Steam is one of man’s dependable servants. More and more in the background, steam is doing more and more work for man. In (1)an electrically heated home steam may be seen only at the teakettle, but the electricity used may be generated by burning coal or gas and producing steam to turn the generator rotors in the power plant. Even if (2) the fuel is uranium and the heat is supplied by nuclear fission, the generation of electricity is still accomplished by(3) first generating steam.
In the U.S.A. the demand for electricity doubles every ten years. For the most part water resources suitable for power generation are already in service, except for emergency standby and peak-load units; approximately 90% of (4) the new generating capacity being installed utilizes steam. The (5) use of steam for electric power generation is expanding in most countries of the world except for the countries where available water power resources are greater than power needs.
Passage 8 Circuit breaker(p300)
Circuit breakers are automatically operated high voltage (or high current) switches. Circuit breakers are required (1) to control electrical power networks by switching circuits on, by carrying load and by switching circuits off under manual or automatic supervision. Their duty is as follows: In closed position the circuit breaker must be a good conductor for the normal load current. In operating the circuit breaker must be able contacts shall have full insulation level. At operation the circuit breaker must close be able (2) to switch the current which can be from a few amperes to many thousands of amperes, and independently of the characteristics from purely inductive to fully capacitive. At closing operation the circuit breaker must close fast and be safe also against full short circuit conditions. They must, therefore, be effective instantaneously when are called upon (3) to perform any switching operation after long periods without movement. As the circuit breaker is the last link in the chain of protective equipment, the circuit breaker reliability is (4) of the highest importance for high reliability of the total electric power system. So the importance (5) of the circuit breaker cannot be over emphasized.
Passage 9 The ration of distance to height of pumped-storage power plant (p301)
For pumped-storage power plant, generally the ratio of distance to height (1) is used to evaluate topographic condition of power plant. Distance-height ratio (2)is the ration of horizontal distance between upper and low reservoir to the drop between upper and low reservoir that is L/H. The head of simple pumped-storage power plant consists of two parts, they are natural topographic elevation difference and elevation difference formed by dam. Of course, the head derived from topographic elevation difference(3)is most economic, so in selecting plant site, the condition that not only a height difference is available but also no very high dam of upper reservoir is necessary should be considered. Smaller L/H value indicates a shorter water transportation line, then the engineering construction cost can be saved. But in many circumstances, topographic condition (4)is not desirable, so we can’t say the smallest L/H value (5) is the best. In newly constructed three large capacity pumped-storage power plants in our country, the L/H value respectively are: 7.12 in Guangzhou, 4.12 in Shisanling, 2.40 in Tian huangping.
Passage 10 Fuel cells(p301)
Fuel cells are devices that when a fuel such as hydrogen or hydrogen-rich compounds and oxygen are
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supplied to materials arranged like the anode and cathode of a conventional battery, combine to convert chemical energy directly into electrical energy. Unlike conventional batteries where the conversion of chemical to electrical energy requires only the closing of a circuit between the cathode and anode to produce and electrical current, (1) there is no storage capacity in a fuel cell. These cells require the fuel and oxygen that will produce the chemical reactions that produce an electrical output. Fuel cells have been made that will supply power in the megawatt range, and (2) further development is under way. Such cells are not yet widely used on power systems, but (3) as they are being developed there have been some applications to supply fuel cell power to isolated loads. As the voltage output of a cell is in the order of to 2.5V, (4) many cells are operated in series to obtain voltages and power out put in ranges that are normally required. Inverters are used to (5) supply alternating current from fuel cells when and AC output is desired.
Passage 11 Substations pace into the Digital Age (p302)
Substations constitute the main part of power transmission and distribution in a power grid. The substations (1) composed of a large number of primary and secondary equipments, with the responsibility for the electric power transmission and distribution in the grid. Over more than a decade of development, (2) substation automation technology has reached a certain level. In the rebuilding and constructing of power grid in rural and urban areas, automation technology is not only used in low-voltage substations to realize unattended operation, but also in the 220kV and above extra-voltage substations, thus greatly enhancing the level of modernization of power grid construction, strengthening the power transmission and distribution and the possibility of power grid dispatching and reducing the total cost of substation construction, (3)all of which has become an indisputable fact. However, the development of technology is never-ending, and the increasing maturation of technology like intelligent switches, photoelectric current and voltage transformers, online condition monitoring of primary equipments,(4)simulation training technology for substation operation, as well as the development and application of high-speed network in real- time system, is bound to have a profound impact upon the existing substation automation technology, and (5)a full-digital automation system is about to emerge.
Passage 12 Overcoming the Problem of Waste
Many large cities are anything but beautiful. Streets are littered with trash. In this trash, however, (1) there is still something that can be useful. If people want to solve the problem of waste, (2) there is no time to be lost. They must work out ways of making use of good things which are just thrown away as waste. When a car gets too old, it may not run any more. But if the metal that the car was made of is still good,(3)it can be put on use again. When a bottle ia empty,it goes out in the trash.But bottle glass can be ground into sand and used to pave streets. Besides, garbage from food can be changed into fertilizer. But first you have to pick out all the glass and metal.
Furthermore, garbage can also be a good source for making building blocks, which are then covered with concrete. Now, (4)more and more machines are designed for this purpose. Some day, people will watch films in a great cinema which has been built out of garbage. Future buildings, roads, and cities may be made from garbage. But so far, (5)building beautiful cities out of garbage is only a dream.
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Passage 13 Hibernation(p303)
The weather in winter can be very cold. Some animals sleep during the winter, for it’s difficult for them to find food then. (1)They can sleep for a long time. For example, their sleep can last from autumn to spring. (2)We call this kind of sleep in wintertime “hibernation”. Animals need food in winter to keep them warm and alive. If they move about more, they need eat more. (3)Hibernating animals don’t need to eat much because they don’t move. Their heart beats slowly. And they breathe slowly. They find a safe place to sleep. (4)So they don’t need to worry about bad weather and other dangerous animals.
In warn winter some animals like bears may wake up. If they see thick no\\\\snow on the ground, they will get back to their sleep. (5)As it gets warmer and warmer in spring, the sleeping animals begin to wake up and look around for food.
Passage 14 Friends
Everyone needs friends. We all like to feel close to someone. (1)It is good to have a friend to talk to, laugh together and do things with. We also need time to be alone. We don’t always want people around. But we may feel lonely if we don’t have any friends.
No two people are the same. Sometimes, people don’t get along well. (2)But they can still like each other. Many places are named after men and women if they are friendly to people in that place. (3)Some libraries and school are also named in this way.
It’s good to be people with friends. They live longer than people without friends. Why? (4)Possibly they are happier. Being happy keeps you healthy. It’s also possible that in this way you know someone cares about you.(5)As a result, you take care of yourself better.
Passage 15 Satellites(p303)
The body that is neatest to the Earth is the moon. (1)It circles the Earth and is therefore know as a satellite of the Earth. This satellite is called natural satellite. Man –made spacecraft which circle the Earth are called artificial satellites. Natural as well as artificial satellites circle the planets in a definite path or orbit. (2)A satellite stays in orbit because the planet gravitational force keeps pulling it into a circle.
All the bodies in space like the moon, the planets and the stars also exert an attractive force on objects.(3)The bigger and heavier a body is ,the greater is its force of gravity. The moon’s gravitational force is only one-sixth that of the Earth’s. The Earth’s gravitational force of pull keeps us and everything else on Earth from floating away to space.
(4)To get out into space we have to overcome the Earth’s gravitational pull. There are many artificial satellites orbiting around the Earth. Some artificial satellites have been sent up to orbit the moon and even the planet Mars.(5)Artificial satellite can be used for various purposes.
Passage 16 Secondary equipment in power system(p304)
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The secondary equipments in power system are 1. the equipments that provide protecting, monitoring, measuring, regulating, controlling primary equipments, the equipments that supply information, working condition for the operator or maintenance persons, and 2. the equipments that send the command signals. The electric circuit connected with the secondary winding of CT or PT is called secondary circuit. The secondary equipment and secondary circuit belong to secondary system.
The voltage and 3.current values in secondary system are: 110V, 22V, DC operating voltages, 100 V rated voltage of the potential transformer secondary winding and 4.5A rated current of the current transformer secondary winding. The related parameter values of secondary system after weak electrification are: 24V, 48V, 60V DC operating voltages, 5.50V rated voltage of PT secondary side, 0.5A rated current of CT secondary side.
Passage 17 Regulation of power grid frequency
The purposes of automatic generation control (AGC) are 1.automatically performing real-time regulation of the active power output of the joined generating units, keeping the system frequency or link line power within the required scope under 2.the precondition of 1 fixed optimized generation cost. AGC includes three control modes:○frequency control mode,
2 fixed power exchange control mode; ○3 frequency and link line power deviation control ○mode.
AGC system is 3.a closed loop control system that is composed of main station, AGC software, data transmission channel, remote data receiving terminal, coordination control system of generating unit and 4.related active power measuring devices, etc. The three key sub-systems of AGC are the generation dispatch control system in general dispatch control center, 5.the intermediate data transmission system and the coordination control system of generating units and power plants.
Passage 18 Switching overvoltage (p304)
A sudden change in the configuration of transmission network caused 1.by the operation of a circuit breaker or by the appearance of a fault can cause a rapid surge known as switching overvoltages, which propagates throughout the network. It is these switching overvoltages which in general 2 determine the insulation requirements of transmission lines. Limitating these overvoltages is of extremely importance, because they have an important effect on both 3.the cost of transmission and line failure rate. These switching overvoltages can occur 4.at both opening and closing of circuit breakers. If non-restriking circuit breakers is not used, it is at the instant of closing, and above all, This switching overvoltage is usually limited 5.by means of circuit breaker closing resistor in 500kV system.
Passage 19 Transmission line and distribution line(p304)
The high voltage power line through which power is (1) transmitted from power plant’s step-up substation to step-down substation is called power transmission line. The voltage classes of 35kV and above are generally used. High voltage distribution line is (2) the power line that transmits the power from step –down substation to distribution transformers.
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Low voltage distribution line is the line that transmits power from distribution transformer to the users. The normal voltage of power distribution line is 380V.
The special sections of (3)the overhead transmission line refer to the sections with large spanning, serious pollution, (4)lightning frequency happening and heavy raining. The overhead line section whose span is 1000m and longer or the height of the pole or tower is 100m and higher is called (5) large spanning line section.
Passage 20 Transformer insulation(p305)
The insulation of a transformer can be categorized into internal insulation and external insulation. (1)The internal insulation refers to the insulation of various parts inside the oil tank. The external insulation is the insulation between the upper part of bushings and the ground and the insulation among the bushings. The internal insulation of transformer can be further subdivided into main insulation and longitudinal insulation.(2)The main insulation refers to the insulation between windings and grounding parts and the insulation between windings. The longitudinal insulation refers the insulation between different parts
of the same winding, such as insulation between different sections, layers and turns of the same winding.
(3)The main insulation materials inside transformer are transformer oil, insulating pressboard, cable paper, corrugated paper, etc. The oil in transformer has two functions, one is insulation, and the other is cooling, (4) The degradation of insulating properties of insulation materials under long period of influence by the factors such as temperature is called insulation aging. (5)If the winding temperature is within the scope of 80-140℃, for every 6℃ temperature rise the insulation life time of the paper used in transformer will decrease by half.
Passage 16 the electricity market(p305)
The aim of introducing market mechanism is to bring various parties more economic benefit through equitable competition ,thereby promote economic development.
Like other markets, it needs B a wholesale market where the sellers and buyers(suppliers) can trade electricity ,and needs D a retail market where the suppliers can sell electricity to final custpmers.as in othert markets ,the participants need A to argee on the price and quantity to be trade.competition is introduced into generation and retail supplied to customers. customers.are free E to choose electricitysuppliers .if a cuseomer chooses to buy electricity in the ratail market ,he should be able to choose electricity supply from one of several competing retailers.if a customer decides to purchase directly from the wholesale market,he will need C to register a market customer.power generation is disaggregated into separate companies to ensure adequate competition.
Passage 17 electric power(p306)
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