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电力专业英语14-电力系统中的高电压技术


电力系统中的高电压技术
High-Voltage Technology In Power Systems

电气工程学院
2013年8月28日1时51分

Topics
? ? ? ? Insulation Grounding Overvoltage Lightning Shielding

/> Insulation 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 the 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 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 level 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 switchyard

500KV变压器 transformer

220KV开关场 switchyard

220KV开关场 switchyard

电力电缆 power cables

电力电缆 power 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 line

Insulation Coordination
? Electric systems’ insulation designers have two options available to them: choose insulation levels for components that would withstand over voltages, devise protective 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.

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
construction of ground grid

Overvoltages
? 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.

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 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 seconds, 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, 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 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. Ionization takes places first at the tips of high buildings, such as church spires, and the top of high trees.

Lighting Stroke

Lighting Stroke

Lighting Stroke

Lighting Strokes

Lighting Conductors

Lighting Conductors

Lightning Shielding of Transmission Lines
? When lightning makes a direct hit on a transmission 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 ionized 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 Transmission 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 Wires

Overhead Ground Wires

Lightning Arresters

Travelling Waves
? The 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.


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