Physics

How we get Electric Power



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Energy is all around. It is part of the environment. Without it the universe would come to a standstill. Another major part of our environment is matter. Matter is anything you can see and touch. The work of Albert Einstein has shown that matter and energy are equivalent. We can change matter into energy.

Energy is found in many forms. Some of these are heat energy, potential energy, chemical energy or mechanical energy. Chemical energy is one way of providing power or current from batteries. The other forms must be converted into mechanical energy before they can be changed to electrical energy.

Heat energy can be produced from the splitting of atoms. It can also be produced by burning. Material such as wood, coal, oil or gas may be burned to release their energy and we can use it raise the temperature of water. Apply enough heat and we get steam. This steam and the pressure it can create may be used to turn machines called turbines. The turbines will then turn other machines called generators. It is in this last machine that the mechanical energy is converted to electricity.

Another form of energy called potential energy, is found when anything is located above the earth. An example is a lake held back by a dam in such a way that the water may be funneled down through tubes to a lower level than the lake. The falling water, acting under the force of gravity, is diverted onto the blades of a turbine where the potential energy of the water is converted into mechanical energy. The turbine then turns a generator. This power source is called hydroelectric.

In order to be of use in a typical power system, the amount of power produced must be large. Energy losses found in every part of a system can represent a sizable amount of power. To be economical, a great number of customers must be served. We must generate a lot of power to provide for these losses and for whatever customers may be attached to the system. To give you an example of what we are talking about, let's get a handle on measurements and quantities.

Voltage is electrical pressure. Think of voltage as compared to water pressure. The water pressure pushes the water (current) along a pipe (conductor), as the voltage "pushes" electric current down a wire (conductor). Electric current is expressed in amperes which is the rate of flow of electricity.

Power, the rate at which work is done, is expressed for an automobile engine in horsepower. For electric power it is expressed in volt-amperes (va). Volt for the pressure and amperes or amps for the current.

Large amounts of power or voltage, are expressed by metric system prefixes. The prefix kilo means 1000. This gives us kilovolts (kv) or 1000 volts and kilovoltamperes (kva) or 1000 va for power. To illustrate, let's select a typical power requirement of 125,000 kva. For comparison an average home might have no more than 20 kva load. Load is a term referring to anything connected to the electrical system that draws current.

Electricity is produced at the relatively low voltage of about 13kv (13,000 volts). Because of the relationship between power and voltage, the current generated must be high for large amounts of power. Numerically the power is related to the current as follows: I = (kva/kv)0.577. In this example the current is 5548 amps.

Such a large amount of current has two distinctive effects on the conductor carrying it. First, the electric current produces heat, similar to what happens in an electric heater (due to power losses). The second effect, a result of these losses, is a reduction of voltage along the conductor.

Every conductor has a physical characteristic called resistance. This produces an opposition to the flow of electricity. Resistance is measured in units called the ohm. Transportation of large amounts of power requires conductors with low resistance to keep the power losses down.

In our example above we have 5548 amps flowing. A typical line resistance might be 0.117 ohms per mile. If the circuit were 5 miles, the voltage lost would be 0.117ohms x 5miles x 5548 amps = 3,245volts lost or 24%. If we could change the voltage from 13kv to 115kv, the amps would become a value of 627. The voltage loss would then be 0.117 x 5 x 627 = 366 volts or 0.3% volt drop.

This calculation serves to illustrate the importance of keeping the current flow down to reasonable levels on the conductor. With the volt loss big enough, the electrical equipment would not operate properly. Think of a water well with a pump (pressure) forcing the water through a long, small pipe. (the smaller the pipe, the greater the resistance). By the time the water gets to the end of the pipe, the pressure at the pump less that lost in moving the water will produce only a trickle at the output.

The idea becomes important when you consider the distance from the generator to your house could be anywhere from a few miles to several hundreds of miles. This could severely limit the distribution of electric power if we were using DC or direct current electricity. In our discussion and in the world today we are using alternating current. DC flows in one direction while AC reverses itself at regular intervals. This is the secret of the enormous versatility of AC.

At the dawn of the electric power industry the great inventor Thomas Edison believed that DC systems were the direction in which the industry should take. Others such as Nicola Tesla and George Westinghouse disagreed. They were in favor of an AC system. Edison's power systems were limited to small areas such as a city block or a small part of a city. The DC current was distributed at the same level of voltage as it was produced. As explained above the voltage available to operate equipment dropped too low in very short distances. Due to the wonderful invention of alternating current and the transformer the current generated could be reduced to levels easily transported to remote locations and increased to levels required by the customers load requirements. The transformer is a marvelous device that can raise or lower voltage or current in an AC system.

Back at the generator station we insert a power transformer (called a step-up transformer) to raise the generated voltage of 13kv to transmission levels. A standard extra high voltage (EHV) line voltage for long distance transmission is 500,000 volts or 500kv. The ratio of 13kv to 500kv is 0.026. At the same time the voltage is raised from 13kv to 500kv, the current level is reduced by the transformer according to the inverse of the same ratio (1/.026 = 38.5). This reduces the 5548 amps (5548 / 38.5) down to 144 amps.

Transformers are made by placing two windings of copper on a steel core. The windings are insulated from each other and the core. Power is passed between them through the magnetic field created by the windings and the current. One winding is called the primary and the other the secondary. The ratio of the number of turns in the primary winding to the number of turns in the secondary winding is equal to the ratio of the primary voltage to the secondary voltage. In our example above the 13kv side is primary and the 500kv side is secondary.

The power produced may travel 100 miles at 500kv, where it goes through a transformer (referred to as a step-down) to a lower voltage, usable by the customers. For local distribution it could be 13kv. The highest voltage you will likely use in your home is 240v. Keep in mind that the power we started off with may be split up into several paths. There could be several 500kv lines from the generator station and there could be several lines from our destination further distributing the power at lower voltages. In each, the voltage and current is stepped up or down as needed through the marvelous transformer.

By the time we get to your neighborhood we find a circuit down the street in front of your house carrying 13kv and perhaps 100 amps. At your house we will find a pole mounted transformer or one secured in an insulated enclosure on the ground. This is called a pad mounted transformer because it sets on a pad type support. The transformer steps the 13kv down to 120/240 volts for your use and maybe 100 amps for a kva power load in your home of about 24kva. The rest supplies other customers and the associated losses which can only be reduced but never eliminated.

Without the generator and the transformer, the electric power system that is largely the basis for our advanced civilization would only be a shadow of what we have now. Given the extreme rate at which technology has been advancing in the last century, there is little doubt that in a few more years major changes will occur in our power system. However, the role of the transformer and the generator should be secure for a long time.

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