Variable-frequency Transformer
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A variable-frequency transformer (VFT) is used to transmit
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
between two ( asynchronous or synchronous)
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
domains. The VFT is a relatively recent development. Most asynchronous grid inter-ties use
high-voltage direct current A high-voltage direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for electric power transmission, in contrast with the more common alternating curre ...
converters, while synchronous grid inter-ties are connected by lines and "ordinary"
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s, but without the ability to control power flow between the systems. It can be thought of as a very high power
synchro A synchro (also known as selsyn and by other brand names) is, in effect, a transformer whose primary-to-secondary coupling may be varied by physically changing the relative orientation of the two windings. Synchros are often used for measuring ...
, or a
rotary converter A rotary converter is a type of electrical machine which acts as a mechanical rectifier, inverter or frequency converter. Rotary converters were used to convert alternating current (AC) to direct current (DC), or DC to AC power, before the adven ...
acting as a
frequency changer A frequency changer or frequency converter is an electronic or electromechanical device that converts alternating current ( AC) of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, ...
, which is more efficient than a
motor–generator A motor–generator (an M–G set) is a device for converting electrical power to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electr ...
of the same rating.


Construction and operation

A variable-frequency transformer is a
doubly fed electric machine Doubly-fed electric machines also slip-ring generators are electric motors or electric generators, where both the field magnet windings and armature windings are separately connected to equipment outside the machine. By feeding adjustable freque ...
resembling a vertical shaft
hydroelectric Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined and ...
generator with a
three-phase Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, Electric power transmission, transmission, and Electric power distribution, distribution. It is a type of polyphase system empl ...
wound
rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator * Helicopter rotor, the rotary wing(s) of a rotorcraft ...
, connected by slip rings to one external power circuit. The
stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric mot ...
is connected to the other. With no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator. A
direct-current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
torque motor is mounted on the same shaft; changing the direction of torque applied to the shaft changes the direction of power flow. The variable-frequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back
HVDC A high-voltage direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for electric power transmission, in contrast with the more common alternating curren ...
, the variable frequency transformer does not demand harmonic filters and reactive power compensation. Limitations of the concept are the
current-carrying capacity Ampacity is a portmanteau for ''ampere capacity'' defined by National Electrical Codes, in some North American countries. Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use ...
of the slip rings for the rotor winding.


Projects

Five small variable-frequency transformer with a total power rate of 25 MVA were in use at Neuhof Substation, Bad Sachsa, Germany for coupling power grids of former East and
West Germany West Germany is the colloquial term used to indicate the Federal Republic of Germany (FRG; german: Bundesrepublik Deutschland , BRD) between its formation on 23 May 1949 and the German reunification through the accession of East Germany on 3 O ...
between 1985 and 1990. Langlois Substation in
Québec Quebec ( ; )According to the Government of Canada, Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is ...
,
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
() installed a 100 MW variable-frequency transformer in 2004 to connect the asynchronous grids in Québec and the
northeastern United States The Northeastern United States, also referred to as the Northeast, the East Coast, or the American Northeast, is a geographic region of the United States. It is located on the Atlantic coast of North America, with Canada to its north, the Southe ...
. This was the first large scale, commercial variable frequency transformer, and was installed at
Hydro-Québec Hydro-Québec is a public utility that manages the generation, transmission and distribution of electricity in the Canadian province of Quebec, as well as the export of power to portions of the Northeast United States. It was established by the ...
Langlois substation and is located electrically near sixteen hydro generators at
Les Cèdres, Quebec Les Cèdres is a municipality located north of the Saint Lawrence River in the Montérégie of Quebec, Canada, near Vaudreuil-Dorion. The population as of the Canada 2021 Census was 7,184. The name means "The Cedars" in French. There is an ex ...
and thirty-six more hydro generators at
Beauharnois, Quebec Beauharnois () is a city located in the Beauharnois-Salaberry Regional County Municipality of southwestern Quebec, Canada, and is part of the Greater Montreal Area. The city's population as of the Canada 2011 Census was 12,011. It is home to t ...
. The operating experience since April 2004 has demonstrated the VFT’s inherent compatibility with the nearby generators AEP Texas installed a 100 MW VFT substation in Laredo, Texas,
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
() in early 2007. It connects the power systems of
ERCOT The Electric Reliability Council of Texas, Inc. (ERCOT) is an American organization that operates Texas's electrical grid, the Texas Interconnection, which supplies power to more than 25 million Texas customers and represents 90 percent of the st ...
(in the United States) to CFE (in Mexico). (Se
The Laredo VFT Project
) Smaller VFTs are used in large land-based
wind turbine A wind turbine is a device that converts the kinetic energy of wind into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW added each year. ...
s, so that the turbine rotation speed can vary while connected to an
electric power distribution Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...
grid Grid, The Grid, or GRID may refer to: Common usage * Cattle grid or stock grid, a type of obstacle is used to prevent livestock from crossing the road * Grid reference, used to define a location on a map Arts, entertainment, and media * News g ...
.


Linden VFT

General Electric General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
installed a 3 × 100 MW VFT substation in
Linden, New Jersey Linden is a city in southeastern Union County, in the U.S. state of New Jersey. It is part of the New York Metropolitan Area, located about southwest of Manhattan and bordering Staten Island, a borough of New York City, across the Arthur Kill ...
, in the United States in 2009. It connects the power systems of PJM & New York Independent System Operator (NYISO). This installation is in parallel with three existing phase-shifting transformers to regulate synchronous power flow.


Economics of energy trading

VFTs provide the technical feasibility to flow power in both directions between two grids, permitting power exchanges that were previously impossible. Energy in a grid with lower costs can be transmitted to a grid with higher costs (higher demand), with energy trading. Power capacity is sold by providers. Transmission scheduling rights (TSRs) are auctioned by the transmission line owners. Financial transmission rights (FTRs) are a financial instrument used to balance energy congestion and demand costs.


See also

*
Induction regulator An induction regulator is an alternating current electrical machine, somewhat similar to an induction motor, which can provide a continuously variable output voltage. The induction regulator was an early device used to control the voltage of elect ...


References


External links


Power Transmission Without the Power Electronics
{{Electric transformers Electric power infrastructure Electric transformers