第五次作业参考答案(完整版)资料.doc

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1、第五次作业参考答案（完整版）资料(可以直接使用，可编辑 优秀版资料，欢迎下载）第五次作业参考答案1、 电力系统设置备用容量的目的是什么？按备用的形式分备用容量有哪几种类型？旋转备用投入速度快，有利于保证电能质量，是否电力系统中旋转备用容量设置的越多越好？为什么？答：电力系统设置备用容量的目的是为了保证电力系统可靠供电和良好的电能质量。按备用形式分，系统备用容量分为热备用（又称旋转备用，指系统中运行机组所具有的备用容量）和冷备用（系统中没有运行的机组所具有可发容量）。不是旋转备用容量设置的越多越好。因为旋转备用（热备用）容量越大，运行机组的负载率越低，运行效率越差。2、某发电厂有两台机组并列运行

2、，当总负荷为时，两台发电机均未达到其额定出力，且有、。当总负荷增大时，哪台发电机应先增加出力？为什么？答：2号机组先增加出力，因为，在增加同样出力的情况下，2号机组增加的燃料消耗比1号机组少；随着2号机组有功出力增加，其耗量微增率增大，当增大到等于1号机组的耗量微增率时，1号机组机组开始按耗量微增率相等原则与2号机组同时增加出力。3、造成电力系统频率波动的原因是什么？什么叫作频率的一次调整、二次调整和三次调整（有功负荷的最优分配）？答：造成电力系统频率波动的原因是有功负荷的变化（负荷波动），因为要维持电力系统在某一频率下运行，就必须保证该频率下的有功功率平衡，当系统有功负荷变化时，打破了系统的

3、在原有频率下的有功功率平衡关系，所以就会引起系统频率变化（波动）。针对第一类负荷波动所引起的频率波动所进行的调整称为频率的一次调整，频率的一次调整通过发电机组调速器改变原动机输入的机械功率，从而改变发电机输出的有功功率来实现。频率的一次调整不能实现无差调节，负荷变化越大，调整结束后的频率偏移越大，当负荷变化较大时（如第二类负荷波动）经一次调整后的频率偏移可能超出允许范围。针对第二类负荷变化引起的频率波动所进行的调整称为频率的二次调整，二次调整通过装设在调频机组上的调频器调整发电机的有功出力来实现，二次调整可以实现无差调节。针对第三类负荷波动所进行的调整称为频率的三次调整，由于第三类负荷变化是可

4、以预测的的，所以针对第三类负荷变化所进行的对发电机出力的调整是由系统调度员根据最优分配原则安排各发电机的有功出力来实现的。4、电力系统有功负荷最有分配的目的是什么？最优分配的原则是什么？答：电力系统有功负荷最优分配的目的是在满足负荷需求的情况下使系统总的能源消耗最小。电力系统有功负荷最有分配的原则是在满足系统有功平衡功率约束条件的情况下，按各机组耗量微增率相等的原则分配各发电机的有功出力。5、水煤换算系数的取值与水电厂允许耗水量之间的关系是什么？答：水煤换算系数的取值与水电厂允许耗水量之间的关系是水电站允许的耗水量越大，则水煤换算系数的取值越小，因为只有这样按等耗量微增率准则（T1T2Tm=m

5、+1m+1=nn）分配时，水轮发电机组分得的有功负荷才比较大，消耗的水量才比较大。6、对调频厂的基本要求有那些？洪水季节宜以什么样的发电厂担负调频任务？枯水季节宜以发电厂担负调频任务？答：对调频厂的基本要求有：调整容量足够大；调整速度足够快；调整范围内经济性能应较好；调整时不至引起系统内部或系统间联络线工作的困难。根据上述基本要求具有调节库容的大容量水电厂作为调频电厂最为适宜。洪水季节，为充分利用水力资源，水电站一般满负荷运行，因而不能再担负调频任务，这种情况下宜采用调节性能较好的中温中压火力发电厂作为调频电厂；枯水季节宜采用调节性能好的大容量水电厂作为调频电厂。7、某发电厂有三台机组并列运行

6、，其耗量特性分别为： （） （） （） 机组功率约束条件为： 求负荷功率为200MW时机组间负荷的最优分配。解：由耗量特性 （） （） （）求得：根据等耗量微增率准则有： 联立求解得：、由于，取将剩余的负荷200MW-50MW=150MW在2#、3#机组之间按等耗量微增率准则重新分配得：、答：负荷功率为200MW时机组间负荷的最优分配方案是： 、8、某电力系统综合负荷的单位调节功率为400MW/HZ，系统负荷增大800MW时，调频电厂经二次调频增发400MW，系统频率变化为，计算系统等值发电机的单位调节功率。解：答：系统等值发电机的单位调节功率为1600MW/HZ。2007年12月全国大学英语

7、四级考试真题和答案 Part II Reading Comprehension (Skimming and Scanning) (15 minutes) Directions: In this part, you will have 15 minutes to go over the passage quickly and answer the questions on Answer Sheet 1. For questions 1-7, choose the best answer from the four choices marked A), B), C) and D). For que

8、stions 8-10, complete the sentences with the information given in the passage. Universities Branch Out As never before in their long history, universities have become instruments of national competition as well as instruments of peace. They are the place of the scientific discoveries that move econo

9、mies forward, and the primary means of educating the talent required to obtain and maintain competitive advantage. But at the same time, the opening of national borders to the flow of goods, services, information and especially people has made universities a powerful force for global integration, mu

10、tual understanding and geopolitical stability. In response to the same forces that have driven the world economy, universities have become more self-consciously global: seeking students from around the world who represent the entire range of cultures and values, sending their own students abroad to

11、prepare them for global careers, offering course of study that address the challenges of an interconnected world and collaborative (合作的) research programs to advance science for the benefit of all humanity. Of the forces shaping higher education none is more sweeping than the movement across borders

12、. Over the past three decades the number of students leaving home each year to study abroad has grown at an annual rate of 3.9 percent, from 800,000 in 1975 to 2.5 million in 2004. Most travel from one developed nation to another, but the flow from developing to developed countries is growing rapidl

13、y. The reverse flow, from developed to developing countries, is on the rise, too. Today foreign students earn 30 percent of the doctoral degrees awarded in the United States and 38 percent of those in the United Kingdom. And the number crossing borders for undergraduate study is growing as well, to

14、8 percent of the undergraduates at Americas best institutions and 10 percent of all undergraduates in the U.K. In the United States, 20 percent of the newly hired professors in science and engineering are foreign-born, and in China many newly hired faculty members at the top research universities re

15、ceived their graduate education abroad. Universities are also encouraging students to spend some of their undergraduate years in another country. In Europe, more than 140,000 students participate in the Erasmus program each year, taking courses for credit in one of 2,200 participating institutions a

16、cross the continent. And in the United States, institutions are helping place students in the summer internships (实习) abroad to prepare them for global careers. Yale and Harvard have led the way, offering every undergraduate at least one international study or internship opportunityand providing the

17、 financial resources to make it possible. Globalization is also reshaping the way research is done. One new trend involves sourcing portions of a research program to another country. Yale professor and Howard Hughes Medical Institute investigator Tian Xu directs a research center focused on the gene

18、tics of human disease at Shanghais Fudan University, in collaboration with faculty colleagues from both schools. The Shanghai center has 95 employees and graduate students working in a 4,300-square-meter laboratory seminars with scientists from both campuses. The arrangement benefits both countries;

19、 Xus Yale lab is more productive, thanks to the lower costs of conducting research in China, and Chinese graduate students, postdoctors and faculty get on-the-job training from a world-class scientist and his U.S. team. As a result of its strength in science, the United States has consistently led t

20、he world in the commercialization of major new technologies, from the mainframe computer and the integrated circuit of the 1960s to the Internet infrastructure (基础设施) and applications software of the 1990s. the link between university-based science and industrial application is often indirect but so

21、metimes highly visible: Silicon Valley was intentionally created by Stanford University, and Route 128 outside Boston has long housed companies spun off from MIT and Harvard. Around the world, governments have encouraged copying of this model, perhaps most successfully in Cambridge, England, where M

22、icrosoft and scores of other leading software and biotechnology companies have set up shop around the university. For all its success, the United States remains deeply hesitant about sustaining the research- university model. Most politicians recognize the link between investment in science and nati

23、onal economic strength, but support for research funding has been unsteady. The budget of the National Institutes of Health doubled between 1998 and 2003, but has risen more slowly than inflation since then. Support for the physical sciences and engineering barely kept pace with inflation during tha

24、t same period. The attempt to make up lost ground is welcome, but the nation would be better served by steady, predictable increases in science funding at the rate of long-term GDP growth, which is on the order of inflation plus 3 percent per year. American politicians have great difficult recognizi

25、ng that admitting more foreign students can greatly promote the national interest by increasing international understanding. Adjusted for inflation, public funding for international exchanges and foreign-language study is well below the levels of 40 years ago, in the wake of September 11, changes in

26、 the visa process caused a dramatic decline in the number of foreign students seeking admission to U.S. universities, and a corresponding surge in enrollments in Australia, Singapore and the U.K. Objections from American university and the business leaders led to improvements in the process and reve

27、rsal of the decline, but the United States is still seen by many as unwelcoming to international students. Most Americans recognize that universities contribute to the nations well-being through their scientific research, but many fear that foreign students threaten American competitiveness by takin

28、g their knowledge and skills back home. They fail to grasp that welcoming foreign students to the United States has two important positive effects: first, the very best of them stay in the States and like immigrants throughout historystrengthen the nation; and second, foreign students who study in t

29、he United States become ambassadors for many of its most cherished (珍视) values when they return home. Or at least they understand them better. In America as elsewhere, few instruments of foreign policy are as effective in promoting peace and stability as welcoming international university students.

30、注意：此部分试题在答题卡1上。 1. From the first paragraph we know that present-day universities have become _. A) more popularized than ever before B) in-service training organizations C) a powerful force for global integration D) more and more research-oriented 2. Over the past decades, the enrollment of oversea

31、s students has increased _. A) at an annual rate of 8 percent B) at an annual rate of 3.9 percent C) by 800,000 D) by 2.5 million 3. In the United States, how many of the newly hired professors in science and engineering are foreign-born? A) 38% B) 10% C) 30% D) 20% 4. How do Yale and Harvard prepar

32、e their undergraduates for global careers? A) They give them chances for international study or internship. B) They arrange for them to participate in the Erasmus program. C) They offer them various courses in international politics. D) They organize a series of seminars on world economy. 5. An exam

33、ple illustrating the general trend of universities globalization is _. A) Yales establishing branch campuses throughout the world B) Yales student exchange program with European institutions C) Yales helping Chinese universities to launch research projects D) Yales collaboration with Fudan Universit

34、y on genetic research. 6. What do we learn about Silicon Valley from the passage? A) It is known to be the birthplace of Microsoft Company. B) It was intentionally created by Stanford University. C) It is where the Internet infrastructure was built up. D) It houses many companies spun off from MIT a

35、nd Harvard. 7. What is said about the U.S. federal funding for research? A) It has increased by 3 percent. B) It doubled between 1998 and 2003. C) It has been unsteady for years. D) It has been more than sufficient. 8. The dramatic decline in the enrollment of foreign students in the U.S. after Sept

36、ember 11 was caused by _. 9. Many Americans fear that American competitiveness may be threatened by foreign students who will _. 10. The policy of welcoming foreign students can benefit the U.S. in that the very best of them will stay and _. Section A Directions: In this section, there is a passage

37、with ten blanks. You are required to select one word for each blank from a list of choices given in a word bank following the passage. Read the passage through carefully before making your choices. Each choice in the bank is identified by a letter. Please mark the corresponding letter for each item

38、on Answer Sheet 2 with a single line through the centre. You may not use any of the words in the bank more than once. Questions 47 to 56 are based on the following passage. As war spreads to many corners of the globe, children sadly have been drawn into the center of conflicts. In Afghanistan, Bosni

39、a, and Colombia, however, groups of children have been taking part in peace education _47_. The children, after learning to resolve conflicts, took on the _48_ of peacemakers. The Childrens Movement for Peace in Colombia was even nominated (提名) for the Nobel Peace Prize in 1998. groups of children _

40、49_ as peacemakers studied human rights and poverty issues in Colombia, eventually forming a group with five other schools in Bogota known as The Schools of Peace. The classroom _50_ opportunities for children to replace angry, violent behaviors with _51_, peaceful ones. It is in the classroom that

41、caring and respect for each person empowers children to take a step _52_toward becoming peacemakers. Fortunately, educators have access to many online resources that are _53_ useful when helping children along the path to peace. The Young Peacemakers Club, started in 1992, provides a Website with re

42、sources for teachers and _54_ on starting a Kindness Campaign. The World Centers of Compassion for Children International call attention to childrens rights and how to help the _55_ of war. Starting a Peacemakers Club is a praiseworthy venture for a class and one that could spread to other classroom

43、s and ideally affect the culture of the _56_ school. 注意：此部分试题请在答题卡2上作答。 A) victims I) forward B) technology J) especially C) role K) entire D) respectively L) cooperative E) projects M) comprehensive F) offers N) assuming G) information O) acting H) images Section B Directions: There are 2 passages

44、in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A), B), C) and D). You should decide on the best choice and mark the corresponding letter on Answer Sheet 2 with a single line through the centre. Passage One Question

45、s 57 to 61 are based on the following passage. In this age of Internet chat, videogames and reality television, there is no shortage of mindless activities to keep a child occupied. Yet, despite the competition, my 8-year-old daughter Rebecca wants to spend her leisure time writing short stories. Sh

46、e wants to enter one of her stories into a writing contest, a competition she won last year. As a writer I know about winning contests, and about losing them. I know what it is like to work hard on a story only to receive a rejection slip from the publisher. I also know the pressure of trying to liv

47、e up to a reputation created by previous victories. What if she doesnt win the contest again? Thats the strange thing about being a parent. So many of our own past scars and dashed hopes can surface. A revelation (启示) came last week when I asked her, “Dont you want to win again?” “No,” she replied,

48、“I just want to tell the story of an angel going to first grade.” I had just spent weeks correcting her stories as she spontaneously (自发地) told them. Telling myself that I was merely an experienced writer guiding the young writer across the hall, I offered suggestions for characters, conflicts and endings for her tales. The story about a fearful angel starting first grade was quickly “guided”