Power Grids

1. 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 • How is it provided to us? • Why do we have “outages”? • How secure is it? • How fragile is the system?

2. 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

3. 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!

4. 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 …

5. 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

6. 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.

7. 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 miles

8. 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

9. 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.

10. 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

11. 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

12. Getting Power to the House • We’re 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

13. 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.

14. GFCI Outlets • Ground 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!!!

15. Major Sources of Electrical Power • Coal-Fired Generators (54%) -> burn coal, heat water, produce steam, run turbine • 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 • 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 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%) -> 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

16. 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 !!!!