The generation of electricity is most commonly achieved by converting chemical energy in fuels or the flowing energy of wind, water, or steam into electrical energy, using a mechanical turbine connected to a generator. The force of the fluid causes the turbine to rotate, which in turn rotates the magnetic field inside the generator to produce electricity.
Typically, a fuel such as coal is burned in a boiler to produce steam. The chemical energy in the fuel becomes heat energy as it burns, forming hot gases. To help protect the environment, these gases are cleaned by special equipment before they are released through the stack.
The steam, under great pressure, rushes through pipe and valves and turns the steam turbine at high speed. The turbine is made up of blades on a shaft and is driven by the steam like wind drives a windmill. Heat energy in the steam is converted to mechanical energy by the turbine.
When the steam leaves the turbine it goes to the condenser. Water from a nearby source is used in the condenser to cool the steam back to water. The water is sent back to boiler to become steam again.
The rotor in the generator is turned by the shaft from the turbine and electricity is produced. The mechanical energy produced in the turbine is changed to electrical energy in the generator.
A steam turbine power generating plant is the most common types of power plant today. This type of plant converts heat into electricity usually using a boiler, and a turbine to drive an electric generator. Large-scale commercial size systems use steam produced from a variety of sources including nuclear reactions, burning fossil fuels and wastes, and even geothermal energy. The most common fuels used at steam turbine plants to produce steam are coal, oil, and natural gas.
The steam boiler is essentially a large tea kettle and the steam turbine acts much like a windmill to turn the generator to make electricity.
After the steam passes through the turbine, it is condensed back into water and pumped back into the boiler to be reheated into steam again.
Coal is an abundant and relatively inexpensive fuel on a dollar per BTU basis in North America. Fifty-Five percent of the United States' utilities' net electric generation comes from coal. However, coal fired power plants are complicated, largely due to the coal handling equipment and strict environmental regulations, and are generally more expensive to build than oil or natural gas.
The coal is most often delivered to the power plant using trucks and stored on a stock pile which usually contains a 30 to 60 day supply. The coal is transported from the storage pile to the plant where it is ground into a fine powder and burned in the boiler.
Coal presents several environmental challenges in that it produces more combustion byproducts than either oil or gas. Burning coal produces four main byproducts which must be carefully controlled in compliance with strict federal regulations. Theses include fly ash, bottom ash, nitrogen oxides, and sulfur oxides. Many utilities sell fly ash to concrete companies to be used as a concrete additive. Bottom ash is collected and stored until it can be ground up and used as a concrete additive and for stabilizing road beds. Scrubbers and other equipment are used to clean and limit the amount of sulfur oxides ad nitrogen oxides released through the stack.
Electricity is unique in that it must be produced the instant it is needed. It just cannot be economically stored in large quantities using today's technology. And, unlike telephone service, power users do not tolerate a busy signal.
The utility normally tries to schedule and operate a series of generators at a given power level known to be the most efficient for the season and time. As the customers' needs for power change, the operators must adjust the amount of electricity produced by various generation units. If the required load is higher than what the generators are currently producing, the system's electrical frequency falls below the desired value of 60 cycles. The generation operators decide which unit is the most economical to increase power output, or they bring an additional generating unit on line to raise the frequency back up to 60 cycles. If the generators are producing too much electricity compared to what the required load is, the system frequency increases above 60 cycles and the generation operators decide which unit's power generation needs to be lowered.
In the event the electric utility doesn't generate the required power when it is needed, the system voltage drops below the minimum set point, and circuit breakers begin to trip to prevent equipment damage. This can cause major power outages for consumers. Fortunately, today's electric utilities have interchange agreements with other power suppliers that minimize the likelihood of this happening.
Electric utilities also plan their power generation to meet widely varying demands during the year, and during any given day. Because of the varied schedules on which customers use electricity, the load varies over the day, the week, and the year. Normally, the load tends to be lowest at night, when most people are asleep, and highest during the day, when the most appliances are in use. Some utilities see a peak at night due to electric space heating in the cooler months, while most utilities see a peak during the hottest days of the summer when air conditioners are working their hardest.
When a utility has its highest demand for power in the winter, it is referred to as winter peaking. When the demand for power is highest in the summer, it is called summer peaking.
As electrical energy is generated, it is transformed and transported instantaneously through a network of wires, substations, and transformers to the consumer. Electricity is generated at a comparatively low voltage at most generating stations. In order to transport this energy great distances, the voltage has to be increased, or stepped up; to values as high as 765,000 volts. This is accomplished through the use of large transformers located near the generation station, and provides an efficient and economical method of transmission. Transmission line voltages can range from 69,000 volts to 765,000 volts.
From the generating station to the final destination, the energy from generated electricity undergoes numerous changes in voltage and direction. Each change requires expert design and handling to provide the consumer with the least expensive, most reliable energy they can buy.
The purpose of the distribution system is to distribute the electricity to each customer's residence, business, or industrial plant. It is primarily composed of the distribution substation and distribution feeders, but also contains many other pieces of equipment including reclosers, sectionalizers, fuses and capacitors.
Electricity is "stepped down" from a high to low voltage by transformers located at the distribution substation. These transformers are just the reverse of those which increase the voltage at the generating station. Electricity enters the primary side coil with the larger number of windings and leaves from the secondary coil with the smaller number of windings. The electricity is reduced to a lower distribution level voltage, usually less than 39,000 volts, and distributed on three phase lines. There are a wide variety of three phase distribution line types and voltages supplied by electric companies across the country. A very common three phase distribution line voltage is 12,000 volts or 12 kV.
The distribution line supplies the final step down transformer at the customer location where the voltage is stepped down or lowered to the service voltage for the customer's electrical system. Then the electricity flows through the service drop to the electrical meter at the service to be measured for billing purposes.
Electric power leaves the distribution substation with voltage ranging typicaly between 4,000 and 36,000 volts. This is still, of course, too high for most typical uses, such as homes, businesses, and even small industrial users. The device used to step this voltage down to service voltage is yet another transformer, the distribution transformer.
Distribution transformers operate just like large transformers in a distribution substation. The transformer steps the voltage down to the service voltage required by the particular customer. A normal residential service utilities two separate voltages, 120 and 240 volts. Several residential services are generally run from one distribution transformer. They are sized to meet the needs of the total load of all the customers connected to it.
Businesses and industrial plants generally do not share transformers with other customers and require larger transformers depending on the type and amount of electrical equipment.
Large power users involving big motors such as heavy manufacturing companies frequently use service voltages higher than 240 volts to reduce the size and cost of their electrical equipment. Most industrial facilities utilize 480 volts but some may require primary line voltages due to very large motors or high energy processing needs.
The wires from the distribution transformer to the customer's service point are called the "service drop". They are commonly installed overhead, however, they can be installed underground. All conductors must be insulated except the neutral, which may be bare in overhead installations. Single phase service drops will have either 2 or 3 wires while three phase service drops will have either 3 or 4 wires.
The National Electrical Code specifies installation requirements on service-drop conductors in terms of wire size and minimum clearances above the ground.
Service Entrance
The service entrance, as the name implies, is where the wires connected to the load side of the meter enter the house or building. The service entrance in a residence is commonly thought of as a breaker or fuse box. In larger more complicated commercial or industrial electrical systems, the service entrance may be a main disconnect panel or a trough where up to six main switches are present.
