问题 单项选择题

关于短路故障时的负序电流,下述说法中正确的是()

A、任何短路情况下,短路电流中都包含有负序分量

B、任何相间短路情况下,短路电流中都包含负序分量

C、只有在发生不对称短路时,短路电流中才会出现负序分量

D、只有在中性点接地系统发生接地短路时,短路电流中才会出现负序分量

答案

参考答案:C

解析:

选C,因为导出负序分量的对称分量法就是针对不对称故障的.其它选项可用对称短路(三相短路)做反例. AB:三相短路无负序分量 D:中性点接地系统也可以发生三相短路

阅读理解

阅读理解。

     Chess must be one of the oldest games in the world. An Arab traveler in the year 900 wrote that it

was played "long long ago". Chess was probably invented in India, and it has been played everywhere

from Japan to Europe since 1400. The name "chess" is interesting. When one player is attacking the

other's king, he says, in English,"check". When the king has been caught and cannot move anywhere,

he says "check mate (将死)". These words come from Persian (波斯人). It means "the king is dead".

That is when the game is over, and one player has won.    

     Such an old game changes very slowly. The rules have not always been the same as they are now.

For example, at one time the queen could only move one square (方块) at a time. Now she is the strongest

piece on the board. It would be interesting to know why this has happened! Chess takes time and thought,

but it is a game for all kinds of people. You don't have to be winner in order to enjoy it. It is not always

played by two people sitting at the same table. The first time the Americans beat the Russians was in a

match played by radio. Some people are able to play many people at the same time. One man played 400

games! It is said that some people play chess by post. This must make chess the slowest game in the

world.

1. Chess _____.[ ]

A. is an old Indian game for travel

B. is the oldest game in the world

C. was played in India long before 900

D. was played in Japan and Europe before 1400

2. "At one time the queen could only move one square at a time.

     Now the queen is the strongest piece on the board."

    This example is used to show _____. [ ]

A. chess is an interesting game

B. chess changes very slowly

C. how this rule has happened

D. there have been some changes of rules in chess

3 "You don't have to be a winner in order to enjoy it." Means _____. [ ]

A. only a winner can enjoy chess

B. you can enjoy chess only you lose a game

C. you can enjoy chess even if you are not a winner

D. you can enjoy chess when you play with a winner

4. The wrier tries to tell us _____. [ ]

A. some interesting facts about chess

B. how the rules of chess have changed

C. how to play chess

D. how people enjoy chess

5. The first time the Americans beat the Russians was in a match played by radio.

    For the first time the Americans ______ the match. [ ]

A. beat

B. got

C. had

D. won

单项选择题 A1型题