TRANSMSSION LINES

                                                       CONTENTS                                 

1.Overview

2.Introduction to transmission lines

3.Component of Transmission Line
   3.1 conductor
   3.2 Earth wire
   3.3 Insulator
   3.4 Transmission Tower
   3.5 Wave trap and other hard war
4. Skin effect
5.Corona effect
6.Ferranti effect

7..Design specifications

8.Types of Transmission Lines
    8.1 Two wire transmission line
    8.2 Co-axial transmission line
    8.3 Wave Guide
    8.4 Micro Strip

 9.  Factors determining transmission line
 10.Economic voltages of transmission of power

 11. Types of conductors
          11.1 AAC : All Aluminium Conductor
          11.2 AAAC : All Aluminium Alloy Conductor
          11.3 ACSR : Aluminision  Conductor, Steel Reinforced
          11.4 ACAR : Aluminium Conductor, Alloy Reinforced

 12.  Factors determining conductor sizes

 13. Performance of Transmission Lines

 14. Poles and types of poles 

 15.Towers and Tower accessories
 16. Protection of tower footing

 17. Earthing

 18.Types of insulator
      18.1 Pin Insulator
      18.2 Suspension Insulator
      18.3 Strain Insulator

 19.General application
 20.General considerations

                                             OVERVIEW

    Ordinary electrical cables suffice to carry low frequency alternating current (AC), such as mains power, which reverses direction 100 to 120 times per second, and audio signals. However, they cannot be used to carry currents in the radio frequency range or higher, which reverse direction millions to billions of times per second, because the energy tends to radiate off the cable as radio waves, causing power losses. Radio frequency currents also tend to reflect from discontinuities in the cable such as connectors and joints, and travel back down the cable toward the source. These reflections act as bottlenecks, preventing the signal power from reaching the destination. Transmission lines use specialized construction, and impedance matching, to carry electromagnetic signals with minimal reflections and power losses. The distinguishing feature of most transmission lines is that they have uniform cross sectional dimensions along their length, giving them a uniform impedance, called the characteristic impedance, to prevent reflections. Types of transmission line include parallel line (ladder line, twisted pair), coaxial cable, stripline, and microstrip. The higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the wavelength of the waves. Transmission lines become necessary when the length of the cable is longer than a significant fraction of the transmitted frequency's wavelength.

   At microwave frequencies and above, power losses in transmission lines become excessive, and waveguides are used instead, which function as "pipes" to confine and guide the electromagnetic waves.Some sources define waveguides as a type of transmission line; however, this article will not include them. At even higher frequencies, in theterahertz, infrared and light range, waveguides in turn become lossy, and optical methods, (such as lenses and mirrors), are used to guide electromagnetic waves. 
   The theory of sound wave propagation is very similar mathematically to that of electromagnetic waves, so techniques from transmission line theory are also used to build structures to conduct acoustic waves; and these are called acoustic transmission lines.

 INTRODUCTION TO TRANSMISSION LINES

A transmission line is a pair of electrical conductors carrying an electrical signal from
one place to another. Coaxial cable and twisted pair cable are examples. The two
conductors have inductance per unit length, which we can calculate from their size
and shape. They have capacitance per unit length, which we can calculate from the
dielectric constant of the insulation. In the early days of cable-making, there would be
current leaking through the insulation, but in modern cables, such leakage is
negligible. The electrical resistance of the conductors, however, is significant because
it increases with frequency. The magnetic fields generated by high-frequency currents
drive those currents to the outer edge of the conductor that carries them, so higher
the frequency, the thinner the layer of metal available to carry the current, and the
higher the effective resistance of the cable. In this discussion, we derive and
demonstrate the equations that govern the propagation of waves down a transmission
line, and show how the frequency-dependent resistance of these cables gives rise to

attenuation and distortion of high-frequency signals.

     Component of Transmission line:

   

CONDUCTOR:

      

     In physics and electrical engineering, a conductor is an object or type of material


that allows the flow of an electrical current in one or more directions. A metal wire is a


common electrical conductor.
   

     In metals such as copper or aluminum, the mobile charged particles are electrons


Positive charges may also be mobile, such as the cationic electrolyte(s) of a battery


or the mobile protons of the proton conductor of a fuel cell. Insulators are


non-conducting materials with few mobile charges that support only insignificant


electric currents.
                                

Overhead conductors carry electric power from generating stations to customers:

Properties:

1.High electrical conductivity(low resistivity).


2.High tensile strength(with stand mech stress).


3.Low specific gravity(weight/volume low).


4.Low cost (use for long distances).


5.The electric field inside the coonductor is zero.


6.The charge density inside the conductor is zero.

Earth wire :

Ground wires are bare conductors supported at the top of transmission towers. They


serve to shield the line and intercept lighting stroke before it hits the current carrying


conductors below. Ground wires normally do not carry current. Therefore, they are


often made of steel. The ground wires are solidly connected to ground at each tower


in transmission and distribution system.

                                                                                                                                       

Insulator:

   An electrical insulator is a material whose internal electric charges do not flow freely

, and therefore make it nearly impossible to conduct an electric current under the

influence of an electric field. This contrasts with other materials, semiconductors and

conductors, which conduct electric current more easily.The property that distinguishes

an insulator is its resistivity; insulators have higher resistivity than semiconductor

A perfect insulator does not exist, because even insulators contain small numbers of

mobile charges (charge carriers) which can carry current. In addition, all insulators

become electrically conductivewhen a sufficiently large voltage is applied that the

electric field tears electrons away from the atoms. This is known as the breakdown

voltage of an insulator. Some materials such as glass, paper andTeflon, which have

high resistivity, are very good electrical insulators. A much larger class of materials,

even though they may have lower bulk resistivity, are still good enough to prevent

significant current from flowing at normally used voltages, and thus are employed as

insulation for electrical wiring andcables. Examples include rubber-like polymers and

most plastics.

                


Transmission Tower:


      A transmission tower or power tower (electricity pylon in the United Kingdom

and parts of Europe) is a tall structure, usually a steel lattice tower, used to support an

overhead power line.


     They are used in high-voltage AC and DC systems, and come in a wide variety of

shapes and sizes. Typical height ranges from 15 to 55 m (49 to 180 ft), though the

tallest are the 370 m (1,214 ft) towers of a 2,700 m (8,858 ft) span of Zhoushan Island

Overhead Powerline Tie. In addition to steel, other materials may be used, including

concrete and wood.

  
    There are four major categories of transmission towers suspension, terminal,

tension, and transposition. Some transmission towers combine these basic functions.

Transmission towers and their overhead power lines are often considered to be a form

of visual pollution. Methods to reduce the visual effect include undergrounding.

Wave trap and other hard wav:

    Wave trap is a parallel tuned inductor - capacitor 'tank' circuit made to be resonant

at the desired communication frequency.It is the effort to utilize the same transmission

line between two substation for the purpose of communications.At this communication

frequencies the tank ckt provides high impedance and does not allow to pass through

them & on to the substation bus & into transformers.

 

                     Skin effect 

 

    The alternating current flowing through the conductor is non-uniformly

distributed throughout its length. This is because the outer strands of the

conductor (i.e., the strands which form the surface of the conductor) carry

more current than the inner strands (i.e., the strands which are closer to

the center of the conductor). This non-uniform distribution of current

causes an increase in the resistance of the conductor to the flow of

alternating current. This effect is known as skin effect and it shown in

 figure.

 This effect is termed because skin effect as the charges flow through the

 surface (skin) and not through the center of the conductor. The skin effect 

is absent when the current is DC this is because the direct current flowing 

through a conductor spreads uniformly throughout the cross—sectional 

area of the conductor.

Why Skin Effect Occurs in Transmission Lines?

     The main cause of skin effect is the non-uniformity of flux linkage. The phenomenon of skin effect can be explained as follows:     

   Consider a multi—stranded conductor composed of ‘n’ number of strand (filaments). The AC current flowing through the inner strands produce flux which links (enclose) the inner strands only. But, the flux produced by the flow of AC current through the outer strands, links (enclose) not only the outer strands but also the inner strands. Thus,,the flux linkage per ampere to the inner strands is higher than the flux linkage per ampere to the outer strands. This causes the inductance (and hence inductive reactances) of inner strands to be much higher than for the outer strands. Thus, higher the impedance lower » is the flow of current. As a result, the outer strands carry more current than the inner strands.

         

 Factors Affecting Skin Effect in Transmission Lines

Skin effect primarily depends on the three important factors,

1. Operating frequencies

2. Size of conductor

3. Type of conductor.

Higher the operating frequency, the greater is the skin effect. The diameter of conductor has a direct relation with skin effect. Bigger the diameter of conductor, higher is the skin effect. The type of conductor also decides the level of skin effect. The skin effect is less incase of a stranded conductor than a solid conductor.

                  Corona Effect

 

     For overhead transmission system the atmospheric air, which is the dielectric medium, behaves practically like a perfect insulator when the potential difference between the conductor is small. If the voltage impressed between the conductor is of alternating nature, the charging current will flow due to the capacitance of the lines. This charging current increases the voltages of the lines and corresponding increase the electric field intensity of the lines.

     When the value of electric field intensity is less than 30kV (disruptive voltage), the flow of current between two conductors of the lines is negligibly small. But when the electric field intensity reaches this critical value or disruptive voltage the airs between the conductors get ionizes and becomes conducting. If the voltage goes on increasing spark is established between the conductors until the complete breakdown of the insulating properties of the material.

This phenomenon of ionization of surrounding air around the conductor due to which luminous glow with hissing noise is rise is called corona effect.

Corona Formation:

   Air is not perfect insulator and even under normal condition, the air contains many free electrons and ions. When an electric field intensity is established between the conductors, these ions and free electrons experience force upon them. Due to this force, the ions and free electrons get accelerated and moved in the opposite direction.

The charged particles during their motion collide with one another and also with the very slow moving uncharged molecules. Thus, the number of charge particles goes on increasing rapidly. This increase the conduction of air between the conductors and a breakdown occurs. Which established the arc or discharge between the conductors.

Factors affecting corona:

 Corona loss depends on a large number of factors, the most important being broadly classified in the following way:

1. Effect of supply voltage –  If the supply voltage is high corona loss is higher in the lines. In low voltage transmission lines the corona is negligible, due to the insufficient electric field to maintain ionization.
2. The condition of conductor surface – If the conductor is smooth, the electric field will be more uniform as compared to the rough surface. The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc. Thus, rough line decreases the corona loss in the transmission lines.
3. Air Density Factor – The corona loss in inversely proportional to air density factor, i.e., corona loss, increase with the decrease in density of air. Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.
4. Effect of system voltage – Electric field intensity in the space around the conductors depends on the potential difference between the conductors. If the potential difference is high, electric field intensity is also very high and hence corona is also high. Corona loss, increase with the increase in the voltage.
5. A spacing between conductors – If the distance between two conductors is much more as compared to the diameter of the conductor than the corona loss occurs in the conductor. If the distance between them is extended beyond certain limits then the dielectric medium between them get decreases and hence the corona loss also reduces.  

                       Ferranti Effect

Definition: A long transmission line has a large capacitance. If such a line is open-circuited or connected to the very light load at the receiving end,  the magnitude of the voltage at the receiving end becomes higher than the voltage at the sending end. This phenomenon is called Ferranti effect.

    Ferranti effect is due to the charging current of the line. When an alternating voltage is applied, the current that flows into the capacitor is called charging current. A Charging current is also known as capacitive current.

 Why Ferranti effect occurs?

     Capacitance and inductance are the main parameters of the lines having a length 240km or above. On such transmission lines, the capacitance is not concentrated at some definite points. It is distributed uniformly along the whole length of the line.

        When the voltage is applied at the sending end, the current drawn by the capacitance of the line is more than current associated with the load. Thus, at no load or light load, the voltage at the receiving end is quite large as compared to the constant voltage at the sending end.

How to reduce Ferranti effect:

     Electrical devices are designed to work at some particular voltage. If the voltages are high at the user ends their equipment get damaged, and their windings burn because of high voltage. Ferranti effect on long transmission lines at low load or no load increases the receiving end voltage.This voltage can be controlled by placing the shunt reactors at the receiving end of the lines.

     Shunt reactor is an inductive current element connected between line and neutral to compensate the capacitive current from transmission lines. When this effect occurs in long transmission lines, shunt reactors compensate the capacitive VAr of the lines and therefore the voltage is regulated within the prescribed limits.

                  DESIGN SPECIFICATIONS

 The towers and conductors of a transmission line are familiar elements in our landscape.However, on closer inspection, each transmission line has unique characteristics that have correspondingly unique implications for the environment. In this section, we list design specifications (line characteristics) that are commonly required to define a transmission line. Many of these specifications have implications for the net environmental effects. For the purpose of this report, a range of values is considered for these specifications, with the exception that a fixed nominal voltage of 500 kV is assumed.

 Overall Descriptive Specification:

   The most basic descriptive specifications include a line name or other identifier, nominal voltage, length of line, altitude range, and the design load district. The line identifier is commonly taken from endpoint names, e.g., Inland−Macedonia on the Cleveland Electric Illuminating Co. system. The endpoint names are generally geographic points, but may be substation names or major industrial facilities. The nominal voltage is an approximation to actual line voltage that is convenient for discussion. Actual voltage will vary according to line resistance, distance, interaction with connected equipment, and electrical performance of the line. For AC lines, the nominal voltage is close to the RMS (root mean square) voltage.The altitude range is a rough surrogate for weather and terrain. This is important, since nearly all aspects of line design, construction, and environmental impacts are linked to weather. The design load district is another surrogate for weather. These districts are defined by the National Electrical Safety Code (NESC) and by some local jurisdictions. These districts include NESC Heavy Loading, NESC Medium Loading, NESC Light Loading, California Heavy Loading, and California Light Loading. The design wind and ice loading on lines and towers is based on the design load district. This affects insulator specifications as well as tower dimensions, span lengths, tower design, and conductor mechanical strength and wind dampening. 

 Tower Specifications:

        The towers support the conductors and provide physical and electrical isolation for energized lines. The minimum set of specifications for towers are the material of construction, type or geometry, span between towers, weight, number of circuits, and circuit configuration. At 500 kV, the material of construction is generally steel, though aluminum and hybrid construction, which uses both steel and aluminum, have also been used. The type of tower refers to basic tower geometry. The options are lattice, pole (or monopole), H-frame, guyed-V, or guyed-Y. The span is commonly expressed in the average number of towers per mile. This value ranges from four to six towers per mile. The weight of the tower varies substantially with height,
duty (straight run or corner, river crossing, etc.), material, number of circuits, and geometry. The average weight of 670 towers for 500-kV lines included in the EPRI survey (EPRI 1982) is 28,000 lb. The range of reported tower weights is 8,500 to 235,000 lb. The type of tower
(specific tower geometry) is very site-dependent, and, for any given conditions, multiple options are likely to exist. The next section provides some illustrations of specific tower types and describes their relative impacts. The number of circuits is generally either one or two. The circuit configuration refers to the relative positioning of conductors for each of the phases. Generally the options are horizontal, vertical, or triangular. The vertical orientation allows for a more compact ROW, but it requires a taller tower. 

 Minimum Clearances:

     The basic function of the tower is to isolate conductors from their surroundings, including other conductors and the tower structure. Clearances are specified for phase-to-tower, phase toground, and phase-to-phase. Phase-to-tower clearance for 500 kV ranges from about 10 to
17 feet, with 13 feet being the most common specification. These distances are maintained by insulator strings and must take into account possible swaying of the conductors. The typical phase-to-ground clearance is 30 to 40 feet. This clearance is maintained by setting the tower height, controlling the line temperature to limit sag, and controlling vegetation and structures in the ROW. Typical phase-to-phase separation is also 30 to 40 feet and is controlled by tower geometry and line motion suppression.

 Insulators:

  Insulator design varies according to tower function. For suspension towers (line of conductors is straight), the insulator assembly is called a suspension string. For deviation towers (the conductors change direction), the insulator assembly is called a strain string. For 500-kV
lines, the insulator strings are built up from individual porcelain disks typically 5.75 inches thick and 10 inches in diameter. The full string is composed of 18 to 28 disks, providing a long path for stray currents to negotiate to reach ground. At this voltage, two to four insulator strings are commonly used at each conductor connection point, often in a V pattern to limit lateral sway.

Lightning Protection:

   Since the towers are tall, well-grounded metallic structures, they are an easy target for lightning. This puts the conductors, other energized equipment, and even customer equipment at high risk. To control the effects of lightning, an extra set of wires is generally strung along the extreme top points of the towers. These wires are attached directly to the towers (no insulation), providing a path for the lightning directly to and through the towers to the ground straps at the base of the towers. The extra wires are called shield wires and are either steel or aluminum-clad steel with a diameter of approximately ½ inch. 

  Conductor Motion Suppression:

    Wind-induced conductor motion, aeolian vibration, can damage the conductors. A variety of devices have been employed to dampen these oscillatory motions. By far, the most common damper style on 500 kV lines is called the Stock bridge damper. These devices look like elongated dumbbells hung close to and below the conductors, a few feet away from the point of attachment of the conductors to the tower. The weighted ends are connected by a short section of stiff cable, which is supported by a clamp to the conductor immediately above. Dampers can prevent the formation of standing waves by absorbing vibrational energy. Typically, a single damper is located in each span for each conductor.  

                 Types of Transmission Lines:

 Two wire transmission line:

        This transmission line consists of a pair of parallel conducting wires separated by a uniform distance .These are used in power systems or telephones lines.

                          

  Coaxial transmission line:

           This consists of an inner and a coaxial outer conducting sheath separated by a dielectric medium . They are used as TV cables, telephones cables and power cables.

 

Parallel plate transmission line or planar line:

           It has two parallel conducting plates separated by a dielectric slab of uniform thickness

                            

Micro strip transmission line:

            It consists of core and cladding . Information passes through the core in the form of totally internal reflected TEM waves.

                      .

 FACTORS DETERMINING TRANSMISSION LINES

some factors to be considered when selecting the transmission line conductors include:

1.Required sag and span between conductors.

2.Tension on the conductors.

3.whether or not the temperature is corrosive.

4.whether or not the line is prone to vibration.

5.power loss allowed on the line.

6.voltage loss allowed on the line.

7.climate at the line location. 

 Economic voltages of transmission power

 While designing any transmission line, economy is one of the most important factors the engineer must consider. An electrical power transmission line must be designed in such a way that the maximum economy is achieved. Economics of electric power transmission is influenced by various factors such as the right of way, supporting structures, conductor size, transmission voltage etc. Transmission voltage closely influences the economics of power transmission. Generally, electric power is transmitted using 3-phase AC system at high voltages. Before studying how to choose economic transmission voltages, one should know the advantages and limitations of high voltage transmission.

Advantages Of High Voltage Transmission

• Efficient transmission of larger amounts of power:
  In a 3 phase AC system, power is calculated as P=√3*VIcosɸ. It is clear that, for a large amount of power to be transmitted at a lower voltage, the amount of current will be very large. Let's take an example, 200 MW of power is to be transmitted at 11kV and consider cosɸ = 0.8 lagging. In this case, the amount of current that will flow through the line would be 200,000,000 / (√3 * 11,000 * 0.8) ≈ 13,122 A. For safely carrying this much large current, a conductor with very large diameter or much more number of conductors in bundled form may be required. And if the same power is transmitted at 220kV, the current would be 200,000,000 / (√3 * 220,000 * 0.8 ) ≈ 656 A. As the power lost in a conductor is given as I²R, you can see large saving in losses can be achieved by transmitting electricity at higher voltages. From this example, it is clearly not feasible and practical to transmit larger power at lower voltages. Also, transmission of electricity at higher voltages is more efficient.
• Saving in conductor material: As shown above, for the same amount of power transmitted at a higher voltage the current will be relatively lower. Current carrying capacity of a conductor depends on the diameter of the conductor (conductor size) along with few other factors. That means, for larger currents to be transmitted, the conductor size must be larger. Hence, transmitting power at higher voltages will reduce the amount of current to be carried and consequently the required conductor size would also be lesser.
• Improved voltage regulation:
  Decreased current will also result in decreased voltage drop across the line. Voltage regulation is defined as (V_S  - V_R)/V_S. As voltage drop is decreased, the difference between sending end voltage and receiving end voltage is also decreased. Thus, voltage regulation is improved.

  Limitations Of High Transmission Voltage

  With increase in the transmission voltage
• cost of insulators increases
• cost of transformers increases
• cost of switchgear increases
• cost of lightning arrestor increases
• cost of support towers increases (as taller towers with longer cross arms are required)

Economic Choice Of Transmission Voltage

From the above advantages and limitations of high voltage transmission, we can say that with increase in transmission voltage the cost of conductor material can be reduced and the efficiency can be increased. But the cost of transformers, insulators, switchgear etc. is increased at the same time. Thus, for overall economy, there is an optimum transmission voltage. The limit to use of higher transmission voltage is reached when the saving in cost of conductor material is offset by the increased cost of transformers, switchgear, insulators etc. The economical transmission voltage is one for which the sum of cost of conductor material, transformers, switchgear, insulators and other equipment is minimum.

If the power to be transmitted and the length of transmission are known, calculations are made for various transmission voltages. Initially, some standard transmission voltage is selected and the relative total cost of equipment is determined. A graph is drawn for the total cost of transmission with respect to various transmission voltages as shown in the figure at right. The lowest point on the curve gives the optimum transmission voltage. As here in the graph, point P is the lowest and the corresponding voltage OA is the optimum transmission voltage.

                                      

  The above method of finding economical transmission voltage very rarely used as it is hard to pre-determine the costs of various equipment. Instead, an empirical formula, according to the American practice, is used. According to this formula, an economical transmission voltage for a 3 phase AC system is given as,

Where, V = line voltage in kV
       P = maximum power per phase (in kW) to be delivered over single circuit
       L = distance of transmission in km

Economical transmission voltage depends on the power to be transmitted and the length of transmission. If the power to be transmitted is large, cost per kW of terminal equipment reduces. This results in increased economic transmission voltage. If the distance of transmission is increased, saving in the cost of conductor material can be significantly increased by increasing the transmission voltage.

 

                 TYPES OF CONDUCTORS

   A conductor is one of the most important components of overhead lines. Selecting a proper type of conductor for overhead lines is as important as selecting economic conductor size and economic transmission voltage. A good conductor should have the following properties:

• high electrical conductivity
• high tensile strength in order to withstand mechanical stresses
• relatively lower cost without compromising much of other properties
• lower weight per unit volume

•   This type is sometimes also referred as ASC (Aluminium Stranded Conductor). It is made up of strands of EC grade or Electrical Conductor grade aluminium. AAC conductor has conductivity about 61% IACS(International Annealed Copper Standard). Despite having a good conductivity, because of its relatively poor strength, AAC has limited use in transmission and rural distribution lines. However, AAC can be seen in urban areas for distribution where spans are usually short but higher conductivity is required.

                                 

      

  AAAC : All Aluminium Alloy Conductor

     These conductors are made from aluminium alloy 6201 which is a high strength Aluminium-Magnesium-Silicon alloy.This alloy conductor offers good electrical conductivity (about 52.5% IACS) with better mechanical strength. Because of AAAC's lighter weight as compared to ACSR of equal strength and current capacity, AAAC may be used for distribution purposes. However, it is not usually preferred for transmission. Also,AAACconductors can be employed in coastal areas because of their excellent corrosion resistance.
  

                                   

  ACSR : Aluminium Conductor, Steel Reinforced

      ACSR consists of a solid or stranded steel core with one or more layers of high purity aluminium (aluminium 1350) wires wrapped in spiral. The core wires may be zinc coated (galvanized) steel or aluminium coated (aluminized) steel. Galvanization or aluminization coatings are thin and are applied to protect the steel from corrosion. The central steel core provides additional mechanical strength and, hence, sag is significantly less than all other aluminium conductors. ACSR conductors are available in a wide range of steel content - from 6% to 40%. ACSR with higher steel content is selected where higher mechanical strength is required, such as river crossing. ASCR conductors are very widely used for all transmission and distribution purposes. 
  
  
  
  
                                    
  
  
      

  aCAR:Aluminium Conductor, Alloy Reinforced

  ACAR conductor is formed by wrapping strands of high purity aluminium (aluminium 1350) on high strength Aluminum-Magnesium-Silicon alloy (6201 aluminium alloy) core. ACAR has better electrical as well as mechanical properties than equivalent ACSR conductors. ACAR conductors may be used in overhead transmission as well as distribution lines.
   .
  
  

  Bundled Conductors

      Transmission at extra high voltages (say above 220 kV) poses some problems such as significant corona loss and excessive interference with nearby communication lines when only one conductor per phase is used. This is because, at EHV level, the electric field gradient at the surface of a single conductor is high enough to ionize the surrounding air which causes corona loss and interference problems. The electric field gradient can be reduced significantly by employing two or more conductors per phase in close proximity. Two or more conductors per phase are connected at intervals by spacers and are called as bundled conductors. The image at right shows two conductors in bundled form per phase. Number of conductors in a bundled conductor is greater for higher voltages.
  
  

     Factors determining conductor sizes

    voltage drop considerations:
        The conductor meets the minimum size requirement but transmits the power with an acceptable loss. It is often expressed as a maximum voltage drop of 5%. The total series impedance is equal to the maximum allowable voltage drop divided by the maximum load current.  

• THERMAL CAPACITY:

•        The conductor should be able to carry the maximum long term load current with out over heating.The conductor is assumed to withstand a temperature of 75 c without decrease in strength.Above this temperature, strength decreases.
  
•   ECONOMIC CONSIDERATIONS:
           The conductor is rarely seized to meet the minimum requirements.The total cost per kilometer or mile must be taken in to account as too the present worth of energy losses associated with the conductor.There must also be some compensation for load growth.
  

                                           POLES 

       The supporting structures for overhead line conductors are various types of poles and towers called line supports.In general, the line supports should have the following properties :
  

  (i) High mechanical strength to withstand the weight of conductors and wind loads etc.

  (ii) Light in weight without the loss of mechanical strength.

  (iii) Cheap in cost and economical to maintain.

  (iv) Longer life.

  (v) Easy accessibility of conductors for maintenance.
  
  

  The line supports used for transmission and distribution of electric power are of various types including wooden poles, steel poles, R.C.C. poles and lattice steel towers.The choice of supporting structure for a particular case depends upon the line span, X-sectional area, line voltage, cost and local conditions.
  
  

  1. Wooden poles : These are made of seasoned wood (sal or chir) and are suitable for lines of moderate X-sectional area and of relatively shorter spans, say upto 50 metres.Such supports are cheap, easily available, provide insulating properties and, therefore, are widely used for distribution purposes in rural areas as an economical proposition.The wooden poles generally tend to rot below the ground level, causing foundation failure.In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil.Double pole structures of the ‘A’ or ‘H’ type are often used to obtain a higher transverse strength than could be economically provided by means of single poles.
  The main objections to wooden supports are : 

  (i) tendency to rot below the ground level

  (ii) comparatively smaller life (20-25 years) 

  (iii) cannot be used for voltages higher than 20 kV

  (iv) less mechanical strength and require periodical inspection.                                                 

   Steel poles :

        The steel poles are often used as a substitute for wooden poles.They possess greater mechanical strength, longer life and permit longer spans to be used.Such poles are generally used for distribution purposes in the cities.This type of supports need to be galvanised or painted in order to prolong its life.The steel poles are of three types (i) rail poles (ii) tubular poles and (iii) rolled steel joints.

                              

   RCC poles 
      The reinforced concrete poles have become very popular as line supports in recent years.They have greater mechanical strength, longer life and permit longer spans than steel poles.Moreover, they give good outlook, require little maintenance and have good insulating properties.Figure shows R.C.C. poles for single and double circuit.The holes in the poles facilitate theclimbing of poles and at the same time reduce the weight of line supports.The main difficulty with the use of these poles is the high cost of transport owing to their heavy weight.Therefore, such poles are often manufactured at the site in order to avoid heavy cost of transportation. 
                 
                              

                    TOWERS 

         A transmission tower or power tower (electricity pylon in the United Kingdom and parts of Europe) is a tall structure, usually a steel lattice tower, used to support an overhead power line.
       They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes. Typical height ranges from 15 to 55 m (49 to 180 ft), though the tallest are the 370 m (1,214 ft) towers of a 2,700 m (8,858 ft) span of Zhoushan Island Overhead Powerline Tie. In addition to steel, other materials may be used, including concrete and wood.
        There are four major categories of transmission towers: suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual effect include undergrounding.
  
  

                TYPES OF TOWERS

           Transmission towers are used to pass signal wires and electrical current from place to place. They are usually made of steel and can run at times for long distances. Transmission towers are most often used when there is a large amount of electrical current to be distributed, usually between 115,000 and 765,000 volts. Several different designs of transmission towers are in wide use in the world today.

  LATTICE STEEL TOWERS

     Lattice steel towers are made up of many different steel structural components connected together with bolts or welded. Many different types of lattice steel towers exist. These towers are also called self-supporting transmission towers or free-standing towers, due to their ability to support themselves. These towers are not always made of steel; they can also be made of aluminum or galvanized steel.
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• TUBULAR STEEL POLES

                                     
  
  
               Tubular steel poles are another of the major types of transmission towers. They are made up of hollow steel poles. Tubular steel poles can be manufactured as one large piece, or as several small pieces which fit together.

  SINGLE AND DOUBLE CIRCUIT TOWERS

                                                                           
  
  
      Both tubular and lattice steel towers can be designed so as to support either one or two circuits of electrical current. Double-circuit towers hold the different conductors stacked atop one another, while in single-circuit towers the conductors are lined up horizontally.

  GUYED TOWERS

                                                 
  
  
     Guyed towers take up a lot of space, and are therefore only used in parts of the world where land use policy allows them. They consist of two masts supported by four guys, or support cables.
                                                  

  SUSPENSION STRAIGHT TOWERS

  Suspension straight towers are a type of self-supporting tower that stands along straight sections of a transmission route. These towers are also sometimes called tangent towers. The only function of these types of towers is to suspend the wires. They do not have to create or regulate tension in any way.
                                      

  SUSPENSION ANGLE TOWERS

                                                
  
  
     Suspension angle towers are built when it is necessary for the route of the electrical current to turn. These angle towers are usually designed so that the axis of the cross-arm bisects the angle of the conductors. This is the most efficient way to use the tower.

  ANCHOR AND ANGLE TENSION TOWERS

  Anchor and angle tension towers are used to sectionalize the routes. They terminate the conductors and they provide containment of possible cascade failures.
                                          
  
  

                    TOWER ACCESSORIS  

   

                      

                      BIRD GUADS

                                                       




                                                    

                                   FONDATION BOLTS

   

         

EARTHING RODS

PROTECTION OF TOWER FOOTING 

        PROTECTION OF TOWER FOOTING

 GENERAL:

     Special measures for protection of foundations shall be taken in respect of locations close to / in nallah, river beds, etc. Protection of foundations is also to be provided in the case of foundations located on the sloping ground of sand dunes or hills. 
      The above is to be done, based on site conditions, by employing any or a combination of the following three methods which are best suited for the site conditions.
  a) Benching.
  b) Protection against cutting of soil by flow of water.
  c) Rivetment.

 BENCHING:

    This method is generally used if the soil  is gently sloping and there is no significant difference in the levels of the soil around the foundation. The soil at the higher level is cut and spread in the lower level so that the soil near the foundation becomes level. 

 PROTECTION AGAINST CUTTING OF SOIL BY FLOW OF WATER:

   This method is generally used where the tower foundation is located at a distance from the edge of river / nalla, etc. The foundation is protected by providing suitable crate of galvanized wire netting and meshing packed with boulders. 

RIVETMENT:

     This method is generally used where the  ground surface is irregular or where there is significant difference in the levels of soil around the tower foundation. The rivetment protection is provided in the form of stone masonry walls around those sides of the foundation where such protection is required.

 Depending on the site conditions, the following are to be decided:
a) The side or sides on which the rivetment is to be provided.
b) Height of the masonry wall.
c) Length of the masonry wall.


                        

EARTHING

      To connect the metallic (conductive) Parts of an Electric appliance or installations to the earth (ground) is called Earthing or Grounding.

      In other words, to connect the metallic parts of electric machinery and devices to the earth plate or earth electrode (which is buried in the moisture earth) through a thick conductor wire (which has very low resistance) for safety purpose is known as Earthing or grounding.

      To earth or earthing rather, means to connect the part of electrical apparatus such as metallic covering of metals, earth terminal of socket cables, stay wires that do not carry current to the earth. Earthing can be said as the connection of the neutral point of a power supply system to the earth so as to avoid or minimize danger during discharge of electrical energy.

                                    

Need of Earthing or Grounding:

      The primary purpose of earthing is to avoid or minimize the danger of electrocution, fire due to earth leakage of current through undesired path and to ensure that the potential of a current carrying conductor does not rise with respect to the earth than its designed insulation.

       When the metallic part of electrical appliances (parts that can conduct or allow passage of electric current) comes in contact with a live wire, maybe due to failure of installations or failure in cable insulation, the metal become charged and static charge accumulates on it. If a person touches such a charged metal, the result is a severe shock.

        To avoid such instances, the power supply systems and parts of appliances have to be earthed so as to transfer the charge directly to the earth.

Basic needs of Earthing:

• To protect human lives as well as provide safety to electrical devices and appliances from leakage current.

• To keep voltage as constant in the healthy phase (If fault occurs on any one phase).
• To Protect Electric system and buildings form lighting.
• To serve as a return conductor in electric traction system and communication.
• To avoid the risk of fire in electrical installation systems.

Different Terms used in Electrical Earthing

• Earth:  The proper connection between electrical installation systems via conductor to the buried plate in the earth is known as Earth.
• Earthed: When an electrical device, appliance or wiring system connected to the earth through earth electrode, it is known as earthed device or simple “Earthed”.
• Solidly Earthed: When an electric device, appliance or electrical installation is connected to the earth electrode without a fuse, circuit breaker or resistance/Impedance, It is called “solidly earthed”.
• Earth Electrode: When a conductor (or conductive plate) buried in the earth for electrical earthing system. It is known to be Earth Electrode. Earth electrodes are in different shapes like, conductive plate, conductive rod, metal water pipe or any other conductor with low resistance.
• Earthing Lead: The conductor wire or conductive strip connected between Earth electrode and Electrical installation system and devices in called Earthing lead.
• Earth Continuity Conductor: The conductor wire, which is connected among different electrical devices and appliances like, distribution board, different plugs and appliances etc. in other words, the wire between earthing lead and electrical device or appliance is called earth continuity conductor. It may be in the shape of metal pipe (fully or partial), or cable metallic sheath or flexible wire.
• Sub Main Earthing Conductor: A wire connected between switch board and distribution board i.e. that conductor is related to sub main circuits.
• Earth Resistance: This is the total resistance between earth electrode and earth in Ω (Ohms). Earth resistance is the algebraic sum of the resistances of earth continuity conductor, earthing lead, earth electrode and earth.

Components of Earthing System

A complete electrical earthing system consists on the following basic components. 

• Earth continuity Conductor

• Earthing Lead

• Earth Electrode  

 Earth Continuity Conductor or Earth Wire

      That part of the earthing system which interconnects the overall metallic parts of electrical installation e.g. conduit, ducts, boxes, metallic shells of the switches, distribution boards,Switches, fuses, Regulating and controlling devices, metallic parts of electrical machines such as, motors, generators, transformers and the metallic framework where electrical devices and components are installed is known as earth wire or earth continuity conductor as shown in the above fig.

        The resistance of the earth continuity conductor is very low. According to IEEE rules, resistance between consumer earth terminal and earth Continuity conductor (at the end) should not be increased than 1Ω. In simple words, resistance of earth wire should be less than 1Ω.

                                

                                        

Earthing Lead or Earthing Joint

The conductor wire connected between earth continuity conductor and earth electrode or earth plate is called earthing joint or “Earthing lead”. The point where earth continuity conductor and earth electrode meet is known as “connecting point” as shown in the above fig


                        .

  Earthing lead is the final part of the earthing system which is connected to the earth electrode (which is underground) through earth connecting point.

     There should be minimum joints in earthing lead as well as lower in size and straight in the direction.

Generally, copper wire can be used as earthing lead but, copper strip is also used for high installation and it can handle the high fault current because of wider area than the copper wire.

Earthing Electrode or Earth Plate

    A metallic electrode or plate which is buried in the earth (underground) and it is the last part of the electrical earthing system. In simple words, the final underground metallic (plate) part of the earthing system which is connected with earthing lead is called earth plate or earth electrode.

   A metallic plate, pipe or rode can be used as an earth electrode which has very low resistance and carry the fault current safely towards ground (earth).

A hard drawn bare copper wire is also used as an earthing lead. In this method, all earth conductors connected to a common (one or more) connecting points and then, earthing lead is used to connect earth electrode (earth plat) to the connecting point.

To increase the safety factor of installation, two copper wires are used as earthing lead to connect the device metallic body to the earth electrode or earth plate. I.e. if we use two earth electrodes or earth plats, there would be four earthing leads. It should not be considered that the two earth leads are used as parallel paths to flow the fault currents but both paths should work properly to carry the fault current because it is important for better safety.

TYPES OF INSULATORS

There are several types of insulators but the most commonly used are pin type, suspension type, strain insulator and shackle insulator. 

 Pin type insulators :

       The part section of a pin type insulator is shown in Figure below.As the name suggests, the pin type insulator is secured to the cross-arm on the pole.There is a groove on the upper end of the insulator for housing the conductor.The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.

                    Pin type insulators are used for transmission and distribution of electric power at voltages up to 33 kV.  Beyond operating voltage of 33 kV, the

pin type insulators become too bulky and hence uneconomical.

Causes of insulator failure :

     Insulators are required to withstand both mechanical and electrical stresses.The latter type is primarily due to line voltage and may cause the breakdown of the insulator.The electrical breakdown of the insulator can occur either by flash-over or puncture.In flash over, an arc occurs between the line conductor and insulator pin (i.e., earth) and the discharge jumps across the air gaps, following shortest distance.Figure below shows the arcing distance (i.e. a + b + c) for the insulator.In case of flash-over, the insulator will continue to act in its proper capacity unless extreme heat produced by the arc destroys the insulator.

            In case of puncture, the discharge occurs from conductor to pin through the body of the insulator.When such breakdown is involved, the insulator is permanently destroyed due to excessive heat.In practice, sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage. The ratio of puncture strength to flash over voltage is known as safety factor i.e           

Advantages of pin-type insulators:

1.Widely used on high voltage distribution lines
2.Having a better anti-fog performance
3.Easily handle and manufacture
4.Can be mounted as necessary, vertically or horizontally

                                                     

  

 Suspension type insulators :

       The cost of pin type insulator increases rapidly as the working voltage is increased.Therefore, this type of insulator is not economical beyond 33  KV. For high voltages (>33 KV), it is a usual practice to use suspension type insulators shown in Figure below.They consist of a number of porcelain discs connected in series by metal links in the form of a string.The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.Each unit or disc is designed for low voltage, say 11 kV.The number of discs in series would obviously depend upon the working voltage.For instance, if the working voltage is 66 kV, then six discs in series will be provided on the string.

 Advantages Of suspension type insulators:

(i) Suspension type insulators are cheaper than pin type insulators for voltages beyond 33 kV.

(ii) Each unit or disc of suspension type insulator is designed for low voltage,usually 11 kV.

Depending upon the working voltage, the desired number of discs can be connected in series.

(iii) If any one disc is damaged, the whole string does not become useless because the damaged disc can be replaced by the sound one.

(iv) The suspension arrangement provides greater flexibility to the line.  The connection at the cross arm is such that insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum.

(v) In case of increased demand on the transmission line, it is found more satisfactory to supply the greater demand by raising the line voltage than to provide another set of conductors.The additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs.

(vi) The suspension type insulators are generally used with steel towers.  As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning.

                               

3. Strain insulators : 

     When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension.In order to relieve the line of excessive tension, strain insulators are used.For low voltage lines (< 11 kV), shackle insulators are used as strain insulators.However, for high voltage transmission lines, strain insulator consists of an assembly of suspension insulators as shown in Figure.The discs of strain insulators are used in the vertical plane.When the tension in lines is exceedingly high, as at long river spans, two or more strings are used in parallel.

Rated System Voltage	Number of disc insulator used in strain type tension insulator string	Number of disc insulator used in suspension insulator string
33KV	3	3
66KV	5	4
132KV	9	8
220KV	15	14

                                   

  

Shackle insulators : 

     In early days, the shackle insulators were used as strain insulators.But now a days,they are frequently used for low voltage distribution lines.Such insulators can be used either in a horizontal position or in a vertical position.They can be directly fixed to the pole with a bolt or to the cross arm.Figure below shows a shackle insulator fixed to the pole.The conductor in the groove is fixed with a soft binding wire.

                                     

   

General applications of transmission lines

 Transferring signals from one point to another Electrical transmission lines are very widely used to transmit high frequency signals over long or short distances with minimum power loss. One familiar example is the down lead from a TV or radio aerial to the receiver.

Signal transfer

   Electrical transmission lines are very widely used to transmit high frequency signals over long or short distances with minimum power loss. One familiar example is the down leadfrom a TV or radio aerial to the receiver.

Pulse generation

      Transmission lines are also used as pulse generators. By charging the transmission line and then discharging it into a resistive load, a rectangular pulse equal in length to twice the electrical length of the line can be obtained, although with half the voltage. A Blumlein transmission line is a related pulse forming device that overcomes this limitation. These are sometimes used as the pulsed energy sources for radar transmitters and other devices.

Stub filters

        If a short-circuited or open-circuited transmission line is wired in parallel with a line used to transfer signals from point A to point B, then it will function as a filter. The method for making stubs is similar to the method for using Lecher lines for crude frequency measurement, but it is 'working backwards'. One method recommended in the RSGB's radio communication handbook is to take an open-circuited length of transmission line wired in parallel with the feeder delivering signals from an aerial. By cutting the free end of the transmission line, a minimum in the strength of the signal observed at a receiver can be found. At this stage the stub filter will reject this frequency and the odd harmonics, but if the free end of the stub is shorted then the stub will become a filter rejecting the even harmonics.

                 General considerations

    Design should satisfy

            1. All the electrical and mechanical considerations

            2. Current carrying capacity(required power transfer can take pace with out 

                 excessive  voltage drop)

            3.Line losses should be slow and line should cope to system voltage.

            4.Line conductors, supports cross arm should cope with worst weather conditions and                   give satisfactory service with out much maintenance.

          5.Tension in the conductor should be below breaking load(SAG)and reasonable FOS  

              should be  used.