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 ...

, 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 ...

,
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, ...

() 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.
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 electr ...
References
External links
Power Transmission Without the Power Electronics
{{Electric transformers
Electric power infrastructure
Electric transformers
{{Commons cat, Transformers
Electromagnetic components
Electric power systems components
Electric power conversion ...