Utility frequency
From Wikipedia, the free encyclopedia
The utility frequency (American English) or mains frequency (British English) is the frequency at which alternating current (AC) is transmitted from a power plant to the end user. In most parts of the Americas, it is typically 60 Hz, and in most parts of the rest of the world it is typically 50 Hz. Precise details are shown in the list of countries with mains power plugs, voltages and frequencies.
During the development of commercial electric power systems in the late 19th and early 20th centuries, many different frequencies (and voltages) had been used. Large investment in equipment at one frequency made standardization a slow process. However, as of the turn of the 21st century, places that now use the 50 Hz frequency tend to use 230 V, and those that now use 60 Hz tend to use 120 V. Both frequencies co-exist today (some countries such as Japan use both) with no technical reason to prefer one over the other and no apparent desire for complete world-wide standardization.
Unless specified by the manufacturer to operate on either 50 or 60 Hz, appliances may not operate efficiently or even safely if used on other than the intended supply frequency.
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[edit] Operating factors
Several factors influence the choice of frequency in an AC system. [1] Lighting, motors, transformers, generators and transmission lines all have characteristics which depend on the power frequency.
All of these factors interact and make selection of a power frequency a matter of considerable importance. The best frequency is a compromise between contradictory requirements. In the late 19th century, designers would pick a relatively high frequency for systems featuring transformers and arc lights, so as to economize on transformer materials, but would pick a lower frequency for systems with long transmission lines or feeding primarily motor loads or rotary converters for producing direct current. When large central generating stations became practical, the choice of frequency was made based on the nature of the intended load. Eventually the improvements in machine design allowed a single frequency to be used both for lighting and motor loads; a unified system improved the economics of electricity production since system load was more uniform during the course of a day.
[edit] Lighting
The first applications of commercial electric power were incandescent lighting and commutator-type electric motors. Both devices operate well on DC, but DC cannot be easily transmitted long distances at utilization voltage and also cannot be easily changed in voltage.
If an incandescent lamp is operated off a low-frequency current, the filament cools on each half-cycle of the alternating current, leading to perceptible change in brightness and flicker of the lamps; the effect is more pronounced with arc lamps, and the later mercury-vapor and fluorescent lamps.
[edit] Rotating machines
Commutator-type motors do not operate well on high-frequency AC since the rapid changes of current are opposed by the inductance of the motor field; even today, although commutator-type universal motors are common in 50 Hz and 60 Hz household appliances, they are small motors, less than 1 kW. The induction motor was found to work well on frequencies around 50 to 60 Hz but with the materials available in the 1890s would not work well at a frequency of, say, 133 Hz. There is a fixed relationship between the number of magnetic poles in the induction motor field, the frequency of the alternating current, and the rotation speed; so, a given standard speed limits the choice of frequency (and the reverse). Once induction motors became common, it was important to standardize frequency for compatibility with the customer's equipment.
Generators operated by slow-speed reciprocating engines will produce lower frequencies, for a given number of poles, than those operated by, for example, a high-speed steam turbine. For very slow prime mover speeds, it would be costly to build a generator with enough poles to provide a high AC frequency. As well, synchronizing two generators to the same speed was found to be easier at lower speeds. While belt drives were common as a way to increase speed of slow engines, in very large ratings (thousands of kilowatts) these were expensive, inefficient and unreliable. Direct-driven generators off steam turbines after about 1906 favored higher frequencies. The steadier rotation speed of high-speed machines allowed for satisfactory operation of commutators in rotary converters. [2]
Direct-current power was not entirely displaced by alternating current and was useful in railway and electrochemical processes. Prior to the development of mercury arc valve rectifiers, rotary converters were used to produce DC power from AC. Like other commutator-type machines, these worked better with lower frequencies.
[edit] Transmission and transformers
With AC, transformers can be used to step down high transmission voltages to lower utilization voltage. Since, for a given power level, the dimensions of a transformer are roughly inversely proportional to frequency, a system with many transformers would be more economical at a higher frequency.
Electric power transmission over long lines favors lower frequencies. The effects of the distributed capacitance and inductance of the line are less at low frequency.
[edit] System interconnection
Generators can only be interconnected to operate in parallel if they are of the same frequency and wave-shape. By standardizing the frequency used, generators in a geographic area can be interconnected in a grid, providing reliability and cost savings.
[edit] History
Very early isolated AC generating schemes used arbitrary frequencies based on convenience for steam engine, water turbine and electrical generator design. Frequencies between 16⅔ Hz and 133⅓ Hz were used on different systems. For example, the city of Coventry, England, in 1895 had a unique 87 Hz single-phase distribution system that was in use until 1906. [3] The proliferation of frequencies grew out of the rapid development of electrical machines in the period 1880 through 1900.
Though many theories exist, and quite a few entertaining urban legends, there is little certitude in the details of the history of 60 Hz vs. 50 Hz. It is known that Westinghouse in the US decided on 60 Hz and AEG in Germany decided on 50 Hz, eventually leading to the world being mostly divided into two frequency camps. In the early incandescent lighting period, single-phase AC was common and typical generators were 8-pole machines operated at 2000 RPM, giving a frequency of 133 cycles per second. Westinghouse Electric decided to standardize on a lower frequency to permit operation of both electric lighting and induction motors on the same generating system. Although 50 Hz was suitable for both, in 1890 Westinghouse considered that existing arc-lighting equipment operated slightly better on 60 Hz, and so that frequency was chosen.[4] Frequencies much below 50 Hz gave noticeable flicker of arc or incandescent lighting. After observing flicker of lamps operated by the 40 Hz power transmitted by the Lauffen-Frankfurt link in 1891, AEG raised their standard frequency to 50 Hz in 1891. [5] The operation of Tesla's induction motor required a lower frequency than the 133 Hz common for lighting systems in 1890. In 1893 General Electric Corporation, which was affiliated with AEG in Germany, built a generating project at Mill Creek, California using 50 Hz, but changed to 60 Hz a year later to maintain market share with the Westinghouse standard.
However, the first generators at the Niagara Falls project, built by Westinghouse in 1895, were 25 Hz because the turbine speed had already been selected before alternating current power transmission had been definitively selected. Westinghouse would have selected a low frequency of 30 Hz to drive motor loads, but the turbines for the project had already been specified at 250 RPM. The machines could have been made to deliver 16⅔ Hz power suitable for heavy commutator-type motors but the Westinghouse company objected that this would be undesirable for lighting, and suggested 33⅓ Hz. Eventually a compromise of 25 Hz, with 12 pole 250 RPM generators, was chosen. [6] Because the Niagara project was so influential on electric power systems design, 25 Hz prevailed as the North American standard for low-frequency AC.
A General Electric study concluded that 40 Hz would have been a good compromise between lighting, motor, and transmission needs, given the materials and equipment available in the first quarter of the 20th Century. Several 40 Hz systems were built. The Lauffen-Frankfurt demonstration used 40 Hz to transmit power 175 km in 1891. A large interconnected 40 Hz network existed in north-east England (the Newcastle-upon-Tyne Electric Supply Company, NESCO) until the advent of the National Grid (UK) in the late 1920's, and projects in Italy used 42 Hz.[7] The oldest continuously-operating commercial hydroelectric power plant in the United States, at Mechanicville, New York, still produces electric power at 40 Hz and supplies power to the local 60 Hz transmission system through frequency changers. Industrial plants and mines in North America and Australia sometimes were built with 40 Hz electrical systems which were maintained until uneconomic to continue. Although frequencies near 40 Hz found much commercial use, these were bypassed by standardized frequencies of 25, 50 and 60 Hz preferred by higher volume equipment manufacturers.
AEG's choice of 50 Hz is thought by some to relate to a more "metric-friendly" number than 60. Since so many frequencies were used it may not have been clear that any one value was desirable. A plethora of frequencies continued in broad use (London in 1918 had 10 different frequencies), and it wasn't until after World War II with the advent of affordable electrical consumer goods that more uniform standards were enacted.
Other frequencies were somewhat common in the first half of the 20th century, and remain in use in isolated cases today, sometimes tied to the standard frequency system via a rotary converter or static inverter frequency changer. Because of the cost of conversion, some parts of the distribution system may continue to operate on original frequencies even after a new frequency is chosen. 25 Hz power was used in Ontario, Quebec, the northern USA, and for railway electrification. In the 1950s, many 25 Hz systems, from the generators right through to household appliances, were converted and standardized. Some 25 Hz generators still exist at the Beck 1 and Rankine generating stations near Niagara Falls to provide power for large industrial customers who did not want to replace existing equipment; and some 25 Hz motors and a 25 Hz electrical generator power station exist in New Orleans for floodwater pumps [1]. Some of the metre gauge railway lines in Switzerland operate at the unusual frequency of 16⅔ Hz, which can of course be obtained from the local 50 Hz 3 phase power grid.
Frequency changers used to convert between 25 Hz and 60 Hz systems were awkward to design; a 60 Hz machine with 24 poles would turn at the same speed as a 25 Hz machine with 10 poles, making the machines large, slow-speed and expensive. A ratio of 60/30 would have simplified these designs, but the installed base at 25 Hz was too large to be economically opposed.
In the United States, the Southern California Edison company had standardized on 50 Hz [8]. Much of Southern California operated on 50 Hz and did not completely change frequency of their generators and customer equipment to 60 Hz until around 1948. Some projects by the Au Sable Electric Company used 30 Hz at transmission voltages up to 110,000 volts in 1914. [9]
In Japan, the western part of the country (Kyoto and west) uses 60 Hz and the eastern part (Tokyo and east) uses 50 Hz. This originates in the first purchases of generators from AEG in 1895, installed for Tokyo, and General Electric in 1896, installed in Osaka.
Utility Frequencies in Use in 1897 in North America [10]
| Cycles | Description |
|---|---|
| 140 | Wood arc-lighting dynamo |
| 133 | Stanley-Kelly Company |
| 125 | General Electric single-phase |
| 66.7 | Stanley-Kelly company |
| 62.5 | General Electric "monocyclic" |
| 60 | Many manufacturers, becoming "increasing common" in 1897 |
| 58.3 | General Electric Lachine Rapids |
| 40 | General Electric |
| 33 | General Electric at Portland Oregon for rotary converters |
| 27 | Crocker-Wheeler for calcium carbide furnaces |
| 25 | Westinghouse Niagara Falls 2-phase - for operating motors |
Even by the middle of the 20th century, utility frequencies were still not entirely standardized at the now-common 50 Hz or 60 Hz. In 1946, a reference manual for designers of radio equipment [11] listed the following now obsolete frequencies as in use. Many of these regions also had 50 cycle, 60 cycle or direct current supplies.
Frequencies in Use in 1946 (As well as 50 Hz and 60 Hz)
| Cycles | Region |
|---|---|
| 25 | Canada (Southern Ontario), Panama Canal Zone(*), France, Germany, Sweden, UK, China, Hawaii,India, Manchuria, |
| 40 | Jamaica, Belgium, Switzerland, UK, Federated Malay States, Egypt, West Australia(*) |
| 42 | Czechoslovakia, Hungary, Italy, Monaco(*), Portugal, Rumania, Yugoslavia, Libya (Tripoli) |
| 43 | Argentina |
| 45 | Italy, Libya (Tripoli) |
| 76 | Gibraltar(*) |
| 100 | Malta(*), British East Africa |
Where regions are marked (*), this is the only utility frequency shown for that region.
[edit] Railways
Other power frequencies are used. Germany, Austria, Switzerland, Sweden and Norway use traction power networks for railways, distributing single-phase AC at 16⅔ Hz. A frequency of 25 Hz was used for the Austrian railway Mariazeller Bahn and some railway systems in New York and Pennsylvania (Amtrak) in the USA. Other railway systems are energized at the local commercial power frequency, 50 Hz or 60 Hz. Traction power may be derived from commercial power supplies by frequency converters, or in some cases may be produced by dedicated generating stations. In the 19th Century frequencies as low as 8 Hz were contemplated for operation of electric railways with commutator motors [12]
[edit] 400 Hz
Frequencies as high as 400 Hz are used in aerospace and some special-purpose computer power supplies and hand-held machine tools. Such high frequencies cannot be economically transmitted long distances, so 400 Hz power systems are usually confined to a building or vehicle. Transformers and motors for 400 Hz are much smaller and lighter than at 50 or 60 Hz, which is an advantage in aircraft and ships.
[edit] Stability
The frequency of large interconnected power distribution systems is tightly regulated so that, over the course of a day, the average frequency is maintained at the nominal value within a few hundred parts per million.[13] While this allows simple electric clocks and motors, based on synchronous electric motors, to keep accurate time, the primary reason for accurate frequency control is to allow the flow of alternating current power from multiple generators through the network to be controlled. The trend in system frequency is a measure of mismatch between demand and generation, and so is a necessary parameter for load control in interconnected systems. Regulation of power system frequency for timekeeping accuracy was not commonplace until after 1926 and the invention of the electric clock driven by a synchronous motor.
Frequency of the system will vary as load and generation change. Increasing the mechanical input power to a synchronous generator will not greatly affect the system frequency but will produce more electric power from that unit. During a severe overload caused by tripping or failure of generators or transmission lines the power system frequency will decline, due to an imbalance of load versus generation. Loss of an interconnection, while exporting power (relative to system total generation) will cause system frequency to rise. AGC (automatic generation control) is used to maintain scheduled frequency and interchange power flows.
Frequency protection relays on the power system network sense the decline of frequency and automatically initiate load shedding or tripping of interconnection lines, to preserve the operation of at least part of the network. Small frequency deviations (i.e.- 0.5 Hz on a 50 Hz or 60 Hz network) will result in automatic load shedding or other control actions to restore system frequency.
Smaller power systems, not extensively interconnected with many generators and loads, will not maintain frequency with the same degree of accuracy. Where system frequency is not tightly regulated during heavy load periods, the system operators may allow system frequency to rise during periods of light load, to maintain a daily average frequency of acceptable accuracy.
[edit] Audible noise and interference
AC-powered appliances can give off a characteristic hum (often referred to as the "60 cycle hum" or "mains hum"), at the multiples of the frequencies of AC power that they use. This often occurs in poorly made amplifiers. Most countries have chosen their television standard to approximate their mains supply frequency. This helps prevent power line hum and magnetic interference from causing visible beat frequencies in the displayed picture.
[edit] See also
- Mains electricity
- Mains power systems
- List of countries with mains power plugs, voltages and frequencies
- Power connector
[edit] Further reading
- Furfari, F.A., The Evolution of Power-Line Frequencies 133⅓ to 25 Hz, Industry Applications Magazine, IEEE, Sep/Oct 2000, Volume 6, Issue 5, Pages 12-14, ISSN 1077-2618.
- Rushmore, D.B., Frequency, AIEE Transactions, Volume 31, 1912, pages 955-983, and discussion on pages 974-978.
- Blalock, Thomas J., Electrification of a Major Steel Mill - Part II Development of the 25 Hz System, Industry Applications Magazine, IEEE, Sep/Oct 2005, Pages 9-12, ISSN 1077-2618.
[edit] References
- ^ B. G. Lamme, The Technical Story of the Frequencies, Transactions AIEE January 1918, reprinted in the Baltimore Amateur Radio Club newsletter The Modulator January -March 2007
- ^ Lamme, Technical story...
- ^ Gordon Woodward ,City of Coventry Single and Two Phase Generation and Distribution, retrieved from http://www.iee.org/OnComms/pn/History/HistoryWk_Single_&_2_phase.pdf October 30,2007
- ^ Owen, E.L, The Origins of 60-Hz as a Power Frequency, Industry Applications Magazine, IEEE, Volume: 3, Issue 6, Nov.-Dec. 1997, Pages 8, 10, 12-14.
- ^ Owen, ..Origins..
- ^ Lamme, Technical Story of the Frequencies
- ^ Thomas P. Hughes, Networks of Power: Electrification in Western Society 1880-1930, The Johns Hopkins University Press, Baltimore 1983 ISBN 0-8018-2873-2 pgs. 282-283
- ^ Central Station Engineers of the Westinghouse Electric Corporation, Electrical Transmission and Distribution Reference Book, 4th Ed., Westinghouse Electric Corporation, East Pittsburgh PA, 1950, no ISBN
- ^ Hughes as above
- ^ Edwin J. Houston and Arthur Kennelly, Recent Types of Dynamo-Electric Machinery, copyright American Technical Book Company 1897, published by P.F. Collier and Sons New York, 1902
- ^ H.T. Kohlhaas, (ed.), Reference Data for Radio Engineers 2nd Edition, Federal Telephone and Radio Corporation, New York, 1946, no ISBN
- ^ B. G. Lamme, The Technical Story of the Frequencies, Transactions AIEE January 1918, reprinted in the Baltimore Amateur Radio Club newsletter The Modulator January -March 2007
- ^ Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07-020974-X, page 16-15, 16-16
