Text book of electrical technology

In the 23rd edition of this textbook, two new chapters have been added on ratingservice capacity and distribution automation. The chapter on electric traction has been revised by adding theory and the electronic control chapter has been amended to take into account the recent concept of digital control.

(ix) CONTENTS CONTENTS 40. D.C. Transmission and Distribution 15691602 Transmission and Distribution of D.C. Power—Two- wire and Three-wire System—Voltage Drop and Transmission Efficiency—Methods of Feeding Distributor— D.C.Distributor Fed at One End— Uniformly Loaded Distributor— Distributor Fed at Both Ends with Equal Voltages — Distributor Fed at Both Ends with Unequal Voltages—Uniform Loading with Distributor Fed at Both Ends— Concentrated and Uniform Loading with Distributor Fed at One End— Ring Distributor— Current Loading and Load-point Voltages in a 3-wire System—Three-wire System— Balancers—Boosters—Comparison of 2-wire and 3-wire Distribution Systems—Objective Tests 41. A.C. Transmission and Distribution 1603 -1682 General Layout of the System— Power Systems and System Networks — Systems of A.C. Distribution-Single-phase, 2- wire System-Single-phase, 3-wire System-Two-phase, 3-wire System-Two-phase, 4-wire System—Three-phase, 3-wire System-Three-phase, 4-wire System—Distribution—Effect of Voltage on Transmission Efficiency —Comparison of Conductor Materials Required for Various Overhead Systems—Constants of a Transmission Line—Reactance of an Isolated Single-phase Transmission Line—Reactance of 3-phase Transmission Line—Capacitance of a Single-phase Transmission Line—Capacitance of a Three-phase Transmission Line-Short Single-phase Line Calculations— Short Three-phase Transmission Line Constants — Effects of Capacitance—Nominal T-method- “Nominal” π- method—Ferranti Effect-Charging Current and Line Loss of an Unloaded Transmission Line—Generalised Circuit Constants of a Transmission Line—Corona-Visual Critical Voltage—Corona Power —Disadvantages of Corona— (x) Underground Cables—Insulation Resistance of a Single-core Cable—Capacitance and Dielectric Stress— Capacitance of 3-core Belted Cables—Tests for Three-phase Cable Capacitance—A.C. Distribution Calculations—Load Division Between Parallel Lines — Suspension Insulators— Calculation of Voltage Distribution along Different Units— Interconnectors—Voltage Drop Over the Interconnector— Sag and Stress Analysis— Sag and Tension with Supports at Equal Levels —Sag and Tension with Supports at Unequal Levels-Effect of Wind and Ice — Objective Tests. 42. Distribution Automation 1683 - 1698 Introduction—Need Based Energy Management (NBEM) — Advantages of NBEM—Conventional Distribution Network—Automated System — Sectionalizing Switches — Remote Terminal Units (RTU’s) — Data Acquisition System (DAS) — Communication Interface — Power line carrier communication (PLCC) — Fibre optics data communication — Radio communication — Public telephone communication — Satellite communication — Polling scheme — Distribution SCADA — Man - Machine Interface — A Typical SCADA System — Distribution Automation — Load Management in DMS Automated Distribution System — Data acquisition unit — Remote terminal unit (RTU) — Communication unit — Substation Automation — Requirements — Functioning — Control system — Protective System — Feeder Automation — Distribution equipment — Interface equipment — Automation equipment — Consumer Side Automation — Energy Auditing—Advantages of Distribution Automation — Reduced line loss —Power quality — Deferred capital expenses – Energy cost reduction – Optimal energy use — Economic benefits — Improved reliability — Compatibility — Objective Tests. 43. Electric Traction 1699 - 1766 General—Traction Systems—Direct Steam Engine Drive — Diesel-electric Drive-Battery-electric Drive- Advantages of Electric Traction—Disadvantages of Electric Traction — Systems of Railway Electrification—Direct Current System—Single- phase Low frequency A.C. System—Three- phase Low frequency A.C. System—Composite System — (xi) Kando System-Single-phase A.C. to D.C. System— Advantages of 25 kV 50 Hz A.C. System—Disadvantages of 25kV A.C. System—Block Diagram of an A.C. Locomotive—The Tramways —The Trolley Bus-Overhead Equipment (OHE) — Collector Gear of OHE—The Trolley Collector—The Bow Collector—The Pantograph Collector — Conductor Rail Equipment—Types of Railway Services — Train Movement—Typical Speed/Time Curve— Speed/Time Curves for Different Services—Simplified Speed/Time Curve—Average and Schedule Speed — SI Units in Traction Mechanics—Confusion Regarding Weight and Mass of a Train—Quantities Involved in Traction Mechanics—Relationship Between Principal Quantities in Trapezoidal Diagram—Relationship Between Principal Quantities in Quadrilateral Diagram —Tractive Effort for Propulsion of a Train—Power Output From Driving Axles— Energy Output from Driving Axles—Specific Energy Output—Evaluation of Specific Energy Output —Energy Consumption—Specific Energy Consumption-Adhesive Weight—Coefficient of Adhesion—Mechanism of Train Movement—General Feature of Traction Motor — Speed— Torque Characteristic of D.C. Motor — Parallel Operation of Series Motors with Unequal Wheel Diameter — Series Operation of series Motor with Uneuqal Wheel Diameter — Series Operation of Shunt Motors with Unequal Wheel Diameter — Parallel Operation of Shunt Motors with Unequal Wheel Diameter — Control of D.C. Motors —Series -Parallel Starting — To find t s , t p and η of starting — Series Parallel Control by Shunt Transition Method — Series Parallel control by Bridge Transition — Braking in Traction — Rheostatic Braking—Regenerative Braking with D.C. Motors — Objective Tests. 44. Industrial Applications of Electric Motors 1767 - 1794 Advantages of Electric Drive—Classification of Electric Drives —Advantages of Individual Drive—Selection of a Motor—Electrical Characteristics —Types of Enclosures— Bearings—Transmission of Power —Noise— Motors of Different Industrial Drives — Advantages of Electrical Braking Over Mechanical Braking — Types of Electric Braking—Plugging Applied to DC Motors—Plugging of Induction Motors—Rheostatic Braking—Rheostatic Braking (xii) of DC Motors—Rheostatic Braking Torque—Rheostatic Braking of Induction Motors — Regenerative Braking— Energy Saving in Regenerative Braking - Objective Tests. 45. Rating and Service Capacity 1795 - 1822 Size and Rating — Estimation of Motor Rating — Different Types of Industrial Loads—Heating of Motor or Temperature Rise—Equation for Heating of Motor — Heating Time Constant — Equation for Cooling of Motor or Temperature Fall — Cooling Time Constant — Heating and Cooling Curves — Load Equalization — Use of Flywheels — Flywheel Calculations — Load Removed (Flywheel Accelerating) — Choice of Flywheel — Objective Tests. 46. Electronic Control of AC Motors 1823 - 1832 Classes of Electronic AC Drives — Variable-Frequency Speed Control of a SCIM—Variable Voltage Speed Control of a SCIM—Speed Control of a SCIM with Rectifier Inverter System—Chopper Speed Control of a WRIM—Electronic Speed Control of Synchronous Motors—Speed Control by Current fed D.C. Link—Synchronous Motor and Cycloconverter— Digital Control of Electric Motors — Application of Digital Control—Objective Tests. 47. Electric Heating 1833 - 1860 Introduction—Advantages of Electric Heating—Different Methods of Heat Transfer — Methods of Electric Heating— Resistance Heating—Requirement of a Good Heating Element—Resistance Furnaces or Ovens—Temperature Control of Resistance Furnaces—Design of Heating Element —Arc Furnaces—Direct Arc Furnace—Indirect Arc Furnace — Induction Heating—Core-type Induction Furnace— Vertical Core-Type Induction Furnace—Indirect Core-Type Induction Furnace—Coreless Induction Furnace—High Frequency Eddy-current Heating—Dielectric Heating — Dielectric Loss-Advantages of Dielectric Heating— Applications of Dielectric Heating — Choice of Frequency — Infrared Heating — Objective Tests. 48. Electric Welding 1861 - 1892 Definition of Welding—Welding Processes—Use of (xiii) Electricity in Welding—Formation and Characteristics of Electric Arc—Effect of Arc Length — Arc Blow—Polarity in DC Welding — Four Positions of Arc Welding— Electrodes for Metal Arc Welding—Advantages of Coated Electrodes—Types of Joints and Types of Applicable Welds—Arc Welding Machines—V-I Characteristics of Arc Welding D.C. Machines — D.C. Welding Machines with Motor Generator Set—AC Rectified Welding Unit — AC Welding Machines—Duty Cycle of a Welder — Carbon Arc Welding — Submerged Arc Welding —Twin Submerged Arc Welding — Gas Shield Arc Welding — TIG Welding — MIG Welding — MAG Welding—Atomic Hydrogen Welding—Resistance Welding—Spot Welding—Seam Welding—Projection Welding —Butt Welding-Flash Butt Welding—Upset Welding—Stud Welding—Plasma Arc Welding—Electroslag Welding—Electrogas Welding — Electron Beam Welding—Laser Welding—Objective Tests. 49. Illumination 1893 - 1942 Radiations from a Hot Body—Solid Angle—Definitions — Calculation of Luminance (L) of a Diffuse Reflecting Surface — Laws of Illumination or Illuminance — Laws Governing Illumination of Different Sources — Polar Curves of C.P. Distribution—Uses of Polar Curves - Determination of M.S.C.P and M.H.C.P. from Polar Diagrams—Integrating Sphere or Photometer — Diffusing and Reflecting Surfaces: Globes and Reflectors—Lighting Schemes —Illumination Required for Different Purposes — Space / Height Ratio— Design of Lighting Schemes and Lay-outs—Utilisation Factor or Coefficient of Utilization [η] — Depreciation Factor (p) —Floodlighting —Artificial Sources of Light — Incandescent Lamp—Filament Dimensions —Incandescent Lamp Characteristics—Clear and Inside—frosted Gas-filled Lamps—Discharge Lamps—Sodium Vapour Lamp—High- Pressure Mercury Vapour Lamp — Fluorescent Mercury— Vapour Lamps— Fluorescent Lamp— Circuit with Thermal Switch —Startless Fluorescent Lamp Circuit — Stroboscopic Effect of Fluorescent Lamps — Comparison of Different Light Sources — Objective Tests. 50. Tariffs and Economic Considerations 1943 - 2016 Economic Motive—Depreciation—Indian Currency — (xiv) Factors Influencing Costs and Tariffs of Electric Supply — Demand — Average Demand — Maximum Demand — Demand Factor—Diversity of Demand—Diversity Factor — Load Factor—Significance of Load Factor — Plant Factor or Capacity Factor— Utilization Factor (or Plant use Factor)— Connected Load Factor—Load Curves of a Generating Station — Tariffs-Flat Rate— Sliding Scale — Two-part Tariff — Kelvin's Law-Effect of Cable Insulation —Note on Power Factor—Disadvantages of Low Power Factor—Economics of Power Factor—Economical Limit of Power Factor Correction — Objective Tests. GO To FIRST " +0)26-4 D.C. TRA NSMISSION AND DISTRIBUTION Learning Objectives ➣➣ ➣➣ ➣ Transmission and Distribu- tion of D.C. Power ➣➣ ➣➣ ➣ Two-wire and Three-wire System ➣➣ ➣➣ ➣ Voltage Drop and Transmission Efficiency ➣➣ ➣➣ ➣ Methods of Feeding Distributor ➣➣ ➣➣ ➣ D.C.Distributor Fed at One End ➣➣ ➣➣ ➣ Uniformaly Loaded Distributor ➣➣ ➣➣ ➣ Distributor Fed at Both Ends with Equal Voltage ➣➣ ➣➣ ➣ Distributor Fed at Both ends with Unequal Voltage ➣➣ ➣➣ ➣ Uniform Loading with Dis- tributor Fed at Both Ends ➣➣ ➣➣ ➣ Concentrated and Uniform Loading with Distributor Fed at One End ➣➣ ➣➣ ➣ Ring Distributor ➣➣ ➣➣ ➣ Current Loading and Load-point Voltage in a 3-wire System ➣➣ ➣➣ ➣ Three-wire System ➣➣ ➣➣ ➣ Balancers ➣➣ ➣➣ ➣ Boosters ➣➣ ➣➣ ➣ Comparison of 2-wire and 3-wire Distribution System (1) Electricity leaves the power plant, (2) Its voltage is increased at a step-up transformer, (3) The electricity travels along a transmission line to the area where power is needed, (4) There, in the substation, voltage is decreased with the help of step-down transformer, (5) Again the transmis- sion lines carry the electricity, (6) Electricity reaches the final consumption points Ç CONTENTS CONTENTS CONTENTS CONTENTS 1570 Electrical Technology 40.1. Transmission and Distribution of D.C. Power By transmission and distribution of electric power is meant its conveyance from the central station where it is generated to places, where it is demanded by the consumers like mills, factories, residential and commercial buildings, pumping stations etc. Electric power may be transmitted by two methods. (i) By overhead system or (ii) By underground system—this being especially suited for densely- populated areas though it is somewhat costlier than the first method. In over-head system, power is conveyed by bare conductors of copper or aluminium which are strung between wooden or steel poles erected at convenient distances along a route. The bare copper or aluminium wire is fixed to an insulator which is itself fixed onto a cross-arm on the pole. The number of cross-arms carried by a pole depends on the number of wires it has to carry. Line supports consist of (i) pole structures and (ii) tower. Poles which are made of wood, reinforced concrete or steel are used up to 66 kV whereas steel towers are used for higher voltages. The underground system employs insulated cables which may be single, double or triple-core etc. A good system whether overhead or underground should fulfil the following requirements : 1. The voltage at the consumer’s premises must be maintained within ± 4 or ± 6% of the declared voltage, the actual value depending on the type of load*. 2. The loss of power in the system itself should be a small percentage (about 10%) of the power transmitted. 3. The transmission cost should not be unduly excessive. 4. The maximum current passing through the conductor should be limited to such a value as not to overheat the conductor or damage its insulation. 5. The insulation resistance of the whole system should be very high so that there is no undue leakage or danger to human life. It may, however, be mentioned here that these days all production of power is as a.c. power and nearly all d.c. power is obtained from large a.c. power systems by using converting machinery like synchronous or rotary converters, solid-state converters and motor-generator sets etc. There are many sound reasons for producing power in the form of alternating current rather than direct current. (i) It is possible, in practice, to construct large high-speed a.c. generators of capacities up to 500 MW. Such generators are economical both in the matter of cost per kWh of electric energy produced as well as in operation. Unfortunately, d.c. generators cannot be built of ratings higher than 5 MW because of commutation trouble. Moreover, since they must operate at low speeds, it necessi- tates large and heavy machines. (ii) A.C. voltage can be efficiently and conveniently raised or lowered for economic transmis- sion and distribution of electric power respectively. On the other hand, d.c. power has to be generated at comparatively low voltages by units of relatively low power ratings. As yet, there is no economical method of raising the d.c. voltage for transmission and lowering it for distribution. Fig. 40.1 shows a typical power system for obtaining d.c. power from a.c. power. Other details such as instruments, switches and circuit breakers etc. have been omitted. Two 13.8 kV alternators run in parallel and supply power to the station bus-bars. The voltage is stepped up by 3-phase transformers to 66 kV for transmission purposes** and is again stepped down to 13.8 kV at the sub-station for distribution purposes. Fig. 40.1 shows only three methods com- monly used for converting a.c. power to d.c. power at the sub-station. * According to Indian Electricity Rules, voltage fluctuations should not exceed ± 5% of normal voltage for L.T. supply and ± 12 1 2 % for H.T. supply. ** Transmission voltages of upto 400 kV are also used. 1571 D.C. Transmission and Distribution 1571 Fig. 40.1 (a) a 6-phase mercury-arc rectifier gives 600 V d.c. power after the voltage has been stepped down to a proper value by the transformers. This 600-V d.c. power is generally used by electric railways and for electrolytic processes. (b) a rotary converter gives 230 V d.c. power. (c) a motor-generator set converts a.c. power to 500/250 d.c. power for 3-wire distribution systems. In Fig. 40.2 is shown a schematic diagram of low tension distribution system for d.c. power. The whole system consists of a network of cables or conductors which convey power from central station to the consumer’s premises. The station bus-bars are fed by a number of generators (only two shown in the figure) running in parallel. From the bus-bars, the power is carried by many feeders which radiate to various parts of a city or locality. These feeders deliver power at certain points to a distributor which runs along the various streets. The points FF , as shown in the figure, are known as feeding points. Power connections to the various consumers are given from this distributor and not directly from the feeder. The wires which convey power from the distributor to the consumer’s premises are known as service mains (S). Sometimes when there is only one distributor in a locality, several sub-distributors (SD) branching off from the distributor are employed and service mains are now connected to them instead of distributor as shown in the figure. Obviously, a feeder is designed on the basis of its current-carrying capacity whereas the design of distributor is based on the voltage drop occurring in it. The above figure shows a motor-generator set. Nowadays, we use solid-state devices, called rectifiers, to convert standard AC to DC current. Back in the olden days, they needed a “motor dynamo” set to make the conversion as shown above. An AC motor would turn a DC Generator, as pictured above 1572 Electrical Technology Fig. 40.2 40.2. Two-wire and Three-wire Systems In d.c. systems, power may be fed and distributed either by (i) 2-wire system or (ii) 3-wire system. Fig. 40.3 [...]... resistance of each is R/2 and total current fed into each distributor is I/2 Hence, drop at the middle point is 1 × I × R = 1 IR = 2 2 2 8 It is 1/4th of that of a distributor fed at one end only The advantage of feeding a distributor at both ends, instead of at one end, is obvious 1 2 The equation of drop at point C distant x units from feeding point A = irlx − irx shows that the 2 diagram of drop of a... one 350 metres long supplying a load of 22 kW and the other 1.5 kilometre long and supplying a load of 44 kW Calculate the cross-sectional area of each cable so that the total weight of copper required shall be minimum if the maximum drop of voltage along the cable is not to exceed 5% of the normal voltage of 440 V at the consumer’s premises Take specific resistance of copper at working temperature equal... gravity of the load system 3 Let us find the drop at any intermediate point like E The value of this drop is = i1R 1 + i2R 2 + i3R 3 + R 3(i4 + i5 + i6 + .)* * The reader is advised to derive this expression himself 1576 Electrical Technology  sum of moments   moment of load beyond   +  E assumed acting at E  =  upto E     In general, the drop at any intermediate point is equal to the sum of. .. Moreover, the cross-section of copper in the cables is decreased in proportion to the increase in voltage which results in a proportionate reduction of the cost of copper in the cables The calculation of drop in a feeder is, as seen from above, quite easy because of the fact that current is constant throughout its length But it is not so in the case of distributors which are tapped off at various places... 60.36 A After knowing the value of x, the drop at A can be calculated as before The alternative method of solution is illustrated in Ex 40.12 40.9 Uniform Loading with Distributor Fed at Both Ends Consider a distributor PQ of length l units of length, having resistance per unit length of r ohms and with loading per unit length of i amperes Let the difference in potentials of the two feeding points be... sum of the moments of each load current about feeding point A 1 Hence, the drop at the far end of a distributor fed at one end is given by the sum of the moments of various tapped currents about the feeding point i.e ν = Σ i R 2 It follows from this that the total voltage drop is the same as that produced by a single load equal to the sum of the various concentrated loads, acting at the centre of gravity... and the distance from one end of the distributor at which p.d is a minimum (London Univ.) [Current in 900 metres: 272.7 A; current in 600 metres: 327.3 A; 545.5 metres from the 900 metre feeder point] 1590 3 4 5 Electrical Technology A pair of distributing mains of uniform cross-section 1,000 metres in length having a resistance of 0.15 Ω each, are loaded with currents of 50, 100, 57.5, 10 and 75 A... Electrical Technology Since 150 A enters the +ve outer but 160 A comes out of the network, it means that a current of 10 A must flow into the neutral at point N Once the direction and magnitude of current in NQ is known, the directions and magnitudes of currents in other sections of the neutral can be found very easily Since PH takes 40 A, currents meeting at P should add up to 40 A As seen, 20 A of CQ... total length of the distributor is 200 metres and loads are tapped off as under : (i) 20 A at 50 metres from A (ii) 40 A at 75 metres from A (iii) 25 A at 100 metres from A (iv) 30 A at 150 metres from A The resistance per kilometre of one conductor is 0.4Ω Calculate the currents in the various sections of the distributor, the minimum voltage and the point at which it occurs (Electrical Technology, ... from F1 while those tapped off between F2 and A will be supplied from F2 The current tapped at point A itself will, in general, be partly supplied by F1 and partly by F2 Let the values of these currents be x and y respectively If the distributor were actually cut off into two at A —the point of minimum voltage, with x amperes tapped off from the left and y amperes tapped off from the right, then potential . Applications of Electric Motors 1767 - 1794 Advantages of Electric Drive—Classification of Electric Drives —Advantages of Individual Drive—Selection of a Motor Electrical Characteristics —Types of Enclosures— Bearings—Transmission. Enclosures— Bearings—Transmission of Power —Noise— Motors of Different Industrial Drives — Advantages of Electrical Braking Over Mechanical Braking — Types of Electric Braking—Plugging Applied to DC Motors—Plugging of Induction. — Calculation of Luminance (L) of a Diffuse Reflecting Surface — Laws of Illumination or Illuminance — Laws Governing Illumination of Different Sources — Polar Curves of C.P. Distribution—Uses of Polar