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Electric power grid

  1. 1. Electric Power Grid Power Transmission & Distribution Infrastructure PPT prepared by Prof. Z. Jan Bochynski EE3824 Course Instructor The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill https://www.osha.gov/SLTC/etools/electric_power/credits.html Sources :
  2. 2. Global Electrical Energy Distribution
  3. 3. Transmission & Distribution System Components: • Power Generation Plants • Transformers Substations • Transmission Lines • Distribution Systems
  4. 4. Power Generation Plants • Fossil fuel plant • Nuclear plant • Hydroelectric plant • Geothermal plant • Solar thermal plant • Wind tower farms
  5. 5. Electrical Energy Resources •Heat (thermal) energy generated from burning fossil fuels: coal, petroleum, natural gas •Thermal energy inside the Earth (geothermal) •Nuclear reaction energy •Potential energy of falling water •Wind kinetic energy •Solar electric from solar (photovoltaic) cells •Thermal energy from the Sun radiation Wind power towers
  6. 6. Power Generation Plants Nuclear power plant Geothermal power plant Hydroelectric power plant Fossil fuel power plant Additional information: The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.
  7. 7. Transformer Substations A transformer substation as a part of the transmission system is a facility where voltage increases (or decreases), but the transmitted power and voltage frequency remain unchanged. A transmission bus is used to distribute electric power to one or more transmission lines. A substation can have circuit breakers that are used to switch generation and transmission circuits in and out of service as needed.
  8. 8. Transformers Substation Functions • Change voltage from one level to another • Regulate voltage to compensate for system voltage changes • Switch transmission and distribution circuits into and out of the grid system • Measure electric power qualities flowing in the circuits • Connect communication signals to the circuits • Eliminate lightning and other electrical surges from the system • Connect electric generation plants to the system • Make interconnections between the electric systems of more than one utility • Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the circuits
  9. 9. Step-up / Step-down Substations A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations. Step-up substationStep-down power transformer
  10. 10. Step-down Substation The step-down substation can change the transmission voltage to a sub- transmission voltage, usually 69 kV. The sub-transmission voltage lines can serve as a source to distribution substations. The specific voltages leaving a transmission substation are determined by the customer needs of the utility supplying power and to the requirements of any connections to regional grids.
  11. 11. Underground Distribution Substation Distribution substation transformers change the subtransmission voltage to the level use by the consumer. Typical distribution voltages vary from 34,500Y/19,920Y volts to 4,160Y/2400Y volts. Underground distribution substations are at the end-users location. Manhole is also called a splicing chamber or cable vault. They are of various sizes usually from 2 to 6 inches in diameter.
  12. 12. Underground System Components Conduits are hollow tubes running from manhole to manhole in an underground transmission or distribution system. Conduits can be made of plastic (PVC), fiberglass, fiber, tile, concrete, or steel. PVC and fiberglass are most commonly used Conduit on a gradeDuct run within conduit showing drainage in both directions Electrical cables are run through ducts. The diameter of a duct should be at least 1/2 to 3/4 inch greater than the diameter of the cable(s) installed in the duct. They can be made of plastic (PVC), fiberglass, fiber, tile, concrete, or steel. PVC and fiberglass are most commonly used.
  13. 13. Underground High-Voltage Underground Cables High-Voltage underground cables are usually shielded cables. They are made with a conductor, conductor-strand shielding, insulation, semi-conducting insulation shielding, metallic insulation shielding, and a sheath. The sheath can be metallic and may then serve as the metallic insulation shielding and be covered with a nonmetallic jacket to protect the sheath. This sheath helps to reduce or eliminate inductive reactance. Such cables are commonly used in circuits operating at 2400 volts or higher
  14. 14. Underground Transformer Vault A transformer vault is a structure or room in which power transformers, network protectors, voltage regulators, circuit breakers, meters, etc. are housed.
  15. 15. Riser for Underground System A riser is a set of devices that connects an overhead line to an underground line. A riser has a conduit from the ground up the pole where potheads are used to connect to the overhead lines. Riser diagramRiser
  16. 16. Transformers in Underground System A vault, pad-mounted, submersible, and direct-buried transformers are used in underground systems. , Pad-mounted transformers are installed on a concrete pad on the surface near the end-user. Vault type transformer in underground vault Underground transformers are essentially the same as an aboveground transformers and are constructed for the particular needs of underground installation.
  17. 17. Transmission Lines Characteristic A transmission line can be either overhead or underground line. Transmission lines carry electric energy from power plants to end-users. They can carry: • Alternating current AC or • Direct current DC or • Both AC and DC voltages. The main characteristics that distinguish a transmission line from a distribution line are: • the transmission line operates at high voltages, • the transmission line transmit large quantities of power • the transmission line transmit the power over much larger distance.
  18. 18. Transmission Lines Transmission Lines Type:  Overhead Transmission Lines  Sub-transmission Lines  Underground Transmission Lines The 3-phase AC transmission line transmitting power of greater voltage from 69 kV to 765 kV and has three wires. Power Transmitting Poles
  19. 19. The DC voltage transmission line (Two wires: positive and negative.) The sub-transmission line carries voltage reduced from the major transmission line system. Overhead high voltage transmission line
  20. 20. Sub-transmission & Distribution Lines Sub-transmission lines carry voltages reduced from the major transmission line system for use in industrial or large commercial facilities. The 34.5 kV - 69 kV voltage is sent to regional distribution substations.
  21. 21. Underground Transmission Lines The underground transmission line run through populated areas and cities. They may be buried with no protection, or placed in conduit, trenches, or tunnels. Transmission lines installed in a trench and different way in tunnels.
  22. 22. Power Distribution System A distribution system originates at a distribution substation and includes the lines, poles, transformers and other equipment needed to deliver electric power to customers. Industrial Customer Transportation Customer Residential Customer Commercial Customer 2.4 – 4.16 kV 14.4 - 7.2 kV
  23. 23. SUBSTATIONS EQUIPMENT Air Circuit Breaker Distribution Bus Potheads Batteries Power-line Carrier Bus Support Insulators Frequency Changers Power Transformers Capacitor Bank Grounding Resistors Rectifiers Circuit Switchers Grounding Transformers Relays High-Voltage Underground Cables SF6 Circuit Breakers High-Voltage Fuses Shunt Reactors Lightning Arresters Steel Superstructures Control Panels Supervisory Control Control Wires Metal-clad Switchgear Suspension Insulators Converter Stations Meters Synchronous Condensers Coupling Capacitors Microwave Transmission Bus Current Transformers Oil Circuit Breakers Vacuum Circuit Breakers Disconnect Switches Potential Transformers
  24. 24. Power Transmission System Lines Representation and Equations Prof. Z. Jan Bochynski
  25. 25. Linear Diagram of AC 3-Phase Power System This power system has two synchronous machines, two loads, two busses, two transformers, and transmission lines to connect the busses together.
  26. 26. One-line Diagram of a Typical Coal-fired Generating Unit
  27. 27. Transmission Lines Dual 345 kV transmission lines on a steel tower Dual 110 kV transmission lines on wooden poles A 13.8 kV distribution lines with the ground wire above the three phases wires. A distribution line with no ground wire. Transmission system voltages are typically from 69KV up to 765KV. Distribution systems typically operate in a voltage range of 4KV to 46KV.
  28. 28. Transmission Lines There are two types of transmission lines: overhead lines and buried cables. An overhead transmission line usually consist of three conductors or bundles of conductors containing the three phases of the power system. The conductors are aluminum cable steel reinforced (ACSR), with a steel core and aluminum conductors wrapped around the core. Cable lines are designed to be placed underground or under the water. In cables conductors are insulated from one another and surronded by the protective sheath. As a rule of thumb, the power handling capability of a transmission line is proportional to the square of the voltage on the line. Therefore, very high voltage transmission lines are used to transmit electric power over long distance. The lower voltage lines are distribution lines used to deliver power to the individual custmers.
  29. 29. Transmission Lines Characteristics Transmission lines are characterized by a series of resistance and inductance and by a shunt capacitance. RDC = 𝝆𝝆 𝒍𝒍 𝑨𝑨 The DC resistance of the line conductor: The transmission line inductance: L = 𝜱𝜱 𝑰𝑰 ρ = resistivity of the conductor A = cross sectional area of the conductor l = length of the conductor Φ = number of flux linkages produced by the current through the line The line capacitance between the pair of conductors in F 𝑪𝑪 = 𝒒𝒒 𝑽𝑽 q = the charges on the conductors in coulombs V = the voltage between the conductors in volts
  30. 30. Skin Effect The AC resistance of a line conductor is always higher than its DC resistance because of skin effect. In AC line as frequency increases, more of the current is concentrated near the outer surface of the conductor. f⋅⋅= πµ ρ δ ρ is wire electrical conductivity μ is magnetic permeability of the flux medium f is frequency
  31. 31. Skin Effect Samples Source: M. R. Patel “Introduction to Electrical Power and Power Electronics” CRC Press, 2013 Two round conductors get current concentration near facing area Two bus bars with facing flats get current concentration neat facing strips f⋅⋅= πµ ρ δ
  32. 32. Internal Inductance of T L For the conductor of a radius r carrying the current I the magnetic field intensity at the distance x from the center of this conductor is: 𝑯𝑯𝑯𝑯 = 𝑰𝑰𝑰𝑰 𝟐𝟐𝝅𝝅𝒙𝒙 Outside a conductor 𝑯𝑯𝒙𝒙 = 𝒙𝒙 𝝅𝝅𝒓𝒓𝟐𝟐 𝑰𝑰 Inside a conductor The flux density Bx at the distance x from the center of the conductor: 𝑩𝑩𝒙𝒙= µ𝑯𝑯𝒙𝒙= µ 𝒙𝒙 𝟐𝟐π𝒓𝒓𝟐𝟐 𝑰𝑰 in Wb The internal inductance per meter of line length: 𝒍𝒍𝒊𝒊 𝒊𝒊𝒊𝒊 = µ 𝟖𝟖𝝅𝝅 H/m For cooper and aluminum (non-ferromagnetic) this inductance is: Lint =0.5 x 10-7 H/m
  33. 33. T L External Inductance The magnetic intensity at the distance x from the center of the conductor: 𝑯𝑯𝒙𝒙 = 𝑰𝑰 𝟐𝟐𝝅𝝅𝒙𝒙 The flux density Bx at a distance x from the center of the conductor: 𝑩𝑩𝒙𝒙 = µ𝑯𝑯𝒙𝒙 = µ 𝑰𝑰 𝟐𝟐𝝅𝝅 𝒙𝒙 [in teslas] The external inductance per meter due to the flux between P1 and P2 : 𝒍𝒍𝒆𝒆𝒆𝒆𝒆𝒆 = µ 𝟐𝟐𝝅𝝅 𝐥𝐥 𝐥𝐥 𝑫𝑫𝟐𝟐 𝑫𝑫𝟏𝟏 [H/m]
  34. 34. Transmission Line Inductance of a Single-Phase Two-Wires Line The two-wire transmission line inductance per line length: l = 𝒍𝒍𝒊𝒊 𝒊𝒊𝒊𝒊 + 𝒍𝒍𝒆𝒆𝒆𝒆𝒆𝒆 = ( µ 𝝅𝝅 + 𝒍𝒍 𝒍𝒍 𝑫𝑫 𝒓𝒓 ) H/m The series inductive reactance of transmission line: X = 𝑗𝑗 𝜔𝜔 𝑙𝑙 = j 2π f l
  35. 35. Inductance of a Transmission Lines Inductance per unit length l of a single-phase, two-wire transmission line is proportional to the ratio D/r , where D is a distance between the centers of the two conductors and r is the wires radius. Therefore: The greater the spacing between the phases of the transmission line, the greater the inductance of the line. A single conductor of a high-voltage line will tend to have a higher inductance than a single-conductor of a low-voltage line. The greater the radius of the conductor in a transmission line, the lower the inductance of the line. The series inductance of buried cables will be much smaller than the inductance of overhead transmission lines. In practical transmission lines do not use conductors of extremely large radius, because they would be very heavy, inflexible, and expensive. Instead, is practiced bundling two, or more conductors together in each phase.
  36. 36. Transmission Lines with Conductors in Bundle Transmission line with two conductors in bundle Transmission line with four conductors in bundle
  37. 37. Capacitance and Capacitive Reactance of a Transmission Line When a voltage is applied to a pair of conductors separated by a nonconducting dielectric medium, opposite electric charges accumulate on the surface of the conductors of equal magnitude that is proportional to the line voltage. q = C V The electric field density at the wire surface is D = εr ε0 E εr is relative permeability of the material is 8,85 x 10-12 F ε0 is one (1) for the air 𝑬𝑬 = 𝒒𝒒 𝟐𝟐 𝝅𝝅 𝜺𝜺 𝒙𝒙 E - the electric field intensity at the point always radially outward from the conductor D = 𝒒𝒒 𝟐𝟐 𝝅𝝅 𝒙𝒙
  38. 38. Capacitance of a Single-Phase Two-Wire Transmission Line The capacitance per unit length between the two conductors of the transmission line: 𝑐𝑐𝑎𝑎𝑎𝑎 = π ε ln ( 𝐷𝐷 𝑟𝑟 ) a b The capacitance to the ground of a single-phase transmission line 𝑐𝑐𝑛𝑛 = 𝑐𝑐𝑎𝑎𝑎𝑎 = 𝑐𝑐𝑏𝑏𝑏𝑏 = 2 π ε ln ( 𝐷𝐷 𝑟𝑟 ) The shunt capacitive admittance of a transmission line depends on the capacitance of the line and the frequency of the power system. Yc = y d = (j2π f c) d y - is the shunt admittance per unit length of the transmission line d – is the length of the line The capacitive reactance is: 𝑋𝑋𝑐𝑐 = 1 𝑌𝑌𝑐𝑐 = −𝑗𝑗 1 (2π𝑓𝑓𝑓𝑓)𝑑𝑑
  39. 39. Transmission Line Capacitance If the transmission line capacitance is proportional to the ratio D/r , therefore :: The greater the spacing between the phases of the transmission line, the lower is the line capacitance. The shunt capacitance of buried cables will be much larger than the shunt capacitance of overhead transmission line because of very small spacing between conductors in a cable. A single-conductor high-voltage line will tend to have a lower capacitance than a single-conductor low-voltage line. The greater the radius of the conductors in a transmission line, the higher the capacitance of the line.
  40. 40. Transmission Line Models A transmission line is characterized by a series resistance per unit length, a series inductance per unit length, and shunt capacitance per unit length. For short transmission line (less than 50 miles), the shunt capacitance can be neglected. For medium-length transmission lines (50 to 150 miles), the shunt capacitance can be devided into two lumped components, one before and one after the series impedances.
  41. 41. The Phasor Diagram of a Short Transmission Line VS = VR + ZI = VR + I(R + j XL) VR = VS – RI - jXLI The phasor diagram of a transmission line Current lagging p.f. cosφ<1, +Q) Unity p.f. =1 (cosφ=1, Q = 0) Current leading p.f. cos φ>1, -Q)
  42. 42. Power Flows in Transmission Line Pin = 3 VS IS cos θS = √3 V S LL IS cos θS VS is the magnitude of the source line-to-neutral voltage VLL is the magnitude of the source line-to-line voltage Pout =3 VR IR cos θR =√3 VR LL IR cos θR Voltage regulation VR: VR = 𝑉𝑉𝑛𝑛𝑛𝑛 − 𝑉𝑉𝑓𝑓𝑓𝑓 𝑉𝑉𝑓𝑓𝑓𝑓 x 100% Vnl no-load voltage Vfl full load voltage Qin = 3 VS IS cos θS = √3VS LL IS cos θS Qout = 3 VR IR cos θR = √3VR LL IR cos θR Reactive power in and out Apparent power Sout = 3 VR IR = √3 VR LL IR Sin = 3 VS IS = √3 VS LL IS Transmission Line efficiency η = 𝑃𝑃𝑜𝑜𝑜𝑜𝑜𝑜 𝑃𝑃𝑖𝑖 𝑖𝑖 x 100%
  43. 43. EMF Overhead T L Electric field under an overhead transmission line Magnetic flux density near overhead transmission line Source: http://www.hpa.org.uk
  44. 44. Magnetic Flux Density around Overhead Line and Cable The magnetic fields arising at/or close to ground level from underground HV cables and overhead lines carrying similar currents. Underground cable (1 meter underground) produces greater flux density above but is decreasing very fast within the distance from its center. Source: http://www.hpa.org.uk
  45. 45. Shielding Electric Fields from Power Lines Electric field from power line can be reduced by building walls, trees and shield made of metal. Source: http://www.hpa.org.uk
  46. 46. Magnetic Fields from Home Appliances Source: http://www.hpa.org.uk/
  47. 47. T L Voltage and Electric Fields Electric fields depend on the magnitude of the voltage and distance from the source. Source: http://www.hpa.org.uk
  48. 48. Magnetic Fields from Appliances Source: http://www.hpa.org.uk
  49. 49. Electric Field and Human Body The electric fields from power line induce electric charges at the body surface. At the highest electric field strength encountered, hair movement and small shocks may be sensed by some people. Electric fields induced currents in the body Source: http://www.hpa.org.uk
  50. 50. Magnetic Field and Human Body Magnetic fields induce circulating currents in the body. Induced currents in large quantity may interfere with the nervous system and flashes of light may be noticed in the eye. People having a pacemaker should avoid the exposure to the magnetic fields. Source: http://www.hpa.org.uk
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