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Power Grids


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 ... – PowerPoint PPT presentation

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Title: Power Grids

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,
  • 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
  • In most cases, the spinning generator is a
  • 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
  • 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,
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
  • 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

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
  • 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
  • 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
  • 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.

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 2-phase 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 2-phase to single phase

Getting Power to the House
  • Were almost there!
  • Remember, the line voltage is 7200V!!!
  • At each house, there is another transformer which
    takes 7200V-gt240V
  • Sometimes, the lines are underground -gt 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

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.

GFCI Outlets
  • Ground Fault Circuit Interrupter
  • Normally, the current flowing on the live wire
    and the ground wire in a circuit are EXACTLY the
  • 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!!!

Major Sources of Electrical Power
  • Coal-Fired Generators (54) -gt burn coal, heat
    water, produce steam, run turbine
  • Advantages U.S. has a lot of coal within its
  • Disadvantages Major greenhouse gas emitter,
    coal is a major air-polluter, non-renewable
    resource, mining of coal is NASTY
  • Natural Gas Generators (17) -gt burn natural gas,
  • 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) -gt build a dam, water
    flows from a large height to power turbine
  • 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) -gt Use the energy released in
    the nuclear fission process to heat water,
    produce steam, run turbine
  • 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
Capital costs of nuclear power are high, but
you recover this if you USE it for everything !!!!
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