Electricity is the main source of power in our daily lives. Convenience and cleanliness are among the most important advantages of using electricity. It is simple to access this virtually unlimited supply of energy at any home in Hong Kong. You simply plug an appliance into the mains supply, turn on the switch and run the appliance as long as you wish. Unlike burning fuels, electricity would not produce any undesirable waste or emission that pollutes the area of use. Without the danger of gas leakage or the storing of inflammable fuels, electricity is also one of the safest power sources.
 |  |
| Fig. 1 Thomas Edison's greatest contribution was perhaps his economically viable model for generating and distributing electric power. | Fig. 2 The fantastic night view of Hong Kong is made possible by an effective electrical transmission and distribution system. (Photo courtesy of HEC) |
With the extensive use of electricity, and the wide geographical distribution of users, an effective transmission and distribution system is essential. It is this extensive network that enables electricity to reach almost every family in Hong Kong from the city centre to remote areas. The history of electricity transmission can be dated back to 1883, when Thomas Edison first introduced an economically viable model for generating and distributing electric power. Edison's greatest achievement was perhaps not the invention of the light bulb or any other single application, but the universally applicable electricity transmission system which has lit up the whole world.
Modern electrical transmission and distribution systems are the result of conscientious efforts and design skills of engineers to ensure high energy efficiency and safety. High energy efficiency means the loss of power through transmission is minimized. In this module, the theories and the design of transmission and distribution systems will be discussed in detail.
| ................ [top] |
Transmission of electricity
Electricity is transmitted mainly through overhead lines or underground cables. Due to the resistance of the transmission wires, there is always some loss of power through the heating effect of current. The electricity transmission systems must be designed in ways which reduce this loss as much as possible.
High transmission voltage
 |
| Fig. 3 High voltage transmission lines on a transmission substation |
Electricity generated in power stations is raised to a very high voltage for transmission. This is an essential way to reduce heat loss. Do you know why?
Consider electrical power transmitted at voltage V and current I . The power transmitted P is given by
| P = IV | (1) |
For a fixed amount of electrical power transmitted, a higher voltage would give a smaller current. To see this relationship more clearly, one can write
| (2) |
Thus for a fixed power transmitted P, the current I is inversely proportional to the voltage V. When the current flows, the power loss by heat Ploss on a segment of wire of resistance R is given by
Ploss = I 2RElectric power transmission or "high-voltage electric transmission" is the bulk transfer of electrical energy, from generating power plants to substations located near population centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. Transmission lines, when interconnected with each other, become high-voltage transmission networks. In the US, these are typically referred to as "power grids" or just "the grid", while in the UK the network is known as the "national grid." North America has three major grids: The Western Interconnection; The Eastern Interconnection and the Electric Reliability Council of Texas (or ERCOT) grid.Historically, transmission and distribution lines were owned by the same company, but over the last decade or so many countries have liberalized the electricity market in ways that have led to the separation of the electricity transmission business from the distribution business.
Transmission lines mostly use three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used only for very long distances (typically greater than 400 miles, or 600 km); submarine power cables (typically longer than 30 miles, or 50 km); or for connecting two AC networks that are not synchronized.[citation needed]
Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost in long distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.
A key limitation in the distribution of electricity is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated system of control is therefore required to ensure electric generation very closely matches the demand. If supply and demand are not in balance, generation plants and transmission equipment can shut down which, in the worst cases, can lead to a major regional blackout, such as occurred in California and the US Northwest in 1996 and in the US Northeast in 1965, 1977 and 2003. To reduce the risk of such failures, electric transmission networks are interconnected into regional, national or continental wide networks thereby providing multiple redundant alternate routes for power to flow should (weather or equipment) failures occur. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available should there be any such failure in another part of the network.
Main article: History of electric power transmission
Due to this specialization of lines and because transmission was so inefficient that generators needed to be near their loads, it seemed at the time that the industry would develop into what is now known as a distributed generation system with large numbers of small generators located nearby their loads.
In 1886 in Great Barrington, Massachusetts, a 1 kV alternating current (AC) distribution system was installed. That same year, AC power at 2 kV, transmitted 30 km, was installed at Cerchi, Italy. At an AIEE meeting on May 16, 1888, Nikola Tesla delivered a lecture entitled A New System of Alternating Current Motors and Transformers, describing the equipment which allowed efficient generation and use of polyphase alternating currents. The transformer, and Tesla's polyphase and single-phase induction motors, were essential for a combined AC distribution system for both lighting and machinery. Ownership of the rights to the Tesla patents was a key advantage to the Westinghouse Company in offering a complete alternating current power system for both lighting and power.
By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to lower energy production cost.[3]
The first transmission of three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt. A 25 kV transmission line, approximately 175 km long, connected Lauffen on the Neckar and Frankfurt.
Voltages used for electric power transmission increased throughout the 20th century. By 1914, fifty-five transmission systems each operating at more than 70 kV were in service. The highest voltage then used was 150 kV.
The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the infrastructure in most industrialized nations. Interconnection of local generation plants and small distribution networks was greatly spurred by the requirements of World War I, where large electrical generating plants were built by governments to provide power to munitions factories. Later these plants were connected to supply civil loads through long-distance transmission.
3D Laser Scan - LiDAR - Electric Towers at D Street Bridge, Petaluma
With LiDAR we captured the towers and the powerlines in complete safety from over 200 feet away. Electrical substations, cellular towers, powerlines, tree interference, clash detection, all this can be achieved remotely......
Alabama Electric Co-op Employee Disaster Relief Fund
The storms are over and the lights are coming back on, but the electric co-op family still needs your help. This video from Alabama Electric shows before and after images of Tuscaloosa, along with line crews working to restore......
Avista Mock Electrical Accident April 2010
In mid-April 2010, Avista Utilities, Spokane, Washington, staged a mock electrical accident in a rural area outside of Addy, Washington. This video, designed for Avista employees, captured events on site. Avista Utilities provides electric service to 355,000 customers. ...
LineScout - 2010 Edison Award Winner
From VisuallyIntelligent: Hydro-Quebec and BCTC received the 2010 Edison Award from the Edison Electric Institute in the International Affiliate category for their partnership on the field deployment and enhancement of a high performance robot called LineScout......
Baltimore Gas & Electric on 2010 Blizzards
From Constellation Energy: BGE reviews the back-to-back blizzards that dumped feet of snow on the BGE service area in central Maryland in 2010. Hats off to BGE's customers for being prepared and patient as the company worked to restore power......
No comments:
Post a Comment