问题 问答题 简答题

电力变压器的额定一次电压,为什么规定有的要高于相应的电网额定电压5%?有的又可等于相应电网额定电压?而其二次电压,为什么规定有的要高于相应的电网额定电压10%?有的又可只高于相应电网额定电压5%?

答案

参考答案:电力变压器一次绕组的额定电压

(1)当变压器直接与发电机相连时,其一次绕组额定电压应与发电机额定电压相同,即高于同级电网额定电压5%。

(2)当变压器不与发电机相连而是连接在线路上时,则可将它看作是线路的用电设备,因此其一次绕组额定电压应与电网额定电压相同。

电力变压器二次绕组的额定电压

(1)变压器二次侧供电线路较长,其二次绕组额定电压应比相连电网额定电压高10%,其中有5%是用于补偿变压器满负荷运行时绕组内部的约5%的电压降,因为变压器二次绕组的额定电压是指变压器一次绕组加上额定电压时二次绕组开路的电压;此外,变压器满负荷时输出的二次电压还要高于电网额定电压5%,以补偿线路上的电压损耗。

(2)变压器二次侧供电线路不长,如为低压电网或直接供电给高低压用电设备时,其二次绕组额定电压只需高于电网额定电压5%,仅考虑补偿变压器满负荷时绕组内部5%的电压降。

单项选择题
阅读理解

阅读理解。

     When I was a child, I often dreamed of the time when I could leave home and escape to the city. We lived

on a farm. In the winter especially, we wear quite out off from the outside world. As soon as I left school, I

packed my bags and moved to the capital. However, I soon discovered that my life has its too.

     One big disadvantage is money. It costs so much to go out, not to mention basics like food and housing.

Another disadvantage is pollution. I suffer from asthma (哮喘), and the air is so that I am afraid to go outside.

Then there is the problem of traveling round. Although I have a car, I seldom use it became of the traffic jams.

One choice is to go by bicycle, but that can be quite dangerous.

     Of course there are advantages. First, there is so much to do in the city, whatever you tastes in culture or

entertainment (娱乐活动). Besides, there are wonderful jobs and greater chances of moving to a more important

job or position. Finally, if you like shopping, the variety of goods is very surprising-and, what is more, shops

are often only a short walk away.

     Is life better then, in the city? Perhaps it is, when you are in your teens (十几岁) or twenties. However, as

you get older, and especially if you have small children, the peace of the countryside may seem preferable. I

certainly hope to move back there soon.

1. What was the writer always thinking about when he was a child?

A. Staying on the farm.

B. Moving to the countryside.

C. Leaving home for the city.

D. Running away from the school.

2. Which of the following is true about the writer?

A. He is very old now.

B. He is in good health.

C. He prefers driving a car.

D. He lives in the city now.

3. In the passage, the writer tries to _____.

A. express his opinions about way of life

B. describe his life in the countryside

C. an interest in the outside world

D. persuade the reader to live in the city

4. How is the passage mainly developed?

A. By inferring.

B. By comparing.

C. By listing examples.

D. By giving explanations.