《电气工程及其自动化专业英语》考查试卷
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翻译：(5×20＝100)
1. In order to reduce over voltages due to line energizing or reclosing, resistors are almost always preinserted in circuit breakers at 345 kV and above. Resistors ranging from 200 to 800 Ω are preinserted when EHV circuit breakers are closed, and subsequently bypassed. When a circuit breaker closes, the source voltage divides across the preinserted resistors and the line, thereby reducing the initial line voltage. When the resistors are shorted out, a new transient is initiated, but the maximum line voltage can be substantially reduced by careful design.
2. To protect equipment such as a transformer against overvoltages higher than its BIL, a protective device, such as that shown in Figure 19.7, is employed. Such protect devices are generally connected in parallel with the equipment from each phase to ground. As shown in Figure 19.5, the function of the protective device is to maintain its voltage at a ceiling voltage below the BIL of the equipment it protects. The difference between the equipment for voltage and the protective device ceiling voltage is the protection margin.
3. There are four classes of surge arresters: station, intermediate, distribution, and secondary. Station arresters, which have the heaviest construction, are designed for the greatest range of ratings and have the best protective characteristics. Intermediate arresters, which have moderate construction, are designed for systems with nominal voltages 138 kV and below. Distribution arresters are employed with lower-voltage transformers and lines, where there is a need for economy. Secondary arresters are used for nominal system voltages below l000 V.
4. In this unit we consider the problem of power system protection. Good design, maintenance, and proper operating procedures can reduce the probability of occurrence of faults, but cannot eliminate them. Given that faults will inevitably occur, the objective of protective system design is to minimize their impact.
5. The second reason involves a characteristic of the load. While there must be enough generating capability available to meet the requirements of the peak-load demand, the load is variable, with daily, weekly, and seasonal variations, and thus has a lower average value.1 The daily variations are roughly cyclic with a minimum value (the base load) typically less than one-half of the peak value.
A typical daily load curve for a utility is shown in Figure 17.2. The (weekly) capability factor for this particular utility is seen to be approximately 65%.
