UTILITY FREQUENCY
The utility
frequency, (power) line frequency (American
English) or mains frequency (British
English) is the frequency of the oscillations of alternating current (AC) in an electric power grid transmitted from a power
plant to the end-user.
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 220-240 V, and those that now use 60 Hz tend to use 100-120 V.
Both frequencies coexist today (Japan uses both) with no great technical reason
to prefer one over the other and no apparent desire for complete worldwide
standardization.
Unless
specified by the manufacturer to operate on both 50 and 60 Hz, appliances
may not operate efficiently or even safely if used on anything other than the
intended frequency.
Operating factors
Several
factors influence the choice of frequency in an AC system.
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
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
When
large central generating stations became practical, the choice of frequency was
made based on the nature of the intended load.
Eventually
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.
Lighting
The first
applications of commercial electric power were incandescent lighting and commutator-type electric
motors. Both devices operate well on DC, but DC could not be easily changed in voltage, and was
generally only produced at the required utilization voltage.
If an
incandescent lamp is operated on 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.
Rotating machines
Commutator-type motors do not operate well on
high-frequency AC because 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 AC electric
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.[3]
The synchronous speed N in RPM is calculated using the formula,
where f is the frequency in Hertz and P is the
number of poles.
|
Synchronous speeds of AC motors
for some current and historical utility frequencies
|
||||||
|
Poles
|
RPM
at 133 1/3 Hz
|
RPM
at 60 Hz
|
RPM
at 50 Hz
|
RPM
at 40 Hz
|
RPM
at 25 Hz
|
RPM
at 16 2/3 Hz
|
|
2
|
8,000
|
3,600
|
3,000
|
2,400
|
1,500
|
1,000
|
|
4
|
4,000
|
1,800
|
1,500
|
1,200
|
750
|
500
|
|
6
|
2,666.7
|
1,200
|
1,000
|
800
|
500
|
333.3
|
|
8
|
2,000
|
900
|
750
|
600
|
375
|
250
|
|
10
|
1,600
|
720
|
600
|
480
|
300
|
200
|
|
12
|
1,333.3
|
600
|
500
|
400
|
250
|
166.7
|
|
14
|
1142.9
|
514.3
|
428.6
|
342.8
|
214.3
|
142.9
|
|
16
|
1,000
|
450
|
375
|
300
|
187.5
|
125
|
|
18
|
888.9
|
400
|
333
1/3
|
266
2/3
|
166
2/3
|
111.1
|
|
20
|
800
|
360
|
300
|
240
|
150
|
100
|
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.
Transmission and transformers
With AC, transformers
can be used to step down high transmission voltages to lower customer
utilization voltage. The transformer is effectively a voltage conversion device
with no moving parts and requiring little maintenance. The use of AC eliminated
the need for spinning DC voltage conversion motor-generators that require
regular maintenance and monitoring.
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.
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.
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. The proliferation
of frequencies grew out of the rapid development of electrical machines in the
period 1880 through 1900. 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.
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.
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.
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.
25 Hz origins
The first
generators at the Niagara Falls project, built by Westinghouse in 1895,
were 25 Hz because the turbine speed had already been set 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.
Because the Niagara project was so influential on electric power systems
design, 25 Hz prevailed as the North American standard for low-frequency
AC.
40 Hz origins
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 1920s, and
projects in Italy used 42 Hz. The oldest continuously operating commercial
hydroelectric
power plant in the United States, Mechanicville Hydroelectric Plant,
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 too 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.
The Ganz Company of Hungary had standardized on 5000
alternations per minute ( 41 2/3 Hz) for their products, so Ganz clients had 41
2/3 Hz systems that in some cases ran for many years.
Standardization
In the early days of electrification, so many
frequencies were used that no one value prevailed (London in 1918 had 10
different frequencies). As the 20th century continued, more power was produced
at 60 Hz (North America) or 50 Hz (Europe and most of Asia). Standardization
allowed international trade in electrical equipment. Much later, the use of standard
frequencies allowed interconnection of power grids. It wasn't until after World War
II with the advent of affordable electrical consumer goods that more
uniform standards were enacted.
In Britain,
a standard frequency of 50 Hz was declared as early as 1904, but significant
development continued at other frequencies. The implementation of the National Grid starting in 1926 compelled the
standardization of frequencies among the many interconnected electrical service
providers. The 50 Hz standard was completely established only after World War
II.
By about 1900, European manufacturers had mostly
standardized on 50 Hz for new installations. The German VDE in the first standard
for electrical machines and transformers in 1902 recommended 25 Hz and 50 Hz as
standard frequencies. VDE did not see much application of 25 Hz, and dropped it
from the 1914 edition of the standard. Remnant installations at other
frequencies persisted until well after the Second World War.
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 power
station exist in New Orleans for floodwater pumps. The 15 kV AC rail networks, used in Germany, Austria, Switzerland,
Sweden and Norway, still
operate at 16⅔ Hz or 16.7 Hz.
In some
cases, where most load was to be railway or motor loads, it was considered
economic to generate power at 25 Hz and install rotary
converters for 60 Hz distribution. Converters for production of DC
from alternating current were available in larger sizes and were more efficient
at 25 Hz compared with 60 Hz. Remnant fragments of older systems may
be tied to the standard frequency system via a rotary converter or static
inverter frequency changer. These allow energy to be interchanged between
two power networks at different frequencies, but the systems are large, costly,
and waste some energy in operation.
Rotating-machine
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
US, Southern California Edison had
standardized on 50 Hz. 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.
In Mexico, areas
operating on 50 Hz grid were converted during the 1970s, uniting the
country under 60 Hz.
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. The boundary between the two regions
contains four back-to-back HVDC substations which convert the
frequency; these are Shin Shinano, Sakuma Dam, Minami-Fukumitsu,
and the Higashi-Shimizu Frequency Converter.
Utility Frequencies in Use in 1897 in
North America
|
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
"increasingly 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
|
Utility Frequencies in Europe to 1900
|
Cycles
|
Description
|
|
133
|
Single-phase lighting systems, UK
and Europe
|
|
125
|
Single-phase lighting system, UK
and Europe
|
|
70
|
Single-phase lighting, Germany
1891
|
|
65.3
|
BBC Bellinzona
|
|
60
|
Single phase lighting, Germany,
1891, 1893
|
|
50
|
AEG, Oerlikon, and other
manufacturers, eventual standard
|
|
48
|
BBC Kilwangen generating station,
|
|
46
|
Rome, Geneva 1900
|
|
45 1/3
|
Municipal power station, Frankfurt
am Main, 1893
|
|
42
|
Ganz customers, also Germany 1898
|
|
41 2/3
|
Ganz Company, Hungary
|
|
40
|
Lauffen am Neckar, hydroelectric,
1891, to 1925
|
|
38.6
|
BBC Arlen
|
|
25
|
Single phase lighting, Germany
1897
|
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 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, Romania,
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.
Railways
Other power frequencies are still used. Germany,
Austria, Switzerland, Sweden and Norway use traction power networks for railways,
distributing single-phase AC at 16⅔ Hz or 16.7 Hz. A frequency of
25 Hz is used for the Austrian railway Mariazeller
Bahn and for Amtrak's 25 Hz traction power
system in the USA. Other AC 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
traction powerstations. In the 19th Century,
frequencies as low as 8 Hz were contemplated for operation of electric
railways with commutator motors Some outlets in trains carry the correct
voltage, but using the original train network frequency like 16⅔ Hz or
16.7 Hz.
400 Hz
Frequencies as
high as 400 Hz are used in aircraft, spacecraft, submarines, server rooms
for computer
power, military equipment, 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. A United States military standard MIL-STD-704 exists for aircraft use of 400 Hz power.
Stability
Long-term stability and clock synchronization
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. Today network operators regulate the
daily average frequency so that clocks stay within a few seconds of 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 the nominal value within a few hundred parts per
million. In the synchronous grid of Continental
Europe, the deviation between network phase time and UTC (based on International Atomic Time) is calculated
at 08:00 each day in a control center in Switzerland.
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 sec × 60 min × 24 hours = 4,320,000 cycles per day. In North America,
whenever the error exceeds 10 seconds for the east, 3 seconds for Texas, or 2
seconds for the west, 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
National Grid one, and an unofficial
one maintained by Dynamic Demand.[24][25]
Real-time frequency data of the synchronous grid of Continental
Europe is available at mainsfrequency.com.
The Frequency Monitoring
Network (FNET) at the University of Tennessee 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.
Smaller
power systems may not maintain frequency with the same degree of accuracy. In
2011, The North American Electric
Reliability Corporation (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 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.
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. Automatic generation control (AGC) is
used to maintain scheduled frequency and interchange power flows. Control
systems in power plants 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. 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.
Frequency protective
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. Portable generators, not connected to a utility system, need not
tightly regulate their frequency because typical loads are insensitive to small
frequency deviations.
Audible noise and interference
AC-powered appliances can give off a characteristic
hum, often called "mains hum", at the multiples of the frequencies of
AC power that they use (see Magnetostriction).
It is usually produced by motor and transformer core laminations vibrating in
time with the magnetic field. This hum can also be evident in
poorly-constructed audio amplifiers, where the power supply is inadequately
filtered.
Most countries chose their television vertical synchronization rate to
approximate 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 analogue receivers, but is of diminishing importance in
modern digital display systems.
