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 describe ...

between two (

asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal wit ...

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 cur ...

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 adv ...

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 elect ...

of the same rating.

Construction and operation

A variable-frequency transformer is a doubly fed electric machine 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 an ...

generator with a

three-phase Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires (or four including an optional neutral ...

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 ring A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure. A slip ring can be used in any electromechanical system that requires rotation while transmitting ...

s 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 m ...

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 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 curre ...

, the variable frequency transformer does not demand harmonic filters and reactive power compensation. Limitations of the concept are the current-carrying capacity 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 Neuhof Substation is a 110 kV substation in Neuhof, an urban part of Bad Sachsa, Lower Saxony. The Neuhof substation went in service in 1985 and was connected with a 110 kV-powerline for three phase alternating current with the Wolkramshausen subs ...

, 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 ...

between 1985 and 1990. Langlois Substation in

Québec Quebec ( ; )According to the 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 one of the thirte ...

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 to ...

() 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 t ...

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 e ...

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 Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

() 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 ...

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 ene ...

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 right Electricity transmission congestion is a condition of the electrical grid that prevents the accepted or forecasted load schedules from being implemented due to the grid configuration and equipment performance limitations. In simple terms, congestion ...

s (FTRs) are a financial instrument used to balance energy congestion and demand costs.

References

External links