ElectricalSystemDesignLaboratory

ListofExperimentasperSyllabus

Sl.No. NameoftheExperiment

1 Designingaheatingelementwithspecifiedwattage,voltageandambienttemperature.

2 Designinganair-coregroundingreactorwithspecifiedoperatingvoltage,nominalcurrent and
faultcurrent.

3 Designingthepowerdistributionsystemforasmalltownship.

4 Designingadoublecircuittransmissionlineforagivenvoltagelevelandpower(MVA) transfer.

5 Wiringandinstallationdesignofamulti-storiedresidentialbuilding(G+4,not less than16

dwellingflatswith aliftandcommonpump)

6 Designingofasubstation

7 DesigninganONANdistributiontransformer.

8 Designingathreephasesquirrelcageinductionmotor.

9 Designingathreephasewoundrotorinductionmotor.

10 Designingasplitphasesquirrelcageinductionmotorforaceilingfanoradomesticpump.
11 Designingapermanentmagnetfractionalhpservomotor.

EXPERIMENT NO -1
 Designing a heating element with specified wattage, voltage and ambient
temperature
INTRODUCTION:

When current is passed through a conductor, it gets heated up due to I2R losses and this
heating characteristic of the electric current is being utilized in industrial and domestic appliances.

Heating is required for domestic purposes such as cooking and heating of buildings whereas
for industrial purposes and heating is required for melting of metals , hardening and tempering and in
welding.

Advantages of electric heating over other systems of heating :

The main advantages of electric heating over other systems of heating (i.e coal, gas,
or oil) heating are:

1. Economical: Electric heating is economical as electric furnaces are cheaper in initial

cost as well as maintenance cost. It does not require any attention so there is a
considerable saving in labour cost over other systems of heating. moreover, the electric
energy is also cheap as it is produced on large scale.

2. Cleanliness: Since dust and ash are completely eliminated in electric heatingsystem,
so it is clean system and cleaning costs are rendered to a minimum.

3. Absence of fuel gases: Since no fuel gases are produced in this system,
the atmosphere around is clean and pollution free.

4. Ease of control: Simple, accurate and reliable temperature of a furnace can be had
with the help of manual or automatic devices. Desired temperature can be had in electric
heating system which is not convenient in other heating systems.

5. Efficiency: It has been practically found that 75 to 100% of heat produced by

electric heating can be successfully utilized as the source can be brought directly to the
point where heat is required there by reducing the losses.

6. Automatic protection: Automatic protection against over currents or overheating can

be provided through suitable switchgears in electric heating systems.

7. Better working conditions: Electric heating system produces no irritation noise and
also the radiation losses are low. Thus working with electric furnaces is convenient and cool.

8. Safety: Electric heating is quite safe and responds quickly.

9. Upper limit temperature: There is no upper limit to the temperature

obtainable except the ability of the material to with stand heat.

10. Special heating requirements: Certain requirements of heating such as uniform

heating of material or heating of one particular portion of the job without effectingothers,
heating of non-conducting materials, heating with no oxidation can be met only in electric heating
system.

Heating element:

The heating effect of electric current can be produced by passing electric current through Heating element

and the material used for heating element must have fallowing properties

1. It should have high specific resistance so that a small length of wire (R = ,p= )
is sufficient to produce the required amount of heat.

2. It should have high melting point so that high temperature can be obtained.

3. It should have low temperature co-efficient since for accurate temperature control, the
resistance should be nearly constant at all temperature and this is possible only if the
resistance does not change with temperature.

4. It should not oxidize at higher temperatures otherwise its life is shortened andneeds
frequent replacement.

The most commonly used heating elements are either alloy of nickel and chromium or nickel-
chromium iron, nickel-chromium-aluminium, nickel-copper. The use of iron reduces the cost but
lowers the life of the element.

Design of Heating Element:

The heating element used for electric heating may be circular or rectangular like a ribbon but
ribbon type of element requires more wattage per unit area. Hence circular heating elements are preferred.

By knowing the electrical input and its voltage the size and length of the heating element required
to produce given temperature can be calculated.
Causes of failure of heating element

There are so many causes are there for the failure of heating element. Some of them are
explained below.

1. Formation of Hot Spots: Hot spots are the points in heating element which are at
higher temperature than the main body of the element. Hot spots may be due to any of
the fallowing causes:

(a) High rate of local oxidation may reduce the cross-section of the element wire
thereby increasing the resistance at that spot. Thus more heat will be produced
locally giving rise to the breakdown of the element.

(b) Shielding of element by supports etc. will reduce the local heat loss by radiation
and causes a rise of temperature of shielded portion of the element therefore
minimum number of supports without producing distortion of the element should
be used.

(c) Due to too high element temperatures, insufficient support for the element or
selection of wrong material, sagging and wrapping of element may result which
may causes uneven spacing of sections there by producing hot spots.

2. Oxidation of intermittency of operation: At high temperature, oxide scale is

formed on the heating element which is continuous and tenacious and is so string
that it prevents further oxidation of inner metal of element. However, if the element
used quite often the oxide layer is subjected to thermal stresses due to frequent
cooling and heating thereby the oxide layer cracks and flakes off exposing further
fresh metal to oxidation thereby producing hot spots.

3. Embrittlement due to grain, growth: All heating alloys containing iron, tend to form
large brittle grains at high temperatures. When cold the elements are very brittle and
liable to rupture easily on slightest handling and jerks.

4. Contamination and corrosion: Elements may be subjected to dry corrosion produced

by their contamination with the gases of controlled atmosphere prevailing in
annealing furnaces or fumes from flux used in brazing furnaces or oil fumes produced
by heat treatment of components contaminated with lubricant.

Modes of Transfer of Heat:

The heat from one body to another body can be transferred by any one of the fallowing methods.

1. Conduction 2. Convection 3. Radiation

 1. Conduction:
In this method, heat travels without the actual movement of practices (molecules). The flow of
heat from one part of the body to other part is dependent upon the temperature differences between these
parts. It is also applicable when two bodies at different temperatures are joined together. The heated
molecules of the substances transfer their heat to the adjacent molecules and this heat flow will invariably
take place so long as there is difference in temperature.

For example when one end of solid is heated, the molecules at that end absorb the heat energy
and begin to vibrate rapidly when these molecules collide with neighboring molecules, some energy is
passed them with in turn begin to vibrate faster and pass some energy to their molecules. Thus heat is
transferred from one molecule to another molecule without their actual movement.

If the heat is to be conducted from one object to another object, the fallowing conditions must be
met.

1. The objects should be bodily in contact with each other.

2. The temperature of the two bodies should be different i.e. temperature gradient
should exixt.
Definition of conduction: The process in which heat is transferred from one particle to another in
direction of fall of temperature without the actual movement of particles of medium is called conduction.

The rate of conduction of heat along a substance depends upon the temperature gradient and is
expressed in Mj/hr/m2/m/c0 or in watts/cm2 in case of electric heating.

In a plate of thickness t meters having X-sectional area of its two parallel feces A sq.meters and
temperature of two faces is T1 and T2 absolute, the quantity of heat transferred through it during T hours is
given by

Q= (T1 – T2) T

Where K is coefficient of thermal conductivity for material in Mj/hr/m2/m/c0

2. Convection:

Def: The process of heat transference in which heat is transferred from one place to another (from
hotter to colder one) by actual movement of particles of medium is calles convection.
For example in case of heater used for heating buildings, the air in contact with a heat radiator
element in a room receives heat from contact with the element. The heated air expands and rises, cold air
flowing into takes place. Thus there is a constant flow of air upwards across the heating elements. Thus in
this way the room gets heated up.
 A similar action takes place in an electric water heater, a continuous floe of water passing
upwards across the immersed heating element, with the result that the whole of the water in the tank
becomes hot.

The quantity of heat absorbed from the heater by convection depends mainlyupon the
temperature of the heating element above the surroundings and upon the size of surface of the heater. It
also depends partly on the portion of the heater.

Heat dissipation is given by the fallowing expression

H = a (T1 – T2) b w/m2
Where a, b - constants whose values depends on the heating surface facilities for heating etc.

T1, T2- temparture of the heating surface and fluid in 0

c.

3. Radiation:
Def: The process of transmission of heat in which heat energy is transferred from hotter body
to colder body without heating the medium in between is called radiation.

Foe example we receive energy from the sun by radiation through there in distance of about 150
million Kms between sun and earth.

Rate of heat radiation is given by stefan’s law according to which: Heat

4
dissipation H = 5.72× 10 Ke –

Where - temperature of source of heat in 0c

- temperature of substance to be heated in 0c

K – constant known as radiant efficiency whose value is 1 for single element

and 0.5 to 0.8 for several elements placed by side by side.

e – emissivity which is 1 for black body. And 0.9 for resistance heating
elements.
Electric heating methods:
Electric heating methods can be classified as:

Electric heating

Power frequency heating high frequency heating

Arc heating Resistance heating Induction heating dielectric heating

Direct resistance heating Indirect resistance heating Infrared heating

Direct arc heating Indirect arc heating

ARC HEATING:
When a high voltage is applied across in air gap, the air in the gap gets ionized under
electrostatic forces and become conducting medium. Current flows in the form of a continuous spark called the
arc. A very high voltage is required to establish an arc across the air gap but to maintain an arc small voltage
should be sufficient.

Alternatively an arc can also be produced by short circuiting the two electrodes momentarily and
then withdrawing them back. Arc between the two electrodes produces heat and has a temperature between
1000 0c and 3500 0c depending upon the material of electrodes used. The use of this principle may be in electric
arc furnace.

Usually arc furnaces are of cylindrical shape but recently conical shaped shells have been used due
to the advantage of a large surface area per unit volume. Moreover the conical shaped furnace consumes less
power, radiation loss and melting time is also reduced.

The arc chamber of the furnace consists of a suitable acid or basic refractory lining supported on
a metal frame. Each furnace is provided with charging door and tap hole for introducing the charge and taking
out the molten metal. The electrodes project through the top or sides of chamber and are arranged for easy
replacement and adjustment. The electrodes used in arc furnace are either made of carbon or graphite. Graphite
is mostly preferred however carbon electrodes are used with small furnaces.

The salient points of the two types of electrodes are:

1. Resistance: owing to lower resistivity of graphite the size of graphite electrode is

reduced to half of the carbon electrode for the same current carrying thus facilitating
easy replacement.
 2. Consumption: graphite begins to oxidize at about 6000c whereas carbon at about
4000c and thus consumption of graphite electrode is about one half of carbon
elevtrode.
3. Evenness of heating: The larger area of carbon electrodes means a greater surface area
of charge covered by the arc and consequently a more uniform distribution of heat, on
the other hand however the arc is brought nearer to the side of furnaces which tend to
shorten the life of refractor lining.
4. Cost: Graphite electrodes cost about twice as much per Kg as carbon electrodes so that
the savings due to their use largely nullified especially if the process is one in which
the electrode consumption is large.
The choice between the two types of electrodes depends upon the application however in general for
processes requiring large quantities of electrodes, the carbon electrode is used.

Types of furnaces:
Arc furnaces may be classifies into two types.

1. Direct arc furnace.

2. Indirect arc furnace.

1. Direct arc furnace:

In this type of furnace, the arc is formed between the electrodes and the charge as shown in figure 1. In
this type of furnace, charge acts as another electrode. There are two carbon or graphite electrodes and the arc
is developed at two places.

Since in direct arc furnace, the arc is in direct contact with the charge and heat is also produced by current
flowing through the charge itself, the can therefore be heated to highest temperature.

In case of single phase arc furnace, two electrodes are taken vertically downward through the roof of
furnace to the surface of charge whereas in case of three phase arc furnace three electrodes put at the corners
of an equivalent triangle are used which produces three arcs, the charge itself thus forms a star point.

The most common application of this type of furnace is for production of steel. This is advantageous
as compared to cupola method for production of steel due to the fallowing reasons.

1. By using this method, purer product can be obtained as referring process can be easily
controlled.

2. Arc furnace can operate on 100% steel scrap which is cheaper than pig iron whereas
cupola requires a proportion of pig iron in cupola charge.
This is the reason, direct arc furnace even being costlier in initial as well as operating costs is preferred.

2.Indirect arc fuarnace:

In this type of furnace, the arc is formed between two electrodes above the charge and the heat is
transmitted to the charge solely by radiation as shown in figure 2.

The temperature of the charge in the indirect-arc furnace is lower than in the direct-arc
 furnace, since heat is transmitted to it solely by radiation. As no current flows through the charge there is no
inherent stirring action, and the furnace must be rocked mechanically; for this reason a cylindrical shape is
adopted, with the electrodes projecting through the chamber at each end along the horizontal axis. This
construction limits the number of electrodes to two and arc is produced by bringing the electrodes into solid
contact and then with drawing them. Power input is regulated by adjusting the arc length by moving the
electrodes.

Due to the indirect heating, the furnace is suitable for comparatively lower melting point such as
melting of non-ferrous metals. They are also used in iron foundries where intermittent supply of molten
metal is required.

Resistance heating:

Direct Resistance:
In this method, current is passed through substance to be heated. The resistence offered by the substance
to flow of current produces ohmic losses I2R which results in heating the substance.

In other words, the material to be heated is taken as resistance and current is passed through it. The
material may be in form of powder, pieces or a liquid. The electrodes in case of
d.c. or single phase a.c. or three electrodes in case of 3-phase a.c. are immerced in the charge and connected to
the supply. The current flows through the charge and heat is produced. This method has high efficiency since
heat is produced in charge itself.

This method of heating is employed in resistance welding, in electrodes boiler for heating water and in
salt bath furnace.

Applications of direct resistance heating:

(a) salt bath furnace:

 They are mainly used for the purpose of tempering, quenching and hardening of steel tools.
The advantages of salt bath heating are rapidly, uniformly and selective localized heating combined with
production from oxidation.

This type of furnace consists of a bath of same salt as sodium chloride and two electrodes
immersed in it. When the current is passed through the electrodes immersed in salt, heat developed and
temperature of salt may vary between 10000c -15000c depending upon type of salt used. In this Bath the material
to be heated is dipped and necessary heat treatment is given to it. As d.c. would cause electrolysis of salt,
therefore alternating current is used. In this method, it must be ensured that the current flows through the salt and
not through the job is used.

Since the voltage required is of order of 20V, therefore a tap changing transformer is used.

(b) Electrode boiler:

It essentially consists of electrodes and water placed in a tank. When the supply is given to the
electrodes the current passes through the electrodes and water and produces heat. Heat is produced due to the
resistance offered by water. The tank in which the water is placed is earthed solidly and connected to earth

D.c supply is not preferable as it results in electrolysis of water which in turn results in evolution of H2 at
negative electrode and oxygen at positive electrode. But passage of a.c. hardly results in evolution of gas, but
heats the water. Thus a.c. is recommended.

Indirect Resistance heating:

In this method, the current is passed through high resistive element known as heating element
which is placed either above or below the substance to be heated. The heat produced by the heating
element due to I2R loss is delivered to the material to be heated by one or modes of transfer of heat. i.e.
conduction or convection or radiation.

In case of industrial heating where a large amount of charge is to be heated the heating element is
kept in cylindrical surrounded by the jacket containing the charge as shown
 This arrangement provides a uniform temperature control can be provided in this case by
presetting the time duration.

This type of heating is used in room heater, immersion heaters, in bimetallic strips and various
types of resistance ovens used in domestic and commercial cooking.

Applications of Indirect resistance heating:

(a) Resistance ovens:

It essentially consists of a high resistive material through which an electric current is
passed placed in a chamber made of heat insulating material. The element may be in the form of strip or wire
and is placed on the top, bottom of the oven depending upon circumstances.

In certain types of ovens, two electrodes project from the opposite walls of the oven and
a high current is passed through these electrodes. This type of ovens is used where high temperature is
desirable. The shape and size of the oven depend upon nature of the job.

Resistance ovens are used for various purposes such as heat treatment of metals, drying,
backing of pottery materials, cooking of food e.t.c.

The temperature of oven (I2Rt) can be controlled by controlling (i) voltage or current (ii)
time and (iii) resistance

Voltage can be varied by using tapped transformer for supply to the oven or by using series
resistance so that some voltage dropped across this series resistaor.

The automatic control of temperature can be obtained by providing thermostat which will operate
a switch to OFF or ON the circuit as soon as the temperature exceeds of fall below the adjusted value.

In order to control the temperature by means of resistance various series and parallel
combinations are used for single phase supply and different star-delta arrangements for three-phase supply.

(b) Immersion water heater:

Most of electric water heating is done by immersion heaters which consists of resistance coils
placed in slotted cylinders of ceramic material. The material used for resistance coils is nichrome wire coated with
magnesium oxide for preventing oxidation of the element which heats up the water due to I2R loss in it.

Radiant Heating or Infrared Heating:

In this method of heating, heating elements consist of tungsten filament lamps together with
reflectors to direct the whole of heat emitted on to charge (material to be heated). The lamps are operated at
23000c there by giving a large amount of infrared radiations and the reflectors are plated with rhodium which
prevents the leakage of heat through the chamber. The lamps used are rated between 250-11,000 watts as
250V.

Radiant heating possess the fallowing advantages:

 1. Rapid heating

2. Compactness of heating units.

3. Flexibility.
And this method of heating founds wide applications in

1. Drying paints of radio cabinets and wood furniture

2. pre-heating of plastics prior to moulding.

3. Softening of thermoplastic sheets.

4. Drying of pottery, paper, textiles, e.t.c.

For obtaining best results, the infrared lamps are located at a distance of 25-30 cm from the object to
be heated

High frequency Heating:

Induction Heating:
Induction heating is based on the principle of a.c. transformers. There is a primary winding
through which an a.c. current is passed which is magnetically coupled to the charge to be heated. When an a.c.
current is passed through primary heating coil, an electric current is induced in the charge and the value of the
induced current is dependent on

1. The magnitude of primary current.

2. The ratio of number of turns in the primary and secondary circuit.

3. Co-efficient of magnetic coupling.

The heat develops depend upon the power drawn by the charge and since P = therefore

to develop heat sufficient to melt the charge the resistance should be low which is possible only with metals
and the voltage must be higher which is obtaining by employing higher flux since the higher the flux linked,
the higher is the voltage induced. Thus magnetic materials are found to be suitable for this type of heating
because of their higher permeability.

In case of charge to be heated is non-magnetic, the heat generated is due to eddy current losses whereas
if it is a magnetic material there will be a hysteresis losses in addition. Eddy current loss is proportional to
frequency and hysteresis loss is proportional to square of frequency and these laws holds good upto a limited
temperature (curie point) since the magnetic materials lose their magnetic properties above curie temperature.

The high frequency require for induction heating is obtained from motor generator set, spark
gap converter and vacuum tube oscillator.

The various factors on which the induction heating depends are

1. Magnitude of primary current Ip since if the primary current is high the flux is
high and hence Is is high and thus heat developed ishigh.

2. Frequency since hysteresis and eddy current loss depends on frequency.

 3. Reciprocal of distance between primary coil and charge because if the distance is
less the magnetic coupling in more and thus heat developed is more.

4. permeability of the charge(metal) and resistivity of the charge.

Magnetic materials generally have high permeability and resistivity than non- magnetic materials
and thus the induction heating is more adaptable and economical for treating magnetic materials.

Characteristics of Induction Heating:

1. Current flows only on the outer surface of the metal and in doing so heats up the
outer surface because if an alternating current is passed through a surface it tends to crowd at
the outer surface(skin effect) because of large inductance at the centre.

2. the current flow is restricted axially to that surface of the metal which is directly
in plane with the primary coil and thus heat produced is restricted to that portion.

3. As the heat is developed directly in side the metal which is to be heated, the the
transfer of heat is very quick.

4. There is no mechanical or chemical contact between the source of energy and the metal
to be heated up. Thus no care is to be paid towards the connection.

5. The temperature attained by this type of heating is extremely high since there is no
medium as heat is produced in the metal itself.

Induction furnaces:

There are basically two types of induction furnaces:

1. Core type or low frequency induction furnace.

2. Core less or high frequency induction furnace.

I. Core type or low frequency induction furnace:

The furnace consists of a circular hearth in the form of trough which contains the charge to be
melted in the form of annular ring. This metal ring is large in diameter is magnetically inter linked with an
electrical winding energized by an a.c. source. The furnace is therefore essentially a transformer in which the
charge to be heated forms a single turn short circuited secondary and is magnetically coupled to primary by an
iron core. The charge is melted due to the heavy current induced in it. When there is no molten metal, no current
will flow in the secondary. Thus to start the furnace, the molten metal is to be poured in the hearth.
 Drawbacks of core type furnace:

1. As the magnetic coupling is between the primary and secondary is poor, since leakage
reactance is high and p.f. is low. To overcome this difficulty the furnace must be designed
foe a low frequency as low as 10Hz which can be achieved by using a frequency changer
which involves extra cost.

2. If normal supply frequency is employed for operation of such furnaces the

electromagnetic lines of force causes turbulence of molten metal and may become severe
unless the frequency is kept low.

3. If the current density exceeds 5 A/m2 , the pinch effect (formation of bubble) due to
electromagnetic forces may cause complete interruption of secondary circuit.

4. The crucible(trough) for the charge is of odd shape and is inconvenient

from metallurgical point of view.

5. For functioning of the furnace the closing of secondary circuit is essentialwhich

necessitates the formation of complete ring of charger around the core.
On account of the above drawbacks, such furnaces have become absolute now-a-days.

II. Core Less Induction Furnace:

The eddy currents developed in the magnetic circuit is given as Eddy

currents α B2 F2

Where B- flux density, f- frequency

In a coreless furnace there is no core and thus flux density will be low. Hence for compensating the
low flux density, the primary current applied to the primary should have sufficiently high frequency. Thus
by applying current of high frequency the core of induction furnace can be eliminated there by reducing its
weight and increasing the flexibility.

The furnace consists of a ceramic crucible cylindrical in shape enclosed within a coil.
Which forms the primary of the transformer and the charge in the crucible, the secondary of the transformer?
The charge is put into the crucible and primary winding coil is connected to high frequency a.c supply. The
flux created by primary winding set up eddy currents in the charge which flow concentrically with those in the
primary winding. These eddy currents heat up the charge to its melting point and set up electromagnetic forces
producing stirring action which is essential for obtaining uniform quality of metal.
 Because of high frequency employed, which is necessary to induce the required voltage in the secondary,
the skin effect in the primary coil increases the effective resistance of the coil and hence copper losses tend to
high and artificial cooling is necessary. Thus the primary winding coils are since made of hallow copper
conductors through which cooling water can be circulated. Insulated supporting structure is employed for such
furnaces as the stray magnetic field due to the current in the primary coil may induce eddy currents in the metal
supporting structures there by leading to the over heating of strictures and reduction of efficiency.

The choice of frequency of primary current can be ascertained by the fallowing penetration formula in
which the secondary current is assumed to be uniformly distributed over a cylindrical layer at the outside edge
of the crucible and having a thickness.

Advantages:

1. They are fast in operation.

2. Precise control of power into the charge can be employed and thus uniform quality of
product obtained is unattainable by any other method.

3. Absence of dirt, smoke, noise e.t.c.

4. Crucible of any shape can be used.

5. The eddy currents in the charge result in automatic stirring.

6. Erection cost of a coreless furnace is less.

7. Operation cost is also low.

8. It is possible to operate coreless induction furnace intermittently as no time is lostin

warming up.
 Applications:

1. These furnaces are used for production of steel and are also used for melting of non-
ferrous metals like brass, bronze, copper, aluminium, magnesium e.t.c.

2. They are also used for specialized applications such as vacuum melting, duplexing
steel, heating of charges of non-conducting materials by use of conducting crucibles.

Applications of induction heating:

1. surface Hardening: The materials used for making parts such as spildle, saw blades,
gears, axles should be hard and tough to withstand the wear which possible with induction
heating since with induction heating it is possible to concentrate the heating effect to
desirableportion.

2. Deep hardening: With the help of induction heating, hardening of material to any depth is
possible and hence this type of heating is used for deep hardening of articles such as screw
driver, tools, drills e.t.c.

3. Tempering: In some mechanical process, the work pieces becomes more hard than required
and may need tempering to loose their hardness for tempering accurate control of heat is
required which is possible only with induction heating.

4. Smalting: Induction heating at high frequency is preferred for extraction of metal from ore
where the process is to be carried out in some protective atmosphere or vacuum.

5. Soldering; For soldering it is essential that required amount of heat is to developed at the
soldering point where as the remaining portion of the solder may remain cold which can be
achieved economically and efficiently by induction heating. With the help of induction it is
possible to melt various metals in suitable furnaces.

High Frequency Eddy Current Heating:

In this method of heating, the machine part to be heated is surrounded by a coil through which an
alternating current at high frequency is passed. The high frequency current carry coil known as the heater coil
or work coil and the material to be heated is knoen as charge load. The electromagnetic field developed in the
coil produces heating due to eddy current set up in the part to be heated.

Since the eddy current loss is proportional to the product of square of supply frequency and flux
density.

Therefore by controlling frequency and flux density, the amount of heat can be controlled.

Induced eddy current is of greatest magnitude at the surface of material to be heated and its value decreases as
we go inside the material due to the skin effect.

Since the depth of penetration of eddy currents into the charge is inversely proportional to supply frequency as
given by

Therefore eddy current heating can be restricted to any depth of the material by judicious selection of
frequency of heating current. This frequency employed is in range of 10,000- 400,000 Hz.
 In case of magnetic material, in addition to eddy current loss, hysteresisloss also contribute of production of
heat.

Advantages:

1. It is quick, clean and convenient method.

2. Amount of heat wasted is less since heat is produced in the body to be heated up.

3. Control of temperature is easy.

4. The can be made to penetrate into metal surface to any desired depth.

5. Unskilled labour can also operate the equipment.

6. The amount of heat produced can be accurately controlled by suitable timing devices.

7. With this type of heating, it is possible to heat many different objects of different
shapes and sizes with the same coil.

Disadvantages:

1. Generation of heat is costly.

2. Efficiency of equipment is quite low.

3. Initial cost of equipment is quite high.

Applications:

1. The high frequency eddy current heating is used for surface hardening in which the
desired depth of penetration of heat can be obtained by judicious selection of frequency
which reduces the cost, labour and time considerably.

2. This method is employed in annealing of metals which saves a lot of time along with
the prevention of scales on metals obtained by conventional methods.

3. Eddy current heating can be economically employed for soldering precisely for high
temperatures.

4. This method of heating is also used for welding, drying of paints, melting ofprecious
metals, sterilization of surgical instruments and forgoing of bolt heads and river heads.

Dielectric Heating:
When non-metallic metals i.e. insulators such as wood, plastic, china glass, ceramics e.t.c. are
subjected to high alternating voltage their temperature willincrease after some time. This increase in temperature
is due to the conversion of dielectric loss to heat. The material to be heated is placed as slab between the metallic
plates or electrodes connected to high frequency a.c. supply.

Dielectric loss is depend upon the frequency and high voltage therefore for obtaining
adequate heating effect high voltage at about 20 Kv and frequency of about 10-30 MHz are usually
employed. High frequency is obtained from valve oscillator.
 The current drawn by the capacitor when connected to an a.c. supply voltage does not lead the
supply voltage by exactly 900 since it is not possible to get a pure capacitor and there is always some resistance
due to which heat is always produced in the dielectric material placed in between the two plates of capacitor.
The electric energy dissipated in the form of heat energy in dielectric material is known as dielectric loss.

Advantages:

1. If the material to be heated is homogeneous and alternating electric field is uniform,

heat is developed uniformly and simultaneously throughout the entire mass of charge.

2. As material to be heated is non-conducting, therefore with the help of other methodsof

heating is not possible to heat the material.

Applications:
The cost of the equipment required for dielectric heating is so high than it is employed where other
methods are impracticable and too slow. Some of the applications of this type of heating are:

(i) synthetics: The raw materials called plastic performs used for synthetics are required
to be heated uniformly before putting them in hot moulds so that the whole mass becomes fluid
at a time, otherwise if the raw material is put directly in the moulds usually heated by steam the
outer surface of the perform will become hot and starts curing while inner surface does not reach
the fluid temperature there by resulting in unequal hardening of plastic.

(ii) Diathermy: Dielectric heating is employed for heating tissues, and bones of
body required for the treatment of certain types of pains and diseases.

(iii) Sterilization: Dielectric heating is quite suitable for sterilization ofbandages,

absorbent cotton, instruments e.t.c.

(iv) Baking of foundary cores: Dielectric heating is more suitable foe baking foundary
cores where thermo setting binders are employed as they set instantaneously when brought to
polymerizing temperature.

(v) Textile industry: In textile industry, dielectric heating is employed for drying
purpose.
 (vi) Food processing: The dielectric heating for food processing is one
of the most modern method and set fourth such processes which are outside.

The dielectric heating is used for;

(a) Heating of general processing such as coffee roasting, chocolate industry.

(b) Cooking of sea foods such as oysters without removing the outer shell.

(c) Dehydration of fruits, milk, cream and eggs.

(d) Defrosting of frozen foods such as meat and vegetables.

(e) For control of bacterial growth and production of germicidal

reactions, the food product are heated and to prevent the product losting the
flavor, they are dielectric heated.

Advantages of High frequency heating( Dielectric, Induction, Eddy current):

High frequency equipments are very costly but are preferred due to the fallowing advantages:

1. The quantity of heat can be accurately controlled with the help of electric
clocks which are function of frequencies as they are run by synchronous motors.

2. The working atmosphere is free from flue gases, smoke and dirt.

3. The equipment used is compact and hence the space required is less.

4. It is easy to maintain high frequency equipment.

5. Operation of high frequency equipments is easy and does not required skilled
labour.

6. As the heat is developed in the material to be heated, loss of heat is less.

7. High frequency equipment can be made automatic.

8. Non-conducting materials can be economically heated by high

frequency heating (Dielectric) which is not possible with other methods.

9. Heat provided by high frequency methods is uniformly and evenly distributed.

10. The quality of product obtained by these methods is improved.

 EXPERIMENT NO – 2
Designing an air-core grounding reactor with specified operating voltage, nominal
current and fault current

Abstract:

Dry type air core reactors are devices which are used at both distribution and transmission
voltages for a variety of applications such as fault current limiting, power flow control,
reactive compensation (shunt reactors), and as the inductive part of tuned harmonic filters.
They may be relatively small devices weighing tens or hundreds of pounds with power
ratings less than 100 kvar, up to very large coils weighing as much as 100,000 lbs. or more,
and having power ratings in the range of 600 Mvar (60 Hz equivalent.)
Although they are well established and proven devices which have been widely used for many
years by electric utilities around the world, much confusion and misunderstanding still exists
in terms of how to properly make reliable electrical connections to air core reactors, how to
properly ground their supports and how to deal with the effects of their stray magnetic fields.
These issues arise as a consequence of the fact that air core reactors do not have a magnetic
core to constrain the magnetic field and as a result the magnetic field is “broadcast” in and
around the reactor. This broadcasted field or so called “stray field” induces eddy currents in
any metallic objects on which it impinges, hysteresis losses in ferromagnetic material, and
potentially large currents in closed loops such as can be formed by concrete reinforcements,
fencing, or improperly arranged support grounding connections. These eddy currents and
closed loop currents can give rise to severe heating problems on terminal connections,
concrete reinforcements and fencing under steady state conditions, and also to damaging
forces during short circuit.

This practical paper illustrates the basic nature of the magnetic field of air core reactors and
the theory behind the formation of eddy currents, closed loop currents and hysteresis losses.
Guidance is given on how to arrange the electrical connection, and the characteristics of
connectors to use, to minimize terminal heating and avoid damaging forces during short
circuit. The concepts of magnetic clearances are explained and alternative methods of
handling concrete reinforcement and fencing are illustrated along with the “Do’s and
Don’ts” of reactor support grounding.
Introduction:

Dry type air core reactors are devices which are used at both distribution and
transmission voltages in both AC and DC systems for a variety of applications such as
fault current limiting, power flow control, reactive compensation (shunt reactors), DC
Smoothing and Coupling reactors, and as the inductive part of tuned harmonic filters,
among others. They may be relatively small devices weighing tens or hundreds of
pounds with power ratings less than 100 kvar, up to very large coils weighing as much as
100,000 lbs. or more, and having power ratings in the range of 600 Mvar (60 Hz
equivalent.) Despite the wide range of applications and sizes, dry type air core reactors
share a basic common structure (with numerous variations in construction details
depending on ratings and application).

1
3

4
5
– winding
– spider
– terminal
6
– glass fiber duct stick 5 – insulator
6 – mounting bracket

Figure 1 - Dry Type Air Core Reactor - Basic Construction

A dry-type air-core reactor (refer to figure 1) consists o f one or more concentric

cylindrical windings (1) which are electrically connected in parallel by welded
connections to aluminum cross arms, commonly referred to as spiders (2), which are
located at the top and bottom of the coil. The individual windings are wound with one or
another type of a variety of specialty cables or individual aluminum wires which are
insulated with insulating tapes or films and encapsulated in a fiberglass epoxy composite.
Each spider carries a terminal (3) for electrical connection of the reactor. The individual
concentric cylindrical windings are radially separated from each other by glass fiber
polyester resin composite duct sticks (4) which form the air ducts which are necessary
for the cooling of the winding. Cooling is provided by natural convection of ambient
air, which enters at the bottom end of the winding and exits at its top end. A protective
paint or coating is applied to the surface of the fiberglass epoxy encapsulated winding to
protect the epoxy from ultraviolet radiation and to improve tracking withstand and
weather resistance. In case of adverse pollution conditions at the site of installation,
reactors for some applications may be equipped with a protective roof or so called “top
hat” (not shown). The reactor is mounted on several base insulators (5) and mounting
 brackets (6). The rating of the insulators depends on the specific system requirements at

I) Magnetic Fields of Air Core Reactors

Since air core reactors do not have a magnetic core to constrain the magnetic field, the
magnetic field is “broadcast” in and around the reactor. The strength of this “broadcasted
field” or more commonly referred to as “stray field” depends on the power rating of the
reactor. In general, the higher the kvar rating of the reactor, the higher the strength of the
stray field. This stray field impinges on both the reactor components themselves such as
windings, spiders, corona electrodes and supports, but also adjacent parts such as
terminals, connectors, connecting bus or cable, bus supports, and in general any adjacent
electrically conductive material.
Figure 2 illustrates the magnetic field distribution of a typical air core reactor, its
magnitude is in milli Tesla [mT]. Note that the stray field of an air core reactor has
rotational symmetry.

As can be seen from the magnetic field lines (Plot d.)), in the mid-plane as well as on
the rotational axis (Plot a.) and c.)), the field is directed in axial direction, whereas in all
other locations the field has components in both the axial and radial direction. In vicinity
of the winding ends of the reactor the magnetic field is predominantly directed in radial
direction (Plot b.)).

Reactor data: reactance (60Hz): inductance: winding length:

114.23 Ohm 303 mH
106in (2700mm)

mean winding diameter:101in (2560mm) number of turns:450

current:418 A(rms)
Figure 2 – Magnetic Field Plot
Biot-Savart's law (1) may be applied to calculate the magnetic field of a cylindrical winding
[9].

dB magnetic flux density of a short

𝑑𝐵
𝐼𝑑𝑙× 𝑟=
^
𝜇0 (1) current filament
|
4𝜋 𝑟 μ0 physical constant (4𝜋10−7 𝐻/𝑚)
𝐼 electric current
𝑑𝑙 infinitesimal length of current
carrying wire filament
𝑟^ unit vector of vector 𝑟
|𝑟| distance between current filament
and field point

The external magnetic field of a dry-type air-core reactor winding at a significant distance
from the winding may be approximated by the field of a current loop as shown in Figure 3.
This approximation holds for coils having a winding length shorter than about three times
the winding diameter. The field produced by a current carrying winding loop in a
distance r of more than around three times the loop diameter may be approximated
according to [5] by the equations (2) and (3). (For locations much closer to the reactor,
numerical techniques are required.)
2
|𝐵| = µ0.𝑛.𝐼.𝐷
ƒ(Θ)
3 ƒ(Θ) = √sin2(Θ) cos2(Θ)
4 (3)
(2) +
8.𝑟

|B| magnitude of the magnetic field

µ0 permeability in
air (µ0=4x10-
7
H/m)
n no. of turns
Figure 3 - loop, equivalent to a reactor I current
winding D loop diameter
(mean winding diameter)
r, Θ coordinates
f(Θ) directivity function as per (3)

Using (2) and (3) in the lateral direction Θ = 0, f(Θ) = 0.5) the magnitude of the magnetic
flux density at moderate distances away from the reactor may be estimated by
2
|𝐵| = 𝜋.𝑛.𝐼.𝐷
10−7 Tesla (4)
4.𝑟 3

As it can be seen from equation (4), the field strength quickly drops off with increasing
distance from the reactor; being inversely proportional to the cube of the distance.
In the case of air core reactors carrying alternating current, the stray magnetic field will
induce eddy currents and associated eddy current losses in any electrically conductive
parts which reside in the stray field, whether they be made of either ferromagnetic or
nonmagnetic material. Where the stray field links closed conducting loops; very large
closed loop currents may be induced, with associated steady state heating and potentially
large forces under short circuit conditions. Finally, where the stray field interacts with
current carrying conductors, such as buswork or cables; oscillatory vibration forces will be
induced under steady state conditions and correspondingly larger forces under short circuit
conditions.
Eddy Current Losses

In order to reach an estimation of the eddy losses in the structural elements of an air core
reactor such as spiders and the terminals as well as the connection bus work an approach
as described in [10] chapter 2 can be chosen.
The terminals and spiders are considered as a plate with thickness d (profile thickness)
with the assumption that the thickness is less than the so called “skin depth” or
“penetration depth”  and the field direction is parallel with the plane of the plate.

The penetration depth is calculated as follows:

 2
  (5)
where:
 penetration depth [m]
 2 f, f in [Hz]
 specific conductance
[S/m] µ permeability,
[H / m]

The skin depth in aluminum at 100°C ( = 23.1 MS/m) is 14.8mm at 50Hz and 13.5mm at 60Hz

The eddy losses can be calculated with some approximation as follows:

B2 3h
p wher  (6)
6 2 e d
0
where 
:
p specific eddy losses [Watt / m]
B magnetic flux density [Tpeak] at the spider or terminal
location d thickness of profile [m]
h height of profile
[m] µ0 4 10-7 H/m
 specific conductance, [S/m]
 penetration depth [m]

Example: A magnetic field of 100 mTpeak 50 Hz in a 100mm x10mm profile results in

specific losses of 95 W/m at 100°C ( = 23.1 MS/m)

The foregoing approximation is acceptably accurate for non-magnetic material

geometries with d <  at fundamental frequency. At high frequencies, or in the case of
ferromagnetic material, the reduced skin depth due to the higher frequency and/or high
magnetic permeability results in significant errors if this approach is used. Hysteresis
losses are also not accounted for in the above formulae.
In practical cases, connector and terminal losses are today normally calculated using FEM methods.
 Induced Currents and Losses in Closed Loops

Faradays law of induction states that the EMF (electromotive force) induced in a
closed loop is proportional to the negative of the time rate of change of the magnetic flux
linked by the loop.

𝐸𝑀𝐹
𝑑ɸ
= −
(7)
𝑑𝑡

Figure 4 - EMF and flux linkage

In the case of a magnetic field 𝐵 the flux linking the loop is:

ɸ = ∫𝐴
𝐵∙ 
𝑑𝐴
where 𝑑𝐴 = 𝑑𝐴. 𝑛 is a vector normal to the small surface 𝑑𝐴 and n is a unit vector
normal to this surface.
From equations (7) and (8) it is clear that the EMF will depend on the size of the loop and
its orientation relative to the 𝐵 field direction; being thus proportional to the component of
𝐵 which is normal to the plane of the loop and to the loop area.
The loop current, I, which results from the induced EMF, will be in accordance with ohms
law where the inductance and resistance of the loop are considered.

|𝐼| = |( 𝐸𝑀𝐹 )| (9)

Z𝑙𝑜𝑜𝑝

2 2
𝑍𝑙𝑜𝑜𝑝 = √(𝑅𝑙𝑜𝑜𝑝 + X𝑙𝑜𝑜𝑝 ) (10)

In order to analyze specific cases, the loop inductances and resistances must be
calculated along with the linking flux and resulting EMF.

Calculation of the flux linking a loop by formulas may be done only for simple
geometries. More complex geometries typically require either purpose built software or
finite element modelling.
Short Circuit Forces
The Lorentz force law states that the force on a current carrying conductor in a time
varying magnetic field is:
𝐹⃗ = 𝐼 𝑙⃗𝑥𝐵̅⃗ (11)
Figure 5 – right hand rule
The direction of the force, in accordance with the well-known right hand rule, is such that it is
directed normal to the plane formed by the B-field vector and the current direction vector.

Another method for calculating the magnetic field forces is to determine the change in the
magnetic energy which is produced by an imaginary change in the configuration of the current
circuit [11]. If any dimension x of a current-carrying circuit is changed by a small distance dx,
magnetic field forces must be overcome (in addition to the elastic stresses). If we denote the
magnetic field force in the direction of x with Fx, a displacement of dx becomes a mechanical
work done.

𝑑W1 = 𝐹𝑥𝑑𝑥 (12)

Assuming the current I is held constant in this change, e.g. by providing a sufficiently high
resistance in the circuit, the total flux of the current circuit,

Φ=𝐿𝐼 (13)

increases with such a change in the amount

∂𝐿
𝑑Φ = 𝐼 𝑑𝐿 = 𝐼 𝑑𝑥 (14)
∂𝑥
If the change takes place in time dt, then a self-induction voltage is obtained
𝑑Ø 6𝐿 𝑑𝑥
𝑢𝐿 = =𝐼 (15)
𝑑𝑡 6𝑥 𝑑𝑡

This requires an electrical work for the current I during the time dt of
∂𝐿
𝑑W2 = 𝐼 𝑑𝑡 = 𝐼2 𝑑𝑥 (16)
∂𝑥
𝑢𝐿
Finally, the energy accumulated in the magnetic field is increased

W𝑚 = 1 𝐿 𝐼2 𝑏𝑦 𝑑W𝑚 = 1 𝐼2 6𝐿
𝑑𝑥 (17)
2 2 6𝑥

Since the energy expended must equal the energy obtained, it follows

𝑑W2 = 𝑑W𝑚 + 𝑑W1 (18)

Or, after inserting the expressions (12), (14) and (15)

1 ∂𝐿
𝐹= 𝐼2 (19)
𝑥
2 ∂𝑥

The force is therefore always directed to increase the inductance. It can be calculated if the
dependence of the inductance of the current circuit of x is known.
In the case of two different circuits 1 and 2, the forces occurring between the circuits can be
found by a similar consideration. The displacement dx of the two current circuits relative to
one another results in a change in the mutual inductance, by which, on the one hand, the
voltages induced in the two current circuits and on the other hand, the change in the field
energy. This follows the force acting in the direction
of the displacement and to bring the current loops into such a position that the mutual
inductance becomes as high as possible

𝐹 = 𝐼 𝐼 ∂𝑀 (20)
𝑥 12
∂𝑥

Additional to the effects of the force to the current carrying structures it should also be noted
that this force has a vibrational nature (in the case of AC systems). Whereby the mechanical
frequency of this force is twice the excitation frequency (fmech=120Hz in case of a 60Hz AC
system). Thus also considerable noise emissions may be triggered due to the vibration of the
affected structures.

II) Clearance Guidelines for Air Core Reactors

The following points serve to summarize the basic installation clearance requirements
associated with air core reactors.

The clearance requirements are of three types:

 Electrical
 Ventilation
 Magnetic

Electrical Clearance:
As the normal air core reactor has exposed live parts at essentially all points on its outer
surface, provision must be made for electrical clearance from the reactor surface to nearby
grounded surfaces and to the surface of other reactors or other live parts in adjacent
phases or circuits. Standard electrical substation clearances to live parts are perfectly
adequate. No special precautions over and above normal substation practices are required.

Ventilation Clearance:
For a typical air core reactor which is mounted such that the cooling ducts are oriented in
the vertical direction, adequate provision must be made for the unimpeded entrance and
exit of cooling air at the bottom and the top of the cooling ducts respectively. In most
cases, the ventilation clearance will be less than the magnetic clearance requirements and
as such it is typically not a decisive factor in the installation arrangement (Refer to Figure
6).

Magnetic Clearances:
The magnetic clearance requirements for air core reactors arise as a consequence of the
interaction of their stray magnetic fields with conductive parts in the vicinity of the
reactor and the resulting currents which may be induced in those parts. The induced
currents are of two types and as such they give rise to two types of clearance requirements:
 Eddy Currents; which are induced in nearby conductive parts give rise to a minimum clearance
required to metallic parts which do not form closed loops. These clearances are typically referred
to as MC1 clearance (Refer to Figure 6).
 Circulating Currents; caused by the coil flux linking a closed electrically conducting loop.
Examples of such loops are those formed by concrete reinforcement rebar, nearby fencing, nearby
structural members in a building, inappropriately arranged grounding connections, or a
combination of the above. These clearances are typically referred to as MC2 Clearance (Refer to
Figure 6).
 MC2

MC1 Clearance to Metallic

Parts Not Forming
Closed Loops

MC2 Clearance to Metallic

Parts Forming Closed
Loops
Note: the designation of MC1 and MC2 distances
as OD/2 and OD from the reactor surface, as
shown here, are a rough approximation to the
values on manufacturers drawings which are based
on calculated field strengths rather than fixed
distances.

Figure 6 - Ventilation and Magnetic Clearances

A very common, though understandable, misconception is that these "magnetic
clearances" are applicable only to ferromagnetic materials. This is simply not the case. The
clearances
MC1 apply to all electrically conductive materials; nonmagnetic or ferromagnetic. It
Ventilation Clearance
is true that ferromagnetic materials will typically present even more severe problems,
particularly in respect of eddy currents, than non-magnetic materials such as aluminum or
copper, however non-magnetic materials may also have very substantial induced eddy
currents and, owing to their generally lower resistivity, even more serious closed loop
currents and short circuit forces when arranged in an inappropriate orientation and/or
location in the reactor's magnetic field. In difficult situations, low resistivity non-magnetic
stainless steel provides the best solution.

The above MC1 and MC2 clearance guidelines are normally provided on manufacturer’s
outline drawings. The manufacturer will typically arrive at theses clearances based upon
the distance at which the field strength has decayed to internal manufacturer specified
values. In approximate terms, the MC2 clearances are generally about double the MC1
clearances and correspond roughly to a distance of a full diameter from the reactor surface,
above and below and to the side of the reactor. MC1 clearances typically then, are in the
range of one half of a diameter from the reactor surface (refer to Figure 6).

A second common misconception is the belief that these magnetic clearance guidelines
are “absolutes”; scientifically calculated values that if respected will lead 100% of the time
to a problem free installation, and if not respected, will always result in problems. This is
not so, in either respect. As hopefully has and will become clear in this paper, the issue of
magnetic clearances is about both the coil magnetic field (magnitude and direction in the
area of interest) as well as about the size, orientation, conductivity and magnetic
permeability of the part or loop which may interact with the
 magnetic field. To perfectly summarize all of this in a very simple and generic “two
number practical clearance distance” (MC1, MC2) on a drawing is impossible, as although
the magnetic field of the reactor can be very accurately calculated by the manufacturer, the
other half of the equation - the very important details of the nearby metallic parts and/or
loops; usually are not known by the reactor designer. This leads to two important
conclusions:

 First, the magnetic clearance guidelines (MC1 and MC2) shown on manufacturer’s drawings are
to a significant extent based on manufacturer’s experience, extensive testing, and on “what
historically works”. They are intended to provide conservative figures that will lead to problem
free installations in most, if not all, circumstances. That said, for very good reasons, there is
guidance in the standards that also recommends, particularly in new installations, to simply avoid
closed loop formation wherever possible (see Annex E, Section E.4 of reference [1]).

 The converse of the above is also true, also as outlined in the same standard. In many instances
magnetic clearance distances which are significantly less than the standard MC1 and MC2 figures
will still result in a satisfactory installation; however such cases should be referred to the
manufacturer for analysis and advice based on the specific circumstances. Such analyses very
often result in being able to fit air core reactors into existing installation locations where the
initial MC1 and/or MC2 clearances on proposal drawings would suggest that they simply do not
fit into the available space. Further examples are shown in reference [7].

III) Practical Implications of the Theory

Terminal Losses and Terminal Heating

The admissible limits for the terminal temperature rise of reactors are given in Table 5 of
IEEE C57.16-2011 “ IEEE Standard Requirements, Terminology, and Test Code for Dry-
Type Air Core Series- Connected Reactors” Section 8.2.4. (The shunt reactor standard,
IEEE C57.21-2008, also references this clause.) A similar table is included in the IEC
reactor standard; IEC Std. 60076-6: 2007 "Power transformers – part 6: Reactors" Table
1. [1][2][3]

Table 5 of IEEE C57.16-2011:

Maximum Value
Temperature rise at
Terminal description
ambient air temperature
Connection, bolted or the equivalent Temperature [°C] not exceeding 40°C
[°C]
Bare-copper, bare-copper alloy ore bare-aluminum alloy
90 50
- in air
Silver-coated or nickel-coated
115 75
- in air
Tin-coated
105 65
- in air

Generally, the temperature rise of terminals of electrical equipment is a function of losses

due to the terminal resistance (including the contact resistance) and the throughput
current. Recommendations for adequate dimensioning of terminals depending on current
may be found in national standards, for example in NEMA CC1-2009. [4] In the case of
air-core reactors however, the eddy losses of the terminals and the accompanying
connectors due to the stray field of the reactor, have to be considered
as well. An intended mitigation of the temperature rise by utilization of higher-ampere-
capacity connectors may actually be counterproductive and may rather increase the
heating of the terminals.

To mitigate eddy current heating of the terminal and the connector it is important to have
geometries that are “streamlined” to the magnetic field, or in other words, which do not
provide a large frontal area to the magnetic field. This is achieved by providing terminals
with rectangular cross section of limited thickness (e.g. 3/8 in (10 mm)) normal to the
radial and axial field direction; like an extension of the spider arm. The required cross
sectional area of the terminal is provided by increasing its width in vertical direction. The
reasons for this are evident when the theory presented in section I.1 is considered (eddy
losses are strongly proportional to the dimension normal to the field direction) and noting
the strongly radial nature of the field in the terminal vicinity, as can be seen in Figure 2
and Figure 7 .
The same concept is applicable to the connector bolted to the terminal. The connector
should have large contact surface area, but should not present a large frontal area to the
radial or axial magnetic field and where multiple cables are involved, should not create a
flux linking loop between the parallel cables.

The factors influencing the eddy current heating of the terminal by the connector attached
to the terminal are demonstrated by a practical example of a high current / high Mvar
power, two coil stacked thyristor controlled shunt reactor (TCR):

Reactor data (single coil):

 winding length 47 inches (1200 mm)
 mean winding diameter 110 inches (2790mm)
 outside diameter 128 inches (3275 mm)
 current (50 Hz) 3160 A rms
 Number of Turns 58.5 (x2)
 Inductance 19.01mH
 MVAR/coil (50 Hz) 59.6
 shape of terminal rectangular, 4 inches x 6 1/4 inch (100 x 160 mm),
radial x vertical)
 cross section of terminal 2.48 inch2 (1600 mm2)
 material aluminum alloy conductivity ~ 28Sm/mm²

The direction and magnitude of the field to which the terminal and the connector are
exposed is shown by the field plot in Figure 7. The flux density lines indicate the direction
of the magnetic field, and the colors, its magnitude in Tesla (rms value) for a current of
3160 A rms.
Figure 7 - Magnetic field plot (contour plot and field line plot) of terminal plus connector
arrangement
As the figure shows, the field drops significantly with distance from the winding so that
the field strength at a distance of about 8 inches (200 mm) from the winding is reduced to
less than 50 % of the field in the immediate vicinity of the winding.

Referring to Figure 8, the connector flag is represented by a plate of 11.8 x 6.3 inches
(300 x 160 mm) (radial x vertical) which is firmly attached to the terminal. The contact
area between flag and terminal is 3.9 x 6.3 inch (100 x 160 mm) (radial x vertical). The
density of eddy currents induced in the connector flag are illustrated by the color scale.
(Note: through current is not considered). As can be seen in the figure, the eddy current
density is concentrated at the end of the flag which is closest to the winding.

Figure 8 - Eddy current density of a connector flag attached to the reactor terminal

Figure 9 shows both the through current I²R losses and the eddy losses of the connector
flag, depending on its thickness. The losses arising from contact resistance are disregarded.
Calculation of eddy losses is done by 3D FE methods.

Figure 9 - Connector losses vs. thickness

The connector losses have a minimum at about 1/2 inch (12 mm) thickness. A similar loss
vs. thickness characteristic is found at 60 Hz with the losses about 20 % higher than those
at 50 Hz.

The losses in the connector plate (and in the terminal) will increase when the terminal
and the attached connector are orientated in horizontal position, due to the increase in
losses contributed by the axial component of the field, while retaining similar losses
resulting from the radial field component.
 Figure 10 - Connector losses at in-plane vs. out-of-plane arrangement
Besides the general question of the type, size, and physical construction of connectors to
be used with air core reactors, another question which often arises relates to their various
possible physical arrangements. The following example, illustrated with actual
measurements of terminal temperature, shows again the same principle at work;
minimizing the frontal area of the connection arrangement presented normally to the
predominant direction of the magnetic field of the reactor and avoiding the creation of flux
linking loops.

The results in Table 1 are showing the measured terminal temperatures resulting from
arranging the connectors axially, one above the other on one side of the terminal, as
compared to arranging the same connectors on opposite sides of the terminal. Clearly the
axial arrangement on one side of the terminal is the better arrangement, with the “opposite
sides configuration” resulting in a 25% higher temperature rise. These results are
completely as one would expect when considering the frontal area the two different
arrangements present to the magnetic field of the reactor, as well as the formation of a
small flux linking loop by the “opposite sides configuration”; all as outlined in earlier
sections of this paper.

Figure 11 - Axial (left) vs “Opposite Sides” (right) configuration of

connection leads Table 1- Measured Temperature Rise on Axially vs.

“Opposite Sides” Arranged Cables

Connection Arrangement Cable Barrel Terminal

Temperature Rise Temperature Rise Temperature Rise
Axial Cables 98 °C 98 °C 98 °C
“Opposite Sides” Cables 123 °C 123 °C 123 °C

Notes: Test Current: 4000 Amp, Cables: 2x1600 MCM each 6 feet in length, 4 hole terminal palm compression
The above example illustrates the issues resulting from a large frontal area to the field and
the formation of closed loops where cable connections are not arranged in an
appropriate manner. The same comments apply in the event of attempting to use bus
tube to make connection to air core reactors of significant power rating. Owing to the
large frontal area which bus tube presents to the field, it is generally problematic to
bring bus tube right up to the reactor terminal, and especially so if it is oriented
tangentially rather than radially to the reactor (refer also to the case study at the end of
this paper).

Recommended Connection Arrangement for Air Core Reactors

Based on all of the foregoing, a connection arrangement as shown in Figure 12 and Figure
13 is recommended for sizeable Mvar ratings and/or reactors with significant short
circuit current ratings. For small kvar ratings and low short circuit currents, the principles
outlined here become less critical, but still represent good practice.

Figure 12 - Connector Design Figure 13 - Connection Arrangement

 The connector flag (as well as the terminal) should be arranged vertically.
 The height of the connector flag should preferably be the same as the height of the terminal and all
holes of terminal should be used for bolting; use stainless steel bolts with Belleville spring
washers on both sides.
 The thickness of the connector flag should not exceed approximately 1/2 inch or 12mm.
 The material of the connector may be either aluminum alloy or copper (in case of copper to
aluminum, a bi-metal plate should be inserted). Nickel plating of the contact surface area may be
considered, in order to allow higher terminal temperature (115°C instead of 90°C).
 The end of the customer connector flag may be configured as a crimped connection (Figure 14) or
prepared for a welded connection as shown in Figure 15 or a clamped connector arrangement as
shown in Figure 16. A minimum distance of this connection point of approximately 8 inches (or
200 mm) from the winding surface is recommended in order that it be located in a reduced field
area.
 Figure 14 - crimped Figure 15 - welded Figure 16 - clamped

 The connecting leads in the vicinity of the reactor shall be placed in a radial direction and
perpendicular to coil vertical axis, to minimize the heating effect and the magnetic force.
 In cases where parallel cables are utilized they should be axially aligned in order to avoid the
induction of closed loop currents between the cables by the axial magnetic field of the reactor.
Refer to Figure 12.
 Connection leads should be provided with sufficient sag so as to provide adequate mechanical
decoupling of the reactor terminals from connecting bus-work.
 For non-plated terminals and connectors, the contact areas must be abraded lightly by using 3M
Scotch-Brite pads which have been saturated with contact grease (e.g. Penetrox-A). Guidance in
this regard is also given in the Trench instruction manual.

Ampacity Selection for Connecting Cables and Bus-bars:

The purpose of this section is not to describe the guidelines for choosing cable ampacities
in the substation. Rather, the focus here is on the last 10 feet or so of cable or bus-bar that
makes the connection to the coil terminals. The desire is not only to have sufficient
conductor cross section to handle the current, but also to have sufficient heat sink capacity
from the connecting cable or bus-bar to aid in maintaining a cool terminal temperature.

The connector geometry and configurations were addressed earlier. Here, we address
strictly the ampacity selection of the cable or bus-bar connected to the reactor. For cable
connections, in order to ensure adequate heat sinking of the terminal connection, the total
cable ampacity (in MCM) connected to a coil terminal should be a minimum of twice the
rated current in amperes. For a bus-bar connection, a current density of not greater than
500 Amps/in2 (0.775 amps/mm2 should be used.

Example: (based on 2000 Amps rated current)

For a cable connection: Total Cable MCM = 2000A x 2 = 4000 MCM (2027 mm2)

For a bus-bar connection: Total bus-bar cross sectional area = 2000 A / 500 A/in2 = 4 in2 (2580
mm2)

III. 2) Circulating Currents in Horizontal Plane Closed loops Such as Foundation Reinforcement

By necessity, concrete foundations and floors very often use steel rebar reinforcement to
satisfy the mechanical loading demands of the foundation or floor. The rebar is typically
tied together with steel wire when assembling the rectangular rebar grid. This creates many
small rectangular short circuited loops which will experience circulating currents, losses
and short circuit current forces. The
 expansion and contraction of the rebar during load cycling of the reactor current can also
cause cracks and degradation of the concrete.

Solutions:
(a) When the foundation or floor already exists, the coil must be installed at MC2 height above the
foundation or floor to minimize the magnetic field effects.
(b) If sufficient height is not available to allow satisfying the MC2 clearance, and also providing the
power rating of the reactor is not too high, aluminum magnetic shields can be placed on the
foundation or floor to keep the magnetic field from penetrating to the rebar. This shield will still
have losses and forces on it but will save the foundation or floor integrity. Note: Such shielding
measures should only be done in consultation with the reactor manufacturer.
(c) If the foundation or floor is not yet constructed, applying short tubes of rubber hose at the rebar
crossover points to isolate them (and also avoiding making electrical connection of the bars via
the steel wires) will eliminate the closed loops. Alternatively, fiber glass reinforcing mesh can be
used in lieu of steel rebar.

Figure 17 - reinforcement using rubber Figure 18 – fiber glass mesh

hose insulation

III. 3) Circulating Currents in Vertical Plane Closed loops Such as Fencing, Large Beams and Columns

In view of the fact that the external surfaces of air core reactors are live parts, common
practices to ensure personnel safety are to either fence the area around the reactors or to
raise the reactors on support structures such that the height of the base of the insulators is
a minimum of 8'6" (2600mm) above grade level. With either of these approaches, care
must be taken in respect of both grounding and, for reactors of sizeable power rating, the
formation of closed flux linking loops near to the coil.

Issues with Fencing:

When traditional metallic fencing is arranged around air core reactors, there are
multiple opportunities to create problematic loops:

 horizontal plane loops formed by the posts and the horizontal crossbars of the fence encircling the
complete reactor arrangement.
 vertical plane loops in individual or multiple fence sections formed by the fence posts and
connecting crossbar(s) or stringer mesh support wires at the top and bottom of the fence
 loops formed by a combination of the vertical plane of a fence section and closing of the loop by
multiple connections to the ground grid.
 Loops created when adjacent sections of fence which are separately grounded become electrically
connected and form a loop through the latch and lock of a closed gate
In most cases the main problems which arise from the presence of such flux linking loops are:
 The existence of points of high resistance or intermittent contact in the path of the loop
current; giving rise to extreme local heating or arcing at the high resistance contact point (eg.
latch and lock of a gate)
 inducing large currents, sometimes hundreds of amps, in the ground grid

The challenge in addressing these issues is to simultaneously avoid creating loops which
are problematic while at the same time satisfying the utility, NESC and IEEE guidelines in
terms of fence grounding, and step and touch potentials. [Refer to references 12 and 13].

Solutions:

First and foremost, whatever solution is employed, it must be ensured to be a safe one and
meet the intent and requirements of the NESC and IEEE 80.

Given the wide variation in situations and fence design specifics, only some general
pointers from the perspective of managing the reactor magnetic field are possible in this
paper. The user/station design engineer must ensure that the result is satisfactory from the
grounding perspective:

 involve the reactor manufacturer in the project at the earliest possible stage. The extent to which
fence loops may be problematic is dependent on the specific reactor ratings and planned physical
arrangement. (Note that reactors of moderate size may present little or no problem.) The
manufacturer should be able to provide helpful guidance in specific cases.

 use double posts at one corner of the fence, thereby avoiding a large horizontal plane loop
 to ensure a safe installation in respect of step and touch potentials, fences are typically connected
to ground at intervals ranging from 5 to 15m along the length of the fence. This can result in large
vertical plane loops involving the ground grid. A possible solution to the involvement of the
ground grid in such loops is to break the fence into multiple sections of suitable length using
multiple posts, and then ground each section at only one location. In cases where it is also
necessary to eliminate loops in the individual fence sections, these can be avoided by isolating the
stringer wire or crossbar at a single point to break the loop. (Refer to Figure 19)
 the problem of the latch and lock of a gate being involved in a high current loop can be avoided in
a few different ways; one of these is to provide a low resistance alternate path for the loop current
through a suitably arranged electrically parallel copper conductor of significant ampacity
Figure 19 - Sectionalizing fencing to allow multiple ground connections
It should also be mentioned that there are very often cases where air core reactors are
raised on support structures to provide, for example, a minimum 8’6” (2600mm) clearance
to live parts, and the reactors are sometimes still fenced as well. In such instances,
especially where there are also long insulators, closed loops in fences which are located
well within MC2 in the lateral direction may not link significant flux due to the coil
elevation above grade level. In such situations, a brief consultation with the reactor
manufacturer may simplify the installation and reduce the necessary size of the fenced
area and/or the need to avoid vertical plane loops in the fence. In addition, in the case of
transmission voltage class air core reactors, the electrical strike distance requirements from
the live reactor surface to an adjacent fence will most often exceed the MC2 distance;
thereby making the electrical clearance the decisive clearance distance parameter.

Of course, an alternative solution is to use non-metallic

fencing. Issues with Large Beams and Columns:

In cases where air core reactors are located in the vicinity of interconnected columns and
beams, similar closed loop and eddy current issues arise as to those with fencing. In this
case however, the beams and columns are often made of structural steel. This exacerbates
the problem as the ferromagnetic nature of the material results in a tendency for portions
of the structure to concentrate the coil flux, leading to even more severe eddy current
heating problems at certain locations. Also as a result of the ferromagnetic material, the
closed loop currents and eddy currents will flow in a thin layer on the surface of the
material resulting in relatively higher resistance values and more severe heating under
most circumstances. Even greater problems may arise when sizeable reactors are placed
in the vicinity of building walls and ceilings which are constructed of metal cladding
which is bolted or riveted to the structural members. In these circumstances it is not
unusual to find that some of the bolted or riveted connections create high resistance
contact points in the path of closed loop currents; resulting in very severe local heating at
the high resistance contact points. The severity of such local heating can be sufficient to
cause bolted or riveted connections to glow red hot and even potentially lead to building
fires if there is combustible material in contact with the hot parts.

Solutions:

The situations described above are some of the most difficult magnetic clearance
problems to deal with, particularly with existing structures. The following techniques can
often provide solutions:

 involve the reactor manufacturer in the project at the earliest possible stage; a full or partial
magnetic clearance analysis will usually provide valuable guidance (over and above the simple
MC1 and MC2 distances) as to the specific issues with a planned installation, along with steps to
take to avoid problems
 wherever possible, avoid the formation of flux linking loops
 as with fencing, take care to avoid the formation of loops involving the ground grid; which can be
created by multiple point grounding of the structure
 avoid connecting metallic fences directly to metallic structures such as buildings by terminating
 the fence at a post which is located a few inches from the building
 observe the magnetic clearance guidelines
 with the support of the reactor manufacturer, in some circumstances problems can be solved
through a magnetic clearance analysis and the judicious use of shielding techniques
III. 4) Circulating Currents in Elevating Support Stands

Personnel protection can be achieved by using elevating support stands to keep live parts
out of reach of substation personnel. This commonly is done instead of fencing within the
station. Such elevating pedestals may be provided by the reactor supplier in the form of
fiberglass pedestals or individual aluminum, steel, or stainless steel columns under each
insulator. Often however, utilities construct lattice type structures, usually made of steel,
which form closed loops under the coil. For small reactors, such loops may not present
difficulties, but for larger Mvar coils they may well lead to problems.

Solutions:

 Steel parts can be isolated from each other to avoid closed loops by using isolated bolting
techniques, however this is not simple, and always presents concerns about whether the isolated
bolting will be correctly installed in the field
 Maintain at least MC2 distance to the support structure parts if they form closed loops
 For moderate size reactors; utilize a single post mounting pole with mounting arms extending
radially outwards from the center to support the insulators, instead of a square frame structure.
(Refer to Figure 20)
 Ground the bases of the structure at a single point only; using star point or daisy-chain connections.
(Refer to Figure 21)

Figure 20 - Single Post Mounting of Reactors Figure 21 - Daisy Chain and Star-Point single
point
grounding
IV) Case
Study: Thyristor Controlled Shunt Reactors (TCR’s) with an
Inappropriate Connection Arrangement:

The following case study illustrates an actual case of an inappropriate connection

arrangement for TCR’s in a Static VAR Compensator (SVC). The connection issue was
identified during the installation phase and tests were subsequently done on a mockup
arrangement at the factory to demonstrate the expected excessive in-service terminal
temperatures which would result if the installation arrangement was not modified.

The subject SVC is comprised of three TC reactor banks: TCR1, TCR2 and TCR3. Each
phase of the TCRs consists of two stacked coils (bottom and top coil) with porcelain
insulators between the coils. Three such stacks form a 3-phase reactor bank.

The reactances per phase (at 60 Hz) and the rated currents and Mvar ratings of the

reactors are: TCR1: 7.955  / 3283 A … 85.7 Mvar/phase

TCR2 and TCR3: 15.91  / 1928 A … 59.1 Mvar/phase

The arrangement of the TC reactors of each bank, TCR1, TCR2 and TCR3 is illustrated in
Figure 22 and Figure 23. All reactors are mounted as 2 coils stacked per phase, phases

side-by-side with a clearance of approx. 2.5 m (8.2 feet) between phases.

Figure 22 - Simplified sketch of the TC reactor arrangement (top view)

Figure 23 - Reactor bank TCR2

The electrical connection of the TC reactors from their terminals to the bus-bars is made
by aluminum cables. Each cable consists of about 91 aluminum strands of 3.74 mm
diameter resulting in a cable diameter of 41.1 mm. The total cross section is 1000 mm²,
the DC resistance is 28.9m/km and the
ampacity is 1305 A. The leads of TCR1 consist of 3 parallel cables and those for
TCR2/TCR3 consist of 2 parallel cables (Figure 24, Figure 25)

Figure 24 - TCR1, cable connection Figure 25 - TCR2 / TCR3, cable connection

arrangement arrangement (loops shown in red
(loop shown in red)
The cables are fixed by angled connectors to the reactor terminals so that the cables are
orientated tangentially to the coil (refer to Figure 24 and Figure 26).

This tangential alignment is in contrast to the recommendations outlined in this paper and
also as stated in the Trench reactor installation and operation manual [8], which requires
that the connection leads to be aligned in the radial direction to minimize heating effects
in the leads due to eddy currents.

Figure 26 - electrical connection showing tangential cable arrangement

The remote ends of the leads are fixed to the bus-bars resulting in about 5 m (16.4 feet)
length of the leads from the coil terminal to the bus-bars. The parallel cables of the leads
are held together by an aluminum fastener placed at a distance of about 1.5 m (4.9 feet)
from the reactor terminal.

As has been outlined in earlier parts of this paper, air core reactors produce a magnetic
field which fringes out from the winding ends. For reactors of significant MVAR rating,
such as those shown in this case study, the fringing fields are of substantial magnitude.
The radial component of this magnetic field which is directed perpendicular to the plane
of the tangential loop created by the cables of the connection leads between the reactor
terminal and the fastener (see Figs. 2, 3, 4, 4a) will, in this case, induce a current in the
loop which is significantly larger than the normal rated current flowing through the leads;
and thereby create an unacceptable terminal heating situation.

These results can be calculated, however for this specific case, it was decided to
construct a mock-up of the in-field situation to demonstrate the expected outcomes
experimentally and arrive at actual measured currents and temperatures.
 B I1

B
B B
BB
B
B

I2

Figure 27 - radial field linking tangentially arranged conductor loops and inducing a
circulating current (loop enlarged for illustration purposes)

Mock-up test Arrangement:

A mock-up test arrangement was prepared. It consisted of very similar aluminum cables,
connector and fastener and a test coil which created a very similar magnetic field to the
on-site reactor. During the heat run test, the cable and terminal temperature-rise as well as
the induced closed loop current in the cables were measured.

The overall test arrangement was as shown in Figure 28 and Figure 29. The cable mock-
up was placed in the magnetic field of the test reactor in such a way that the magnetic flux
density in the reactor's radial direction was equivalent to the original setup at the SVC
site, having a value of 30.5 mT rms (the same as the on-site TCR1 at rated current). The
length of the mock-up loop was 1.5 m (4.9 feet), the center-center distance between
adjacent cables was 100 mm (3.9 inches). These critical parameters are virtually identical
to the on-site figures.

Figure 28 - test setup with connector cables arranged tangentially to reactor surface
 thermocouples

Figure 29 - model representing the on-site cable arrangement

Measurement Results:
 measured induced loop current in cables at radial magnetic flux density 30.5 mTRMS (note that this
is the induced closed loop current only, and does not include through current)
 top cable: 4523 ARMS
 middle cable: 94 ARMS
 bottom cable: 4406 ARMS
 temperature rise in cables after 30min heat run
 top cable: 209 K
 middle cable: 25 K
 bottom cable: 225 K

As can be seen from the results above, the induced current in the cable loops was on the
order of 4500 Amps. As expected, the middle cable carried very little net induced current
due to the cancelling effect of the upper loop current and lower loop currents flowing in
opposing directions in the center cable. The temperatures in the upper and lower cables
were approximately 250 degrees C after allowing for a 25 degree C ambient. Clearly a
modified arrangement was necessary.

For comparison purposes, an alternative arrangement utilizing the typically recommended

radial connection to the coil was also tested using the same test coil and field strengths.

Mock-up test with a radial cable arrangement:

Figure 30 - connector cables arranged radially to reactor surface

Measurement Results:
 measured current in cables at radial magnetic flux density 30.5 mTRMS
 top cable: 23.5 ARMS
 middle cable: 5.8 ARMS
 bottom cable: 35.3 ARMS
  temperature rise in cables after 90min heat run
 top cable: 19 K
 middle cable: 16 K
 bottom cable: 19 K
With the radial arrangement, the induced currents and temperatures were, as expected, reduced to
negligible values. (The small induced currents which were observed were due to the cables not being
arranged exactly one above the other throughout their length, and therefore the resulting small loop linked
some of the axial coil field in the region.)

VI) Conclusion:

For those who are unfamiliar with the peculiarities of air core reactors; applying them, and installing them
can initially seem to be a confusing and difficult task. However, to a significant extent this is more a
matter of familiarity than fundamental difficulty. With a basic understanding that results from applying
simple electromagnetic concepts as are outlined in this paper, the principles become clear. With this
understanding, it can be seen that applying reactors, making reliable connections, implementing
appropriate grounding, and dealing with their stray magnetic fields are all relatively straight forward
issues that can most often be easily solved. In the more complex cases, the reactor supplier should be able
to provide support in finding a clear way forward.
 EXPERIMENT NO – 3
Designing the power distribution system for a small township
INTRODUCTION
Electricity is the most convenient and useful form of energy. Without it the present social
infrastructure cannot be feasible. The increasing per capital consumption of electricity
throughout the world reflects a growing standard of living of people (Pablo, 2000) and the
optimum utilization by society of this form of energy can be ensured by effective supply and
distribution system. Distribution system differs from transmission system in several ways. Apart
from voltage magnitude, the number of branches and source is much higher in distribution
system and the general structure topology is different. Transmission is normality implied, the
bulk transfers of power by high voltage between main load centers. Distribution on the other
hand is mainly concerned with the conveyance of power of consumer by means of lower voltage
network.
Due to expansion in the use of electricity, the demand on the distribution become greater
and more complex.Therefore, the distribution network are designed to be able to carry the load
imposed upon it without causing excessive heating in the consumers conductor and consequent
damages to the insulator. The voltage drop through the network must be kept to minimum so as
to maintain the voltage at the customer terminal within specified units (i.e 6% of the nominal
value) whatever the loading conditions [NKPA
electricity distribution manual 1977]. Electrical wiring is one of the major parts of building construction.
Electrical wiring is the connection of electrical accessories such as:sockets, lamp holder, distribution
boards, fuses or cutouts, ceiling rose, etc. with electrical wire or cable of the appropriate rating.
According to Electrical Engineering Portal (EEP), wires used in electrical wiring are normally coded
with colour codes for easy identification. In electrical wiring, current entered a circuit through the hot
(live) wire (usually red color) and returned along neutral wire (Uguru, and Obukoeroro, 2020). An
electrical circuit is a continuous loop, which carries electricity from the mains (e.g. distribution line),
throughout the house, then returns it back to the mains. Switches and other electrical appliances are
usually connected to a single electrical circuit (EEP, 2020). Electrical wiring are done by trained
professionals, but in some countries like Nigeria, due to lack of skill workforce, people with informal
education, commonly called “engineers” within the locality, are mainly employed to carry out the
electrical wiring of buildings. The utilization of substandard electrical materials or wrong connection of
the circuit is very dangerous, as it can lead to electrical fires or breakdown of the system. Electrical fires
are fires comprising the potential energization of electrical appliances and accessories.Electrical fires
are mainly caused by either over-loading of the circuit or short-circuiting the system (Fair, 2014). Safely
of human lives and materials in residential, administrative or commercial buildings is of high priority to
every country. Electrical fire has become a serious threat to the life and materials in residential and
commercial buildings (Madueme, 1997).
The paper is aimed at designing an electrical distribution network for the newly completed 500
housing estate. Toward the development of the area, power demand is necessary. Consideration is given
to the future expansion of the area, also the maximum load demand of the Varian load centers and the
effective maximum demand (i.e the total maximum load demands required by the consumers) are
conserved for successful.
Methodology
This chapter deals with how the electrical distribution network for 500 housing unit was
designed, the distribution network was also designed to be overhead type which technically suit the
area.The type of line support and their accessories are also designed to the technical standard.
Distribution substation is also designed to receive energy from a higher voltage system, convert into a
form suitable for local distribution.
LINE SUPPORTS
For the purpose of this project, a steel reinforced concrete pole was designed, the poles were
designed to be 8.5m length for the low voltages and 10m length for the high voltage line.
The span length for the low voltage lines was designed to be 40m while that of high voltage line
was designed to be 50m, this is to avoid the difficulties of terrains, urban development and natural
hazards.
The reinforced concrete pole have the advantage of longer life, shattering tendency when hit by
vehicles and can be used in areas that have high humidity
 ACCESSORIES OF OVERHEAD LINES DESIGNED FOR THESE PROJECT
ARE:
i. CROSS-ARM
A hot dip galvanized cross - arms are designed for the project, the length of the cross
- arm should be 1.63m long and, the bolts and nuts of 5mc and 8mc respectively should be used.
The hot dip galvanized has the following advantages:
a. It has longer life.
b. They cannot be attack by termites
c. They are stronger than the wood types.
ii. INSULATORS
For the insulation of the distribution lines, use shall be made of porcelain insulators as specified
in B.S. 137.
The Disc type insulator of approximately 254mm in diameter, 6.3kg In weight and mechanical
failing load of 10.6KN should be used for the high voltage lines.
Also for the low - voltages a single groove type shackle insulator of 76mm in diameter, 0.4kg in
weight with a mechanical failing land of 19KIM should be used.
iii. CONDUCTORS
Conductors used are aluminum conductors (AAC), conductors shall be 50mm 2 and 100mm2 in
size for the high voltage and the low voltage respectively. Base conductors shall be used in normal
conditions, conductors shall be hard-drawn aluminum twisted wires made of aluminum for electric
purposes. However, it major advantage is that it is cheaper than copper.
Design of Electrical Installation for Buildings
The total load was used to determine the actual size of the transformer required for the area.
According to IEE Regulation A30 - 36, that the cables supplying lighting load need only be rated for
50% of the full load current.
The diversity factor is taken into consideration due to the fact that the likely-hood of all the
domestic installation been ON at the same time is remote, hence it reduces imbalance in the lighting
loads. (George G. 1975).
Finally, the diversity factors suggested in the I.E.E regulations A27 & A28 are:
1. Lighting circuit is 65%
2. 13A socket outlet is 65%
3. 15A socket outlet is 80%
 4. Electric Cooker is 65%
The breakdowns of the load demand are as follows:-
The total lumens is given by
θ = E x A/n x p (1)
Where E = is the illumination in Im/m2
A = is the Area of working plane to be illuminated in m2
p = Maintenance factor n
= utilization factor.
Using the standard table of installation (T. G. Francis 5th edition).
P = 0.8 for bedrooms and living room. n
= 0.5 for bed rooms and living room.
E = 40lm/m2 for Bedrooms and living room. Lumen/watt for (4 x 4)m2 size
= 10lm/w.
Now, for Bedrooms;
a) Two Bedrooms Flat: There are 350 two bedroom flats, the load demand of single 2-
bedroom was calculated and multiplied by 350 to give the total load demand of the flats.
i. For the bedroom with size (3.5 x 3.5)m2, the following standard were obtained using
electrical installation IEE standard table (M. A. Laughton, 2003). As stated in (A) above.

P = 0.7
n = 0.5
E = 40lm/m2

E ×A
θ = n×p
40×(3.5×3.5) = 1400𝑙𝑚
= 0.5×0.7

The total watt required = 1400lm/12lm/w = 116.7w

Therefore, 116.7/60 = 1.9 = 2 lamps of 60w for each bedroom
ii. For the living room with size (4.5 x 4.5)m2, it has the following data.
Lumen/watt = 13lm/w, n = 0.5, E = 40lm/m2, p=0.7

E ×A
θ = n×p 40×(4.5×4.5) = 2314𝑙𝑚
= 0.5×0.8

The total wattage required = 2314.3lm/13lm/w = 178w

Therefore, 178/60 = 3 lamps of 60w.
iii. For the kitchen, toilet, Veranda and security light, eight numbers of lamps was designed.
That is, totaling to 15 lamps.
iv. Six number of 13A socket, each of 100w was designed.
v. Two number of ISA socket, each of 1500w was designed.
vi. Three number of ceiling fan, each of 80w was designed.
vii. One number of electric cooker with 1200w rating.
viii. One number of water heaters with 8000w rating.

b) One Bedroom Flat: there are 150 one bedroom flats, the load demand of single one
bedroom was calculated and multiplied by 150 to give the total load demand of the all
flats.
i. For the bedroom with size (3.5 x 3.5)m2, the total number of wattage required was the
same with that of two bedroom, since they have same size, hence the same illumination
was required.
ii. Therefore, two lamps of 60w each was designed.
ii. For the living room; with size (4.2 x 4.2)m2, the following data's were
extracted from the standard installation table.
Lumen/watt = 17im/w, n = 0.5, E = 40lm/m2, P = 0.8, A
= (4.2 x 4.2)m2.
From:

E ×A
θ = n×p 40×(14.2×4.3) = 1764𝑙𝑚
= 0.8×0.5

The total wattage required = 1764lm/17lm/w = 103.76w Therefore,

103.76w/60 = 3 lamps.
Hence, the living room was designed with two lamps of 60w each for it illumination.
iii. For the kitchen, toilet, veranda and security lights, the number of lamps designed was
8 lamps of 60w each.
iv. Five number of 13A socket outlet, each of 100w was designed.
v. One number of 15A socket outlets, each of 1500w was designed.
vi. Two number of ceiling fan, each of 80w was designed.
vii. One number of electric cooker, rating 8000w was designed.
 viii. One number of water heater ratings.
Design of Distribution Transformer
The distribution transformer were design based on the load demand of the area and the breakdown
of the load demand are presented below.

Table 1: Maximum required for single two bedrooms.

Diversity
Load Wattage (W) Total (Kw) Maximum load
factors (%)
Demand

(KW)
Lightings 15 x60 0.9 65 0,59

13A socket outlets 6 x 100 0.6 65 0.3

15A socket outlets 2 x 1500 3.0 80 2.4

Ceiling Fan 3 x 80 0.24 - 0.24

Cooker 1 x 8000 8.0 65 5.2

Water heater 1 x 1200 1.2 - 1.2

Maximum load = 9.9kw

Therefore, the total maximum load demand for the 350 two bedroom flats will be; 9.9 x 626
= 3465kw.
Sub-total (1): 3465kw

Table 2: Maximum required for single one bedrooms.

Load Wattage (W) Total (Kw) Diversit Maximum load

y factors demand (KW)
(%)
Lightings 13 x 60 0.78 65 0.51
13A socket 5 x 100 0.5 65 0.325
outlets
 ISA socket
1 x 1500 1.5 80 1.2
outlets

Ceiling Fan 2 x80 0.16 - 0.16

Cooker 1 x 8000 8.0 65 5.2
Water heater 1 x 1200 1.2 - 1.2

Maximum demand = 8.595kw.

Therefore, the total maximum load demand for the 150 one bedroom flats will be;
8.595 x 150 = 1288.5kW
Sub-total (2) =1288.5kW

c) Load Requirement of the Other Building

v. For the primary school, the required maximum load is 7.5kw i.e subtotal (3) =
7.5kw.
vi. For the Boreholes, the required maximum load is 180kw. i.e subtotal (4) =
180kw.
vii. For the street light the required maximum load is 15kw. i.e sub-total (5) = 15kw.
vii. For the fire-service office, the required maximum demand is 2.5kw.sub-total (6) =
2.5kw
Therefore, the total maximum demand of the whole units will be total maximum demand
of all load centres, i.e Total maximum demand of sub (1) + sub (2) + Sub (3) + Sub (4) + Sub (5)
+ Sub (6) = 3465kw + 1288.5kw + 7.5kw + 180kw + 15kw + 2.5kw = 11220.47kw.
Grand total = 4958.5kw.
Now, power = 3VI Cos θ where θ is the power factor and is taken as 0.85.
Therefore, IV = Total Power in kw (2)
3θ
VI = 4958.5/3×0.85 = 1944.51kw
Where VI, is the KVA which determines the sizes of transformers to be used.
The total KVA = 1944.51kw. Now, it is designed to use 4 x 500KVA transformers.
 TRANSFORMER LOCATION AND ITS LOADING CONDITION.
Distribution transformers are output rated, then can deliver their rated KVA without exceeding
temperature rise limits when following condition are applied (ABB, 1995).
a. The secondary voltage should not exceed 105% of rating.
b. The load factor should be > 80%.
c. The frequency should not be > 95% of rating.
Transformers are to he located at the centre of load so as to minimized the looses and
maintained quality supply to a consumer.
Each transformer is designed to carry 440.0184 KVA, therefore it should be placed at the load
centre even though the loads are not evenly distributed.
SUBSTATION
i. TRANSFORMER
A step-down transformers of ground mounted outdoor type was designed. The high voltage sides
and low voltage side of the transformers shall be fitted with cable boxes in accordance to B.S 2562 part
equipment.
They shall allow connection of 50mm2 paper insulated copper cables on high voltage side and
150mm2 PVC insulated copper cables on low voltage side.
The Cable size was designed considering the rating of the transformer. According to IEE
regulation 7M, 50mm2 copper cables should be connected on the high voltage side and 150mm2 copper
cables should be connected on the low voltage side.
ii. LIGHTING ARRESTERS
For each transformer, there lighting arresters shall be installed in order to protect the system
from high voltage surge due to lighting.
They should be attached to the high voltage side of this transformer and these arrangement will
send any over voltage which falls on the system due to lighting to the ground directly.
iii. DROPOUT FUSES
The dropout fuses shall be of cross arm mounted type with such construction as shall allow
opening and closing operation of the contact safely by an operating rod. Its construction shall be such
that after this operation of the fuses shall construct as shall allow opening and closing operating of
the contract safely by an operating after the operation of the fuses, the primary and secondary will be
disconnected from each other and complete insulation will be maintained. Fuse intended for the
dropout fuse switch shall be mounted on the switch and shall meet the rating capacity of
100MVA in the case of short circuit failing on the transformer side.
 iv. EARTHING:
According to (Anthony J., 2006), the neutral conductors of low voltage shall be earthed at the
supply point and each and every terminal as well as at interval of 250m along the distribution line
route.The ear thing shall be covered copper conductor and it cross-section shall be less than 25mm
earthing rods shall be used to serve as earthing electrode, each ear thing rod 2.5cm in diameter and 1.8m
in length.
v. FEEDER PILLAR REQUIRED FOR THE LOW VOLTAGE LINE
Four feeders shall be provided for the four low voltage line. The feeder pillars should be water
proved and installed. Each pillar should contain:
i. 3-fuse set, 3-phases and a neutral for overhead line feeders,
ii. 1-fuse set, 3-phases and a neutral for input cable from transformer.
According to (Frigsby L. L. 2001), the incoming and outgoing cable should allow connection to
150mm copper cables on the transformer side and 70mm copper cable on the side feeder side.
Therefore, each transformer shall be provided with it feeder pillar.
CONSTRUCTION OF SUBSTATION
According to (NEPA Manual, 1977), the foundation which the transformer is to be installed shall
be composed of an upper concrete base 20 to 25cm in thickness and wider than the transformer bottom
by 10cm, reinforced by the steel wire 10mm in diameter placed cross-wise at a depth of 5mm below the
surface.
A lower concrete base wider than the upper one by 10cm shall be constructed, there under a
layer of stones of 150 to 200mm in diameter, 30cm thickness shall also be placed. Substation shall be
constructed away from any such places;
i. Along heavy traffic road where motor cars are likely to collide.
ii. Where the soil is soft,
iii. Where people are frequent,
iv. As is likely to be flooded.

PROTECTION REQUIREMENTS
(i) According to I.E.E Regulation A8-A10, every consumers installation supplied from a
external source shall be adequately controlled by protection equipment accessible to
consumer, the protection equipment should incorporate:
i. Means of Isolation
ii. Means of excess current protection.
Circuit breakers was designed for the protection of excess current every conductor in the
installation is to be protected by a circuit breakers fitted at the origin of the circuit.
The current rating of the circuit breaker should not exceed the current rating of the lowest rated
conductor in the protected circuit.
(ii) Earthing: According to IEE Regulation D1, every conductor shall be prevented from
giving rise to earth leakage current by earthing of exposed metal parts. For this purpose
of this project, it was designed that all conductor are earth to ground using earth
electrode.
3.0 Results and discussion
Based on the design of this project, the following results were obtained.
Table 2: Results for overhead line Design

S/N ITE TYP SPECIFICATION

M E
1 Line Support Concrete Poles 10m long -H.T
8.5m long - LT.
2 Cross-Arm Hot-Dip Galvanized Cross-arm 1.63m Long

3 Insulator Disc Insulator Shackle insulator 254mm diam. 76mm diam.

4 Conductor AAC 50mm2 - H. T. 100mm2 - L.

T.

From table 2 above, it can be seen that concrete poles were designed for the project, wooden
poles are rather short in service life because they are inapplicable in place that have higher humidity and
liable to be affected by insects or animals. It can be concluded that concrete poles have advantage over
wooden poles.
Although pin type insulators are also available but because of it low level of insulation, disc type
was used as specified by B.S 137.
All aluminium conductors of 50mm2 were designed for the high voltage line while 100mm2 was
designed for the low voltage line. Aluminium was used because of it cheaperness and easy to machine
handle.
Standard cross-arms shall have a size of either 1.63 or 2.24m. for places that require span length
size cross-arm will be used, but for the purpose of this project which the span length is not more 80m,
1.63 size was designed.
Table 3: Maximum load demand for whole unit.

S/N BEDROOM FLAT/OTHER TOTAL LOAD DEMAND (KW)

3. Two Bedrooms 3465

4. One Bedrooms 2388.5

 5. Primary school 7.5

6. Boreholes 180
7. Street light 1.5

8. Fire services 2.5

9. Transformer Designed 4 x 500KVA

The total load demand for each unit was calculated in table 1.6 above.
This load demand was added to have maximum load demand for the whole unit.
The transformers designed for the whole unit was obtained by considering this
maximum load demand of the area. It can be concluded that ten number of
500KVA transformers was designed for the purpose of this project.
CONCLUSION
The objective of the design is to provide electrical power supply to 500
housing units, because the optimum utilization by society of electrical energy
can be ensured by effective supply and distribution systems.Also, electricity is
the most convenient and useful from of energy, without it the present social
infrastructure cannot be feasible, hence there is a need to electrify the area.
The designed network of the area can carry the load imposed upon it
without excessive heating in the conductor and consequently damages to the
insulation. The voltage at the consumer terminal is kept within the specified
limit (i.e 6% of the nominal value). (NEPA Manual, 1975)
The system as designed can meet the load variations which are likely to
arise in near future and provide continuity of supply and should a fault occur on
the system, interruption in the supply to the consumers should last for a shortest
possible time.The system is very simple to maintain and operate and routine
maintenance should be carried out with minimum interruption to power supply.
From the above results, it can be concluded that the system is quite
reliable. Since the reliability of any good designed system depends on efficient
control equipment such as circuit breakers, lighting arresters, fuses etc., hence it
becomes necessary to incorporate in this design to obtain reliable system. The
overhead lines distribution and the electrical installation designed for this
project will suite the area, because it is cheaper and easy to maintain.
 EXPERIMENT NO – 4
Designing a double circuit transmission line for a given voltage level and power
(MVA) transfer.

1. INTRODUCTION
Nowadays, power systems are extensively interconnected requiring the huge
transfer of electric power. Considering that a typical transmission line with a certain
voltage level, can only carry a limited capacity, to carry an enormous power it is required
to construct extra high voltage (EHV) transmission lines [1]. Due to the vastness of Iran
and energy demand increase, using 765kV EHV transmission lines can be a good choice to
meet the needs.
765 kV transmission lines have been established in some countries including South
Korea, India and the United States of America [1, 2] and different standards are provided
for these transmission lines by IEEE, BS and ANSI [3]. However, to make in parameters
these standards with respect to Iran’s current conditions, such as for weather conditions,
require the combination and analysis of international standards and national standards
which are available for other voltage levels.
In this paper, using IEEE and ANSI standards related to 765kV transmission lines,
and 230kV and 400kV standards, electrical design of all parts of a 765kV transmission line
is presented.

2. DETERMINING THE TYPE AND NUMBER OF CONDUCTORS PER PHASE

Reviews on a variety of transmission lines in United States of America, South
Korea and some other countries shows that the type of conductors in EHV transmission
lines are of Rail or Curlew. Further, from Table (1) it can be understood that Curlew
conductors has lower resistance and higher minimum tear compared to the Rail conductors
which shows a better characteristics. However, in high power transmission, to reduce
losses and limit the corona phenomenon, it is needed to increase the number of
conductors per
bundle. Therefore, Curlew conductor can not be used due to excessive weight. Rail
conductors, hence, have a low electrical resistance; high mechanical strength and low weight
are selected.

Table 1. Rail and Curlew conductors’ characteristics

Conductor structure Approximate
Cross Conductor weight of Maximum electrical Minimum Packing
Conductor conductor 0 tear
section AL. St. (No- diameter resistance in 20 C Areas
(mm2) AL. St. strength
(No-mm) mm) (mm) (Ω/km) (m)
(No- (No- (kN)
mm) mm)
Martine 684.84 54*4.02 7*2.41 36.17 1906 679 0.04259 205.8 1000
Rail 483.42 45*3.70 7*2.47 29.61 1339 260 0.05994 115.6 1000
Curlew 523.68 54*3.52 7*3.52 31.68 1451 530 0.05531 162.7 1000

Considering the issues related to high power transmission and corona phenomenon
in EHV lines the use of several conductors in a bundle is essential [4]. Studies on a variety
of existing 765kV transmission lines show that these lines are designed to transfer a power
of 4000 MVA (or even more), for which line current is calculated using Eq. (1).

S
I (1)
3.V

According to the analysis of 765 kV transmission lines, the transferred power of

S=5000MVA is considered for a single circuit line has six conductors per phase [4]. And
by considering the voltage V = 765 kV, rated current value is equal to a 3773 A. Therefore,
regarding the following cases, the mentioned transmission line is chosen to have six
conductors per phase [5]:
 To balance the weight, the number of conductor bundles must be even (four or six).
 The four-wire bundled Rail Conductor has a tolerable current equal to 3940A when compared with
the rated current of 765kV transmission line, it is inferred that the former will has a lower loading
margin.
 An example of six-wire transmission line is shown in Figure 1.

Figure 1. Six-wire transmission line

3. APPROPRIATE CROSS SECTION FROM THE VIEWPOINT OF SHORT CIRCUIT
Besides the numerous factors that are involved in determining the cross-section of
conductors, short circuit current is another key aspect in proper selection of the conductors.
Conductor cross section is determined according to the rated current, and then based on the
level of short circuit test. Eqs. (2) and (3) assess the minimum cross section required to
withstand the heat generated due to the short circuit [6].
S  I SC .t (2)
K
W .C.
K
0.24  (3)

Where:
S: Cross section of conductor (mm)
ISC: Standard short circuit current (A)
t: The persistence time of short circuit current (s)
k: Constant coefficient related to the conductor material which is dependent to the
following parameters:
w: Specific weight of the conductor (gr/cm3)
C: Specific heat of conductor metal (Calory/g-oc)
∆ϴ : The conductor temperature rise (0c)
: Specific resistance of the conductor (ohm.m/mm2).

K value for ACSR conductors is 85, Isc value for 765kV line is 70kA, and the t is
0.5s. Inserting these values in Eq. (2), the required cross section is obtained to be 582.32
mm2 which is able to withstand short circuit level compared to the six-wire bundled Rail
conductor.

4. DETERMINING THE APPROPRIATE DISTANCE FROM THE BUNDLE, GMD AND GMR
The studied transmission line is a horizontal single-circuit line with six
conductors per bundle.
According to the standards, maximum distance between conductors in bundled lines is
457mm.
By studying various transmission lines, phase distance from the next phase is about
18m while this distance in lateral phases is about 36m [7].
Then geometric mean radius (GMR) and the geometric mean distance (GMD) are
calculated using Eqs. (4) and (5) [8]:
6
GMR  6D. d 5 (4)
GMD  D12  D13  D 23
3
(5
)

Where:
GMR : Geometric mean
radius (m) GMD: Geometric
mean distance (m)
d: Distance between the conductors in the bundle (m)
D: Distance between the phases (m).
Inserting d = 0.430 m and D = 18 m, GMR and GMD values are obtained as follows:

GMD=22.678
m
GMR=0.371
m
 By calculating the GMR and GMD, the inductance and the capacitance values of
transmission line are obtained using Eqs. (6) and (7).

2 0
C  (6)
ln( GMD GMR )

2 7 GMD
L  10 ln
1 / 4 (7)
e GMR

Where:
C : The line capacitance (F/ m)
ε0 : The dielectric coefficient of vacuum which is
8.85*10-12 (F/m) GMD : Geometric mean distance (m)
GMR : Geometric mean radius (m).
 By placing GMR and GMD values, inductance and capacitance values are obtained
as follows:

L  0.8725 H
m
C  0.01351 nF
m

Thus, regarding network frequency f=50Hz, reactance and susceptance values of

the transmission line are obtained using Eqs. (8) and (9).

X L  2. f .L (8)
B  2. f .C
(9
)

By inserting L, C and f values,


X L  0.274 km
6 1
B  4.24241 10
.km

Rail conductors’ permitted current value is equal to 985A. Also, regarding the line
characteristic for which the transmitted power is considered about 5000 MW, it can be
claimed that the amount of current passing through each phase with respect to line voltage
765kV is 3770A. And given that there are six wires for each phase, therefore for each
bundled conductors’ current passing is 630A in steady state.
As a result, Rail conductors can easily pass constant current through transmission
line.

5. DETERMINING THE PROPER CROSS SECTION FOR FITTINGS

One of the problems occurred in fittings due to short circuit current is in welded
joint corrosion, such as connecting two parts together or celebrating with a hole between
the bolts and nuts. Considering the amount and time of current passing through lines, a
cross section can be obtained using Eqs. (2) and (3) for existing connections in the line
which are of steel and aluminum type such that short circuit current and in turn the
generated heat do not deform them.
According to the standard for metal alloys, the values of W, C, ∆θ, ρ, t and Isc for
steel are 7.8, 0.11, 0.096, 0.5 and 70, respectively, while the mentioned values for
aluminum are 2.7, 0.215, 0.028, 0.5 and 70. Inserting these values in Eqs. (2) and (3)
results in proper cross sections of 580 mm2 and 330 mm2 for
aluminum and steel, respectively.

6. DETERMINING THE NUMBER OF INSULATORS

Insulators used in designed transmission line are 160kN insulators with tolerable
voltage of 10kV. To calculate the number of insulators in insulator string, voltage
distribution should be checked along insulator string and finally obtain the required
number of insulators.
In practice, due to the capacitance between metal parts of insulator string, towers
and earth, voltage distribution on insulators is not uniform. The maximum and minimum
voltages on insulator strings are on the insulator connected to the conductor and the
insulator connected to the tower, respectively.
To calculate the voltage distribution along the insulator string, the capacitance
between insulators themselves and the tower should be determined. Although the
capacitance of all insulators is not same, however, considering the short length of insulator
string respect to the tower height and its uniformity, capacitance of whole insulators is
same to be C1, and the capacitance between insulators and tower is to be C2. Accordingly,
by calculating α using Eq.(7), voltage on to ends of insulator string is obtained by Eq.(8).

(
C 2 )0.5
C1 (7
)

V V Sinh( .K )
kg
. Sinh( .n ) (8)
ng
 Where, C1 and C2 values given in [9] are the capacitance between the metal part and
earth, and insulator capacitance, respectively. Consequently, having the value of α,
distributed voltage in two ends of insulators is obtained. If the voltage distribution curve is
considered to be linear along insulators, in this case, C1/C2=12 and α=0.2887. In addition,
K, n, Vng and Vkg are insulator numbers we can calculate the voltage across it, the total
number of insulators, phase voltage of transmission line, and Kth insulator voltage,
respectively. Inserting all of these parameters in Eq. (8), the number of insulators is obtained
to be 35.

7. CALCULATING THE VOLTAGE GRADIENT

Voltage gradient around the conductor and fittings can play an important role in the
phenomenon of corona and the resulted losses. According to Ref. [7] the voltage gradient
for Six-bundled Rail conductors in each phase is obtained using Eqs. (9) to (11).

gmax 18.C . 2( n  1 )r sin(  / n )

[1  GMR (9)
V ]
nr
0.02413
C GMD (10)
Log( )
GMR
GMR  [ r.n[
BS ]
n 1
]
1
n
(11)
2 sin( 180 / n )

Where:
gmax: The maximum voltage gradient at the surface of
conductors (kV/cm) V: Line phase voltage (KV)
n: The number of bundled conductors
per phase r: Radius of conductor (cm)
C: Line capacitance (F/ km)
GMR: Geometric mean radius of the bundled
conductors (cm) Bs: Distance from the bundle
conductors (cm).
According to the Standard of Power Ministry [7], g max value should not exceed the
critical voltage gradient g0=15.9 kV/cm, which the performed calculations also are in the
desired range. However to limit the amount of voltage gradient in the surrounding insulator
strings and fittings, corona rings can be used [10].

8. CALCULATING CRITICAL VOLTAGE

In a transmission line, if the applied voltage reaches to the critical value, the
surrounding air begins to be ionized. Corona losses and communication disturbances to be
in a certain level, it is required that according to Ref. [7] the applied voltage to the
conductor does not exceed 1.8 times of the critical voltage.
Critical voltage value is a function of the line physical features and environmental
conditions which is calculated using Eqs. (12) to (14).

gv  g 0.3
.(1 ) (12)
0 
 .r
298P
  T (13)
V C  gv .m..r.ln( GMD / r ) (14)
Where:
gv : Critical voltage gradient (kV/cm)
g0 : The threshold breakdown voltage
(KV / cm) r : Radius of conductor (cm)
δ : Relative
density P : Air
pressure (At.)
T : Air temperature (K◦)
m : Coefficient of conductor surface roughness
GMD : Geometric mean distance between
conductors (cm).
 Due to different climatic conditions existing in Iran, the temperature of 40 0C and
height of 1000m is considered which can be a good condition in summer in most parts of
the country. The value of m for ACSR conductors is 0.85. It is noteworthy that the values
of g0 and GMD are mentioned formerly.
By inserting the mentioned parameters in Eqs. (12) to (14), the critical voltage is
VC=267.843 kV.
V ph 441.686
  1.65 p 1.8
VC 267.843

9. THE AMOUNT OF CORONA LOSSES

The main disadvantage of corona phenomenon is the resulted losses which may be
increased to ten times on rainy or snowy days. In a typical EHV transmission line, the
losses can be of a significant amount. Therefore, in designing transmission line the corona
losses should be also calculated [11].
There are two methods for calculating corona losses. One of these methods is
presented by Pik which is formulated according to Eq. (15).

PC  0.00314.F.( V
log( GMD / r ) ) (15)

Where:
PC : Corona losses (kW / km)
V : Effective-phase voltage (kV)
GMD : Geometric mean distance between
conductors (cm). r : Radius of conductor (cm)
F: Constant coefficient and the critical voltage
Studies show that when corona losses are low, the mentioned equation has no good
accuracy, and at this situation using Peterson method would has good accuracy as shown in
Eq. (16). r
GMD
0.545
 (16)
PC 
(
V V C ) .

In which;
Pc: Corona losses (kW / km)
V : Effective-phase voltage
(kV) Vc : A critical voltage
(KV)
GMD : Geometric mean distance between
conductors (cm). r : Radius of conductor (cm)
Given that the amount of Vph/VC is less than 1.8, therefore, to calculate corona losses
Peterson equation is used as follows:
Pc=1.44 kW/km
Corona losses in a 1000km transmission line are almost 1.5MW/km which is a good
value.

10. CORONA RING DESIGN

 Figure 2 shows an example of the 765 kV insulator strings with 35 insulators
divided into four branches (to withstand tensile forces), in which to limit the voltage
gradient along the insulator string ring located in proper position and with appropriate
dimensions. In addition to reduction of the voltage gradient around insulator string causes
to reduction of losses.
In corona ring design, three main parameters should be determined [12],
1) Diameter profiles
2) Radius of the ring
3) Position of ring along the insulator strings
For this purpose, an insulator strings along with a corona ring are simulated in Quick Field
simulation software. Ring radius and profile diameter are changed in the range of different
to obtain optimal dimensions for corona ring. The corona ring diameter of 7cm and radius
of 150cm are the best optimized to reduce the size of the voltage gradient around
insulators.
 Figure 2. A typical corona ring

After determining the corona ring dimensions, the height of the corona ring is also
examined along insulator strings. Finally, with the simulation software and changing the
height respect to the first insulator, it was determined that the height of 4cm above the first
insulator to corona rings that is placed along with insulators, and the second corona ring on
the connections of end clamp create the best position to decrease the voltage gradient [10,
12].

11. VOLTAGE REGULATION PERCENTAGE

Voltage regulation percentage is the change of voltage from zero to the rated
voltage divided to the nominal value which is expressed by Eq. (17).

VR% 
V s V r 100
V (17)
r

Where:
VR%: Line voltage regulation
percentage Vs : Sending end
voltage (kV)
Vr : Receiving end voltage (kV)
Vr is the line rated voltage. And Vs is obtained in long transmission lines as follows:
V s   A B  V r
  (18)
    
C D
 I s     I r 

Where:
Is : The sending end current (A)
Ir : The receiving end current (A). Values of A, B, C and D are obtained from
Eq. (19) to (22).
(19)
A  cosh l 
(20)
B Z c sinh l.
1
C
Z C sinh l  (21)
D  cosh l (22)

 In
which:
(23)
  z.y
z
ZC  y (24)

Where:
y: Parallel Admittance of transmission line (Ψ/ Km)
z: Series impedance of transmission line (Ω/ Km).
Inserting the parameters obtained from section three, voltage regulation percentage will be:

VR%= 0.042

Given that the voltage changes from ±10% in line is allowed, the amount of voltage
regulation percentage is appropriate.

12. SIGNIFICANTLY REDUCING INSTALLATION PERMITTED DISTANCE AND COST OF TRANSMISSION LINES
One of the most striking features of the transmission lines are the permitted
distance which is directly related to the cost of transmission lines installations [2].
Permitted distances requirements depend on several transmission line factors which are
considered during calculations. These factors include: electric and magnetic fields, and the
safety distance required for each voltage level, considering the distance required between
conductors in the line to prevent the effects of wind and storm, planning and future
development of lines due to the increase in consumer demand, and mechanical
considerations and considering the permitted distance between the line conductors to
prevent the occurrence of galloping in two circuits transmission line [5]. In most cases, the
allocation of extra line distances in designing and constructing the line leads to better
control of the line and thereby increase network reliability. However, it is impossible in
many cases due to lack of space, expensive lands, and/or a favorable ground for the
construction of this line. So the best solution to solve the permitted distance problem is the
use of high voltage lines, which in this case by replacing high voltage lines instead of low
voltage lines will lead to a significant reduction in the size and permitted distance.
As it is depicted in Figs. (3) and (4), for construction of a 765 kV single circuit
transmission line with 6 conductors per phase about 200 feet permitted distance is required,
while for a 400kV two circuits transmission this value is about 150 feet. Thus, for
transmitting the equal electric power by 400 kV power transmission line, it is required to
have three 400 kV transmission lines of two circuits or six single-circuit transmission line.
In this case, the permitted distance is increased 450 feet (2.5 times) for two-circuit line and
900 feet (4.5 times) for single-circuit line. Also, as it can be seen from Figs. (3) and (4), by
the voltage reduction the problem of permitted distance is resolved. Given the importance
of construction of high voltage lines, it is clear that this will result in lower costs [13, 4].
Figure 3. Comparison between 345kV and 765 kV permitted distance with equal
transmission capacity
 Figure 4. Comparison between 132kV and 765 kV permitted distance with equal transmission capacity

13. 765 KV NETWORK RELIABILITY RESPECT TO THE LOWER VOLTAGE LEVELS

In studies conducted in various types of transmission lines of different voltage levels and the
comparison with 765 kV voltage reveals that the transmission lines of 765kV level are inherently more reliable
than the other lower voltage levels lines. When 765 kV transmission lines with six conductors in each phase is
utilized, due to the increase in geometric mean radius, and the uniform distribution field, the level of corona
effects and noises are acceptable [13, 4].
According to statistics results obtained in [3], 765 kV transmission lines have one outage in every 100
miles per year, while for 500kV transmission lines the outage is 1.4. Further, because these lines are of single-
circuit type, the faults are single phase to the tower and the two-phase as well as three-phase faults are rare
resulting in less outage. Also, using single circuit line causes no Galloping phenomenon, also due to the far
distance between phases the fault occurrences by phases are reduced [14].

14. CONCLUSION
Given the growing need for electrical energy, appropriate measures are essential to overcome the
problems of electric power transmission lines, reducing permitted distances, increasing network reliability from
the view point of less outage, power loss reduction especially corona losses, communication disturbances
reduction and many other issues. In this paper, the importance of increasing voltage of transmission lines and
the benefits that resolve many of the problems are studied, and finally, a single circuit 765kV transmission line
with six conductors per phase were compared with lower voltage levels transmission lines and the results were
analyzed.