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?

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

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!

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

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

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.

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

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

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.

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

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

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

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 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!!!

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

Major Sources of Electrical Powerwww.nuclearfaq.caCapital costs of nuclearpower are high, but yourecover this if you USE itfor everything !!!!