Title: Generation,transmission and distributions of electrical energy
1Generation,transmission and distributions of
electrical energy
2Brief 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.
3Brief 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.
4Generation 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
5Generation of Electricity (Ctd.)
6Generation 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.
7Generation 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.
8Generation 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
9Generation 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.
10Generation 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.
11Transmission 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.
12Transmission and distribution of Electricity
13Transmission 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
14Transmission 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-
15Transmission of Electricity
High-voltage transmission lines
16Transmission 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.
17Transmission 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
18Distribution 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
19Distribution 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.
20Distribution 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.
21Distribution of Power (cont.)
22The 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.
23The Distribution Grid
Typical Small substation
24The 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.
25The Distribution Grid
Here is a view of the transmission lines entering
the substation and passing through the switch
tower
26The Distribution Grid
Here is a view of the switch tower and the main
transformer
27The Distribution Bus
The power goes from the transformer to the
distribution bus
28The 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.
29The 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.
30The 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.
31What 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.
32At 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-
33At 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