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1、电力力专业英英语14-电力系力系统中的高中的高电压技技术TopicsInsulationGroundingOvervoltageLightning Shielding2021/5/222Insulation 1Insulation 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 the particular application a
2、nd by cost.2021/5/223Insulation 2The 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.2021/5/224Insulation 3Overhead lines are supported on porcelain insulators.Between the support
3、s 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.2021/5/225Insulation 4The cost of equipment and the system as a whole is closely related to the insulation level wh
4、ich 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.2021/5/226500KV开关场 switchyard2021/5/227500KV变压器 transformer2021/5/228220KV开关场 switchyard2021/5/229220KV开关场 sw
5、itchyard2021/5/2210电力电缆 power cables2021/5/2211电力电缆 power cable2021/5/2212钢芯铝绞线 aluminum conductor steel reinforced 2021/5/2213架空输电线overhead transmission line2021/5/2214架空输电线overhead transmission line2021/5/2215Insulation CoordinationElectric systems insulation designers have two options available t
6、o them:choose insulation levels for components that would withstand over voltages,devise protective devices that could limit over voltages there.2021/5/2216GroundingGrounding is defined as a conducting connection,whether intentional or accidental,by which an electric circuit or equipment is connecte
7、d to the earth,or to some conducting body of relatively large extent that serves in place of the earth.2021/5/2217GroundingThe 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:Person
8、nel safety To assure protective device operation Noise control2021/5/2218接地网施工 construction of ground grid2021/5/2219接地网施工 construction of ground grid2021/5/2220OvervoltagesThe overvoltages can be divided into external overvoltages and internal overvoltages according to the locations of sources.Exte
9、rnal overvoltages are generated by sources that external to the power system network.Their magnitude is essentially independent of the system.2021/5/2221External OvervoltagesLightning overvoltages are characterized by very high peak currents and relatively low energy content.They are responsible for
10、 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 lightning surges.2021/5/2222Internal OvervoltagesInternal overvoltages are principally determined by the system configuration and its parameters.They
11、are mainly due to switching operations and following fault conditions.According to their duration,which ranges from hundreds of microseconds to several seconds,three main categories of internal overvoltages can be defined:2021/5/2223Internal Overvoltages1 Transient Switching Surges.2 Temporary Overv
12、oltages.3.Steady-state Overvoltages.2021/5/2224Lightning StrokesDuring stormy weather,by a process not yet fully understood,a charge separation takes place inside clouds,so that positive charges move to the upper part of the cloud while negative charges stay below.This transfer of electric charge se
13、ts up an electric field within the cloud.2021/5/2225Lightning StrokesThe negative charge at the base of the cloud repels the free electrons on the ground below.Consequently,the surface of the ground becomes positively charged,by induction.2021/5/2226Lightning StrokesAs more and more positive charges
14、 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.Ionization takes places first at the tips of high buildings,such as church spires,and the top of high trees.2021/5/2227
15、Lighting Stroke2021/5/2228Lighting Stroke2021/5/2229Lighting Stroke2021/5/2230Lighting Strokes 2021/5/2231Lighting Conductors 2021/5/2232Lighting Conductors 2021/5/2233Lightning Shielding of Transmission LinesWhen lightning makes a direct hit on a transmission line,it deposits a large electric charg
16、e,producing an enormous overvoltage between the line and the ground.The dielectric strength of air is immediately exceeded and a flashover occurs.2021/5/2234Lightning Shielding of Transmission LinesThe arc between the line and the ground produces a highly ionized path which behaves like a a conducti
17、ng 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.2021/5/2235Lightning Shielding of Transmission LinesDirect hits on a trans
18、mission 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.2021/5/2236Overhead Ground Wires2021/5/2237Overhead Ground Wires2021/5/2238Lightning Arresters
19、2021/5/2239Travelling WavesThe concentrated charge immediately divides into two waves that swiftly move in opposite directions at close to the speed of light.The height of the impulse wave represents the magnitude of the surge voltage that exists from point to point between the line and the ground.The peak voltage may attain one or two million volts.2021/5/2240谢谢！