4th-Sem Elect Generation Transmission Distribution Lecture Notes PDF

KIIT POLYTECHNIC LECTURE NOTES ON Generation, Transmission & Distribution Compiled by Mr. Sunil Kumar Bhatta (Lecturer in Department of Electrical Engineering, KIIT Polytechnic BBSR) ...

KIIT POLYTECHNIC LECTURE NOTES ON Generation, Transmission & Distribution Compiled by Mr. Sunil Kumar Bhatta (Lecturer in Department of Electrical Engineering, KIIT Polytechnic BBSR)  CONTENTS Sl. Chapter Name Page No. No. 1 GENERATION OF ELECTRICITY 1-16 2 TRANSMISSION OF ELECTRIC POWER 17-23 3 OVER HEAD LINE 24-32 4 PERFORMANCE OF SHORT & MEDIUM LINES 25-44 5 EHV TRANSMISSION 45-49 6 DISTRIBUTION SYSTEM 50-58 7 UNDERGROUND CABLE 59-69 8 ECONOMICS ASPECTS 70-81 9 TYPES OF TARIFF 82-85 10 SUBSTATION 86-90  KIIT POLYTECHNIC CHAPTER-1 GENERATION OF ELECTRICITY HYDRO ELECTRIC POWER STATION The power station which convert the kinetic energy of water in electrical energy are called hydroelectric power station. ELEMENTS OF HYDRO ELECTRIC POWER PLANT: Catchment Area The whole area behind the clam training into a stream as river across which the dam has been built at suitable place is called catchments area Reservoir A reservoir is employed to store water which is further utilized to generate power. Dam A dam is a barrier which confines or raises water for storage or diversion to create a hydraulic head. Dam’s are generally made of concrete, Stone masonry Rock fill or Timber Turbine and generator 1 Generation, Transmission & Distribution  KIIT POLYTECHNIC Turbine & Generator is the most important part of any power plant This combination is known as THE HEART OF THE POWER PLANT. TURBINE: - Turbine is a very light fan like structure having many numbers of blades. Ithas an ability to rotate on its axis when water passes through it. GENERATOR :- Generator is a device in which converts mechanical energy of turbine intoelectrical energy Draft tube It allows the turbine to be set above tail water level without loss of head, to facilitateinspection and maintenance. It regains by diffuser action, the major portion of the kinetic energy delivered to it from therunner. It increases the output power. It increases the efficiency of Hydro Power Plant. 2 Generation, Transmission & Distribution  KIIT POLYTECHNIC Penstock Penstock is the connecting pipe between the dam & the turbine house. Penstock is made up of a very strong material which can sustain the high pressure of water. Spill ways Spill Way’s is a kind of canal provided besides the dam. Spill Way’s is used to discharge the excess of accumulation of water on the dam because excess accumulation of water may damage the dam structure Surge tank When there is a sudden close of gates or change in pressure due to sudden change in rate of water flow. This creates a high pressure zone in the penstock due to which it may burst. For releasing excess amount of water there is a tank. This tank is called as Surge Tank Initially the water of the river is in Catchments Area From catchments area the water flows to the dam. At the dam the water gets accumulated. Thus the potential energy of the water increases due to the height of the dam.When the gates of the dam are opened. Then the water moves with high Kinetic Energy into the penstock. Through the penstock water goes to the turbine house. Since the penstock makes water to flow from high altitude to low altitude, Thus the Kinetic Energy of the water is again raised the turbine house the pressure of the water is controlled by the controlling valves as per the requirements. The controlled pressurized water is fed to the turbine. 3 Generation, Transmission & Distribution  KIIT POLYTECHNIC Due to the pressure of the water the light weight turbine rotates Due to the high speed rotation of the turbine the shaft connected between the turbine and the generator rotates Due to the rotation of generator the ac current is produced This current is supplied to the power house From powerhouse it is supplied for the commercial Purpose Advantages No fuel charges. Less supervising staff is required. Maintenance & operation charges are very low. Running cost of the plant is low. The plant efficiency does not change with age. It takes few minutes to run & synchronize the plant. No fuel transportation is required. No ash & flue gas problem & does not pollute the atmosphere. These plants are used for flood control & irrigation purpose. Long life in comparison with the Thermal & Nuclear Power Plant Disadvantages The initial cost of the power plant is very high. Takes long time for construction of the dam. Generally, such plants are located in hilly areas far away from load center & thus they require long transmission lines & losses in them will be more. Power generation by hydro power plant is only dependent on natural phenomenon of rain. Therefore at the time of drought or summer session the Hydro Power Plant will not work. VALVE & GATES: Generally these are fitted at Entrance to the turbine during in section & repairing these are shut off. 4 Generation, Transmission & Distribution  KIIT POLYTECHNIC TAIL RACE: The water from turbine is discharged to the tail race generally tail race may be same stream or another one but design & size of tail race should be search that water are free exist. PRIME MOVERS / WATER TURBINES: In hydro power plant water turbines are used as prime movers which convert kinetic Energy of water into mechanical energy which is further utilized to drive the alternators generating electric at energy. SELECTION OF SIDE FOR HYDRO ELECTRIC POWER PLANT: These are some factors which are taken in to consideration for the selection of site for hydroelectric power plant i.e. 1. Availability of water: Hydroelectric power plant should be built where there adequate water available at goof head or huge quantity of water is flowing across a given point 2. Water Storage: For continuous supply of water. The water storage in suitable reservoir at height or building of dam across the river is essential so convient accommodation for the erection of a dam per Reservoir must be available. 3. Water Head: It has a considerable effect on the cost & economy of power generation i.e. an increasing effective head reduces the quantity of storage water & handle by pen stock screens & turbine resulting reduction in cost. 4. Distance from Load Centre: Generally these plants locate far away from load center so roots & distances affects oneconomical transmission. 5 Generation, Transmission & Distribution  KIIT POLYTECHNIC Thermal power plant A generating station which converts heat energy of coal combustion into electrical energy is known as a steam or thermal power station 6 Generation, Transmission & Distribution  KIIT POLYTECHNIC Layout of steam power plant It is basically works on rankine cycle. Steam produced in boiler by utilizing the heat energy of coal combustion. The steam is expanded in prime mover (i.e steam turbine) and it is condensed in condenser to be fed into boiler again. The steam turbine drives the alternator which converts mechanical energy of turbine into electrical energy Major components of thermal power plant Coal and ash handling plant The coal is stored in coal storage plant and from coal storage plant coal is delivered to coal handling plant where it pulverized (i.e crushed into small pieces) in order to increase surface exposure and rapid combustion. The coal is burnt in the boiler and the ash produced after complete combustion of coal is removed to the ash handing plant and then delivered to the ash storage plant for disposal 7 Generation, Transmission & Distribution  KIIT POLYTECHNIC Boiler The heat of combustion of coal in the boiler is utilized to co0nvert water into steam at high temperature and pressure. The flue gases from boiler make their journey through superheated economizer, air pre-heater and finally exhausted to atmosphere through the chimney Super heater The steam produced in boiler is wet and is passed through the superheater where it is dried and superheated. The superheated steam from the superheater is fed to steam turbine through main valve Economizer An economizer is essentially a feed water heater and derives heat from flue gases for this purpose. The feed water is fed to the economizer before supplying to the boiler. The economizer extracts a part of heat of flue gases to increase the feed water temperature Air pre –heater An air pre-heater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Alternator The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer, circuit breaker, and isolators Condensers A condenser is a device which condenses the steam at the exhaust of turbine. The condensed steam is used as feed water for boiler Cooling arrangement The hot water coming out from the condenser is discharged at a suitable location down the river. During the scarcity of water cooling towers are used Water treatment plant Boiler require clean and soft water for longer life and better efficiency so here the water is softened by removing temporary and permanent hardness through different chemical process Advantages The fuel used is quite cheap Less installation cost Less space required compare to the hydroelectric power station 8 Generation, Transmission & Distribution  KIIT POLYTECHNIC Running cost is lesser than diesel power plant 9 Generation, Transmission & Distribution  KIIT POLYTECHNIC Disadvantages It pollutes the atmosphere Running cost is more compared to hydroelectric plant SELECTION OF SITE FOR THERMAL POWER PLANT:- There are different factors which govern the site selection. 1. Near to the load center:- It locates near the load center resulting low transmissions cost & loss. 2. Supply of water:- Large quantity of water is required 1. To raise steam in boilers. 2. For cooling 3. For carrying disposal of Ash. 4. For drinking 3. Availability of coal:- It required huge amount of coal so plants are located near the coal mines to avoid the transport of coal & ash. 4. Load requirement - Land is requires not only for setting of plant but also other purposes for staff colony, coal storage ash disposal etc. 10 Generation, Transmission & Distribution  KIIT POLYTECHNIC NUCLEAR POWER STATION A generating station in which nuclear energy is converted into electrical energy is known as a NuclearPower Station. Basically, Nuclear power station is based on nuclear fission which is carried out between the two heavy elements such as Uranium (U 235) or Thorium (Th 232) in a special apparatus known as Nuclear reactor. The heat energy then released is utilized in raising steam at high temperature and pressure. The heated steam is then utilized to run the steam turbine which converts steam energy into mechanical energy. Lastly the turbine drives the alternator which converts mechanical energy into electrical energy. SCHEMATIC ARRANGEMENT OF NUCLEAR POWER STATION: The schematic arrangement of a nuclear power station is shown in fig. The whole arrangement can bedivided in to the following stages: 11 Generation, Transmission & Distribution  KIIT POLYTECHNIC 1. Nuclear Reactor 2. Heat Exchanger 3. Steam turbine 4. Alternator 12 Generation, Transmission & Distribution  KIIT POLYTECHNIC NUCLEAR REACTOR: It is an important apparatus in which nuclear fuel (U 235) is subjected to nuclear fission. It controls the chain reaction that starts once the fission is done. If the chain reaction is not controlled, the result will be an explosion due to the fast increase in the energy released. Construction: A nuclear reactor is a cylindrical stout pressure vessel and houses fuel rods of Uranium, moderator and control rods. The fuel rods constitute the fission material and release huge amount of energy when bombarded with slow moving neutrons. The moderator consists of graphite rods which enclose the fuel rods. The moderator slows down the neutrons before they bombarded the fuel rods. The control rods are of cadmium and are inserted into the reactor. Function: The fission material is on fuel rods on which the bombardment of slow moving neutron is done. The moderator which houses the fuel rods slows down the neutron before they bombard the fuel rods. After the fission, free neutron in released which is observed by Cadmium and thus regulates the supply of neutrons for fission. HEAT EXCHANGER: The coolant gives up heat to the heat exchanger which is utilized in raising the heat of steam after giving upheat, the coolant is again comes to the reactor. 13 Generation, Transmission & Distribution  KIIT POLYTECHNIC STEAM TURBINE: The steam produced in the heat exchanger is led to the steam turbine through a valve. After doing a useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump. ALTERNATOR: The steam turbine drives the alternator which converts mechanical energy into electrical energy. The output from the alternator is delivered to the bus-bars through transformer circuit breakers and isolators. ADVANTAGES: Fuel required is less It requires less space as compared to any other type of the same capacity Low running charges Very economical for production of bulk electric power Reliable Can be located near the load centers DISADVANTAGES: The fuel used is expensive Capital cost is very high The erection and commissioning of plant require high technical know-how. The produced waste is radio-active and may cause a dangerous amount of radioactive pollution. Maintenance charge is high. Not well suited for varying loads REFERENCE LINK https://www.youtube.com/watch?v=8uwrMLrqQlU SELECTION OF SITE FOR NUCLEAR POWER PLANT: The factor to be considered while selecting a site for nuclear power plant for economicaldeficient generation. Availability of water supply:- 1. It requires more water i.e. two times of thermal power plant of same rating. So it located 14 Generation, Transmission & Distribution  KIIT POLYTECHNIC near the river, sea side or lake. 2. Distance from populated area:- Generally these are located for away from populated area due to danger of radio activity. 3. Nearness to load center:- Those plants can be located near the load center because of absence of transportation. 4. Availability of space for disposal of water:- There should have adequate space & arrangement for the disposal of radio activity waste. 5. Types of land: The land should be strong enough to support the heavy reactor i.e. 10,000 tones weightwith imposed boarding pressure around 50 tones /m2 Introduction to Solar Power Plant The solar power plant is also known as the Photovoltaic (PV) power plant. It is a large-scale PV plant designed to produce bulk electrical power from solar radiation. The solar power plant uses solar energy to produce electrical power. Therefore, it is a conventional power plant. Solar energy can be used directly to produce electrical energy using solar PV panels. Or there is another way to produce electrical energy that is concentrated solar energy. In this type of plant, the radiation energy of solar first converted into heat (thermal energy) and this heat is used to drive a conventional generator. This method is difficult and not efficient to produce electrical power on a large scale. Hence, to produce electrical power on a large scale, solar PV panels are used. In this article, we will explain details about solar PV plants and PV panels. Below is the layout plan of photovoltaic power plant. Components of Solar Power Plant The major components of the solar photovoltaic system are listed below. 1. Photovoltaic (PV) panel 15 Generation, Transmission & Distribution  KIIT POLYTECHNIC 2. Inverter 3. Energy storage devices 4. Charge controller 5. System balancing component Photovoltaic (PV) Panel PV panels or Photovoltaic panel is a most important component of a solar power plant. It is made up of small solar cells. This is a device that is used to convert solar photon energy into electrical energy. Inverter The output of the solar panel is in the form of DC. The most of load connected to the power system network is in the form of AC. Therefore, we need to convert DC output power into AC power. For that, an inverter is used in solar power plants. For a large-scaled grid-tied power plant, the inverter is connected with special protective devices. And a transformer is also connected with the inverter to assure the output voltage and frequency as per the standard supply. Energy storage devices The batteries are used to store electrical energy generated by the solar power plants. The storage components are the most important component in a power plant to meet the demand and variation of the load. This component is used especially when the sunshine is not available for few days. Charge Controller A charge controller is used to control the charging and discharging of the battery. The charge controller is used to avoid the overcharging of the battery. The overcharging of a battery may lead to corrosion and reduce plate growth. And in the worst condition, it may damage the electrolyte of the battery. REFERENCE LINK OF NPTEL https://www.youtube.com/watch?v=JFnXlY8MVL8&list=PLwdnzlV3ogoUtaGiq-lVJc4CC6x_czs9D&index=16 16 Generation, Transmission & Distribution  KIIT POLYTECHNIC CHAPTER-2 TRANSMISSION OF ELECTRIC POWER Draw layout of transmission and distribution scheme. Generally generating stations are situated for away from load centre hence a power supply network connects the generating stations to load centre. As followed Here power system generally composed of transmission system and distribution system, which again divided in to primary and secondary (or sub) Generally voltage is generated in 33kv ( in advanced countries) 11 kV ( in India) which if transmitted results heaving current and power 17 Generation, Transmission & Distribution  KIIT POLYTECHNIC loss. 18 Generation, Transmission & Distribution  KIIT POLYTECHNIC Which if transmitted results heavy current and power losses. So there are stepped up to a higher value i.e. 66kv 132kv 220/230kv and 400kv step up transformer present near generating station and transfer to primary transmission system. Where it step down and transmitted to secondary transmission. Again voltage is step down to 33kv or 11kv and transmitted to primary distribution system from where medium large consumers connected and finally voltage step down to 400v at secondary distribution system and fed to small consumers. Voltage regulation: It is defined as the difference of supply end voltage and relieving end voltage to the receiving end voltage in other word. The voltage drop i.e. difference of secondary end voltage and receiving end voltageexpressed as a percentage of receiving end voltage called regulation. Mathematically % voltage regulation = VS - VR/VR X 100 VS = Supply end voltage VR= Receiving end voltage EFFICIENCY: It is defined as the ratio of power delivered at the receiving end to the power sent from the sending end. Mathematically η T = PR/PS X 100 = PR/PR + PL X 100 Where PR= VR.IR Costr = Receiving end power PS = VS.IS.CosФs = Sending end power An overhead 3-d Transmission line delivers 5000kw at 22kv at 0.8 p.f lagging with resistance and reactance of 4-2 and 6-2 respectively. Calculate (1) percentage regulation (2) efficiency. 19 Generation, Transmission & Distribution  KIIT POLYTECHNIC Solution: VR = 22KV VPH = 22/√3 = 12700V ZPH = 4+6J Here sending voltage / phase VS = Vph + Izph Here I = 500x103/√3x22x103x0.8 = 164A I = 164 -36.86 So VS = 12700 + 164 – 3686 X 7.2 56.31 = 12700 + 1180.8 19.45 = 12700 + 1113.41 + j 393.6 = 13813.4 + J 393.6 = 13819 1.6 Voltage regulation = VS-VR/VRx100 = 13819-12700/12700x100 = 8.825% Efficiency = PR/PR+3I2R = 5000 x100/5000 + (3x1642x4) = 93.94% Pl = Line Losses Kelvin’s Law: The law states that the most economical area of conductor is that for which the total annual cost of transmission is minimum Generally annual cost splitted into two parts a. Annual charge on capital out lay i.e. P1 + P2 a b. Annual cost of energy wasted i.e. P3/a Where P1, P2 and P3 are constants and, A is area of X- section of the conductor. 20 Generation, Transmission & Distribution  KIIT POLYTECHNIC So, total annual cost (C) = P1+P2a+P3/a for minimum DC/DA = O i.e. D/DA (P2a + P1 +P3/A)= 0 =P2-P3/a2 = 0 P2.a = P3/a i.e. variable part of annual charge = Annual cost of energy wasted. In other word it can be stated that the most economical area of conductor is that for which the variable part of annual charge is equal to the cost of energy losses per year. Graphically: Limitation: (i) Difficult to estimate the energy loss in the line without load curve, which is not available at the time of estimation. 21 Generation, Transmission & Distribution  KIIT POLYTECHNIC (ii) Annual charge on capital out day i.e. P1+P2 a not true Eg.- neither the cost of cable dielectric and heath in underground cables the cost of laying vary in this manner. (iii) Current density, mechanical strength, corona loss are not considered. (iv) By this low conductor size may be too small to carry the necessary current safely. (v) Interest and depreciation not determined accurately. CORONA: The phenomenon of violet glow, hissing noise and production of zone gas in an overhead transmission line is known as corona. The phenomenon of corona is accompanied by a hissing sound and production of ozone gas due to corona also increases. In case of polished and smooth, the corona glow will be uniform throughout the length of the conductor; otherwise the rough points will appear brighter. In case of D.C. Supply the TVC wire has uniformly glow. While the -ve conductor has spotty glow. Formation of corona: Under normal condition, the air around the conductors contain some ionized particles like free electrons and +ve ions and neutral molecules. Due to altra-violet ray and radioactivity applied, the potential gradient is setup in the air. When the potential gradient at the conductor surface reaches abut 30 kv per cm the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force to dislodge one or more electrons from it. This produces another ion and one or more free electrons. Those electrons again collide with the other neutral molecules and produce other dons. This process of ionization is cumulative. As a result corona is formed. Factors affecting corona: 1. Atmosphere: In stormy weather corona occurs more than in normal weather due to more ions than the normal weather. 2. Conductor size: 22 Generation, Transmission & Distribution  KIIT POLYTECHNIC The rough and irregular surface will produce more corona. 3. Spacing between conductors: If the spacing between the conductors is made very large as compare to their diameter,there may not be any corona effect 4. Line voltage : In low voltage no corona is formed Critical disruptive voltage: It is the minimum phase to neutral voltage at which corona occurs. Visual critical voltage: It is the minimum phase neutral voltage at which corona glow appears all along the line conductors. Power loss due to corona: Due to corona electric energy is lost. This is dissipated in the form of light, heat, sound and chemical action. Power loss due to corona is P= 242.2 (F+25/6) √R/D (V-VX)2 x 10-5 KW/KM/PH VC = disruptive voltage /Phase. a. Advantages: i. Reduces electro static stress between the conductors ii. Reduces transient due to surges. b. Disadvantages: i. It reduces transmission efficiency ii. Corrosion of conductor occurs due to ozone iii. Inductive interference due to non- sinusoidal voltage c. Methods reducing corona: 1. Large dia conductor 2. Hollow conductors 3. Bundled conductor 23 Generation, Transmission & Distribution  KIIT POLYTECHNIC CHAPTER-3 LINE SUPPORTS The line supports should have the following properties. 1. High mechanical strength to withstand the weight of conductor and wind loads. 2. Light in weight 3. Long life 4. Cheap in cost 5. Easy accessibility of conductors’ maintenance. The line supports used for transmission and distribution of electric power are made in either wood , steel, R.C.C or lattice steel. WOODEN POLE: Wooden poles are made of woods. These types of poles are used for shorterspans up to 50 meters. These are mainly used in rural areas. 24 Generation, Transmission & Distribution  KIIT POLYTECHNIC 2. SHACKLE INSULATORS: For low voltage line less than 11KV shackle insulators are used as strain inhalators. This is also known as spool insulators. it is used in low voltage distribution limes. 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 cross arm. The conductor in the groove is fixed with soft binding wire. Coesote oil is impregnated in the ground level of the wooden pole for better foundation. These wooden poles have smaller life 20 to 25 years. Also it has less mechanical strength. it can’t be used for voltage higher than 20kv. Steel poles: The steel poles are used generally for the distribution purpose in the city areas it has high mechanical purpose in the city areas. It has high mechanical strength with longer life poles. The outer surface of the pole is painted for longer life. These poles are three types. 1. Rail poles 2. Tabular poles 3. Rolled steel joints 25 Generation, Transmission & Distribution  KIIT POLYTECHNIC R.C.C. Poles: These poles are very popular in recent days. They provide longer span than steel and wood poles. They have greater mechanical strength with conger life. These type of pole require law maintenance it has good insulating properties. These poles are heave weights. So the cast of transportation is high. Steel Towers:- For long distance transmission at higher voltage steel towers are used. These steel towers have longer life, great mechanical strength. It is suitable for severe climate condition these type of steel towers are used for conger spans. Tower footings are grounded by driving rods in to the earth. This minimizes the lightning troubles as each tower acts as a lightning conductors it can also used as double circuit tower which allow continuity of supply in case of one circuit break down. TYPES OF INSULATOR INSULATORS:- An electrical insulator is a material whose internal electric charges do not flow freely. In overhead transmission or distribution lines insulators are used to insulate the current flowing in the conductor to earth through the poles or towers. The most commonly used materials for insulator of over head line is porcelain. Types of insulator in over head lines:- There are mainly three types of insulator used overhead lines for high voltage. 26 Generation, Transmission & Distribution  KIIT POLYTECHNIC 1. Pen type insulator 2. Suspension type insulator 3. Strain type insulator There is two other type insulators are used for low voltage over head lines. These are. 1. Stay type insulators 2. Shackle type insulators 1. Pen type insulators: This type of insulator is popularly used in power network up to 33kv system. The pen type insulator is secured to the cross arm on the pole. The live conductor attached to the top of the pen insulator the shortest distance between conductor and earth, surrounding the insulator body along which electrical discharge may take place through air is known as flash over distance. When the insulator is wet, its outer surface becomes almost conducting. Hence the flesh over distance of insulator is decreased. So the design of the electrical insulator (pin type) is umbrella type on the upper part, so that it can protect the rest lower part of the insulator from rain. 2. Suspension type insulator:- This type of insulator is used beyond the voltage 33kv. The pin type insulator is not economical for beyond the voltage 33kv. In this type of insulator number of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator because of their disc shape. Each disc is desig n for 11kv the number of disc in series 27 Generation, Transmission & Distribution  KIIT POLYTECHNIC would depend upon the working voltage. It any of the disc is damage, then that disc can be replaced. There is no need to change the whole string. If the line voltage will increase, then that can be insulated by adding more disc in string. This type of insulators are mainly used with steel towers. As the conductor run bellow the earthed cross arm of the tower, therefore this arrangement provides partial protection from lighting the spacing between the conductor is more when suspension type insulator is used in overhead lines. 3. Strain insulator : When there is a dead end of the line or there is a sharp carve the line is subjected to greater tension. In order to relive the line of excessive tension, strain insulators are used. Strain insulator consists of an assembly of suspension insulators. It has considerable mechanical strength. LOW VOLTAGE APPLICATION INSULATORS : 1. Stay insulator : For low voltage range lines, the stays are to be insalated from ground at a height. Stay Insulator This insulator used in the stay wire is called as the stay insulator. Calculation of sag: The sag should be so adjusted that tension in the conductor is within safe limits. The tension is governed by conductor weight, effects of wind, ice loading and temperature variations. 28 Generation, Transmission & Distribution  KIIT POLYTECHNIC 1. When supports are at equal levels : The above figure shows a conductor is placed between two equilevel supports A and B. The point O is the lowest point. If L= Length of span W = Weight per unit length of conductor T= Tension in the conductor Consider point „P‟ on the conductor let the distance between poin „O‟ and p in horizontal projection is X. There are two forces acting on the conductor portion „OP‟ 1. The weight Wx of the conductor acting at a distance X/2 from „O‟ 2. Tension T acting at „O‟ If we equate the two forces on the conductor at Point „O‟. We get Ty = Wx (x/2) Y = WX2/2T If the sag is represented by the value of „Y‟ At support „A‟ X= L/2 and Y=S 29 Generation, Transmission & Distribution  KIIT POLYTECHNIC Then sag S = W (L/2)2/2T S= WL2/8T CALCULATION OF SAG When supports are at unequal levels :- Unequal level supports are normally found in hilly areas. The above figure shows a conductor suspended between two supports A and B at different levels. Consider the lowest point is „O‟ Let L = Span Length H= Difference in levels between two supports X1 = Distance of support at lower level from „O‟ X2 = Distance of support at higher level from „O‟ T = Tension in the conductor. If “W” is the weight per unit length of the conductor, then Say S1 = WX12 /2T (Y= WX2/2T) Sag S2 = WX22/2T From the figure X1+X2 = L By subtracting S2 from S1 S2-S1 = W/2T (X22-X12) S2-S1 = W/2T (X2+X1) X (X2-X1) We know X2+X1 =L 30 Generation, Transmission & Distribution  KIIT POLYTECHNIC Then S2-S1 = WL/2T (X2-X1) From the figure S2-S1 = H Equation-3 becomes S2-S1 = H = WL/2T (X2-X1) X1-X2 = 2TH/WL By salving eqn.-3 & 5, we will get X1 –L/2 – TH/WL and X2 = L/2 + TH/WL Patting the values of X1 and X2 in Eqn. 1&2 We can calculate sag S1 and S2 THE EFFECT OF WIND AND ICE LOADING ON THE CALCULATION OF SAG: In some cases the conductor may have ice coating and simultaneously subjected to wind pressure – At this time the weight of the ice acts vertically down word as the same direction as the weight of conductor the force due to the wind is assumed to act horizontally as shown in fig. „b‟ 31 Generation, Transmission & Distribution  KIIT POLYTECHNIC The total force on the conductor due to wind and ice coating is the vector sum of horizontal and vertical forces as shown in fig „C‟ If W= Weight of the conductor for unit length = Conductor material density x Volume per unit length. Wi= Weight of ice per unit length = density of ice x volume office per unit length. Density of ice x at (d+t) Ww = wind force per unit length = wind pressure per unit area x projected area per unit length. = wind pressure x [ (l+2t)x1] Then Wt = (w+wi)2 + (WW)2 If the conductor sets it self in a plane at an angle @ to the vertical then Tan = Ww/W+We Then the sag of in the conductor is given By S = Wt L2/2T The vertical say = S cosΦ NPTEL Link for Insulator https://nptel.ac.in/courses/108102047 32 Generation, Transmission & Distribution  KIIT POLYTECHNIC CHAPTER-4 Performance of Short & Medium Lines 33 Generation, Transmission & Distribution  KIIT POLYTECHNIC 34 Generation, Transmission & Distribution  KIIT POLYTECHNIC 35 Generation, Transmission & Distribution  KIIT POLYTECHNIC 36 Generation, Transmission & Distribution  KIIT POLYTECHNIC 37 Generation, Transmission & Distribution  KIIT POLYTECHNIC 38 Generation, Transmission & Distribution  KIIT POLYTECHNIC 39 Generation, Transmission & Distribution  KIIT POLYTECHNIC 40 Generation, Transmission & Distribution  KIIT POLYTECHNIC 41 Generation, Transmission & Distribution  KIIT POLYTECHNIC 42 Generation, Transmission & Distribution  KIIT POLYTECHNIC 43 Generation, Transmission & Distribution  KIIT POLYTECHNIC 44 Generation, Transmission & Distribution  KIIT POLYTECHNIC CHAPTER-5 EHV TRANSMISSION EXPLAIN EHV AC TRANSMISSION: Generally the voltages of 300kv to 765kv are considered as extra high voltage (EHV) transmission line. Generally EHV AC lines are selected for long distances 250km and above and high power 500mw and above. Electrical 3-ⱷ EHV – AC line as follows. In EHV-AC line series inductance (L) and shunt capacitance C influence the performance of line (Voltage regulation power flow, stability etc.) Significantly and they can‟t be neglected. Here the surge impedance of the line is as follows Zs = √(L/C) Ω Eg. 400kv- EHV-AC line has been installed in India in 1974 but recently 765 EHV AC line installed i.e. in 1992 EXPLAIN REASONS FOR ADOPTION OF EHV AC TRANSMISSION : There are different reasons for adaption of EHV-AC transmission line as follows. a. Increase in size of generating units : In order to reduce the investment cost per kw, The size of generating units has been constantly increased, thus this increased large amount of power over long distances can be transmitted technically and economically in EHV transmission. 45 Generation, Transmission & Distribution  KIIT POLYTECHNIC b. Pithead steam plants and remote hydro plants:- Generally steam and hydro-power plants are located accordingly their favorable characteristics like availability of coal, water, land ,labor, transportation etc. Generally these are very far away from load center. Hence transmission of large amount of power over long distances can only be technically and economically possible by EHV – Transmission. c. Surge impedance loading :- Surge impedance loading is the power that line carries when each phase terminated by a load equal to surge impedance of the line i.e.ZC=√(L/C) where L= Series inductance per unit length C= shunt capacitance per unit length. Hence, SIL = 3V2/ZC Therefore voltage at higher value makes large power transmission at economic. d. Transmission: - The number of circuits and the land requirement for transmission decreases with adoption of EHV AC transmission. E.g. for transmitting 600MW over 250km at 480 line using two conductor per phase and requiring a right of way of about 40m giving 600/40 = 15mw per meter right of way but if it transmitted by two double circuit lines at 200kv then it requires a right of 60m giving 600/60 = 10 mw per meter right of way. So double circuit 400kv line gives this ratio as about 30mw per meter right of way. Hence EHV line reduces right of way requirement quite substantially. e. Line cost : The line installation cost per MW per km decreases with increase in voltage level. Moreover the impact of the cost of losses on the overall transmission costs can be substantially different at different voltage levels. The total line cost including the cost of losses per mw per km decreases considerably by use of EHV AC transmission. PROBLEMS INVOLVED IN EHV TRANSMISSION : There are different major problems associated with EHV transmission line as follows: a. Corona loss and Radio Interference: Generally corona appears in High voltage transmission line which not only a source of power loss but it is also a source of interference with radio and television. 46 Generation, Transmission & Distribution  KIIT POLYTECHNIC b. Heavy supporting structure and erection difficulties:- Generally EHV –AC transmission uses bundle conductor, which results large mechanical loading on tower. Similarly large air and ground clearances, dynamic forces due to broken conducts etc makes large mechanical load. Hence strength of tower should be heavy. c. Insulation requirement:- Generally EHV – AC transmission requires high insulation to withstand the voltage surges due to internal sources i.e. switching operation or due to external sources i.e lightning etc. which produces very high voltage generally 2-3 times of normal voltage. Hence insulation level depends upon switching over- voltages, temporary over- voltages and atmospheric over voltages. HVDC TRANSMISSION: Generally high voltage DC transmission requires converter station at both sending end and receiving end. The converter station are transformers and thyristor valves. At sending end thyristor valves act as rectifier to convert ac in to dc which is transmitted over the line, whereas at the receiving end thyristor valves act as inverter to convert dc into ac which is utilized at receiving end. Each converter can function as rectifier or inverter. Thus power can be transmitted in either direction. It has following line diagram. 47 Generation, Transmission & Distribution  KIIT POLYTECHNIC The different HVDC links are as follow:- ADVANTAGES AND LIMITATIONS OF HVDC TRANSMISSION SYSTEM: ADVANTAGES: HVDC transmission has many technical and Economics advantages over ac transmission as follows. 1. Cheaper in cost: Bipolar HVDC Transmission lines require two pole conductors which is very cheap. No skin effect : As there is uniform current distribution in dc so no skin effect in HVDC. 2. Lower transmission losses : HVDC Transmission system needs only two conductors and therefore the power losses in a dc line is less. 3. Voltage regulation : Due to absence of inductance it has better voltage regulation. 4. Line loading : Generally loading on AC line limited by transient stability limit and line reactance but no such limit in HVDC lines. It also has no SIL with greater reliability. 5. Low short circuit current : It has no short circuit. 6. Lesser corona loss and radio interference : It has lesser corona loss due to zero frequency. 7. Higher operating voltages and no reactive power compensation : Generally HVDC system has high operating voltage with absence of reactive power compensation. 8. No stability limit : There is no stability limit in HVDC transmission system. 48 Generation, Transmission & Distribution  KIIT POLYTECHNIC LIMITATIONS OF HVDC TRANSMISSION : HVDC Transmission system has following limitations as follows: 1. Costly terminal equipment : The converters are used in HVDC are very costly along with the converters produce lot of harmonics both on dc and ac sides, which requires filtering and smoothing equipment resulting extra additional expense. It also require complex cooling system and circuit breaker, which again adds cost. 2. More maintenance of line insulators : It requires more maintenance for in insulation 3. Circuit breaking in multi terminal dc system is difficult and costlier. 4. Voltage Transformer: Voltage transformation is not easier in case of dc and hence it has to be accomplished on the ac side of the system. DC System can‟t be employed for distribution sub transmission and back bone transmission. 49 Generation, Transmission & Distribution  KIIT POLYTECHNIC CHAPTER-6 DISTRIBUTION SYSTEM INTRODUCTION TO DISTRIBUTION SYSTEM : An electric power distribution system is the final stage in the delivery of electric power. It carries electricity from transmission system to individual consumer. Distribution lines mainly sub divided in to two types. a. Primary distribution lines b. Secondary distribution lines a. Primary distribution lines : Carry the medicine voltage power to distribution trans formers located near the customer‟s premises. b. Secondary distribution lines : carry the step down voltage from the transformer located near the customer‟s premises to the customers. The figure shows the line diagram of a distribution system with the connection of feeders, distributor and service mains. 50 Generation, Transmission & Distribution  KIIT POLYTECHNIC 1. Feeders : It is a conductor which conduct electricity from substation to the area where the power is to be distributed. The feeders carries same current because no tapping are taken from the feeder. The feeder is designed according to the current carrying capacity. 2. Distributor : Consumers take supply from the distributor by tapping. In the above figure PQ, QR, RS, and PS are the distributor. The voltage drop along its length is considered before designing a distributor. 3. Service mains : It is a cable which connects the distributor to the consumer‟s energy meter. Radial Electrical Power Distribution System In early days of electrical power distribution system, different feeders were radially come out from the substation and connected to the primary of distribution transformer directly. Radial Distribution System But radial electrical power distribution system has one major drawback that in case of any feeder failure, the associated consumers would not get any power as there was no alternative path to feed the transformer. In case of transformer failure also, the power supply is interrupted. In other words the consumer in the radial electrical distribution system would be in darkness until the feeder or transformer was rectified. 51 Generation, Transmission & Distribution  KIIT POLYTECHNIC RING MAIN ELECTRICAL POWER DISTRIBUTION SYSTEM The drawback of radial electrical power distribution system can be overcome by introducing a ring main electrical power distribution system. Here one ring network of distributors is fed by more than one feeder. In this case if one feeder is under fault or maintenance, the ring distributor is still energized by other feeders connected to it. In this way the supply to the consumers is not affected even when any feeder becomes out of service. In this way, supply to the consumers connected to the healthy zone of the ring, can easily be maintained even when one section of the ring is shutdown. The number of feeders connected to the ring main electrical power distribution system depends upon the following factors. 1. Maximum demand of the system : If it is more, then more numbers of feeders feed the ring. 2. Total length of the ring main distributors : If it length is more, to compensate the voltage drop in the line, more feeders to be connected to the ring system. 3. Required voltage regulation : The number of feeders connected to the ring also depends upon the permissible allowable, voltage drop of the line. 52 Generation, Transmission & Distribution  KIIT POLYTECHNIC The sub distributors and service mains are taken off may be via distribution transformer at different suitable points on the ring depending upon the location of the consumers. Sometimes, instead of connecting service main directly to the ring, sub distributors are also used to feed a group of service mains where direct access of ring distributor is not possible. INTER CONNECTED SYSTEM: In this type of system two to more than two generating station or substation are energizing the feeder ring. In this system any area from one generating station during peak load hours can be fed from other generating station which increase the efficiency of the system. Now a days electrical energy is generated, transmitted and distributed in the form of a.c. But for certain applications D.C. Supply is necessary. The D.C. Supply from the substations fed to the load through distributors. TYPES OF D.C. DISTRIBUTORS – IN SHORT The most general method of classifying d.c. distributors is the way they are fed by the feeders. On this basis, d.c. distributors are classified as : Distributor fed at one end Distributor fed at both ends Distributor fed at the centre Ring distributor DISTRIBUTOR FED AT ONE END: In this type of feeding, the distributor is connected to the supply at one end and loads are taken at different points along the length of the distributor. 53 Generation, Transmission & Distribution  KIIT POLYTECHNIC The current in the various sections of the distributor away from feeding point goes on decreasing. Thus current in section AC is more than the current in section CD and current in section CD is more than the current in section DE. The voltage across the loads away from the feeding point goes on decreasing. In case a fault occurs on any section of the distributor the whole distributor will have to be disconnected from the supply mains. Therefore, continuity of supply is interrupted. DISTRIBUTOR FED AT BOTH ENDS. In this type of feeding, the distributor is connected to the supply mains at both ends and loads are tapped off at different points along the length of the distributor. Here, the load voltage goes on decreasing as we move away from one feeding point say A, reaches minimum value and then again starts rising and reaches maximum value when we reach the other feeding point B. The minimum voltage occurs at some load point and is never fixed. It is shifted with the variation of load on different sections of the distributor. If a fault occurs on any feeding point of the distributor, the continuity of supply is maintained from the other feeding point. 54 Generation, Transmission & Distribution  KIIT POLYTECHNIC In case of fault on any section of the distributor, the continuity of supply is maintained from the other feeding point. The area of X-section required for a doubly fed distributor is much less than that of a singly fed distributor. DISTRIBUTOR FED AT THE CENTRE. In this type of feeding, the centre of the distributor is connected to the supply mains. It is equivalent to two singly fed distributors, each distributor having a common feeding point and length equal to half of the total length. RING MAINS: In this type, the distributor is in the form of a closed ring. It is equivalent to a straight distributor fed at bot h ends with equal voltages, the two ends being brought together to form a closed ring. The distributor ring may be fed at one or more than one point. USE OF DC DISTRIBUTION D.C. supply is required for the operation of variable speed machinery (e.g. d.c. motors), electrochemical work and electric traction. For this purpose, a.c. power is converted into d.c. power at the sub-station by using converting machinery e.g. mercury arc rectifiers, rotary converters and motor–generator sets. The d.c. supply from the sub-station is conveyed to the required places for distribution. 55 Generation, Transmission & Distribution  KIIT POLYTECHNIC METHODS OF SOLVING A.C. DISTRIBUTION PROBLEMS :- The current in different sections of the distributor will be the vector sum of load currents and not the arithmetic sum in a.c. distribution. Hence the power factor of various load currents have to be considered when solving A.C. distribution problems. 1. Power factor referred to receiving and voltage : AB is an AC distributor where I1 and I2 are the loads taken from the point „C‟ and „B‟ as shown in fig-I. VB is the receiving end voltage. The lagging power factor at Point „C‟ is cos ⱷ1 and the lagging power factor at Point B is cos ⱷ2 w.r.t. VB. The section AC has impedance = R1+jx1 and the section CB has the impedance = R2 + jx2. The load current at point „C‟ = I1=I1(cos ⱷ1 – j sin ⱷ1).The load current at point „B‟ is equal to current in section CB is equal to ICB = I2 =I2 (cos ⱷ2-j sin ⱷ2) The current in section AC = I1 +I2 = I1 (cos ⱷ1 – j sin ⱷ1)+ I2 (cos ⱷ2-j sin ⱷ2) The current in section AC = I1 +I2 = I1 (cos ⱷ1 – j sin ⱷ1)+ I2 (cos ⱷ2-j sin ⱷ2) The load current at point „B‟ is equal to current in section CB is equal to ICB = I2=I2 (CosQ2 – JSINQ2) The current in sectio n AC = I1+I2 = I1 (Cos Q1 – Jsin Q1) + I2 (CosQ2-JsinQ2) The voltage drop in section CB = VCB = ICB ZCB = I2 (as Q2-JsinQ2) (R2+Jx2) 56 Generation, Transmission & Distribution  KIIT POLYTECHNIC Voltage drop in section AC, VAC = IAC ZAC = (I1+I2) ZAC = [(I1cosQ1-JsinQ1) + (i2CosQ2-JsinQ2)] x (R1+Jx1) The sending end voltage VA= VB +VCB +VAC Sending end current = IA =I1+I2 The vector diagram of a.c. distribution when power factor referred to receiving end voltage is shoun in figure -2 VC = VB+ (I2R2) 2 +(I2x2)2 VA= VB + (Iac R1)2 + (iACx1) Power factor referred to respective load voltage : If power factors of loads are referred to their respective load voltages. Then Q1is the phase angle between VC and I1and Q2 is the phase angle between VB and I2 the vector diagram under this condition is shown in figure -2 The voltage drop in section CB = I2ZCB =I2 (asQ2-jsinQ2) (R2+jx2) Voltage at point „C‟ – VB + Drop in section CB = VC LR I1 = I1LQ1 w.r.t. voltage VC I1 = I1 L- (Q1-R) w.r.t. voltage VB I1 =I1 as (Q1-R) – Jsin (Q1-L) IAC = I1+I2 57 Generation, Transmission & Distribution  KIIT POLYTECHNIC = I1 as (Q-L_)-I sin (Q1-L) +I2 [ as Q2-JsinQ2] Volt drop in section AC = IAC ZAC Voltage at point A = VB+ Vol+dropin CB + Vol + dropin AC 3- PHASE, 4-WAIR STAR CONNECTED SYSTEM:- In this system 4- wires are connected in a circuit. There are three phase wire and a neutral wire. The phases are electrically 120degree apart from each other. All the 3- Phases meeting at a point known as neutral point. The wire connected to the neutral point is known as neutral wire. The voltage between any one phase and neutral line is known as phase voltage (Vph). The voltage between the two line is known as line voltage (VL) in a star connection the line voltage is √3 times greater than phase voltage. But the line current is same as the phase current. Most of the A.C. machines are star connected. 58 Generation, Transmission & Distribution  KIIT POLYTECHNIC CHAPTER-7 UNDERGROUND CABLES INTRODUCTION Since the loads having the trends towards growing density. This requires the better appearance, rugged construction, greater service reliability and increased safety. An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover. The interference from external disturbances like storms, lightening, ice, trees etc. should be reduced to achieve trouble free service. The cables may be buried directly in the ground, or may be installed in ducts buried in the ground. ADVANTAGES & DISADVANTAGES Advantages  Better general appearance  Less liable to damage through storms or lighting  Low maintenance cost  Less chances of faults  Small voltage drops Disadvantages :  The major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with equivalent overhead system. Properties of insulation materials for cables :- The properties are 1. High resistivity 2. High mechanical strength 3. High dielectric strength 4. Not affected by acid or alkalis 59 Generation, Transmission & Distribution  KIIT POLYTECHNIC 5. Non- inflammable 6. High tensile strength 7. Low cost INSULATING MATERIALS USED IN UNDER GROUND CABLES :- The insulating materials used in underground cables are. 1. Rubber 2. VIR 3. Impregnated paper 4. PVC 5. Varnished cambric 1. Rubber : It is the most commonly used insulating material. Natural rubber is produced from the latex of the rubber tree. Synthetic rubber is produced from alcohol or oil products. It absorbs moisture slightly and is soft. Therefore pure rubber cannot be used as an insulating material. VULCANIZED INDIA RUBBER (VIR): It is used for low voltage power distribution systems only. It is prepaired by mixing pure rubber with mineral matter such as sulphur, zink oxide, red lead etc. it has greater mechanical strength than pure rubber. The advantages of using this type of rubber for cable is that, the cable becomes strong and more curable before using VIR as insulation, the copper conductor must be tunnel well because it attacks copper. (Reacts copper) its use is limited because of its low melting point and short span of life. IMPREGNATED PAPER : It has low capacitance, high dielectric strength and economical. The paper is manufactured with wood pulp, rags or plant fiber by a suitable chemical process. It has high resistance due to high resistivity center dry condition. It absorbs a small amount of moisture only, which reduced the insulation resistance. For this drawback it requires some sort of protective covering. It is impregnated in insulating oil before used. 60 Generation, Transmission & Distribution  KIIT POLYTECHNIC POLYVINYL CHLORIDE (PVC): It is a synthetic compound material. It is obtained from the polymerization of acetylene and is in the form of white power. It is combined chemically with a plastic compound and is used over the conductor as an insulation cover. It has high insulation resistance, good dielectric strength.PVC insulated cables are usually employed for medium and low voltage domestic insulation. VARNISHED CAMBRIC:- It is the cotton cloth impregnated and coated with varnish. The cambric is lapped on the conductor in the form of a tape and its surface are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent. CLASSIFICATION OF CABLES: Mainly underground cables are classified in two ways. 1. According to the type of insulating materials used in their manufacture 2. The voltage for which they are manufacture 3. No of conductors used in cable 4. Power to be handled in cable. CLASSIFICATION OF CABLES Things may be classified on many aspects and similar is the case with cables. There are several ways of classifying cables. These includes classification of cables on the basis of No. of Conductors in Cable Voltage Rating of Cable Insulation Used in Cable Power To Be handled in Cable No. of Conductors in Cable 61 Generation, Transmission & Distribution  KIIT POLYTECHNIC On the basis of number of conductors in the cable, cables are of two types viz Single core cables and 3 core cables. Single core cable have only one conductors and the diagram is shown in Underground Cable Basics while three core cable has three conductors and they have bedding and filler too. so on the basis of number of conductors cables are of two types Single Core Cable 3 Core Cable Voltage Rating of Cable Cables may be classified on the basis of voltage rating also LT Cables : Cables up to 1000 Volts or l kV are called LT (Low Tension) or LV (Low Voltage) cables HT Cables : Cables from 1001 volts to 11000 Volts or 11kV are called HT (High Tension) or HV (High Voltage) cables. ST Cables : Cables from 11001 volts to 33000 Volts or 33kV are called ST (Super Tension) cables. EHV Cables : Cables from 33001 volts to 66000 Volts or 66kV are called EHT (Extra High Tension) or EHV Oil & Gas Filled Cables : From 66 kV to 132 kv Oil and Gas Filled cables are used. EST Cables : Cables used above 132 W are called EST (Extra Super Tension) cables. Insulation Used in Cable Cables are also classified on the basis of insulation provided in the cable. Following are the type of cables on the basis of insulation used in the cable - PIC or PILC : Paper Insulated Cable - Paper is used as insulation to the conductor. PVC : Poly Vinyle Chloride Cable - PVC is used as insulation to the conductor. 62 Generation, Transmission & Distribution  KIIT POLYTECHNIC PE : Poly Ethylene - Poly Ethylene is used as insulation to the conductor PTFE : Poly Tetra Fluoro Ethylene - PTFE is used as insulation to the conductor XLPE : Cross Linked Poly Ethylene - Cross Linked Poly Ethylene is used as insulation. This is most commonly used cable in Industry. Power to Be handled in Cable On the basis of amount of power to be transferred through the cable. cables are classified into two categories. These are Power Cables : If large amount of power is to be transferred then these are called power cables. These are further classified on the basis of voltage at which power is to be delivered. 1. Low Voltage Power Cable : If the voltage in the cable is less then 1000 Volts or I kV then the cable is called Low Voltage Power Cable. 2. High Voltage Power Cable : If the voltage in the cable is above 1000 Volts or l kV then the cable is called High Voltage Power Cable. Control Cables: If the cable is used to carry very low power signal generally for controlling equipments then the cables are called control cables. 1. Low tension cables (L.T. cables) : These are used for the voltage levels up to 6.6kv. The electrostatic stresses in L.T. cables are not severe hence no special construction is used for L.T. cables. The paper is used as an insulation in these cables. Sometimes resin is also used which increases the viscosity and helps to prevent drainage. 63 Generation, Transmission & Distribution  KIIT POLYTECHNIC The Fig. 5.2 shows the cross-section of a single core L.T. cable. It consists of a circular core of stranded copper or aluminum. The conductor is insulated by impregnated paper. Over the paper insulation, the lead sheath is provided. Then a layer of compounded fibrous material is provided. Then armoring is provided and finally covered again with a layer of fibrous compounded material. Many a times. L.T. cabbies are not provided with armoring, to avoid excessive sheath losses. The simple construction and the availability of more copper section are the advantages of L.T. single core cable. H.T. Cable :- H.T. cables are the cables which are used for voltage levels up to 11kv. Construction :- The cores are not circular in shape. The cores are insulated from each other by layers of impregnated papers. It has 3 – cores. The three cores are grouped together and belted with the help of a paper belt. The gaps are filled with fibrous materials like jute. This gives circular cross sectional shape to the cable. The belt is covered with lead sheath which protect cable from moisture and also give mechanical strength. The belted type construction is suitable only for low and medium voltages. Under high voltage cases, the cumulative effect of tangential electrical stress is to form spaces inside the cable due to leakage currents. Such air spaces formed inside the insulation is called void formation. Under high voltage, spaces are ionized which deteriorate the insulation which may lead to the breakdown of the insulation. Hence belt cables are not used for the high voltage levels. 64 Generation, Transmission & Distribution  KIIT POLYTECHNIC METHODS OF CABLE LYING : There are three main methods used for laying underground cables. 1. Direct laying. 2. Draw-in system 3. Solid system 2. Direct laying : The above figure shows how the cable is laying in this method. A trench of about 1.5 meter deep and 45cm wide is dug. Then the trench is covered with a layer of fine sand, about 10cm thick the cable is laid over the sand. The sand does not allow the entry of moisture from the ground. Then the cable is covered with another layer of sand of about 10cm thickness. After that the trench is covered with bricks and other materials to protect the cables from mechanical injury. When more than one cable is to be laid in the same trench, a horizontal or vertical inter axial spacing of at least 30cm is provided in order to reduce the effect of mutual heating. This method of laying under ground cables is simple and cheap. 3. Draw- in system : This method of cable laying suitable for congested areas where excavation is inconvenient. In this method a line of conduits, ducts or tubes made of either iron, glazed stoneware clay or cement concrete are laid in ground with manholesat suitable positions along the cable rotate. The cables are then pulled in to position. Separate pipes and ducts are provided for each cable laid in the same duct. Care must be taken that where the duct line changes direction, depths, dine so that a large cable may be pulled easily between the manholes. The distance between the manholes should not be too long so as to simplify the pulling in the cables. 65 Generation, Transmission & Distribution  KIIT POLYTECHNIC SOLID SYSTEM : In this method the cable is laid in open pipes or troughs dug out in earth along the cable route. The toughing is of cast iron, stoneware, asphalt or treated wood. After laying the cable in position, the toughing is filled with a bituminous and covered over. METHODS OF LOCATION OF CABLE FAULTS: Several methods are used for locating the faults in underground cables. The most popular methods for locating faults in underground cables known as loop tests. These are as follows. 1. Murray loop test 2. Varley loop test 1. Murray loop test : This test is the common and accurate method of locating earth fault or short – circuit fault in underground cables. a. Earth fault : The figure shows the circuit diagram of locating the earth fault by Murray loop test. Fault by Murray loop test. Here AB is the sound cable and CD is the faulty cable and the earth fault occurring at the point F. The end of fault cable at point „D‟ is connected with the point „B‟ of the sound cable with a low resistance. Two variable resistances P and Q are connected to ends A and C Let R= Resistance of the conductor loop from the test end to the point „F‟ where the fault occur X = Resistance from fault point „F‟ to the rest of the faulty cable „C‟ The resistance P and Q are varied till the galvanometer indicates zero. The balanced position of the Wheatstone bridge where P,Q,R & X are the four arms is P/Q = R/X Or P/Q +1 = R/X +1 66 Generation, Transmission & Distribution  KIIT POLYTECHNIC Or P+Q/Q = R+X/X If „R‟ is the resistance of each cable Then R+X = 2r So P+Q/Q = 2r/X X= Q/P+Q x 2r If „L‟ is the length of each cable in meter Then resistance per meter length of cable =r/L Distance of fault point from test end is D= x/(r/l) Putting the values of x in the above equation D = Q/P+Q x 2r x L/r = Q/P+Q x 2L [ D = Q/P+Q x loop length in meter ] B SHORT – CIRCUIT FAULT : In the circuit shown P,Q,R & X are the four arms of the bridge. The fault resistance is in the battery circuit. the bridge is balanced by adjusting the resistances „P‟ and „Q‟ in balanced condition. P/Q = R/X P/Q+1 = R/X +1 P+Q/Q = R+X/X 67 Generation, Transmission & Distribution  KIIT POLYTECHNIC If „R‟ is the resistance of each cable Then R+X = 2r So P+Q/Q = 2r/X X = Q/P+Q X 2r If „L‟ is the length of each cable in meter the resistance per meter length of cable = r/L Distance of short-circuit fault point from test end is VARLEY LOOP TEST : It is also used for locating earth fault or short- circuit fault in underground cables. This test can be done with the help of Wheatstone bridge principle. In this test P and Q are the fixd resistances and the balance can be done by varring the variable resistance S. For earth fault or short circuit fault, the key „K2‟ is connected to position-I. After that the variable resistance S is varied till the bridge is balanced for resistance value S1. So P/Q = R/X+S1 , P/Q+1 =( R/X+S1 )+1 P+Q/Q =( R+X+S1)/(X+S1) X=Q (R+X)-PS1/P+Q…................................................ (1) 68 Generation, Transmission & Distribution  KIIT POLYTECHNIC Then the key K2 is connected to position – 2 and the bridge is balance to a new value of resistance P/Q = R+X/S2 , (R+X) Q = PS2.................................................................... (2) From equation 1 & 2 we will get X = P(S2-S1)/P+Q The loop resistance = R+X = (P/Q)S2 If „r‟ is the resistance of the cable per meter length. The distance of fault from the test end is D = X/r meters. 69 Generation, Transmission & Distribution  KIIT POLYTECHNIC CHAPTER-8 CAUSES OF LOW POWER FACTOR The main cause of low Power factor is Inductive Load. As in pure inductive circuit Current lags 900 from Voltage, this large difference of phase angle between current and voltage causes zero power factor. Basically, all those circuit having Capacitance and inductance (except resonance circuit (or Tune Circuit) where inductive reactance = capacitive reactance (XL = Xc), so the circuit becomes a resistive circuit), power factor would be exist over there because Capacitance and inductance causes in difference of phase angle (ⱷ) between current and voltage. there are a lot of disadvantages of low Pf and we must improve Pf. FOLLOWING ARE THE CAUSES OF LOW POWER FACTOR: 1. Single phase and three phase induction Motors (Usually, Induction motor works at poor power factor i.e. at: Full load, Pf= 0.8 - 0.9 Small load, Pf = 0.2 - 0.3 No Load, Pf may come to Zero (0). 2. Varying Load in Power System (As we know that load on power system is varying. During low load period, supply voltage is increased which increase the magnetizing current which cause the decreased power factor) 3. Industrial heating furnaces 4. Electrical discharge lamps (High intensity discharge lighting) Arc lamps (operate a very low power factor) 5. Transformers 6. Harmonic Currents 70 Generation, Transmission & Distribution  KIIT POLYTECHNIC You may also read about: Power Factor Active, Reactive, Apparent and Complex Power. Simple explanation with formulas. Disadvantages of Low Power Factor Power Factor improvement Methods with Their advantages & Disadvantages Advantages of Power factor improvement and Correction How to Calculate the Suitable Capacitor Size in Farads & kVAR for Power factor improvement (Easiest way ever) How to Convert Capacitor Farads into kVAR and Vice Versa (For Power factor improvement) DISADVANTAGES OF LOW POWER FACTOR: Power factor play an important role in AC circuits and power dissipation depends on this factor. For instant, we know that; Power in a Three Phase AC Circuit = P = √3 V x I Cosⱷ And Current in a Three Phase AC Circuits = I = P / (3 V x Cosⱷ) 1α1 /Cosⱷ …..(1) Also, Power in a Single Phase AC Circuits = P = V x I Cosⱷ And Current in a Three phase AC Circuits = I = P / (V x Cosⱷ) 1α1 /Cosⱷ …..(2) It is clear from both equations (1) and (2) that Current “I” is inversely proportional to Cosⱷ i.e. Power Factor. 71 Generation, Transmission & Distribution  KIIT POLYTECHNIC In other words, When Power Factor increases, Current Decreases, and when Power Factor decreases, Current Increases. Now, in case of Low Power Factor, Current will be increased, and this high current will cause to the following disadvantages. 1.) Large Line Losses (Copper Losses): We know that Line Losses is directly proportional to the square of Current I2 Power Loss = I2xR i.e., the larger the current, the greater the line losses i.e. I»Line Losses In other words, Power Loss = I2xR = 1/Cosⱷ2 …………Refer to Equation "1α1 /Cosⱷ "…….(1) Thus, if Power factor = 0.8, then losses on this power factor =1/Cos ⱷ2= 1/0.82 = 1.56 times will be greater than losses on Unity power factor. 2.) Large kVA rating and Size of Electrical Equipments: As we know that almost all Electrical Machinery (Transformer, Alternator, Switchgears etc) rated in kVA. But, it is clear from the following formula that Power factor is inversely proportional to the kVA i.e. Cos ⱷ = kW / kVA Therefore, The Lower the Power factor, the larger the kVA rating of Machines also, the larger the kVA rating of Machines, The larger the Size of Machines and The Larger the size of Machines, The Larger the Cost of machines. 3.) Greater Conductor Size and Cost: In case of low power factor, current will be increased, thus, to transmit this high current, we need the larger size of conductor. Also, the cost of large size of conductor will be increased. 72 Generation, Transmission & Distribution  KIIT POLYTECHNIC 4.) Poor Voltage Regulation and Large Voltage Drop: Voltage Drop = V = IZ. Now in case of Low Power factor, Current will be increased. So the Larger the current, the Larger the Voltage Drop. Also Voltage Regulation = V.R = (VNo Load – Vfull Load )/ Vfull Load In case of Low Power Factor (lagging Power factor) there would be large voltage drop which cause low voltage regulation. Therefore, keeping Voltage