.The subtransmission system transmits power in small q
8、uantities from the transmission substations to the distribution substations.Large industrial customers are commonly supplied directly from the subtransmission system.In some systems,there is no clear demarcation between subtransmission and transmission circuits.As the system expands and higher volta
9、ge levels become necessary for transmission,the older transmission lines are often relegated to subtransmission function.The distribution system represents the final stage in the transfer of power to the individual customers.The primary distribution voltage is typically between 4.0 kV and 34.5 kV.Sm
10、all industrial customers are supplied by primary feeders at this voltage level.The secondary distribution feeders supply residential and commercial customers at 120/240V.Small generating plants located near the load are also connected to the subtransmission or distribution system directly.Interconne
11、ctions to neighboring power systems are usually formed at the transmission system level.The overall system thus consists of multiple generating sources and several layers of transmission networks.This provides a high degree of structural redundancy that enables the system to withstand unusual contin
12、gencies without service disruption to the customers.Knowledge about translation 动词的 V-ed形式与 be 结合构成被动态，与 have(had)结合构成完成时，因此 V-ed形式本身含有被动与完成的意思，它可在句子中担任定语、状语等，保留了动词性，表示这一动作是已完成的或所修饰名词所（被动）接受的。1.V-ed作定语 单个 V-ed作定语一般放在名词前面(也可以放在后面)，V-ed短语作定语一般放在名词之后。在作定语时，分词在意思上接近一个定语从句。The work done is the product of
13、 the force and the distance.2 Heat is the energy produced by the movement of molecules.A semiconductor diode consists of a PN junction made of semiconductor material.2.V-ed作状语 V-ed作状语可表示动作发生的背景和情况。All metals are fairly hard,compared to nonmetals.Tonight,lit by countless electric lights,all the halls
14、 are as bright as day.3.V-ing与 V-ed的区别 正如前面已提到的，V-ing通常有主动的意思，V-ed通常有被动的意思，在阅读翻译时要注意这一点，才能区分动作的发出者和对象。试比较：I heard someone opening the door.I heard the door opened.The switched(controlled)current goes between emitter and collector,while the controlling current goes between emitter and base.1 In citie
15、s and other areas where open lines create a safety hazard or are considered unattractive,insulated underground cables are used for distribution.2 Each utility that agrees to share gains an increased reserve capacity,use of larger,more efficient generators,and the ability to respond to local power fa
16、ilures by obtaining energy from a linking grid.3 Inductance and capacitance react with a tendency to nullify one another.4 Metering values such as demand and peak are saved within the meter to create historical information about the activity of the power system.5 Substation integration refers to com
17、bining data from the IEDs local to a substation so that there is a single point of contact in the substation for all of the I&C data.6 The communications industry uses the term client/server for a device that acts as a master,or client,retrieving data from some devices and then acts as a slave,or se
18、rver,sending this data to other devices.How Power Grids Work？Electrical power is a little bit like the air you breathe:You dont really think about it until it is missing.Power is just there,meeting your every need,constantly.It is only during a power failure,when you walk into a dark room and instin
19、ctively hit the useless light switch that you realize how important power is in your daily life.You use it for heating,cooling,cooking,refrigeration,light,sound,computation,entertainment.Without it,life can get somewhat cumbersome.Fig 6.1 Power travels from the power plant to your house Power travel
20、s from the power plant to your house through an amazing system called the power distribution grid（see in Fig 6.1）.The grid is quite public-if you live in a suburban or rural area,it is right out in the open and chances are for all to see.Your brain likely ignores all of the power lines because it ha
21、s seen them so often.In this article,we will look at all of the equipment that brings electrical power to your home.3 1.The Power Plant Electrical power starts at the power plant.In almost all cases,the power plant consists of a spinning electrical generator.Something has to spin that generator-it m
22、ight be a water wheel in a hydroelectric dam,a large diesel engine or a gas turbine.But in most cases,the thing spinning the generator is a steam turbine.The steam might be created by burning coal,oil or natural gas,or the steam may come from a nuclear reactor.No matter what it is that spins the gen
23、erator,commercial electrical generators of any size generate what is called 3-phase AC power.To understand 3-phase AC power,it is helpful to understand single-phase power first.2.Alternating current Single-phase power is what you have in your house.You generally talk about household electrical servi
24、ce as single-phase,220-volt AC service.If you use an oscilloscope and look at the power found at a normal wall-plate outlet in your house,what you will find is that the power at the wall plate looks like a sine wave,and that wave oscillates between-311 volts and 311 volts(the peaks are indeed at 311
25、 volts;it is the effective(rms)voltage that is 220 volts).The rate of oscillation for the sine wave is 50 cycles per second.Oscillating power like this is generally referred to as AC,or alternating current.3.The power plant produces AC.The power plant produces three different phases of AC power simu
26、ltaneously,and the three phases are offset 120 degrees from each other.There are four wires coming out of every power plant:the three phases plus a neutral or ground common to all three.If you were to look at the three phases on a graph,they would look like this（Fig 6.2）relative to ground:Fig 6.2 Th
27、e waveform of 3-phase AC 4.Transmission Substation The three-phase power leaves the generator and enters a transmission substation at the power plant.This substation uses large transformers to convert the generators voltage(which is at the thousands of volts level)up to extremely high voltages for l
28、ong-distance transmission on the transmission grid.You can see at the back several three-wire towers leaving the substation.Typical voltages for long distance transmission are in the range of 155,000 to 765,000 volts in order to reduce line losses.A typical maximum transmission distance is about 483
29、 km.High-voltage transmission lines are quite obvious when you see them.They are normally made of huge steel towers（see in Fig 6.1）.5.The Distribution Grid For power to be useful in a home or business,it comes off the transmission grid and is stepped-down to the distribution grid.This may happen in
30、several phases.The place where the conversion from transmission to distribution occurs is in a power substation.A power substation typically does two or three things:It has transformers that step transmission voltages(in the tens or hundreds of thousands of volts range)down to distribution voltages(
31、typically less than 10,000 volts).It has a bus that can split the distribution power off in multiple directions.It often has circuit breakers and switches so that the substation can be disconnected from the transmission grid or separate distribution lines can be disconnected from the substation when
32、 necessary.6.Distribution Bus The power goes from the transformer to the distribution bus:In this case,the bus distributes power 4 to two separate sets of distribution lines at two different voltages.The smaller transformers attached to the bus are stepping the power down to standard line voltage（72
33、00V）for one set of lines,while power leaves in the other direction at the higher voltage of the main transformer.The power leaves this substation in two sets of three wires,each headed down the road in a different direction:The three wires at the top of the poles are the three wires for the 3-phase
34、power.The fourth wire lower on the poles is the ground wire.In some cases there will be additional wires,typically phone or cable TV lines riding on the same poles.7.Taps and at the house A house needs only one of the three phases,so typically you will see three wires running down a main road,and ta
35、ps for one or two of the phases running off on side streets.Past a typical house runs a set of poles with one phase of power(at 7,200 volts)and a ground wire(although sometimes there will be two or three phases on the pole,depending on where the house is located in the distribution grid).At each hou
36、se,there is a transformer drum（Fig 6.1）attached to the pole，The transformers job is to reduce the 7,200 volts down to the 220 volts that makes up normal household electrical service.The 220 volts enters your house through a typical watt-hour meter.About the IEEE(Institute of Electrical and Electroni
37、cs Engineers)The IEEE(Eye-triple-E)is a non-profit,technical professional association of more than 377,000 individual members in 150 countries.The full name is the Institute of Electrical and Electronics Engineers,although the organization is most popularly known and referred to by the letters I-E-E
38、-E.Through its members,the IEEE is a leading authority in technical areas ranging from computer engineering,biomedical technology and telecommunications,to electric power,aerospace and consumer electronics,among others.Through its technical publishing,conferences and consensus-based standards activi
39、ties,the IEEE produces 30 percent of the worlds published literature in electrical engineering,computers and control technology,holds annually more than 300 major conferences and has nearly 900 active standards with 700 under development.The benefits of IEEE membership include these offerings:Member
40、ship in one or more of 37 IEEE societies and four technical councils spanning the range of electrotechnologies and information technologies More than 300 local organizations worldwide for member networking and information sharing Educational opportunities to ensure engineers technical vitality More
41、than 1,150 student branches at universities worldwide Educational opportunities to ensure engineers technical vitality More than 1,150 student branches at universities worldwide Special cost-saving and value-added benefits for MEMBERS ONLY Special cost-saving and value-added benefits for MEMBERS ONL
42、Y Prestigious awards and recognition of technical and professional achievements Opportunities for volunteering,leadership and participation in a variety of IEEE activities Industrial Robots An industrial robot is a precision manipulator designed to interact with the real world while being monitored
43、and controlled by an electronic“brain”.As one expert put it,”a robot is the muscle of a computer”.They spray-paint cars,they pick up,insert,and screw in dash-panel light bulbs,they assemble small components,they perform inspections,and they handle a variety of other assignments.A modern robot is,in
44、effect,a teleoperator in which the brain of the human being has replaced by a 5 programmable microprocessors.Most robots are equipped with a single hand and a single arm with five or six articulated joints.Some have joints that swivel in smooth arcs mimicking the behavior of the human shoulder,wrist
45、,and elbow;others are rigged for straight-line movements resembling those of an overhead crane.A robot rarely has two arms,and even if it does,they do not coordinate their movements the way our arms do:one stops while the other moves.A few advanced robots are rigged with two hands often on the end o
46、f a single arm.Most industrial robots do not move along the factory floor.Exceptions include delivery robots waddling up and down the halls of office buildings delivering mail and supplies,and cartlike robots that carry linens and other items to patients and nursing station in convalescent homes and
47、 hospitals.Todays robots are more efficient than human workers in many situations,but they require expensive items of hardware to pre-position the objects they are assigned to grasp and manipulate.Typically,the cost of the necessary fixtures and jigs will approximately match the cost of the robot it
48、self,which ranges somewhere between$10,000 and$120,000.These heavy installation costs are required because robots tend to be insensitive to their surroundings.If an object is out of position,the robot might miss it entirely or fail to pick it up.A few centimeters of offset or misalignment can be dis
49、astrous for a robot,whereas a human worker in quite tolerant of errors of this type.Choose the Best Answer 1)Which of the following is true？A.A robot is designed to have a brain.B.A robot is designed without a brain.C.A robot is designed with programmable microprocessors as a brain.D.A robot is desi
50、gned not to interact with the real world.2)According to the passage,most robots cant A.Make intelligent decisions B.Assemble components C.Perform inspections D.Grasp objects 3)Which of the following is not true?A.Most robots have only one hand instead of two.B.The two hands of some advanced robots a