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 possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

between two ( asynchronous or synchronous)

alternating current Alternating current (AC) is an electric current that 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 w ...

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

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 uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages betwe ...

converters, while synchronous grid inter-ties are connected by lines and "ordinary"

transformer In electrical engineering, 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 coil of the transformer produces ...

s, but without the ability to control power flow between the systems, or with phase-shifting transformers with some flow control. It can be thought of as a very high power synchro, or a

rotary converter A rotary converter is a type of electrical machine which acts as a mechanical rectifier, Power inverter, inverter or frequency converter. Rotary converters were used to convert alternating current (AC) to direct current (DC), or DC to AC power, ...

acting as a

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

, which is more efficient than a

motor–generator A motor–generator (an MG 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 generation, electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other Renewable energ ...

generator with a

three-phase Three-phase electric power (abbreviated 3ϕ) is a common type of alternating current (AC) used in electricity generation, Electric power transmission, transmission, and Electric power distribution, distribution. It is a type of polyphase system ...

wound

rotor ROTOR was an elaborate air defence radar system built by the British Government in the early 1950s to counter possible attack by Soviet bombers. To get it operational as quickly as possible, it was initially made up primarily of WWII-era syst ...

, connected by

slip ring A slip ring is an electromechanical device that allows the transmission of Electric power, power and electrical Signal, signals from a stationary to a rotating structure. A slip ring can be used in any Electromechanics, electromechanical system ...

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 (such as bacterial flagella or ATP synthase). Energy flows through a stator to or from the rotat ...

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 uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages betwe ...

, 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 was the common English name for the Federal Republic of Germany (FRG) from its formation on 23 May 1949 until German reunification, its reunification with East Germany on 3 October 1990. It is sometimes known as the Bonn Republi ...

between 1985 and 1990. Langlois Substation in Québec,

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

() 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 List of regions of the United States, census regions United States Census Bureau. Located on the East Coast of the United States, ...

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

Hydro-Québec Hydro-Québec () is a Canadian Crown corporations of Canada#Quebec, Crown corporation public utility headquartered in Montreal, Quebec. It manages the electricity generation, generation, electric power transmission, transmission and electricity ...

Langlois substation and is located electrically near sixteen hydro generators at Les Cèdres, Quebec and thirty-six more hydro generators at Beauharnois, Quebec. 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 (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

() in early 2007. It connects the power systems of ERCOT (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 wind power, converts the kinetic energy of wind into electrical energy. , hundreds of thousands of list of most powerful wind turbines, large turbines, in installations known as wind farms, were generating over ...

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 Power delivery, delivery of electricity. Electricity is carried from the Electric power transmission, transmission system to individual consumers. Distribution Electrical substation, substatio ...

grid.

Linden VFT

General Electric General Electric Company (GE) was an American Multinational corporation, multinational Conglomerate (company), conglomerate founded in 1892, incorporated in the New York (state), state of New York and headquartered in Boston. Over the year ...

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, 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 boro ...

, 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