HV_UNIT1.pptx

7/27/2019 HV_UNIT1.pptx

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High VoltageEngineeringUNIT I


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In modern times, high voltages are used for a wide variety of applications likethe power systems, industry, and research laboratories such as in nuclear

research, in particle accelerators, and Van de Graaff generators.

For transmission of large bulks of power over long distances, high voltages areindispensable. Also, voltages up to 100 kV are used in electrostaticprecipitators, in automobile ignition coils, etc.

X-ray equipment for medical and industrial applications also uses highvoltages. Modern high voltage test laboratories employ voltages up to 6 MV or

more.

A high-voltage, direct current (HV) electric power transmission system usesdirect current for the bulk transmission of electrical power, in contrast withthe more common alternating current systems.

For long-distance distribution, HV systems are less expensive and suffer lowerelectrical losses. For shorter distances, the higher cost of DC conversionequipment compared to an AC system may be warranted where other benefitsof direct current links are useful.


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High ac voltage of one million volts or even more are required for testingpower apparatus rated for extra high transmission voltages (400KV system andabove).

High impulse voltages are required testing purposes to simulate over voltagesthat occur in power systems due to lighting or switching surges.

Main concern of high voltages is for the insulation testing of variouscomponents in power system for different types of voltages namely powerfrequency, ac high frequency, switching or lightning impulses.

Voltages upto 100KV are used in electrostatic precipitators.

Expansion of transmission systems has led to the development of Flexible A.C.Transmission Systems (FACTS) which are based on newly developing high-power electronic devices such as GTOs and IGBTs. Examples of FACTS systems

include Thyristor Controlled Series Capacitors and STATCOMS.

The FACTS devices improve the utilization of a transmission system byincreasing power transfer capability.


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Advantages of very high voltages

for transmission

1) Reduces volume of conductor material:

We know that I = P/(3 VCos ) But R = L / a

Where = resistivity of transmission line

L = length of transmission line in meters

A = area of cross section of conductor material

Hence Total Power Loss,

W = 3 I2 * R

= 3 (P/(3 VCos )) 2 L / a

A = P2 L / (W V2 Cos2 )

Therefore Total Volume of conductor = 3 * area * length

= 3 P2 L2 / (W V2Cos2 )

From the above equation, the volume of conductor material is inverselyproportional to the square of the transmission voltage. In other words, thegreater the transmission

voltage , lesser is the conductor material required.


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2) Increases Transmission efficiency:

Input power = P + total losses

= P + P2 L / ( V2 Cos2 a)Let J be the current density, therefore a = I/ J

Then input power = P P2 L J / (V2 Cos2 ) 1/I

Transmission efficiency = Output Power / Input Power

= P / (P * 13 J L/ V cos +)

Since J,L are constants, therefore transmissions efficiency increases when linevoltage is increased.

3) Decrease percentage line drop:

Line drop = IR = I * L / a

= I * L * J/I = L J% line drop = J L / V * 100

As J, and L are constants, therefore percentage line drop decreases when thetransmission voltage increases.


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a number of applications HV is more effective than AC transmission.

Examples include:

Undersea cables, where high capacitance causes additional AC

losses. (e.g., 250 kmBaltic Cable between Sweden and Germany,the 600 km NorNed cable between Norway and the Netherlands,and 290 km Basslink between the Australian Mainland andTasmania)

Endpoint-to-endpoint long-haul bulk power transmission withoutintermediate 'taps', for example, in remote areas

Increasing the capacity of an existing power grid in situationswhere additional wires are difficult or expensive to install

Power transmission and stabilization between unsynchronised ACdistribution systems Connecting a remote generating plant to thedistribution grid, for example Nelson River Bipole


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Stabilizing a predominantly AC power-grid, without increasing prospectiveshort circuit current

Reducing line cost. HV needs fewer conductors as there is no need to support

multiple phases. Also, thinner conductors can be used since HV does not sufferfrom the skin effect

Facilitate power transmission between different countries that use AC atdiffering voltages and/or frequencies

Synchronize AC produced by renewable energy sources

HV can carry more power per conductor because, for a given power rating, theconstant voltage in a DC line is lower than the peak voltage in an AC line.

In AC power, the root mean square (RMS) voltage measurement is consideredthe standard, but RMS is only about 71% of the peak voltage.

The peak voltage of AC determines the actual insulation thickness andconductor spacing.

Because DC operates at a constant maximum voltage, this allows existingtransmission line corridors with equally sized conductors and insulation tocarry 100% more power into an area of high power consumption than AC,which can lower costs.


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Application of High Voltage

Engineering in Industry Electrostatics is currently found at the basis of many major

industries related to environment preservation, communications,processing of mineral ore resources, and so on.

In the majority of these industries, the unique properties of high-

voltage electrostatic fields and forces are utilized to collect, direct,deposit, separate, or select very small or lightweight particles.

electrostatic precipitation, electrostatic separation, electrostaticpainting, electrostatic spraying of pesticides in orchards,

electrostatic imaging, electrostatic printing, electret transducers,transport of light materials, paper manufacture, smoke detection,electrostatic spinning, electrostatic pumping, electrostaticpropulsion, air cleaning from gaseous pollutants, ozone generation,and biomedical applications.


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Electrostatic precipitation

Electrostatic precipitation is essentially the charging of dust particlesin a gas and their subsequent separation under the effect of theelectric field

These processes may occur within a single zone or be distributedover two zones, where the first zone-the charging zone-is intendedto charge the particles, and the second zone-the collecting zone-isdesigned to settle the particles.

The stream of ions charging the dust particles is produced by meansof a corona discharge in an inhomogeneous electric field.

These ions interact with the particulates entrained in the gas andimpart charge to the dust, which then experiences a force towardsthe collecting electrode where it is held by electrostatic forces untilit is removed by mechanical rapping (Fig. 19.1).


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Two systems of electrodes are used in

electrostatic precipitators to obtain an

inhomogeneous electric field:

a wire conductor enclosed in acylindrical pipe (pipe-type precipitators),

or a row of wire conductors located

between plates (plate- or duct-type

precipitators).

The electrodes around which a corona

discharge is formed are termed the

discharge electrodes, whereas the

electrodes receiving the charged dust

particles, deposited by the action of the

electric field, are the collecting

electrodes.


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Figure 19.2 displays schematically the charging of dust particles in a precipitator.The particles are charged as a result of bombardment by ions energized in the

electric field, for particles larger than 1 pm, and through collision with ions

which participate in the continuous thermal motion of molecules without the

aid of an electric field, for particles in the sub-micron range.


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A dust cake on a collecting electrode consists of only 10-50% dust,depending on the size of the particles, the remaining part being

channels (cracks) filled with gas. Because of a difference in the values

of relative permittivities of dust and gas, the lines of electric field

concentrate inside the channels (Fig. 19.3).


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When the voltage is high, an electric breakdown takes place across the dust cake, and thegas inside the channels is ionized. The outlet of each channel acts then as a point dischargeon the collecting electrode. This phenomenon is termed back corona .

High resistivity particles cause back corona to be formed in the precipitated layer and thishas a number of deleterious effects.

The most significant is the release of ions which move countercurrent with respect to thedust particles and partly neutralize their charges, so that dust collection is adverselyaffected and current intensity" in the precipitator greatly increases.

This is in addition to a reduction in the sparkover voltage, which consequently reduces theprecipitator's performance.

Pulse charging was proposed as an alternative solution to back discharge. A train of pulses,usually superimposed on a DC level value, provides a high peak electric field for particlecharging. However, the mean collector current in this case depends not only on the peakvoltage but also on the pulse duty cycle.

In this way the voltage and current can be decoupled by changing the mark/space ratio ofthe applied pulses, and high charging fields can be established without back discharge.Pulse charging can produce a uniform distribution of ionic current on both the dischargeand collecting electrodes


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The disadvantage of pulse charging is the ion shortage that occurs when thecurrent must be lowered extremely to cope with high dust resistivity.

This results in a low charging rate which impairs the performance even if backdischarge is avoided. To overcome this shortcoming, prechargers have beenproposed.

The fundamental aim of all precharger systems is to separate the charging

and collecting processes into two separate stages.

In the charging stage, different techniques are used to eliminate or reducethe effects of back corona, whereas the collecting stage is similar for allprechargers and comprises only parallel plates so that no back corona can beformed.

Different types of precharger have been developed and tested at pilot-scalestage, including the tri-electrode charger, high-intensity charger, boxercharger, and cold-pipe charger Precipitator efficiency can reach or evenexceed 99%, depending on several design parameters, including thedimensions and geometry of the gas duct, gas temperature and velocity,

average size and resistivity of the particulates, and corona discharge intensity.


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Electrostatic SeparationElectrostatic separation is the selective sorting of solid species by means of

utilizing forces acting on these species in an electric field. The main items ofthe separator are a charging mechanism in the charging zone, an externalelectric field in the separating zone, and a feeding and product collectionsystem.

The charging of two different species entering the separating zone results in:

(a) particles bear electric charges of opposite sign; or (b) only one type ofparticle bears an electric charge; or (c) particles bear the same sign of charge,but the magnitude of the electric charge is significantly different. Althoughthere are many ways to charge solid particles, the most commonmechanisms are as follows.

1. Charging by contact and frictional electrification is the mechanism mostfrequently used to selectively charge and electrostatically separate twospecies of different materials such as phosphate and quartz. The ore,composed of small particles of quartz and phosphate, is vibrated on its wayfrom the hopper to the forming chute (Fig. 19.4a)


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Phosphate particles enter the separating zone with a net positive chargewhile the quartz particles bear a net negative charge. A coal beneficiationsystem has been developed on the basis of electrostatic separation, wherethe external field in the separating zone deflects the coal-rich and ash-rich

particles in opposite directions towards the collecting-plate compartments.

2. Charging by ion or electron bombardment is used, in which solid particlespass through a corona discharge from a fine wire or a series of needle pointspositioned parallel to a grounded rotor of the separator.

The particles are charged by bombardment with the corona ions. The chargedparticles rapidly share their charge with the grounded rotor and are thrownfrom the rotor in a trajectory determined by centrifugal force, gravity, and airresistance.

The dielectric or poorly conducting particles lose their charge slowly and arethus held to the surface of the rotor by the image force associated with theirsurface charge. The well conducting particles are thrown free of the rotor by acombination of centrifugal force and gravity (Fig. 19.4b).


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3. Charging by conductive induction is a charging mechanism suitable forseparating well conducting particles from well insulating particles. A grounded

rotor is located close to a positive drum (Fig. 19.4c). When conductive particles

coming from the hopper pass over the rotor they become negatively charged

and attracted toward the positive drum. However, insulating particles fall down

by gravity.


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Electrostatic PaintingThis is a kind of electrostatic precipitation of powder or liquid paint on the surface of

an object to be coated (or painted)

In liquid paint, a liquid jet issues from the reservoir and extends along the axis of a

concentric charging cylinder where a potential V is applied between the jet and the

cylinder. The jet charges and breaks up into charged droplets while it is in the cylinder.

The electric field plus space charge effects between the jet and the grounded object

deposit the paint droplets on the surface of the object to be painted, not only on thefront side but also on the back side of the object (Fig. 19.5).


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In powder painting, the paint particles are charged by bombardment withcorona ions moving under the influence of the prevailing electric fieldbetween the corona electrode and the object being grounded (Fig. 19.6).

The paint precipitation stops at a certain thickness called the limitingthickness because of back discharge which results in craters that impair thequality of finished coat.

In its simplest form, an electrostatic coating operation is visualized as takingplace in the following manner. The object to be coated is grounded andsupported so that its surface can be approached without obstruction.


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The coating material, after being charged, is sprayed into the space above the

surface in the form of finely divided particles.

There is an attraction of the particles to the surface and, as a result, they move

toward and accumulate on the surface to form the coating.

The various electrostatic coating applications are somewhat sophisticated

modifications of this simple situation.

They differ from one another in the manner in which the particles are forced,

the means by which they are charged, etc.

Electrostatic painting is widely used in the continuous coating lines of

automobiles, electric appliances, furniture, and so on


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Electrostatic Printers

Many typewriters and computer line printers use so-called impact printing, i.e.,

characters are cast on metal type, which makes an impact on an ink ribbon to

produce prints on paper. New requirements for performance and speed exceed

the capabilities of most impact printing technologies.

Electrostatic printing is an alternative method. Electrostatic printers areclassified into ink-jet and ink-spray printers

The printing ink is formed into macroscopic droplets which are imaged by

electric field, external to the form (print surface). The form serves only as a

receptor for the imaged ink droplets and, upon contact, a visible image isinstantly produced.

This technique is viable on standard forms (sheets of paper), and no image

development or fixing is required.

Electrostatic printers are usually known as nonimpact printers.


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The ink-jet printing technique produces instantly visible images on standardforms by the electrostatic deflection of charged ink droplets into electricallyprogrammable dot matrix patterns.

This principle is analogous to the deflection of electrons in a cathode raytube. The electrically conducting ink is forced through a nozzle to form a thinjet, which then breaks up into droplets under the influence of surface tensionand the mechanical vibrations in the nozzle.

At the point where droplets are forming, a high-voltage, field controlled by a

computer is applied to the jet to give the newly formed droplets an electriccharge related to the signal (Fig. 19.9).

This is why the ink-jet printer is known as the charge modulation type of inkprinter. The drops then move into a deflection region where a steadytransverse electric field deflects them by an amount depending on theiracquired charge.

This deflection causes them to strike the print surface (usually a piece ofpaper) at different points, creating an image.


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In the ink-spray printing technique, a reservoir containing the ink is pressurized at a

low level (a few centimeters of water) sufficient to form a convex meniscus of ink at

the opening of a vibrating nozzle but not high enough to cause an outflow of ink. The

conductive ink, maintained at ground potential, is attracted by the electric field of the

gate.

When the electrostatic attraction force exceeds the surface tension of the meniscus,

droplets are produced. Expressed in terns of potential V, the droplets are produced

when the potential between the gate electrode and the meniscus is

where y= ink surface tension, E = ink permittivity, d= nozzleorifice diameter, and I) = distance between orifice and gate.

The droplets are accelerated and then imaged into the desired dot-matrix format by

electrostatic deflection. Since the droplets are produced with a nearly uniform specific

charge, programmable deflection is obtained by varying the magnitude of the

deflection field. This is accomplished by electronically controlling the voltage appliedto the deflection electrodes (Fig. 19.10).


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This is why the ink-spray printer is known as the field-modulation type of ink printer.

Two sets of electrodes are employed to obtain both horizontal and vertical deflection,

to allow for printing on stationary forms. Since the droplets are very small they can be

quickly accelerated and are able to produce high-quality hard copy much faster thanelectronic (impact) printers, There are commercially available electrostatic printers with

capabilities of printing in excess of 30,000 lines/minute.

With the current concern for noise from impact printers, electrostatic printers have a

definite advantage as the level of noise is only that generated by the hardly visible

droplets of ink landing on the paper.

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