问题 单项选择题

某2×300MW火力发电厂厂用电系统采用6kV和380V两级电压,其中每台机组设A、B两段6kV工作段,全厂设01A、01B两段6kV公用段,考虑公用段工作电源取自1、2号机组6kV工作段,备用电源取自启动/备用变压器。见示意图。每台机组设一台电动给水泵,都接于机组6kV工作A段。高压厂用分裂变压器容量为:40/25-25MVA(无载调压)。

条件同上题,且1号机6kVA段计算负荷:19520kVA(2号机相同)电动给水泵额定容量为5000kW;B段计算负荷:20360kV6A(2号机为20450kVA);试确定高压厂用变压器的阻抗电压(%)范围(启动电压计算只考虑正常最大一台电动机启动,最大电动机ηdcosφd=0.8)为()。

A.13%~17%;

B.14%~18%;

C.15%~19~6;

D.16%~20%a

答案

参考答案:D

解析:

按启动电压计算确定变压器阻抗的上限。根据题意,计算过程只考虑正常最大一台电动机启动。按规程,最大一台电动机启动时,高压厂用母线电压不应低于80%。

电动给水泵是机组最大电动机,它接于6kV工作A段,因此按A段计算。

电动给水泵起动前A段已带负荷标么值S1=14520/25000=0.581

电动给水泵起动容量标么值Sq=(KqPe)/(S2eηcosφd)=(6.5×5000)/(25000×0.8)=1.625

合成负荷标么值S=0.581+1.625=2.206

变压器阻抗标么值XT=[(Uo/Um)-1] /S=((1.05/0.8)-1/2.206=0.142

阻抗(%):Ud%=(100XTST)/(1.1S2T)=(100×0.142×40)/(1.1×25)=20.65%

取:Ud%=20%

按(1)和(2)的计算值,变压器阻抗电压可在16%~20%间选择。

阅读理解

Is there anything more important than health? I don’t think so.“Health is the greatest wealth(财富),” wise people say. You can’t be good at your studies or well when you are ill.

If you have a headache, toothache, backache, earache or bad pain in the stomach, if you complain of a bad cough, if you run a high temperature and have a bad cold, or if you suffer from high or low blood pressure, I think you should go to the doctor.

The doctor will examine your throat, feel your pulse, test your blood pressure, take your temperature, sound your heart and lungs, test your eyes, check your teeth or have your chest X-rayed. After that he will advise some treatment, or some medicine. The only thing you have to do is to follow his advice.

Speaking about doctor’s advice, I can’t help telling you a funny story.

An old gentleman came to see the doctor. The man was very ill. He told the doctor about his weakness, memory loss and serious problems with his heart and lungs. The doctor examined him and said there was no medicine for his disease.

He told his patient to go to a quiet place for a month and have a good rest. He also advised him to eat a lot of meat, drink two glasses of red wine every day and take long walks. In other words, the doctor advised him to follow the rule: “Eat at pleasure, drink with measure and enjoy life as it is.” The doctor also said that if the man wanted to be well again, he shouldn’t smoke more than one cigarette a day.  

A month later the gentleman came into the doctor’s office. He looked cheerful and happy. He thanked the doctor and said that he had never felt a healthier man.

“But you know, doctor,” he said, “it’s not easy to begin smoking at my age.”

小题1:The writer thinks that     .

A.health is more important than wealth

B.work is as important as studies

C.medicine is more important than pleasure

D.nothing is more important than money小题2:The doctor usually tells his patient what to do     .

A.without examining the patient

B.after he has examined the patient

C.if the patient doesn’t take medicine

D.unless the patient feels pain小题3:The underlined part means “    ”.

A.he was feeling better than ever

B.he wasn’t a healthy man

C.he was feeling worse than before

D.he will be well again小题4:This passage is developed by _____________.

A.giving time order

B.giving examples

C.giving tips

D.giving cause and effect小题5:Which of the following is TRUE?

A.Wealth is more important .

B.The doctor does not examine his patients carefully

C.If you have a fever, just stay at home.

D.The doctor’s advice usually works.

单项选择题