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Generation,transmission and distributions of electrical energy

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Title: Generation,transmission and distributions of electrical energy


1
Generation,transmission and distributions of
electrical energy
2
Brief History of the Electricity Supply
  • Electrical energy cannot be stored in large
    quantities, and bulk supplies of electrical
    energy must therefore be fed directly from the
    generating station through a transmission and
    distribution system to the consumer.
  • The public supply of electricity was not
    available until 1879.
  • The publication of the results of Faradays
    famous 1831 experiment, many practical generators
    were constructed.
  • The aim was to produce a supply of direct current
    (DC) that would take the place of the battery as
    a source of electrical energy.

3
Brief History of the Electricity Supply
  • The first DC generator or dynamo were produced
    for experimental purpose and for supplying the
    arc lamp in lighting.
  • After that the arc lamp was being used for
    lighting public places.
  • Before 1880, the primary purpose of most
    electrical sources was to supply the electrical
    energy for arcs lamp.
  • Gradually, as alternator, motors, transformaer
    and equipment progressively improved, 3-phase
    A.C. generation, transmission and distribution
    was adopted on a worldwide basis.

4
Generation of Electricity
  • Electrical power starts at the power plant.
  • In almost all cases the power plant consists of a
    spinning electrical generator.
  • Something has to spin that generator -- it might
    be a water wheel in a hydroelectric dam, a large
    diesel engine or a gas turbine.
  • But in most cases the thing spinning the
    generator is a steam turbine.
  • The steam might be created by burning coal, oil
    or natural gas. Or the steam may come from a
    nuclear reactor like at the Shearon Harris
    nuclear power plant

5
Generation of Electricity (Ctd.)
6
Generation of Electricity (Ctd.)
  • All commercial electrical generators of any size
    generate what is called 3-phase AC power.
  • To understand 3-phase AC power it is helpful to
    understand single-phase power first.
  • Single-phase power is what you have in your
    house. You generally talk about household
    electrical service as single-phase 240 volt AC
    service.
  • The alternative to AC is DC, or Direct Current.
    Batteries produce DC a steady stream of
    electrons flow in one direction only from the
    negative to the positive terminal of the battery.

7
Generation of Electricity (Ctd.)
  • Advantages of AC compared with DC
  • Large electrical generators happen to generate AC
    naturally, so conversion to DC would involve an
    extra step.
  • Transformers must have alternating current to
    operate, and we will see that the power
    distribution grid depends on transformer.
  • It is easy to convert AC to DC but expensive to
    convert DC to AC, so if you were going to pick
    one or the other AC would be the better choice of
    the two.

8
Generation of Electricity (Ctd.)
  • The power plant, therefore, produces AC.
  • However, it produces three different phases of
    power simultaneously, and the three phases are
    offset 120 degrees from each other.
  • Out of every power plant come four wires the
    three phases plus a neutral or ground common to
    all three.
  • The three phases on a graph, would look like this
    relative to ground

9
Generation of Electricity (Ctd.)
  • Why 3-Phase??? Why not 1 or 2 or 4 Phase??
  • One big advantage that 3-phase power has over
    1-phase or 2-phase power is the fact that, at any
    given moment, one of the three phases is nearing
    a peak.
  • In 1-phase and 2-phase power there are 120
    moments per second when the sine waves cross zero
    volts.
  • High-power 3-phase motors (used in industrial
    applications) and things such as 3-phase welding
    equipment therefore have even power output.

10
Generation of Electricity (Ctd.)
  • The power company essentially uses the earth as
    one of the wires in the power system.
  • The earth is a pretty good conductor and it is
    huge, so it makes a good return path for
    electrons.
  • Car manufacturers do something similar they use
    the metal body of the car as one of the wires in
    the car's electrical system and attach the
    negative pole of the battery to the car's body.
  • "Ground" in the power distribution grid is
    literally "the ground" all around you when you
    are walking outside.

11
Transmission and distribution of Electricity
  • It is more efficient to transmit electrical
  • power (P IV) at high voltage and low current.
  • The losses along the transmission lines are
  • reduced compared to transmission at low V.

12
Transmission and distribution of Electricity
13
Transmission of Electricity
  • The 3-phase power leaves the generator and enters
    a transmission substation at the power plant .
  • This substation uses large transformers to
    convert the generator's voltage (which is at the
    thousands of volts level) up to extremely high
    voltages for long-distance transmission on the
    transmission grid.
  • Here is a typical substation at a power plant

14
Transmission of Electricity
  • At the back of the image several three-wire
    towers leaving the substation.
  • Typical voltages for long distance transmission
    are in the 155,000 to 765,000 volt range in order
    to reduce line losses.
  • A typical maximum transmission distance is about
    300 miles. High-voltage transmission lines are
    quite obvious compare with low voltage
    transmission lines.
  • They are normally made of huge steel towers like
    the picture below-

15
Transmission of Electricity
High-voltage transmission lines
16
Transmission of Electricity
  • All power towers like this always have three
    wires for the three phases.
  • Many towers, like the ones shown before, have
    extra wires running along the tops of the towers.
  • These are ground wires and are there primarily in
    an attempt to attract lightning.

17
Transmission of Electricity
  • From power station to consumer

Power Station
Final low-voltage distribution to consumers
High Voltage Distribution
Generation
Primary Transmission
Secondary Transmission
330kV, 500kV
60kV, 132kV
11kV, 33kV
Typical Voltage
11kV, 17kV, 22kV, 23kV
230/400V
Yard transformer
Regional Substation
Zone Substation
DistributionSubstation
18
Distribution of Power
  • High Voltage and low voltage distribution system

Consumer 1 and 2 230V two wire Consumer 3
230/400V three wire Consumer 4 230/400V four
wire
Bulk Supply Consumer
4
Delta/Star transformer
Line 1(A)
Line 2(B)
Three phase, high voltage distribution
Line 3(C)
Neutral
Three phase four wire distribution low voltage
230/400V
Three Phase Star System
19
Distribution of Power (cont.)
  • The system described above is known as Three
    phase star system due to the manner in which the
    transformer phase windings are interconnected it
    is also term wye(Y) system, and the supply
    transformer is called a D-Y-connected
    transformer.
  • Between any two line wires there is a voltage of
    400V, and between any one line and the neutral
    the voltage is 230V.
  • Line and neutral voltages are for the supply of
    single phase equipment such as jugs, irons,
    lamps, small motors and hot water systems.
  • If the single phase load become too heavy for one
    phase (the energy distributor decides on the
    current permissible per phase), another line is
    brought in, the same neutral being used as a
    common return wire.

20
Distribution of Power (cont.)
  • Still heavier single phase loads might require
    balancing over three phases for example, in a
    block of 30 flats the could be 10 flats connected
    to each of the three phase.
  • Three phase supply would also be necessary if
    three phase equipment such as three phase motors
    or instantaneous water heaters were installed.
  • Some equipment with a 400V single phase rating
    such as X-ray machines and 400V welders require
    two active conductors.

21
Distribution of Power (cont.)
  • Three Phase Delta System

22
The Distribution Grid
  • For power to be useful in a home or business, it
    comes off the transmission grid and is
    stepped-down to the distribution grid.
  • This may happen in several phases. The place
    where the conversion from "transmission" to
    "distribution" occurs is in a power substation. A
    power substation typically does two or three
    things
  • It has transformers that step transmission
    voltages (in the tens or hundreds of thousands of
    volts range) down to distribution voltages
    (typically less than 10,000 volts)
  • It has a "bus" that can split the distribution
    power off in multiple directions.
  • It often has circuit breakers and switches so
    that the substation can be disconnected from the
    transmission grid or separate distribution lines
    can be disconnected from the substation when
    necessary.

23
The Distribution Grid
Typical Small substation
24
The Distribution Grid
  • The box in the foreground is a large transformer.
  • To its left (and out of the frame but shown in
    the next shot) are the incoming power from the
    transmission grid and a set of switches for the
    incoming power.
  • Toward the right is a distribution bus plus three
    smaller transformers.

25
The Distribution Grid
Here is a view of the transmission lines entering
the substation and passing through the switch
tower
26
The Distribution Grid
Here is a view of the switch tower and the main
transformer
27
The Distribution Bus
The power goes from the transformer to the
distribution bus
28
The Distribution Bus
  • In this case the bus distributes power to two
    separate sets of distribution lines at two
    different voltages.
  • The smaller transformers attached to the bus are
    stepping the power down to standard line voltage
    (usually 7,200 volts) for one set of lines, while
    power leaves in the other direction at the higher
    voltage of the main transformer.
  • The power leaves this substation in two sets of
    three wires, each headed down the road in a
    different direction.

29
The Distribution Bus
  • Three wires at the top of the poles are the three
    wires for the 3-phase power. The fourth wire
    lower on the poles is the ground wire.
  • As mentioned above, this particular substation
    produced two different voltages.
  • The wires at the higher voltage need to be
    stepped down again, and that will often happen at
    another substation or in small transformers
    somewhere down the line.

30
The Regulator Bank
  • Regulator banks located along the line either
    underground or in the air.
  • They regulate the voltage on the line to prevent
    undervoltage and overvoltage conditions.
  • Up toward the top are three switches that allow
    this regulator bank to be disconnected when
    necessary for maintenance.

31
What does a House Need?
  • A house needs only one of those phases, so
    typically you will see three wires running down a
    main road, and taps for one or two of the phases
    running off on side streets.
  • Here is a 3-phase to 2-phase tap, with the two
    phases running off to the right.

32
At the House
  • A typical house runs a set of poles with one
    phase of power (at 7,200 volts) and a ground wire
    (although sometimes there will be two or three
    phases on the pole depending on where the house
    is located in the distribution grid).
  • At each house there is a transformer drum
    attached to the pole, like this-

33
At the House (cont.)
  • The transformer's job is to reduce the 7,200
    volts down to the 240 volts that makes up normal
    household electrical service.
  • The 240 volts enters your house through a typical
    watthour meter, so that the power company can
    charge you for putting up all of these wires
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