问题 问答题 简答题

变压器反充电有什么危害?

答案

参考答案:

变压器出厂时,就确定了其作为升压变压器使用还是降压变压器使用,且对其继电保护整定要求作了规定。若该变压器为升压变压器,确定为低压侧零起升压。如从高压侧反充电,此时低压侧开路,由于高压侧电容电流的关系,会使低压侧因静电感应而产生过电压,易击穿低压绕组。若确定正常为高压侧充电的变压器,如从低压侧反充电,此时高压侧开路,但由于励磁涌流较大(可达到额定电流的6~8倍)。它所产生的电动力,易使变压器的机械强度受到严重的威胁,同时,继电保护装置也可能躲不过励磁涌流而误动作。

阅读理解

For generations here in the deepest South, there had been a great taboo(禁忌): publicly crossing the color line for love. Less than 45 years ago, marriage between blacks and whites was illegal, and it has been forbidden for much of the time since.

So when a great job about an hour’s drive north of the Gulf Coast attracted him, Jeffrey Norwood, a black college basketball coach, had reservations. He was in a serious relationship with a woman who was white and Asian.

“You’re thinking about a life in South Mississippi?” his father said in a skeptical voice, recalling days when a black man could face mortal(致命的) danger just being seen with a woman of another race, regardless of intentions. "Are you sure?"

But on visits to Hattiesburg, the younger Mr. Norwood said he liked what he saw: growing diversity. So he moved, married, and, with his wife, had a baby girl, who was counted on the last census(人口普查) as black, white and Asian. Taylor Rae Norwood, three, is one of thousands of mixed-race children who have made this state home to one of the nation's most rapidly expanding multiracial populations, up 70 percent between 2000 and 2010, according to new data from the Census Bureau.

In the first comprehensive accounting of multiracial Americans since statistics were first collected about them in 2000, reporting from the 2010 census, made public in recent days, shows that the nation’s mixed-race population is growing far more quickly than many researchers had estimated, particularly in the South and parts of the Midwest. That conclusion is based on the bureau’s analysis of 42 states; the data from the remaining eight states will be released soon.

In North Carolina, the mixed-race population doubled. In Georgia, it grew by more than 80 percent, and by nearly as much in Kentucky and Tennessee. In Indiana, Iowa and South Dakota, the multiracial population increased by about 70percent.

Census officials estimated the national multiracial growth rate was about 35 percent since2000 according to the known result, when seven million people ----- 2.4 percent of the population ------ chose more than one race.

小题1:If a black man married a white woman 50 years ago, the worst result was that _____.

A.he was sentenced to death

B.he was considered to be immoral

C.he was criticized by the public

D.he was treated as a lawbreaker小题2:The underlined word “serious” in Paragraph 2 can be replaced by “____”.

A.stable

B.bad

C.mixed

D.dangerous小题3:What can we infer from Paragraph 4?

A.Jeffrey Norwood was born in Hattiesburg and grew up there.

B.Taylor Rae Norwood’s mother is a white-Asian.

C.70 percent of the people in Mississippi are multiracial.

D.Mississippi has the largest multiracial population in the US.小题4:Which of the following states had the fastest growth rate of mixed-race population?

A.Georgia.

B.Tennessee.

C.North Carolina.

D.South Dakota.

单项选择题