1893 Chicago Worlds fair. The
Hammond Organ
The Hammond organ is an electric organ invented by Laurens Hammond and John M. Hanert and first manufactured in 1935. Multiple models have been produced, most of which use sliding drawbars to vary sounds. Until 1975, Hammond organs generated s ...
also depends on a synchronous AC clock motor to maintain the correct speed of its internal "tone wheel" generator, thus keeping all notes pitch-perfect.
Today, AC power network operators regulate the daily average frequency so that clocks stay within a few seconds of the correct time. In practice the nominal frequency is raised or lowered by a specific percentage to maintain synchronization. Over the course of a day, the average frequency is maintained at a nominal value within a few hundred parts per million. In the
synchronous grid of Continental Europe
The synchronous grid of Continental Europe (also known as Continental Synchronous Area; formerly known as the UCTE grid) is the largest synchronous electrical grid (by connected power) in the world. It is interconnected as a single phase-locked ...
, the deviation between network phase time and
UTC (based on
International Atomic Time
International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 atomic ...
) is calculated at 08:00 each day in a control center in
Switzerland
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
. The target frequency is then adjusted by up to ±0.01 Hz (±0.02%) from 50 Hz as needed, to ensure a long-term frequency average of exactly 50 Hz × 60
s/
min
Min or MIN may refer to:
Places
* Fujian, also called Mǐn, a province of China
** Min Kingdom (909–945), a state in Fujian
* Min County, a county of Dingxi, Gansu province, China
* Min River (Fujian)
* Min River (Sichuan)
* Mineola (Am ...
× 60 min/
h × 24 h/
d = cycles per day. In
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
, whenever the error exceeds 10 seconds for the
Eastern Interconnection
The Eastern Interconnection is one of the two major alternating-current (AC) electrical grids in the North American power transmission grid. The other major interconnection is the Western Interconnection. The three minor interconnections are ...
, 3 seconds for the
Texas Interconnection
The Texas Interconnection is an alternating current (AC) power grid – a wide area synchronous grid – that covers most of the state of Texas. The grid is managed by the Electric Reliability Council of Texas (ERCOT).
The Texas Inter ...
, or 2 seconds for the
Western Interconnection
The Western Interconnection is a wide area synchronous grid and one of the two major alternating current, alternating current (AC) power grids in the North American power transmission grid. The other major wide area synchronous grid is the East ...
, a correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and end either on the hour or on the half-hour.
Real-time frequency meters for power generation in the United Kingdom are available online – an official one for the National Grid, and an unofficial one maintained by Dynamic Demand. Real-time frequency data of the synchronous grid of Continental Europe is available on websites such as . The
Frequency Monitoring Network (FNET) at the
University of Tennessee
The University of Tennessee (officially The University of Tennessee, Knoxville; or UT Knoxville; UTK; or UT) is a public land-grant research university in Knoxville, Tennessee. Founded in 1794, two years before Tennessee became the 16th state, ...
measures the frequency of the interconnections within the North American power grid, as well as in several other parts of the world. These measurements are displayed on the FNET website.
US regulations
In the United States, the
Federal Energy Regulatory Commission
The Federal Energy Regulatory Commission (FERC) is the United States federal agency that regulates the transmission and wholesale sale of electricity and natural gas in interstate commerce and regulates the transportation of oil by pipeline in ...
made time error correction mandatory in 2009. In 2011, The
North American Electric Reliability Corporation
The North American Electric Reliability Corporation (NERC) is a nonprofit corporation based in Atlanta, Georgia, and formed on March 28, 2006, as the successor to the North American Electric Reliability Council (also known as NERC). The original N ...
(NERC) discussed a proposed experiment that would relax frequency regulation requirements for electrical grids which would reduce the long-term accuracy of clocks and other devices that use the 60 Hz grid frequency as a time base.
Frequency and load
The primary reason for accurate frequency control is to allow control of the flow of alternating current power from multiple generators through the network. The trend in system frequency is a measure of mismatch between demand and generation, and is a necessary parameter for load control in interconnected systems.
The frequency of the system will vary as load and generation change. Increasing the mechanical input power to any individual synchronous generator will not greatly affect the overall system frequency, but will produce more electric power from that unit. During a severe overload caused by 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 will cause system frequency to increase upstream of the loss, but may cause a collapse downstream of the loss, as generation is now not keeping pace with consumption.
Automatic generation control
In an electric power system, automatic generation control (AGC) is a system for adjusting the power output of multiple generators at different power plants, in response to changes in the load. Since a power grid requires that generation and load ...
(AGC) is used to maintain scheduled frequency and interchange power flows. Control systems in power stations detect changes in the network-wide frequency and adjust mechanical power input to generators back to their target frequency. This counteracting usually takes a few tens of seconds due to the large rotating masses involved (although the large masses serve to limit the magnitude of short-term disturbances in the first place). Temporary frequency changes are an unavoidable consequence of changing demand. Exceptional or rapidly changing mains frequency is often a sign that an electricity distribution network is operating near its capacity limits, dramatic examples of which can sometimes be observed shortly before major outages. Large generating stations including
solar farm
Solar may refer to:
Astronomy
* Of or relating to the Sun
** Solar telescope, a special purpose telescope used to observe the Sun
** A device that utilizes solar energy (e.g. "solar panels")
** Solar calendar, a calendar whose dates indicate t ...
s can reduce their average output and use the headroom between operating load and maximum capacity to assist in providing grid regulation; the response of solar inverters is faster than generators because they have no rotating mass. As variable resources such as solar and wind replace traditional generation and the inertia they provide, algorithms have had to become more sophisticated. Energy storage systems such as batteries are fulfilling the regulation role to an expanding degree as well.
Frequency
protective relay
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detecti ...
s on the power system network sense the decline of frequency and automatically initiate
load shedding
Demand response is a change in the power consumption of an electric utility customer to better match the demand for power with the supply. Until the 21st century decrease in the cost of pumped storage and batteries electric energy could not be ...
or tripping of interconnection lines to preserve the operation of at least part of the network. Small frequency deviations (e.g., 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, system operators may allow system frequency to rise during periods of light load to maintain a daily average frequency of acceptable accuracy. Portable generators, not connected to a utility system, need not tightly regulate their frequency because typical loads are insensitive to small frequency deviations.
Load-frequency control
Load-frequency control (LFC) is a type of
integral control that restores the system frequency and power flows to adjacent areas back to their values before a change in load. The power transfer between different areas of a system is known as "net
tie-line power".
The general control algorithm for LFC was developed by
Nathan Cohn in 1971. The algorithm involves defining the term area control error (ACE), which is the sum of the net tie-line power error and the product of the frequency error with a frequency bias constant. When the area control error is reduced to zero, the control algorithm has returned the frequency and tie-line power errors to zero.
Audible noise and interference
AC-powered appliances can give off a characteristic hum, often called "
mains hum
Mains hum, electric hum, cycle hum, or power line hum is a sound associated with alternating current which is twice the frequency of the mains electricity. The fundamental frequency of this sound is usually double that of fundamental 50/60 Hz, '' ...
", at the multiples of the frequencies of AC power that they use (see
Magnetostriction
Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field change ...
). It is usually produced by motor and transformer core laminations vibrating in time with the magnetic field. This hum can also appear in audio systems, where the power supply filter or signal shielding of an amplifier is not adequate.
Most countries chose their
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
vertical synchronization
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog s ...
rate to be the same as the local mains supply frequency. This helped to prevent power line hum and magnetic interference from causing visible beat frequencies in the displayed picture of early analogue TV receivers particularly from the mains transformer. Although some distortion of the picture was present, it went mostly un-noticed because it was stationary. The elimination of transformers by the use of
AC/DC receivers, and other changes to set design helped minimise the effect and some countries now use a vertical rate that is an approximation to the supply frequency (most notably 60 Hz areas).
Another use of this side effect is as a forensic tool. When a recording is made that captures audio near an AC appliance or socket, the hum is also incidentally recorded. The peaks of the hum repeat every AC cycle (every ms for 50 Hz AC, or every ms for 60 Hz AC). The exact frequency of the hum should match the frequency of a forensic recording of the hum at the exact date and time that the recording is alleged to have been made. Discontinuities in the frequency match or no match at all will betray the authenticity of the recording.
See also
*
Mains electricity
Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to h ...
*
Network analyzer (AC power)
From 1929 to the late 1960s, large alternating current power systems were modelled and studied on AC network analyzers (also called alternating current network calculators or AC calculating boards) or transient network analyzers. These special-pu ...
*
Telechron
Telechron is the name of a U.S. company that manufactured electric clocks between 1912 and 1992. "Telechron" is derived from the Greek word''tele'' meaning "far off," an''chronos'' "time," thus referring to the transmission of time over long dista ...
Further reading
*Furfari, F.A., ''The Evolution of Power-Line Frequencies to 25 Hz'', Industry Applications Magazine, IEEE, Sep/Oct 2000, Volume 6, Issue 5, Pages 12–14, .
*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, .
References
{{Electric clock technology
Electric power