Transcript Power Grids Lecture

Power Grids
• Electrical 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
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How is it provided to us?
Why do we have “outages”?
How secure is it?
How fragile is the system?
Power Plants
• Electrical power starts
at the power plant
• No matter what you use
as “fuel”, the heart of
the power plant is the
spinning electrical
generator
• In most cases, the
spinning generator is a
turbine
• The most common
turbine is a STEAM
TURBINE, such as are
used in coal-fired
generators
AC power
• The 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!
AC power
• If 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 …
Advantages ofAC power
• Large electrical generators generate AC
naturally – making DC would involve an extra
step
• The 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 expensive
• Easy to convert large power line voltages to
much smaller voltages used in homes
Mathematics of AC Power
The voltage oscillates between
+170V and -170V, 60 times each
second
The average voltage, Vrms=120V
In 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.
Transmission Substation
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The 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 losses
At 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
The Distribution Grid
• Once 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
distribution
• Note that there are TWO
sets of outgoing three phase
lines.
• One is at about 7200 Volts,
and is for local consumption
• The second set is at a higher
voltage, and will be stepped
down to the usual 7200 Volts
later on, at a different
substation
The Distribution Grid
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In 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
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Remember that this substation
produces two different voltages
- The wires at the higher
voltage need to be stepped down
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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.
Getting Power to the House
• In your house, you only
need a single phase …
• This is accomplished
using taps
• The first takes 3-phase
to 2-phase
• The second takes 2phase to single phase
Getting Power to the House
• In your house, you only
need a single phase …
• This is accomplished
using taps
• The first takes 3-phase
to 2-phase
• The second takes 2phase to single phase
Getting Power to the House
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We’re almost there!
Remember, the line voltage is
7200V!!!
At each house, there is another
transformer which takes
7200V->240V
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Sometimes, the lines are
underground -> green transformer
boxes at each house.
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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
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This allows the use of 120V and
240V appliances
Inside the house …
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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.
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On each line going out of this panel
to some room in the house, there is a
circuit breaker switch
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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.
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Because it is heat sensitive, no
damage occurs (like in a fuse), and
you can reset the breaker switch if
it opens.
GFCI Outlets
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Ground Fault Circuit Interrupter
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Normally, the current flowing on the live
wire and the ground wire in a circuit are
EXACTLY the same
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But, if something bad happens, there will
be a current surge on the live wire
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GFCI outlets sense the DIFFERENCE in
current between the live and ground wire
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Therefore, it can react MUCH faster
than a breaker switch, which requires the
total current to become very large before
it trips.
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Sensitive to current differences of 4-5
milliamps, and will react in 1/30th of a
second!!!
Major Sources of Electrical
Power
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Coal-Fired Generators (54%) -> burn coal, heat water, produce steam, run turbine
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Natural Gas Generators (17%) -> burn natural gas, etc.
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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
turbine
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Advantages – U.S. has a lot of coal within it’s borders
Disadvantages – Major greenhouse gas emitter, coal is a major air-polluter, non-renewable
resource, mining of coal is NASTY
Advantages – RENEWABLE resource!!! At first glance, seems like a non-polluter
Disadvantages – 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 turbine
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Advantages – Major amount of energy for a small amount of ore, non-polluter!!!!
Disadvantages – Have to deal with nuclear waste, public fear, possible nuclear accidents
Major Sources of Electrical
Power
www.nuclearfaq.ca
Capital costs of nuclear
power are high, but you
recover this if you USE it
for everything !!!!