Variable frequency transformer
Encyclopedia
A variable frequency transformer is used to transmit electricity
between two (asynchronous or synchronous) alternating current domains. The VFT is a relatively recent development (first deployed in 2004). Most asynchronous grid inter-ties use high-voltage direct current
converters, while synchronous grid inter-ties are connected by lines & "ordinary" transformers, but without the ability to control power flow between the systems.
Five small variable frequency transformers 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 between 1985 and 1990.
For instance, the Langlois Substation in Québec
, Canada
45°17′13.76"N 74°0′56.07"W has installed a 100 MW variable frequency transformer to connect the asynchronous grids in Québec and the north-eastern United States.
AEP Texas installed a 100 MW VFT substation in Laredo
, Texas, United States
27°34′13.64"N 99°30′34.98"W in early 2007. It connects the power systems of ERCOT (in the United States) to CFE (in Mexico). See The Laredo VFT Project
GE installed a 3 x 100 MW VFT substation in Linden, New Jersey, United States
40°38′0.03"N 74°12′46.04"W in 2009. It connects the power systems of PJM & NYISO (in the United States). This installation is in parallel with three existing phase shifting transformers to regulate synchronous power flow. (See http://www.geenergyfinancialservices.com/LindenVFT_Inauguration.asp The Linden VFT Project])
Smaller VFTs are used in large land-based wind turbine
s, so that the turbine rotation speed can vary while connected to a distribution grid.
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
, 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.
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
between two (asynchronous or synchronous) alternating current domains. The VFT is a relatively recent development (first deployed in 2004). Most asynchronous grid inter-ties use high-voltage direct current
High-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
converters, while synchronous grid inter-ties are connected by lines & "ordinary" transformers, but without the ability to control power flow between the systems.
Five small variable frequency transformers with a total power rate of 25 MVA were in use at Neuhof Substation
Neuhof Substation
Neuhof Substation is a 110 kV substation in Neuhof, an urban part of Bad Sachsa. Neuhof substation went in service in 1985 and was connected with a 110 kV-powerline for three phase alternating current with Wolkramshausen substation in former GDR. It was therefore one of the few substations in...
, Bad Sachsa, Germany for coupling power grids of former East and West Germany between 1985 and 1990.
For instance, the Langlois Substation in Québec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
45°17′13.76"N 74°0′56.07"W has installed a 100 MW variable frequency transformer to connect the asynchronous grids in Québec and the north-eastern United States.
AEP Texas installed a 100 MW VFT substation in Laredo
Laredo, Texas
Laredo is the county seat of Webb County, Texas, United States, located on the north bank of the Rio Grande in South Texas, across from Nuevo Laredo, Tamaulipas, Mexico. According to the 2010 census, the city population was 236,091 making it the 3rd largest on the United States-Mexican border,...
, Texas, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
27°34′13.64"N 99°30′34.98"W in early 2007. It connects the power systems of ERCOT (in the United States) to CFE (in Mexico). See The Laredo VFT Project
GE installed a 3 x 100 MW VFT substation in Linden, New Jersey, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
40°38′0.03"N 74°12′46.04"W in 2009. It connects the power systems of PJM & NYISO (in the United States). This installation is in parallel with three existing phase shifting transformers to regulate synchronous power flow. (See http://www.geenergyfinancialservices.com/LindenVFT_Inauguration.asp The Linden VFT Project])
Smaller VFTs are used in large land-based wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
s, so that the turbine rotation speed can vary while connected to a distribution grid.
Construction and operation
A variable frequency transformer is a doubly fed electric machine resembling a vertical shaft hydroelectric generator with a three-phase wound rotor, connected by slip rings to one external power circuit. 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; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator.The variable frequency transformer behaves as a continuously adjustable phase-shifting transformer
Quadrature booster
A phase-shifting transformer, also quadrature booster , is a specialised form of transformer used to control the flow of real power on three-phase electricity transmission networks....
. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back HVDC
High-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
, 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.