TRANSMISSION 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 : Aluminium  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. Clearances
 20. BOQ of 33 kv transmission line
 21.Types of fault in electrical power system
 22.Testing and Commissioning

 23.General application

 24..General considerations 

                                                 1.  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 wave guides are used instead, which function as "pipes" to confine and guide the electromagnetic waves.Some sources define wave guides as a type of transmission line; however, this article will not include them. At even higher frequencies, in the terahertz, infrared and light range, wave guides in turn become loss, 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.

      2. 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.

                     3. Component of Transmission line

3.1 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 common electrical conductor.

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

                                                      
                                                 

 3.2  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 carry in 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.

3.3 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 semi conductor 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 conductive when 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 and Teflon, 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 and cables. Examples include rubber-like polymers and most plastics.

                                       
   

3.4 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 and 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 f t), though the tallest are the 370 m (1,214 f t) towers of a 2,700 m (8,858 f t) span of Zhoushan Island Overhead Power line 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 under grounding.                                                   

                                                                                        

                                   

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3.5 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.

                              
                                        
                                 

                              4. 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 conductor.

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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 in case of a stranded conductor than a solid conductor.

                         5. 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.

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Factors affecting corona:

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

  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.

   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.

    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.

    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.

    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 

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                              6. 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.

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 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. Itis 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  of the lines and therefore the voltage is regulated within the prescribed limits.

                    7. 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 to ground, 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.  

                8. Types of Transmission Lines

 8.1 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.

                                      
                                     

  8.2 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.

                                    
                                      

8.3 Parallel plate transmission line or planar line:

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

  8.4 Micro strip transmission line:

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

                                         

  

 

              9.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. 

                  10. Economic Choice Of Transmission Voltage

     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 (VS  - VR)/VS. 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

·        . 1. cost of insulators increases

·         2.cost of transformers increases

·         3.cost of switch gear increases

·         4.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,

formula for economic choice of transmission voltage

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.
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                       11.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:

·        1. high electrical conductivity

·         2.. high tensile strength in order to withstand mechanical stresses

·          3.  relatively lower cost without compromising much of other properties

·               lower weight per unit volume
 

1   11.1 ASC :  Aluminium Standard Conductor

      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.

 

                    

 

11.2 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.

                          

                                

 11.3 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.

                                 

                             

   11.4 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.

   11.5 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.

                                          

            12.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.

                                 13.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.

       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.

                                  
    

                                                14.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

Types of Transmission Tower:

    According to different considerations, there are different types of transmission towers.  The transmission line goes as per available corridors. Due to unavailability of shortest distance straight corridor transmission line has to deviate from its straight way when obstruction comes. In total length of a long transmission line there may be several deviation points.

    According to the angle of deviation there are four types of transmission tower-

A – type tower – angle of deviation 0 to 2deg.

B – type tower – angle of deviation 2 to 15deg.

C – type tower – angle of deviation 15 to 30deg.

D – type tower – angle of deviation 30 to 60deg.

   As per the force applied by the conductor on the cross arms, the transmission towers can be categorized in another way-

    Tangent suspension tower and it is generally A - type tower.Angle tower or tension tower or sometime it is called section tower. All B, C and D types of transmission towers come under this category.

A part from the above customized type of tower, the tower is designed to meet special usages listed below, 

                                             

                                                

These are called special type tower

River crossing tower

Railway/ Highway crossing tower

Transposition tower

Based on numbers of circuits carried by a transmission tower, it can be classisfied as-

Single circuit tower

Double circuit tower

Multi circuit tower.

             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.


                                            .

    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.

                                                    

                            15.TOWER ACCESSORIS

                 

                       

BIRD GUARDS

                                           

FONDATION BOLTS

                              

           

                                

EARTHING RODS
                                                    

      16.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 / nallah, 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.

                                                       17. 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.

·         1.Earth continuity Conductor

·        2. Earthing Lead

·         3.Earth Electrode  

     Earth continuity 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 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 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.


                                             

                   18. TYPES OF INSULATORS

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

18.1 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, thepin 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.

18.2 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.

18.3 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.

                               

18.4 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.


                                                19.CLEARANCES

  Ground Clearance:

   Ground clearence means the height of middle of line between two towers from ground... it is so designed as the sag is minimum and stress on transmission line is also minimum.

    Clearance above ground of the lowest conductor (including guard – wires) is to meet the following conditions:

 1. No conductor of an overhead line including service lines erected across a street shall be at any part there of be at a height less than:

 a) For low & medium voltage line (i.e. up to 650 volts) – 5.791 mts

 b) For high voltage lines (up to 33 KV) – 6.096 mts

 2. Along a street: a) For low& medium voltage lines _5.486 mts

 b) For high voltage lines(up to 33 KV) _ 5.794 mts

3. Elsewhere other than 1,2 above

 a) For high voltage up to 11KV - 4.572 mts

     Clearance from Building, Structure etc a. The vertical clearance above building (from the highest point) on the basis of maximum sag shall not be less than (2.439 m) 8 ft for low and medium voltage line (up to 650 volts) and (3.64 m) 12 ft for 11 and 33 KV line. b. The horizontal clearance is to be 4 ft (1.219 m) upto 11 KV line and 6 ft (1.820 m) for 33 KV line.

i)                    It may be mentioned that for Railway crossing, rules as prescribed by Railways are to be followed

ii)                   ii) Similarly the lines crossing or in proximity to the telecommunication lines, the overhead line is to be protected as per code of practice laid down by PTCC coordination committee.

                         

CLEARANCE ABOVE GROUNDS

(Clause 77 of Indian Electricity Rules)

	m. m.
33 KV	5100
66 KV	5490
132 KV	6100
220 KV	7015
400 KV	8840

                                                                               

                     

The minimum clearances of conductor over rivers, which are not navigable, shall be kept 3.05 m over maximum flood level.

The minimum clearances between the conductors of a power line and telecommunication cable shall be:

132 kV	2.44 m
220 kV	2.74 m
400 kV	4.88 m

The minimum spacing between power lines shall be:

132 kV	2.75 m
220 kV	4.55 m
400 kV	6.00m

The spacing of conductors is determined by considerations, which are partly electrical and partly mechanical. Usually conductors will swing synchronously (in phase) with the wind, but with long spans and small size of conductors, there is always possibility of the conductors swinging non- synchronously, and the size of the conductor and the maximum sag at the centre of the span are factors, which should be taken into account in determining the phase distance apart at which they should strung. As a rule of thumb, minimum horizontal spacing between conductors should not be less than 1% of the span length in order to minimize the risk of phases coming into contact with each other during swing.

There are number of empirical formula in use, deduced from spacing, which have successfully operated in practice:

NESC, USA formula

Horizontal spacing in cm,

Where A = 0.762 cm per kV line voltage

S = Sag in cm, and

L = Length of insulator string in cm

Swedish formula

Horizontal spacing in cm,

Where S = Sag in cm and

E = Line voltage in kV

French formula

Horizontal spacing in cm,

Where S = Sag in cm

L = Length of insulator string in cm

E = Line voltage in kV

Tower top clearance

Tower top clearance is the vertical clearance between earthwire and top conductor, which is governed by the angle of shielding. The shield angle varies from about 250 to 300,depending on the configuration of conductors. Tower top clearance shall be taken 1.5 and 2.25 m for 132 kV and 220 kV respectively for 00 swing.

Clearances from building:

(a) Where a high or extra high voltage over head line passes above or adjacent to any

building or part of a building, it shall have on the basis of maximum sag, a vertical

clearance above the highest part of the building immediately under such line, of not

less than :

1	for high voltage line upto and including.      33 KV	3.658 mtrs. (12 ft.)
2	for extra high voltage lines	3.685 mtrs. (12 ft.) plus                                                                                             0.305 mtrs. (1 ft.) for                                                                every additional 33 KV or part thereof.

(    

(b) The horizontal clearance between the nearest conductor and any part of such

building shall on the basis of maximum deflection due to wind pressure, be not less

than :

1	for high voltage line up  to andincluding11KV	1.219 mtrs. (4ft.)
2	for high voltage line above 11 KV and up to and including 33 KV	1.829 mtrs. (6ft.)
3	for extra high voltage line	1.829mtrs.(6ft)plus                                                                                           0:305 mtrs, (1 ft.)for                                                                    every additional 33 KV  or part thereof.

                                               

FOREST CLEARANCE WHERE NO TREES ARE REQUIRED TO BE FELLED

   Only following information shall be required to be submitted in the prescribed format for obtaining forest clearance in case of those transmission lines, which do not involve felling of trees :—

i)   Geographical location of transmission in along with Index map.

ii)   Purpose for which forest land is required to be used.

iii) Area of forest land to be used.

iv)   Legal status of the forest land.

v)    Whether forest land is Ear-marked for any National Garden/Park wild life, part                     reserved   wild life, vegetation or rehabilitation which are in danger of extinction.

vi)  Whether there is any other alternative route to save forest area, and whether the forest area being used, is minimum required for the purpose. In this regard a certificate of the Regional Forest Officer shall have to be submitted along with the proposal.

vii) Compensatory A forestation Schemes.

viii) A certificate clearly stating that no felling of tree is required in the proposal.

ELECTRICAL CLEARANCE:

      The minimum distance permitted between fixed structures and parts energized at contact line voltage is said to be a electrical clearance.

 As per 1SS 162—1961 minimum electrical clearance from live part to earth and safety clearance in case of different voltage must be kept as follows :

VOLTAGE	ELECTRICALCLEARANCE(mm)       PhaseEarth/ PhasePhase	SAFETY CLEARANCE  IN  SIS (mm)
KV
33	381                   432	2740
66	658                   786	3050
132	1127                 1473	3810
220	2082                 2388	4570
4000	350                   4000	6100

 

MINIMUM CLEARANCE BETWEEN POWER LINES:

"Power line crossing" means an electrical overhead line or under-ground cable placed across railway track(s) for the transmission and/or distribution of electrical energy. It may also be referred to as a "Crossing" in these Regulations.

Nominal System Voltage of line to be crossed:

KV                   11               33        66        132      220        400

11                     2.44            2.44     2.44     3.05     4.58     6.10

33                                        2.44      2.44     3.05     4.58     6.10

66                                                     2.44      3.05     4.58     6.10

132                                                                3.05     4.58     6.10

220                                                                            4.58     6.10
 
400                                                                                        6.10
 

CLEARANCE FROM RAILWAY TRACKS:

(As per Regulation for Electrical Crossing of Railway Tracks 1963)

  The relevant provisions for the crossings of Railway Tracks by the power lines are as under: The minimum height above rail level of the lowest portion of any conductor under conditions of maximum sag are as follows in accordance with the Regulations for Electrical Crossings of Railway Tracks, 1963:

(i)  FOR UNELECTRIFIED TRACKS OR TRACKS ELECTRIFIED ON 1500 VOLTS D.C.

                                                     Broad Gauge             Meter and Narrow Gauge

                                                    Inside    Outside            Inside  Outside

                                                    station   station               station  station

                                                    limits      limits                limits  limits

                                                     (mm)     (mm)                (mm)    (mm)

66 KV                                        10,300    7,900                  9,100  6,700

132 KV                                      10,900    8,500                   9,800  7,300

 220 KV                                      11,200   8,800                   10,000 7,600

440 KV*                                      13,600  11,200             12,400 10,000

(ii) TRACKS ELECTRIFIED ON 25 KV A.C.

                                                      For Broad, Meter and Narrow Gauges

                                                                        Inside        Outside

                                                                         station      station

                                                                          limits       limits

                                                                           (mm)       (mm)

66 KV                                                              13,000      11,000

132 KV                                                            14,000      12,000

220 KV                                                            15,300     13,300

440 KV*                                                          16,300     14,300

* Tentatively assumed.                

     

No conductor of an extra high voltage overhead line crossing a tramway or trolley bus using trolley wires should have a clearance less than 3050 mm above the trolley line.

The provisions of the above Regulations must be kept in mind while carrying out the patrolling of Transmission lines. Any deviation noticed should be reported / attended on top-priority

CLEARANCE OF RIGHT OF WAY:

An electric transmission line right-of-way (ROW) is a strip of land used by Electrical utilities to construct operate, maintain and repair the transmission line facilities.

The width of a right-of-way depends on the voltage of the line and the height of the structures. The right-of-way generally must be clear of unauthorized structures that could interfere with a power line.

 The width of right of way for the various line voltages is repeated below.

Line Voltage	Width of Right of Way
132 kV	27 metres
220 kV	35 metres
400 kV	52 metres

 

i)          A drawing showing the requirements of line clearance within the right of way is given atAppendix – A.

ii)                  Cutting of trees, shrubs, bushes, etc. in the right of way is to be got done as shown in thedrawing above. All trees, shrubs, bushes, etc. which infringe on the clearances are to becut.

iii)                Small bush growth, shrubs and trees whose height is not expected to rise beyond 3 metersmay be allowed to remain.

iv)                Grass growth on the boundary walls (Dola) of agriculture fields which can grow to a heightsuch as to infringe on the clearance are to be cut.

v)                  Trees outside right of way but of such height as may infringe on line clearance are to betrimmed accordingly.

vi)                Trees or bushes growing inside or very close to the legs of towers shall be cut / removed



                                      
    


                           

                                     

 




                     20.BOQ OF 33 KV TRANSMISSION LINE

Discription of work

   1.Supply and erection of M+6 Towers, Excavation of pit and setting of  stubsposition duly filling it with CC mix ( 20mm HBG metal) and complete erection of  M+6 Towers

                                  

	Qty	Unit
Material	3	EA
Labour	3

                

2 Supply of 7/4 RS Joist 12Mtr Box poles, Excavation of pit and erection of pole into position by mass concreteing below the ground level and coping of the pole above the ground level                                        Qty              Unit

                        Material           17                EA           

                        Labour             17                EA

3. Supply of 100Sqmm AAA Doc Conductor and stringing 100Sqmm  AAA conductor.
                                                  Qty             Unit

                        Material           6.25             KM

                        Labour             6.25             KM   

                                   

4 Supply of 33KV 100x50mm V Cross arms with back clamps, top fitting completed nuts & bolts                       

                                               Qty             Unit     

                        Material          55               No         
                        Labour            55               No       


         

                                            

                       

5. Supply of 33KV 100x50mm Channel cross arms with back clamp

                                                Qty           Unit                       

                        Material           5               No            

                        Labour             5               No   
                                                    
  

6. Supply and erection of 33KV Pin Insulators with pins

                                                 Qty           Unit           

                        Material           125            No                                   

                        Labour             125            No    

   
                             

                          

7. Supply and erection 33KV Polymer Disc Insulators with metal parts                                                                                                   Qty          Unit

                        Material            50            No            

                        Labour              50            No             

                                 

8. Supply and Erection of 40 mm GI Earth flat        

                                                Qty           Unit

                        Material            7              No            

                        Labour              7              No      

                          

9. Supply & Painting two coat red oxide and two coat Aluminium Painting towers and RS Joist Pols, V X Arms, Channels, Back Clamps, top clamps  

                                                            Qty          Unit

                        Material                       LS                  

                        Labour                         LS         

   

               21.Types of Faults in Electrical Power Systems

Types of Faults

Electrical faults in three-phase power system mainly classified into two types, namely open and short circuit faults. Further, these faults can be symmetrical or unsymmetrical faults. Let us discuss these faults in detail.

Open Circuit Faults

These faults occur due to the failure of one or more conductors. The figure below illustrates the open circuit faults for single, two and three phases (or conductors) open condition.

The most common causes of these faults include joint failures of cables and overhead lines, and failure of one or more phase of circuit breaker and also due to melting of a fuse or conductor in one or more phases.

Open circuit faults are also called as series faults. These are unsymmetrical or unbalanced type of faults except three phase open fault.

Consider that a transmission line is working with a balanced load before the occurrence of open circuit fault. If one of the phase gets melted, the actual loading of the alternator is reduced and this cause to raise the acceleration of the alternator, thereby it runs at a speed slightly greater than synchronous speed. This over speed causes over voltages in other transmission lines.

Thus, single and two phase open conditions can produce the unbalance of the power system voltages and currents that causes great damage to the equipments.

                      

Causes

Broken conductor and malfunctioning of circuit breaker in one or more phases.

Effects

1.      Abnormal operation of the system

2.      Danger to the personnel as well as animals

3.      Exceeding the voltages beyond normal values in certain parts of the network, which further leads to insulation failures and developing of short circuit faults.

Although open circuit faults can be tolerated for longer periods than short circuit faults, these must be removed as early as possible to reduce the greater damage.

Short Circuit Faults

A short circuit can be defined as an abnormal connection of very low impedance between two points of different potential, whether made intentionally or accidentally.

These are the most common and severe kind of faults, resulting in the flow of abnormal high currents through the equipment or transmission lines. If these faults are allowed to persist even for a short period, it leads to the extensive damage to the equipment.

Short circuit faults are also called as shunt faults. These faults are caused due to the insulation failure between phase conductors or between earth and phase conductors or both.

The various possible short circuit fault conditions include three phase to earth, three phase clear of earth, phase to phase, single phase to earth, two phase to earth and phase to phase plus single phase to earth as shown in figure.

The three phase fault clear of earth and three phase fault to earth are balanced or symmetrical short circuit faults while other remaining faults are unsymmetrical faults.

Causes

These may be due to internal or external effects.

  Internal effects include breakdown of transmission lines or equipment, aging of insulation, deterioration of insulation in generator, transformer and other electrical equipments, improper installations and inadequate design.

2.      External effects include overloading of equipments, insulation failure due to lighting surges and mechanical damage by  public.

Effects

1.        Arcing faults can lead to fire and explosion in equipments such as transformers and circuit breakers.

2.      Abnormal currents cause the equipments to get overheated, which further leads to reduction of life span of their insulation.

3.      The operating voltages of the system can go below or above their acceptance values that creates harmful effect to the service rendered by the power system.

4.      The power flow is severely restricted or even completely blocked as long as the short circuit fault persists.

      

                                

                                 

Symmetrical and Unsymmetrical Faults

As discussed above that faults are mainly classified into open and short circuit faults and again these can be symmetrical or unsymmetrical faults.

Symmetrical Faults

    A symmetrical fault gives rise to symmetrical fault currents that are displaced with 1200 each other. Symmetrical fault is also called as balanced fault. This fault occurs when all the three phases are simultaneously short circuited.

  These faults rarely occur in practice as compared with unsymmetrical faults. Two kinds of symmetrical faults include line to line to line (L-L-L) and line to line to line to ground (L-L-L-G) as shown in figure below.

   A rough occurrence of symmetrical faults is in the range of 2 to 5% of the total system faults. However, if these faults occur, they cause a very severe damage to the equipments even though the system remains in balanced condition.

 The analysis of these faults is required for selecting the rupturing capacity of the circuit breakers, choosing set-phase relays and other protective switchgear. These faults are analyzed on per phase basis using bus impedance matrix or Thevenins’s theorem.

                             

Unsymmetrical Faults

The most common faults that occur in the power system network are unsymmetrical faults. This kind of fault gives rise to unsymmetrical fault currents (having different magnitudes with unequal phase displacement). These faults are also called as unbalanced faults as it causes unbalanced currents in the system.

Up to the above discussion, unsymmetrical faults include both open circuit faults (single and two phase open condition) and short circuit faults (excluding L-L-L-G and L-L-L).

The figure below shows the three types of symmetrical faults occurred due to the short circuit conditions, namely phase or line to ground (L-G) fault, phase to phase (L-L) fault and double line to ground (L-L-G) fault.

 A single line-to-ground (LG) fault is one of the most common faults and experiences show that 70-80 percent of the faults that occur in power system are of this type. This forms a short circuit path between the line and ground. These are very less severe faults compared to other faults.

A line to line fault occur when a live conductor get in contact with other live conductor. Heavy winds are the major cause for this fault during which swinging of overhead conductors may touch together. These are less severe faults and its occurrence range may be between 15-20%.

In double line to ground faults, two lines come into the contact with each other as well as with ground. These are severe faults and the occurrence these faults is about 10% when compared with total system faults.

Unsymmetrical faults are analyzed using methods of unsymmetrical components in order to determine the voltage and currents in all parts of the system. The analysis of these faults is more difficult compared to symmetrical faults.

This analysis is necessary for determining the size of a circuit breaker for largest short circuit current. The greater current usually occurs for either L-G or L-L fault.

                

Protection Devices against Faults

When the fault occurs in any part of the system, it must be cleared in a very short period in order to avoid greater damage to equipments and personnel and also to avoid interruption of power to the customers.

The fault clearing system uses various protection devices such as relays and circuit breakers to detect and clear the fault.

Some of these fault clearing or faults limiting devices are given below.

Fuse

It opens the circuit whenever a fault exists in the system. It consists of a thin copper wire enclosed in a glass or a casing with two metallic contacts. The high fault current rises the temperature of the wire and hence it melts. A fuse necessitates the manual replacement of wire each time when it blows.

Circuit Breaker

It is the most common protection device that can make or break the circuit either manually or through remote control under normal operating conditions.

There are several types of circuit breakers available depending on the operating voltage, including air brake, oil, vacuum and SF6 circuit breakers. For more information on circuit breakers, follow the link attached.

                      
                      
                   

Protective Relays

These are the fault detecting devices. These devices detect the fault and initiate the operation of the circuit breaker so as to isolate the faulty circuit. A relay consists of a magnetic coil and contacts (NC and NO). The fault current energizes the coil and this causes to produce the field, thereby the contacts get operated.

Some of the types of protective relays include

1.      Magnitude relays

2.      Impedance relays

3.      Directional relays

4.      Pilot relays

5.      Differential relays

Lighting Arrestor

Surges in the power system network caused when lightning strikes on transmission lines and equipments. This causes high voltage and currents in the system. These lighting faults are reduced by placing lighting arrestors at transmission equipments.

                                 

            22.TESTING AND COMMISSIONING

TESTING:

    After final checking is carried out and there are no defects / shortcomings in the work of the transmission line, the line is considered as having been completed and clear forenergizing.

    Pamphlets bearing the warning notice as shown in Appendix – A are got circulated in thearea through which the transmission line is passing.

    The concerning Executive Engineer shall be present at the time of charging the transmissionline who shall ensure that all testing and checking has been done and approval of therelevant authorities has been obtained.

    Before commissioning the line, the tests given below are to be carried out.

 INSULATION RESISTANCE TEST:

      This test is carried with a motor driven megger / insulation tester of at least 5 kV rating.This test is carried out to ascertain the insulation condition of the line.

        Measures for ensuring safety from induced high voltages in the lines should be taken before carrying out this test.

        The line is kept open at the other end. The insulation resistance is measured between each phase and ground, and between the phases. The ambient temperature and weather conditions are noted for future reference.

         The insulation resistance values are dependent on the ambient temperature and weather conditions prevailing at the time when these are measured. Therefore no comparable values are prescribed. However, the values should not be ZERO. The observed values are recorded.

 Conductor Continuity Test:

          The electrical resistance of the conductors is to be measured with a Wheatstone Bridge or other suitable instrument. This test is carried out to verify that each conductor of the transmission line is properly connected electrically. This is verified by comparing with the electrical resistance of a continuous conductor of the same size and length after correcting it to the temperature at which measurement has been made.

          Measures for ensuring safety from induced high voltages in the lines should be taken before carrying out this test.

           For measurement of the resistance, the line is got earthed at the other end. The resistance of each phase to ground is measured. The ambient temperature is noted for reference. The observed values are recorded and compared as given at para below.

            The maximum values of electrical resistance at 20°C of the conductors used in the transmission lines are given below as per IS 398 (Part 2) – 1996 / IS 398 (Part 5) – 1992

No	Transmission line voltage	Code name of conductor	Resistance ohms per km	Resistance ohms per km per phase
1	400 KV	ACSR Moos	0.05552	0.02776
2	220 kV	ACSR Zebra	0.06868	0.06868
3	132 kV	ACSR Panther	0.13900	0.13900

     23. 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 lead from 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.

                      24.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.  

Testing:.........