什么是二次调频？
参考答案：
二次调频的作用是在电网负荷发生变化时，达到新的供求平衡以维持频率稳定。这主要是靠调整调速汽门的开度变化，来改变发电机组的功率以恢复电网正常频率。
一次调频是暂态的，即电网负荷变化后，二次调频还来不及充分保证电网功率的供求平衡时，暂时由一次调频来保证频率不致变化过大而造成严重后果。
当二次调频跟上后，使电网频率恢复正常，这时一次调频卸掉，其作用消失。
阅读下面短文，根据所读内容在文章后表格中的空格里填入最恰当的单词。(注意：每空填1个单词。) Celebrating sleep World Sleep Day, observed （庆祝）on 21 March, was created to bring attention to the growing number of sleep problems worldwide. Sleep is a natural part of human life, but many people in the modern world ignore its importance. As a result, sleep problems are increasingly common. In 2002, scientists at the University of Athens in Greece conducted a survey of over 35,000 people in 10 countries around the world, including China, Belgium and South Africa. Of the people, 24 percent said that they did not sleep well, and based on their responses, 31.6 percent were diagnosed (诊断)with insomnia（失眠）and another 17.5 percent were diagnosed with borderline insomnia. In addition, 30.7 percent of survey respondents reported visiting a doctor for help with sleep problems and 11.6 percent said that they felt very sleepy during the day. Overall, the survey found that many people suffer the effects of poor quality sleep, some examples of which are being unable to fall asleep and waking up in the middle of the night. The main factors causing these problems are pressure from school or work and a fast pace of life, but noise pollution and light pollution also disturb sleep and contribute to poor quality sleep. The survey’s findings are particularly troubling because large numbers of people were found to have difficulty sleeping at night. The short-term effects of this are tiredness and trouble concentrating（专心）. If this happens for a long time, the risk of putting on weight and having a heart attack may increase. There are many things you can do to improve the quality of your sleep. First, you should have regular sleep habits. Try to go to bed and wake up around the same time each day. You also need to make your bedroom a cool, dark, quiet place to sleep so that heat, light and noise do not disturb you. At night, you should also avoid things that contain caffeine（咖啡因）, such as chocolate, coffee, tea and many soft drinks, as caffeine may keep you from sleeping. Getting a good night’s sleep is important for everyone. By following the advice above, you can rest better at night and work better during the day. Title: Celebrating Sleep
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