ELE 2303 Power Generation and Transmission Past Paper PDF

Power Generation and Transmission ELE-2303 Monday, July 1, 2024 ELE 2303: Power Generation and Transmission CLO3 : Describe the different methods used for insulation and protection of the transmission lines 2  Power Generation...

Power Generation and Transmission ELE-2303 Monday, July 1, 2024 ELE 2303: Power Generation and Transmission CLO3 : Describe the different methods used for insulation and protection of the transmission lines 2  Power Generation and Transmission ELE 2303 CLO3 Describe the different methods used for insulation and protection of the transmission lines 3.1 Describe and sketch the characteristics of a ceramic insulator as used for suspension of a medium voltage 3.2 Determine the maximum voltage that may be carried by wooden support poles 3.3 Explain the process of cleaning insulators on transmission lines 3.4 Describe the structure of transmission line towers, explaining the maximum spacing, and acceptable slack 3.5 Explain the requirements of grounding wires and tower grounding 3.6 Sketch and Label the main components of a medium /high voltage circuit breaker 3.7 Describe and differentiate different types of circuit breakers; this includes air, vacuum, oil, SF6 circuit breaker, moulded-case circuit breaker and miniature circuit breakers 3.8 Describe the principal components of electrical installation systems 3 3.1 Describe and sketch the characteristics of a ceramic insulator as used for suspension of a medium voltage An insulator is one kind of material where the internal electric charge of this does not run freely; insufficient electric current will run through it in the power of an electric field. An insulator gives support to the overhead line conductors on the poles to prevent the current flow toward earth. 4  In the transmission lines, insulator plays an essential role in its operation. The designing of an insulator can be done using different materials like rubber, wood, plastic, mica, etc. The special materials used in the electrical system are glass, ceramic, porcelain, PVC (Poly Vinyl Chloride) and polymers. 5 Types of Insulators Insulators are used in transmission & distribution system where each insulator consists of several insulating discs. These are classified into different types based on their ratings. Some are discussed below:  Pin Insulator  Suspension Insulator  Strain Insulator  Shackle Insulator 6 Types of Insulators Pin Insulator This kind of insulator is used in distribution systems. The voltage capacity of this insulator is 11kV. It is designed with a high mechanical strength material. These are connected in vertical as well as horizontal positions. The construction of this insulator is simple and needs less maintenance as compared with other types. 7 Types of Insulators Suspension Insulator  This also called disc insulator and the designing of these insulators can be done using materials like ceramic, porcelain or glass.  The voltage capacity of suspension insulator ranges from 11 kV to 765 kV.  It is used in overhead transmission lines by providing more flexibility.  It uses various discs based on the level of voltage.  It is connected by using the steel tower so it needs more height to support all the discs.  These insulators are most helpful compare with other insulators because; if one disc in the insulator gets damage then remaining all the discs will work properly. So the damaged disc can be replaced with others. 8 Types of Insulators Strain Insulator  This is similar to suspension type insulators because it is used in an overhead transmission system but Rated System Number of disc Number of disc its specifications and working are Voltage insulator insulator somewhat different. used in used in strain type suspension  The voltage capacity of the strain tension insulator insulator is 33kV. insulator string string  Mostly in the transmission line, it is placed in bend otherwise arm 33KV 3 3 place. 66KV 5 4 132KV 9 8 220KV 15 14 9 Types of Insulators Shackle Insulator  These insulators are small in size, used in overhead distribution systems.  The connection of this insulator can be done by using a metallic strip.  The voltage capacity of this insulator is 33 kV and works in the positions of bend or circular turn.  Shackle insulators are used in a vertical position or horizontal position.  These are connected to the pole using bolt otherwise cross arm. 10 Insulators Voltage Capacity and Use 11 Ceramic Insulator Characteristics The major developments in ceramic insulating materials have been made since World War I. These developments parallel the growth of the electrical and electronic industries. The pattern of these developments has been to tailor the properties of the ceramic insulators to specific requirements. The most important electrical properties are  dielectric constant  power factor  dielectric strength  Resistivity  The effect of frequency and temperature. 12 Ceramic Insulator Characteristics In the common insulator ceramics the dielectric constant will vary from 1.5 for a porous ceramic to approximately 10.5. Dielectric constants of 5.5 to 6.5 are most common. The power factor of ceramics may vary between.02 for porcelain to.000015. Most of the ceramics, however, have power factors in the range of.004 to.0005. The room temperature resistivity of ceramics is between 10^+14 and 10^+16 ohms cm. Dielectric strength varies from 190 to 350 volts per mil. When examining ceramic materials with frequency as a variable it is found that these materials stand up very well to about 10 megacycles. Higher frequencies cause the loss factor to increase slowly. 13 Ceramic Insulator Characteristics The mechanical and physical characteristics which are common to all ceramics are:  Brittleness  Low tensile strengths  High compressive strengths  High temperature stability  Chemical stability  Low thermal conductivities https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7473192 https://www.youtube.com/watch?v=m4bZaLkwfis 14 3.2 Determine the maximum voltage that may be carried by wooden support poles Electric pole made of wood are usually used in 400 volt and 230-volt low tension lines. The cost of a pole made of wood is less than that of other poles. In other poles, the cost of a foundation is more which can be said to be equal to that of the wood pole. These poles can serve for a long period of time. 15 Voltage Distribution Over A Suspension Insulator String and String Efficiency The figure shows a 3-disc string of suspension insulator. As each disc lies in between two metal links, it forms a capacitor. This capacitance is known as self capacitance or mutual capacitance. Moreover, air capacitance is also present between metal links and the earthed tower. This is known as shunt capacitance. The figure illustrates the equivalent circuit of a 3-disc suspension insulator (assuming that shunt capacitance is some fraction of self-capacitance i.e shunt capacitance = k * self-capacitance). 16  If there were only mutual capacitances, then the charging current would have been the same through all the discs. In this case, the voltage would have been uniformly distributed across the string, i.e. voltage across each disc would have been the same. But, due to the shunt capacitances, charging current is not the same through all the discs. From the above equivalent circuit, applying Kirchoff's current law to node A, I2 = I1 + i1 V2ωC = V1ωC + V1ωkC V2 = V1 + V1k V2 = (1 + k)V1...... eq.(i) 17  Applying Kirchoff's current law to node B, I3 = I2 + i2 V3ωC = V2ωC + (V2 + V1)ωkC V3 = V2 + (V1 + V2)k V3 = kV1 + (1 + k) V2 V3 = kV1 + (1 + k)2 V1...... from eq.(i) V3 = V1 [k + (1 + k)2] V3 = V1 [k + 1 + 2k + k2] V3 = V1 (1 + 3k + k2)...... eq.(ii) Now, voltage between the conductor and the earth tower is, V = V1 + V2 + V3 V = V1 + (1 + k)V1 + V1 (1 + 3k + k2) V = V1 (3 + 4k + k2)...... eq.(iii) 18  From the above equations (i), (ii) & (iii), it is clear that the voltage across the top disc is minimum while voltage across the disc nearest to the conductor is maximum, i.e. V3 = V1 (1 + 3k + k2). As we move towards the cross arm, voltage across the disc goes on decreasing. Due to this non-uniform voltage distribution across the string, the unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. 19 String Efficiency The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is called as string efficiency String efficiency = Voltage across the string / (number of discs X voltage across the disc nearest to the conductor). Greater the string efficiency, more uniform is the voltage distribution. String efficiency becomes 100% if the voltage across each disc is exactly the same, but this is an ideal case and impossible in practical scenario. However, for DC voltages, insulator capacitances are ineffective and voltage across each unit would be the same. This is why string efficiency for DC system is 100%. Inequality in voltage distribution increases with the increase in the number of discs in a string. Therefore, shorter strings are more efficient than longer string insulators. 20 Problem Solving Question An overhead 3-phase transmission line, which has phase voltage 30 kV, is hanging from a three insulators suspension strain. The capacitance between the links and the earth is 0.2 C, where C is the capacitance of an insulator. a. Voltage across each disc b. String Efficiency 21 Solution: a. V2 = (1 + k)V1 =1.2V1 V3 = V1 (1 + 3k + k2) =1.64 V1 V = V1 + V2 + V3 30000=3.84 V1 V1 =7812.5 V V2 = (1 + k)V1 =1.2V1 =9375 V V3 = V1 (1 + 3k + k2) =1.64 V1 =12812 V b. String efficiency=30000/(3 V3 )=0.7804=78.04% 22 Repeat the above problem when the capacitance between the links and the earth is 1. 0.1 C 2. 0.0 C 3. 0.3 C 4. 0.4 C where C is the capacitance of an insulator. 23 3.3 Explain the process of cleaning insulators on transmission lines If the insulators are not cleaned, a flashover may occur causing blackouts and damaged equipment. Hot Line Washing is basically a procedure where insulators on electrical lines are cleaned with water while the electricity is energized (on). White vinegar can also be used to remove water residue and accumulation from years of the insulator weathering on the pole or lying in a field. Pour white vinegar in a deep bowl, and let the insulator soak overnight. Brush its teeth and threads the next morning. Rinse with clear water to remove the vinegar. 24 3.3 Explain the process of cleaning insulators on transmission lines Oxalic acid can also be used for cleaning insulators due to its availability, cheapness, safeness and excellent cleaning. https://www.youtube.com/watch?v=iaMSlF7goO4 25 3.4 Describe the structure of transmission line towers, explaining the maximum spacing, and acceptable slack Structure of transmission line towers Explained in LO2 slides 16-22 Spacing and Slack 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. 26 Spacing and Slack considerations, power conductors along the route of the transmission line should maintain requisite clearances to ground in open country, national highways, rivers, railways, tracks, telecommunication lines, other existing power lines. The ground clearance for different voltages, which generally applicable are: 5.66 kV 6.5 m at +650 C conductor temperature 6.132 kV 7.0 m at +650 C conductor temperature 7.220 kV 7.5 m at +800 C conductor temperature The minimum clearances of conductor over rivers, which are not navigable, shall be kept 3.05 m over maximum flood level. 27 Spacing and Slack 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 6.00 m 28 Spacing and Slack In hot weather, power lines can overheat. The lines are often heavily loaded because of increased power consumption, and the conductors, which are generally made of copper or aluminum, expand when heated. That expansion increases the slack (not held tightly in position) between transmission line structures, causing them to sag further. 29 Spacing and Slack Tension is a force that exist in a string that is under the action of two forces in opposite direction. Thus a cable hanging on a pole is under tension and would be under more tension if the cables are to made tight which would make the cables to cut easily when little contraction or expansion occurs. 30  SUB OUTCOME 3.5 3.5 Explain the requirements of grounding wires and tower grounding 31 GROUND WIRES AND THEIR REQUIREMENTS Ground wires or earth wires are bare conductors supported at the top of transmission towers. They serve to shield the line and intercept lightning stroke before it hits the power lines. 32  GROUND WIRES AND THEIR REQUIREMENTS Ground wire is a conductor run parallel to the main conductors of the line. It is supported on the same towers, is placed higher than the main conductors and is adequately grounded at every tower. https://www.electricaltechnology.org/2012/04/what-is-purpose-of-ground-wires-in- 33 over.html#:~:text=Ground%20wires%20or%20earth%20wires,are%20often%20made%20of%20steel. GROUND WIRES AND THEIR REQUIREMENTS For horizontal arrangement of conductors, there are two ground wires to provide effective shielding to power conductors from direct lightning strokes whereas in vertical configuration of conductors there is only one ground wire. 34 GROUND WIRES AND THEIR REQUIREMENTS 1. The ground wire used should be mechanically strong and should be so located that they provide sufficient shielding. 2. There should be sufficient clearance between the power conductors and the tower structure. 3. There should be an adequate clearance between the line conductors and the ground wires, particularly at the mid-span, so as to avoid flashover to the power conductor up to the protective voltage level used for the line design. 4. The tower footing resistance should be as low as permissible. 5. The ground wire is made up of galvanized steel or ACSR conductors. 6. The protective angle is in the region of 20o to 45o. (" Angle between the vertical line passing through the ground wire and the line passing through the outermost power conductor is called the protective angle“) 35 TOWER EARTHING https://www.electrical4u.com/earthing-of-electrical-transmission-tower/ 36 TOWER EARTHING The tower earthing system is provided by an electrically interconnected system of conductors and rods, connectors, foundation and the local soil. Individual tower earthing design must consider the electrical performance of the line and the individual tower. Different tower earthing designs can occur from tower to tower due to the variation in parameters and conditions along the length of the line. 37 TOWER EARTHING Each tower of an electrical transmission line should be earthed. The footing resistance of each tower should be measured. The footing resistance of tower should be taken in dry season. In any circumstances footing resistance of the tower will not be more than 10 ohms. Pipe earthing should be used for the electrical transmission line tower. In case of river crossing and railway crossing towers, we provide earthing at diagonally opposite two legs of a tower. 38  SUB OUTCOME 3.6 3.6 Sketch and Label the main components of a medium /high voltage circuit breaker 39 What is Circuit Breaker (CB) ? A CB (Circuit breaker) is a device which: Control (make or break) a circuit manually or by remote control under normal and fault conditions. Break a circuit automatically under fault conditions like overcurrent and short circuit etc. 40 WORKING PRINCIPLE OF CIRCUIT BREAKER A circuit breaker essentially consists of fixed and moving contacts, called electrodes. These contacts are placed in the closed chamber containing a fluid containing medium (either liquid or gas) which quenches the arc formed between the contacts. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. When a fault occurs on any part of the system, the trip coils of the breaker get energized. The moving contacts are pulled apart by some mechanism, thus opening the circuit. https://studyelectrical.com/2018/12/circuit-breaker-operating- principle-and.html 41 WORKING PRINCIPLE OF CIRCUIT BREAKER 42 WORKING PRINCIPLE OF CIRCUIT BREAKER When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The main problem in a circuit breaker is to extinguish the arc within the shortest possible time. So that heat generated by it may not reach a dangerous value. During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts 43 Factors for Arc Phenomenon The arc resistance depends upon the following factors: The degree of ionization – the arc resistance increases with the decrease in the number of ionized particles between the contacts. Length of the arc – the arc resistance increases with the length of the arc i.e. separation of contacts. Cross-section of arc – the arc resistance increase with the decrease in the area of cross-section of the arc. https://www.slideshare.net/merajulmridul/arc-phenomena 44 Methods of Arc Extinction in Circuit Breaker Methods of Arc Extinction in Circuit Breaker There are two methods of extinguishing the arc in circuit breakers 1.High resistance method 2.Low resistance or current zero method. 45 46 COMPONENTS OF THE CIRCUIT BREAKER 1. Actuator lever: Used to ON and OFF the breaker. Sometimes, a current trip off while the lever is at the ON position. Such is termed as trip-free. 2. Actuator mechanism: Opens and closes the contact. 3. Contacts: Allow current flow when closed and break the current when open. 4. Terminals: This is where breakers connect to the entire circuit. 5. Bimetallic strip: This separates the contacts when there is a high- voltage or short circuit. 6. Calibration screw: It is what the manufacturers used to set the trip current after assembly. 7. Solenoid: Separate contacts when there is over current. 8. Arc extinguisher, or divider: This separates an arc each time a breaker interrupts light. https://cupdf.com/document/what-is-a-circuit-breakerppt.html 47 48  MEDIUM VOLTAGE & HIGH VOLTAGE CIRCUIT BREAKERS Voltage level from 1KV-69KV is categorized under medium & 69KV-230KV is categorized as High Voltage. Circuit Breakers operating in these voltage ranges are known as Medium & High Voltage CBs respectively. VACUUM CIRCUIT BREAKER Vacuum Circuit Breaker or VCB is used for Medium Voltage applications. In VCB the contacts operation & arc quenching takes place inside bottles where Vacuum is present. SF6 CIRCUIT BREAKER SF6 circuit breakers are also used mainly in medium voltage applications. In this breaker SF6 gas is used for arc quenching due to its ability of quenching the arc very efficiently. SF6 Breakers being highly efficient in arc quenching are still not preferred much as SF6 being a poisonous gas, is dangerous to environment & humans. OIL CIRCUIT BREAKER Oil Circuit Breakers were also used on high voltages & Oil was used as the arc quenching medium. 49  LABEL THE COMPONENTS OF DIFFERENT TYPE CIRCUIT BREAKERS –VACUUM CIRCUIT BREAKER 50 LABEL THE COMPONENTS OF DIFFERENT TYPE CIRCUIT BREAKERS-OIL CIRCUIT BREAKER 51 LABEL THE COMPONENTS OF DIFFERENT TYPE CIRCUIT BREAKERS-SF6 CIRCUIT BREAKER 52  SUB OUTCOME 3.7 3.7 Describe and differentiate different types of circuit breakers; this includes air, vacuum, oil, SF6 circuit breaker, moulded-case circuit breaker and miniature circuit breakers 53 54 55 56 57 58 59 60 61 62 https://electricalnaukri.com/sf6-circuit-breaker-construction- working-principle-application/ 63 64 65 LOW VOLTAGE CIRCUIT BREAKER- MINIATURE CIRCUIT BREAKER (MCB) MCB stands for Miniature Circuit Breaker. It automatically switches OFF electrical circuit during any abnormal condition in the electrical network such as overload & short circuit conditions. However, fuse may sense these conditions but it has to be replaced though MCB can be reset. The MCB is an electromechanical device which guards the electric wires &electrical load from overcurrent so as to avoid any kind of fire or electrical hazards. Handling MCB is quite safer and it quickly restores the supply. When it comes to house applications, MCB is the most preferred choice for overload and short circuit protection. MCB can be reset very fast & don’t have any maintenance cost. 66 LOW VOLTAGE CIRCUIT BREAKER- MCCB (MOULDED CASE CIRCUIT BREAKER) MCCB stands for Molded Case Circuit Breaker. It is another type of electrical protection device which is used when load current exceeds the limit of a miniature circuit breaker. The MCCB provides protection against overload, short circuit faults and is also used for switching the circuits. It can be used for higher current rating and fault level even in domestic applications. The wide current ratings and high breaking capacity in MCCB find their use in industrial applications. MCCB can be used for protection of capacitor bank, generator protection and main electric feeder distribution. 67 68 Circuit Breaker Size Calculation for Single Phase Supply To determine the appropriate size of circuit breaker for single phase supply, it depends on multiple factors like type of load, cable material and environment temperature etc. The general rule of thumb is that circuit breaker size should be 125% of the capacity of cable and wire or the circuit which has to be protected by the CB. 69 EXAMPLE QUESTIONS Suppose, a 12 gauge wire is used for 20 amperes lighting circuit having 120V single phase supply. What is the best size of circuit breaker for that 20 A circuit? Solution: Circuit Current: 20A Circuit Breaker Size: ? CB size should be 125% of the circuit current. = 125% x 20A = 1.25 x 20A Circuit Breaker Size = 25A 70 Exercise Questions 2. What is the appropriate size of circuit breaker for 2000W, single phase 120V Supply? 3. What is the suitable size of circuit breaker for 230V, 1840kW load single phase circuit? 71 Circuit Breaker Size Calculation for Three Phase Supply To find the breaker size for three phase supply voltage, we must know the exact kind of load as there are many factors affecting the load current. In other words, same rule won’t apply to the different types of loads i.e. light, motor, inductive or capacitive load as motor takes initially very high current during the starting process as well as power factor involvement. For residential use, we may follow the same formula as above for single phase with taking the √3 (1.732) due to three phase power formula. 72 EXAMPLE and EXERCISE QUESTIONS Example 1: Which size circuit breaker is needed for 6.5kW, three phase 480V load? Power in Three Phase: P = V x I x √3 Current: P / V x √3 I = 6.5kW / (480V x 1.732) … (√3 = 1.732) I = 6.5kW / 831.36 I = 7.82A The recommended size of circuit breaker is 1.25 x 7.82A = 9.77A The next closest standard of circuit breaker is 10A. Exercise Question: Find the appropriate size of CB for 3-Phase 415V, 17kW load? 73  SUB OUTCOME 3.8 Describe the principal components of electrical installation systems 74 PRINCIPAL COMPONENTS OF THE ELECTRICAL INSTALLATION SYSTEMS https://www.slideshare.net/catherinelindsay/components- 75 used-in-electrical-installations 76 77 78 79 80 81 82