多项选择题

在火力发电厂工程中，下列哪些厂内通信直流电源的设置原则是正确的？（）

A.应由通信专用直流电源系统提供，其额定电压为DC48V
B.通信专用直流电源系统为不接地系统
C.应设置两套独立的直流电源系统，每套均由一套高频开关电源，一组（或二组）蓄电池组成
D.单组蓄电池的放电时间4～6h

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One-room SchoolsOne-room schools are part of the heritage of the United States, and the mention of them makes people feel a vague longing for 'the way things werE.' One-room schools are an endangered species, however. For more than a hundred years, one-room schools have been systematically shut down and their students sent away to centralized schools. As recently as 1930 there were 149,000 one-room schools in the United States. By 1970 there were 1,800. Today, of the nearly 800 remaining one-room schools, more than 350 are in NebraskA.The rest are scattered through a few other states that have on their road maps wide-open spaces between towns.Now that there are hardly any left, educators are beginning to think that maybe there is something yet to be learned from one-room schools, something that served the pioneers that might serve as well today. Progressive educators have come up with progressive-sounding names like 'peer-group teaching' and 'multi-age grouping' for educational procedures that occur naturally in the one-room schools. In a one-room school the children teach each other because the teacher is busy part of the time teaching someone elsE.A fourth grader can work at a fifth-grade level in math and a third-grade level in English without the stigma associated with being left back or the pressures of being skipped aheaD.A youngster with a learning disability can find his or her own level without being separated from the other pupils. In larger urban and suburban schools today, this is called 'mainstreaming.' A few hours in a small school that has only one classroom and it becomes clear why so many parents feel that one of the advantages of living in Nebraska is that their children have to go to a one-room school.It is implied in the passage that many educators and parents today feel that one-room schoolsA.need to be shut down.B.are the best in NebraskA.C.are a good example of the good old days.D.privide good education

A.'
B.
Now
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It
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F.
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H.
D.privide

多项选择题

下列哪几种故障属于电力系统安全稳定计算的Ⅱ类故障类型？（）

A.发电厂的送出线路发生三相短路故障
B.单回线路发生单相永久接地故障重合闸不成功
C.单回线路无故障三相断开不重合
D.任一台发电机组跳闸

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