电力专业英语14-电力系统中高电压技术

1、*电力系统中的高电压技术电气工程学院 High-Voltage Technology In Power SystemsTopics Insulation Grounding Overvoltage Lightning ShieldingInsulation 1 Insulation is required to keep electrical conductors separated from each other and from other nearby objects. The choice of material is dictated by the requirements of t

2、he particular application and by cost.Insulation 2 The coils of generators and motors may be insulated with various tapes. The insulation on power transformer windings is commonly paper tape and pressboard operated under oil.Insulation 3 Overhead lines are supported on porcelain insulators. Between

3、the supports air serves as insulation. Porcelain is chosen because of its resistance to deterioration when exposed to the weather, its high dielectric strength, and its ability to wash clean by rain.Insulation 4 The cost of equipment and the system as a whole is closely related to the insulation lev

4、el which has been adopted for the network. The choice of insulation level should take into account both the probability of occurrence and the severity of stress that the system may be subjected to. 500KV开关场 switchyard500KV变压器 transformer220KV开关场 switchyard220KV开关场 switchyard电力电缆 power cables电力电缆 pow

5、er cable钢芯铝绞线 aluminum conductor steel reinforced 架空输电线overhead transmission line架空输电线overhead transmission lineInsulation Coordination Electric systems insulation designers have two options available to them: choose insulation levels for components that would withstand over voltages, devise protect

6、ive devices that could limit over voltages there.Grounding Grounding is defined as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth.

7、Grounding The most important reason for grounding is safety. Two important aspects to grounding requirements with respect to safety and one with respect to power quality are follows: Personnel safety To assure protective device operation Noise control接地网施工 construction of ground grid接地网施工 constructi

8、on of ground gridOvervoltages The overvoltages can be divided into external overvoltages and internal overvoltages according to the locations of sources. External overvoltages are generated by sources that external to the power system network. Their magnitude is essentially independent of the system

9、.External Overvoltages Lightning overvoltages are characterized by very high peak currents and relatively low energy content. They are responsible for nearly half of all short circuits on lines in systems of 300kv and above. Up to 300kv the system insulation has to be designed primarily to withstand

10、 lightning surges.Internal Overvoltages Internal overvoltages are principally determined by the system configuration and its parameters. They are mainly due to switching operations and following fault conditions. According to their duration, which ranges from hundreds of microseconds to several seco

11、nds, three main categories of internal overvoltages can be defined:Internal Overvoltages 1 Transient Switching Surges. 2 Temporary Overvoltages. 3. Steady-state Overvoltages.Lightning Strokes During stormy weather, by a process not yet fully understood, a charge separation takes place inside clouds,

12、 so that positive charges move to the upper part of the cloud while negative charges stay below. This transfer of electric charge sets up an electric field within the cloud.Lightning Strokes The negative charge at the base of the cloud repels the free electrons on the ground below. Consequently, the

13、 surface of the ground becomes positively charged, by induction.Lightning Strokes As more and more positive charges are move upward within the cloud, the electric field below the cloud becomes more and more intense. Ultimately, it reaches the critical ionization level where air begins to breakdown.

14、Ionization takes places first at the tips of high buildings, such as church spires, and the top of high trees. Lighting StrokeLighting StrokeLighting StrokeLighting Strokes Lighting Conductors Lighting Conductors Lightning Shielding of Transmission Lines When lightning makes a direct hit on a transm

15、ission line, it deposits a large electric charge, producing an enormous overvoltage between the line and the ground. The dielectric strength of air is immediately exceeded and a flashover occurs. Lightning Shielding of Transmission Lines The arc between the line and the ground produces a highly ioni

16、zed path which behaves like a a conducting short-circuit. Consequently, the normal AC line voltage immediately delivers a large AC current that follows the ionized path. This follow-through current may sustain the arc until the circuit breakers open at the end of line.Lightning Shielding of Transmis

17、sion Lines Direct hits on a transmission line are rare. More often, lightning will strike the overhead ground wire that shields the line. In the latter case, a local charge shill accumulates on the line, producing a very high local overvoltage. Overhead Ground WiresOverhead Ground WiresLightning ArrestersTravelling Waves The concentrated charge immediately divides into two wa