A variable-frequency transformer (VFT) is used to transmit

electricity Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (Olivia Newton-John song), "Physi ...

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 with ...

or synchronous)

alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...

frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

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 Electric power transmission is the bulk movement of electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. When used loosely, ...

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 Electrical network, circuits. A varying current in any one coil of the transformer produces a varying magnetic flux ...

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 A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple Electrical network, circ ...

synchro

, or a

rotary converter A rotary converter is a type of electrical machine In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electr ...

acting as a

frequency changer A frequency changer or frequency converter is an Electronics, electronic or electromechanical device that converts alternating current (Alternating current, AC) of one frequency to alternating current of another frequency. The device may also cha ...

, which is more efficient than a

motor–generator A motor–generator (an M–G set) is a device for converting electrical power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. Whe ...

of the same rating.

Construction and operation

A variable-frequency transformer is a

doubly fed electric machineDoubly-fed electric machines also slip-ring generators are electric motors or electric generator In electricity generation, a generator is a device that converts motive power ( mechanical energy) into electrical power for use in an external ci ...

resembling a vertical shaft

hydroelectric Hydroelectricity, or hydroelectric power, is electricity produced from hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water Water (chemical formula H2O) is a ...

generator with a

three-phase Three-phase electric power (abbreviated 3φ) is a common type of alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική ...

wound

rotor Rotor may refer to: Science and technology Engineering *Rotor (electric) The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between t ...

, connected by

slip ring A slip ring is an electromechanical In engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline ...

s to one external power circuit. The

stator The stator is the stationary part of a rotary system, found in electric generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities ...

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 the one directional flow of electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charge: ''posit ...

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 Electric power transmission is the bulk movement of electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used ...

, 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, Bad Sachsa, Germany for coupling power grids of former East and

West Germany West Germany is the common English name for the Federal Republic of Germany (FRG; german: Bundesrepublik Deutschland , BRD) between its formation on 23 May 1949 and the German reunification German reunification (german: Deutsche Wieder ...

between 1985 and 1990. Langlois Substation in

Québec Quebec ( , sometimes ; french: Québec, link=no )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 ...

Québec

Canada Canada is a country in the northern part of North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, ...

Canada

() 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 American Northeast, the Northeast, and the East Coast) is a geographical region In geography, regions are areas that are broadly divided by physical characteristics (physical geography), ...

. This was the first large scale, commercial variable frequency transformer, and was installed at

Hydro-Québec Hydro-Québec is a public utility A public utility company (usually just utility) is an organization that maintains the infrastructure Infrastructure is the set of fundamental facilities and systems that support the sustainable functionalit ...

Langlois substation and is located electrically near sixteen hydro generators at Les Cèdres, Quebec and thirty-six more hydro generators at

Beauharnois, Quebec Beauharnois () is a city located in the Beauharnois-Salaberry Regional County Municipality of southwestern Quebec ) , image_map = Quebec in Canada 2.svg , Label_map = yes , coordinates = , offici ...

. 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 Texas (, ) is a state in the South Central United States, South Central region of the United States. It is the second largest U.S. stat ...

(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 Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a social ...

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 Delivery may refer to: *Delivery (commerce), of goods, e.g.: **Pizza delivery **Milk delivery Film and television *Delivering (film), ''Delivering'' (film), a 1993 short film by Todd ...

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 gri ...

Linden VFT

General Electric General Electric Company (GE) is an American Multinational corporation, multinational Conglomerate (company), conglomerate incorporated in New York State and headquartered in Boston. Until 2021, the company operated through GE Aviation, aviat ...

installed a 3 × 100 MW VFT substation in

Linden, New Jersey Linden is a City (New Jersey), city in southeastern Union County, New Jersey, Union County, New Jersey, United States. It is part of the New York Metropolitan Area, located about southwest of Manhattan and bordering Staten Island, a borough of ...

, 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.

References

External links

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