专业外语测试翻译材料班级 学号 姓名 成绩 REACTIVE POWER AND VOLTAGE CONTROLFor efficient and reliable operation of power systems, the control of voltage and reactive power should satisfy the following objectives:(a) Voltages at the terminals of all equipment in the system are within acceptable limits. Both utility equipment and customer equipment are designed to operate at a certain voltage rating. Prolonged operation of the equipment at voltages outside the allowable range could adversely affect their performance and possibly cause them damage.(b) System stability is enhanced to maximize utilization of the transmission system. As we will see later in this section and in Chapters 12 to 14, voltage and reactive power control have a significant impact on system stability.(c) The reactive power flow is minimized so as to reduce RI2 and XI2 losses to a practical minimum (see Chapter 6,Section 6.3).This ensures that the transmission system operates efficiently, i.e., mainly for active power transfer.The problem of maintaining voltages within the required limits is complicated by the fact that the power system supplies power to a vast number of loads and is fed from many generating units. As loads vary, the reactive power requirements of the transmission system vary. This is abundantly clear from the performance characteristics of transmission lines discussed in Chapter 6.Since reactive power cannot be transmitted over long distances, voltage control has to be effected by using special devices dispersed throughout the system active power balance. The proper selection and coordination of equipment for controlling reactive power and voltage are among the major challenges of power system engineering.We will first briefly review the characteristics of power system components from the viewpoint of reactive power and then we will discuss methods of voltage control.1 Production and Absorption of Reactive PowerSynchronous generators can generate or absorb reactive power depending on the excitation. When overexcited they supply reactive power, and when underexcited they absorb reactive power. The capability to continuously supply or absorb reactive power is, however, limited by the field current, and end-region heating limits, as discussed in Chapter 5 (Section 5.6). Synchronous generators are normally equipped with automatic voltage regulators which continually adjust the excitation so as to control the armature voltage.Overhead lines, depending on the load current, either absorb or supply reactive power. At loads below the natural (surge impedance) load, the lines produce net reactive power; at loads above the natural load, the lines absorb reactive power. The reactive power characteristics of transmission lines are discussed in detail in Chapter 6.Underground cables, owing to their high capacitance, have high natural loads. They are always loaded below their natural loads, and hence generate reactive power under all operating conditions. Transformers always absorb reactive power regardless of their loading; at no load, the shunt magnetizing reactance effects predominate; and at full load, the series leakage inductance effects predominate.Loads normally absorb reactive power. A typical load bus supplied by a power system is composed of a large number of devices. The composition changes depending on the day, season, and weather conditions. The composite characteristics are normally such that a load bus absorbs reactive power. Both active power and reactive power of the composition loads vary as a function of voltage magnitudes. Loads at low-lagging power factors cause excessive voltage drops in the transmission network and are uneconomical to supply. Industrial consumers are normally charged for reactive as well as active power, this gives them an incentive the load power factor by using shunt capacitive.Compensating devices are usually added to supply or absorb reactive power and thereby control the reactive power balance in a desired manner. In what follows, we will discuss the characteristics of these devices and the principles of application.2 Methods of Voltage ControlThe control of voltage levels is accomplished by controlling the production, absorption, and follow of reactive power at all levels in the system. The generating units provide the basic means of voltage control; the automatic voltage regulators control field excitation to maintain a scheduled voltage level at the terminals of the generators. Additional means are usually required to control voltage throughout the system. The devices used for this purpose may be classified as follows.(a) Sources or thinks of reactive power, such as shunt capacitors, shunt reactors, synchronous condensers, and static var compensators (SVCs).(b) Line reactance, compensators, such as series capacitors.(c) Regulating transformers, such as tap-changing transformers and boosters.Shunt capacitors and reactors, and series