A Textbook of Electrical Technology Volume III -Transmission and Distribution

454 Pages·2007·14.83 MB·english

by  B.L.Theraja

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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
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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—
(ix)
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.
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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.
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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 —
(x)
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, t and η of starting — Series
s p
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.
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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
(xi)
of DC Motors—Rheostatic Braking Torque—Rheostatic
Braking of Induction Motors — Regenerative Braking—
Energy Saving in Regenerative Braking - Objective Tests.
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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.
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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.
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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.
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Definition of Welding—Welding Processes—Use of
(xii)
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.
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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.
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Economic Motive—Depreciation—Indian Currency —
(xiii)
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.
(xiv)
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Learning Objectives
D.C.
➣➣➣➣➣ Transmission and Distribu-
tion of D.C. Power TRANSMISSION
➣➣➣➣➣ Two-wire and Three-wire
System
AND
➣➣➣➣➣ Voltage Drop and
Transmission Efficiency
➣➣➣➣➣ Methods of Feeding DISTRIBUTION
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 (cid:1) (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:8)(cid:11)(cid:9)(cid:12)(cid:4)(cid:6)(cid:7)(cid:13)(cid:14)(cid:7)(cid:15)(cid:4)(cid:9)(cid:16)(cid:7)(cid:4)(cid:17)(cid:18)(cid:19)(cid:7)(cid:10)(cid:4)(cid:17)(cid:6)(cid:13)(cid:20)(cid:9)(cid:21)(cid:4)(cid:1)(cid:22)(cid:3)(cid:4)(cid:23)(cid:9)(cid:15)
➣➣➣➣➣ Three-wire System (cid:14)(cid:18)(cid:6)(cid:9)(cid:13)(cid:24)(cid:7)(cid:4)(cid:11)(cid:15)(cid:4)(cid:11)(cid:20)(cid:8)(cid:10)(cid:7)(cid:13)(cid:15)(cid:7)(cid:25)(cid:4)(cid:13)(cid:9)(cid:4)(cid:13)(cid:4)(cid:15)(cid:9)(cid:7)(cid:17)(cid:26)(cid:27)(cid:17)(cid:4)(cid:9)(cid:10)(cid:13)(cid:20)(cid:15)(cid:28)(cid:18)(cid:10)(cid:29)(cid:7)(cid:10)(cid:21)
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➣➣➣➣➣ Balancers
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➣➣➣➣➣ Boosters (cid:9)(cid:16)(cid:7)(cid:4)(cid:15)(cid:27)!(cid:15)(cid:9)(cid:13)(cid:9)(cid:11)(cid:18)(cid:20)(cid:21)(cid:4)(cid:14)(cid:18)(cid:6)(cid:9)(cid:13)(cid:24)(cid:7)(cid:4)(cid:11)(cid:15)(cid:4)(cid:25)(cid:7)(cid:8)(cid:10)(cid:7)(cid:13)(cid:15)(cid:7)(cid:25)(cid:4)(cid:19)(cid:11)(cid:9)(cid:16)(cid:4)(cid:9)(cid:16)(cid:7)(cid:4)(cid:16)(cid:7)(cid:6)(cid:17)
➣➣➣➣➣ Comparison of 2-wire and (cid:18)(cid:28)(cid:4)(cid:15)(cid:9)(cid:7)(cid:17)(cid:26)(cid:25)(cid:18)(cid:19)(cid:20)(cid:4)(cid:9)(cid:10)(cid:13)(cid:20)(cid:15)(cid:28)(cid:18)(cid:10)(cid:29)(cid:7)(cid:10)(cid:21)(cid:4)(cid:1)"(cid:3)(cid:4)#(cid:24)(cid:13)(cid:11)(cid:20)(cid:4)(cid:9)(cid:16)(cid:7)(cid:4)(cid:9)(cid:10)(cid:13)(cid:20)(cid:15)(cid:29)(cid:11)(cid:15)(cid:26)
3-wire Distribution System (cid:15)(cid:11)(cid:18)(cid:20)(cid:4)(cid:6)(cid:11)(cid:20)(cid:7)(cid:15)(cid:4)(cid:8)(cid:13)(cid:10)(cid:10)(cid:12)(cid:4)(cid:9)(cid:16)(cid:7)(cid:4)(cid:7)(cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:8)(cid:11)(cid:9)(cid:12)(cid:21)(cid:4)(cid:1)$(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:8)(cid:11)(cid:9)(cid:12)
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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
1
L.T. supply and ± 12 % for H.T. supply.
2
** Transmission voltages of upto 400 kV are also used.
D.C. Transmission and Distribution 11557711
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. The above figure shows a motor-generator set. Nowadays, we
power for 3-wire distribution use solid-state devices, called rectifiers, to convert standard AC
to DC current. Back in the olden days, they needed a “motor
systems.
dynamo” set to make the conversion as shown above. An AC
In Fig. 40.2 is shown a schematic
motor would turn a DC Generator, as pictured above
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.
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