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Power GridsElectrical power, in our homes and in industry, is
really the life blood of our existence
Used for heating, cooling, cooking, refrigeration, light, sound,
computation, entertainment
How is it provided to us?Why do we have outages?How secure is
it?How fragile is the system?
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Power PlantsElectrical power starts at the power plantNo matter
what you use as fuel, the heart of the power plant is the spinning
electrical generatorIn most cases, the spinning generator is a
turbineThe most common turbine is a STEAM TURBINE, such as are used
in coal-fired generators
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AC powerThe power that modern power plants produce is called
three phase AC power.In order to understand this, we need first to
understand regular single phase AC power this is actually what it
in our homes!
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AC powerIf we were to look at the VOLTAGE in a typical home wall
outlet, we would find that it is NOT a constant, like you get from
a battery. The voltage oscillates between +170 Volts and 170
Volts.The frequency of oscillation is 60Hz, or 60 times per second.
Electrons, therefore, actually flow back and forth in both
directions through devices that are connected to this voltage
source.Light bulbs actually flash on and off 60 times every second.
Our eyes are not sensitive to flashes that are this fast
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Advantages ofAC powerLarge electrical generators generate AC
naturally making DC would involve an extra stepThe power grid
relies on TRANSFORMERS to work. Transformers require AC to function
It is easy to convert AC to DC, but hard and expensive to go the
other way better to deliver AC, and then convert it if you need
to.Delivery of DC power over long distances is impractical and
expensiveEasy to convert large power line voltages to much smaller
voltages used in homes
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Mathematics of AC PowerThe voltage oscillates between+170V and
-170V, 60 times eachsecond
The average voltage, Vrms=120VIn THREE PHASE AC power, there are
THREE WIRES, plus the ground wire.
Each of the three live wires are offset in time from one
another. We say that they have different phases.
Notice that at any given time, there is always one of the wires
that has a LARGE POSITIVE voltage advantages for large electrical
motors, welding machines, etc.
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Transmission SubstationThe generator of the power plant produces
large amount of electrical current, at a voltage of about a few
thousand volts.This may seem high, but in fact it is way too low to
minimize transmission lossesAt the transmission substation,
transformers convert the power produced by the plant to a much
higher voltage as high as 750,000 Volts!!!Power can then be
transferred via three phase power lines over distances of many
hundreds of miles
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The Distribution GridOnce the power lines have reached the city,
the first step in the distribution grid is to connect to the high
voltage lines, and step-down to a lower voltage for
distributionNote that there are TWO sets of outgoing three phase
lines. One is at about 7200 Volts, and is for local consumptionThe
second set is at a higher voltage, and will be stepped down to the
usual 7200 Volts later on, at a different substation
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The Distribution GridIn the typical scene pictured here, the
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
Remember that this substation produces two different voltages -
The wires at the higher voltage need to be stepped down
You will often see a large green box (perhaps 6 feet/1.8 meters
on a side) near the entrance to a subdivision. It is performing the
step-down function for the subdivision.
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Getting Power to the HouseIn your house, you only need a single
phase
This is accomplished using taps
The first takes 3-phase to 2-phaseThe second takes 2-phase to
single phase
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Getting Power to the HouseIn your house, you only need a single
phase
This is accomplished using taps
The first takes 3-phase to 2-phaseThe second takes 2-phase to
single phase
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Getting Power to the HouseWere almost there!Remember, the line
voltage is 7200V!!!At each house, there is another transformer
which takes 7200V->240V
Sometimes, the lines are underground -> green transformer
boxes at each house.
Note that there are three wires that run out of the transformer
1 ground wire, two live wires each 120V, but exactly 180 degrees
out of phase
This allows the use of 120V and 240V appliances
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Inside the house The wires from the street connect into a panel
in your house, which is like a little mini-substation it is
responsible for the distribution to the rest of the house.
On each line going out of this panel to some room in the house,
there is a circuit breaker switch
This switch is normally closed, and allows current to flow. But,
if the current gets too high, the switch will HEAT UP. At a certain
temperature, the switch will open, and stop current from
flowing.
Because it is heat sensitive, no damage occurs (like in a fuse),
and you can reset the breaker switch if it opens.
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GFCI OutletsGround Fault Circuit Interrupter
Normally, the current flowing on the live wire and the ground
wire in a circuit are EXACTLY the same
But, if something bad happens, there will be a current surge on
the live wire
GFCI outlets sense the DIFFERENCE in current between the live
and ground wire
Therefore, it can react MUCH faster than a breaker switch, which
requires the total current to become very large before it
trips.
Sensitive to current differences of 4-5 milliamps, and will
react in 1/30th of a second!!!
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Major Sources of Electrical PowerCoal-Fired Generators (54%)
-> burn coal, heat water, produce steam, run turbineAdvantages
U.S. has a lot of coal within its bordersDisadvantages Major
greenhouse gas emitter, coal is a major air-polluter, non-renewable
resource, mining of coal is NASTY
Natural Gas Generators (17%) -> burn natural gas,
etc.Advantages Much cleaner than coal, U.S. has lots, and its
spread around over a larger region of the country.Disadvantages
non-renewable resource, greenhouse gas emitter
Hydroelectric Power (7%) -> build a dam, water flows from a
large height to power turbineAdvantages RENEWABLE resource!!! At
first glance, seems like a non-polluterDisadvantages dams have a
very finite lifetime, major wildlife impact upstream, and now,
studies indicate that decay of vegetation in the flood plain
upstream of the dam may produce even MORE greenhouse gases than
coal-fired generators!!!
Nuclear Power (20%) -> Use the energy released in the nuclear
fission process to heat water, produce steam, run turbineAdvantages
Major amount of energy for a small amount of ore,
non-polluter!!!!Disadvantages Have to deal with nuclear waste,
public fear, possible nuclear accidents
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Major Sources of Electrical Powerwww.nuclearfaq.caCapital costs
of nuclearpower are high, but yourecover this if you USE itfor
everything !!!!