15-01-2013, 04:27 PM
POWER GRID
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ABSTRACT
Electrical power is a little bit like the air in your breathe: You don't really think about it until it is missing. Power is just "there," meeting your every need, constantly. It is only during a power failur, when you walk into a dark room and instinctively hit the useless light switch, that you realize how important power is in your daily life. You use it for heating, cooling, cooking, refrigeration, light, sound, computation, entertainment...
Power travels from the power plant to your house through an amazing system called the power distribution grid.
The Power Plant
Electrical power starts at the power plant. In almost all cases, the power plant consists of a spinning electrical generator. Something has to spin that generator -- it might be a water wheel in a hydroelectric dam, a large diesel engine or a gas turbine. But in most cases, the thing spinning the generator is a steam turbine. The steam might be created by burning coal, oil or natural gas. Or the steam may come from a nuclear reactor like this one at the Shearon Harris nuclear power plant near Raleigh.
The Power Plant: Alternating Current
Single-phase power is what you have in your house. You generally talk about household electrical service as single-phase, 230-volt AC service. If you use an oscilloscope and look at the power found at a normal wall-plate outlet in your house, what you will find is that the power at the wall plate looks like a sine wave, and that wave oscillates between -325 volts and 325 voltss (the peaks are indeed at 325 volts; it is the effective (rms) voltage that is 325 volts). The rate of oscillation for the sine wave is 60 cycles per second. Oscillating power like this is generally referred to as AC, or alternating current. The alternative to AC is DC, or direct current. Batteries produce DC: A steady stream of electrons flows in one direction only, from the negative to the positive terminal of the battery.
AC has at least three advantages over DC in a power distribution grid:
1. Large electrical generators happen to generate AC naturally, so conversion to DC would involve an extra step.
2. Transformers must have alternating current to operate, and we will see that the power distribution grid depends on transformers.
3. It is easy to convert AC to DC but expensive to convert DC to AC, so if you were going to pick one or the other AC would be the better choice.
The power plant, therefore, produces AC. On the next page, you'll learn about the AC power produced at the power plant. Most notably, it is produced in three phases.
The Power Plant: Three-phase Power
The power plant produces three different phases of AC power simultaneously, and the three phases are offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral or ground common to all three. If you were to look at the three phases on a graph
The Distribution Grid
For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. This may happen in several phases. The place where the conversion from "transmission" to "distribution" occurs is in a power substation. A power substation typically does two or three things:
• It has transformers that step transmission voltages (in the tens or hundreds of thousands of volts range) down to distribution voltages (11 kv).
• It has a "bus" that can split the distribution power off in multiple directions.
• It often has circuit breakers and switches so that the substation can be disconnected from the transmission grid or separate distribution lines can be disconnected from the substation when necessary.
CONCLUSION:-
As the technology is advancing the consumption of power is steadily rising. Now recent trail is going on how to meet the huge demand of power. So now we are trying to generate power from renewable sources. I hope this recent trial will become the reality in the near future.