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Avariable-frequency transformer (VFT) is used to transmitelectricity between two (asynchronous or synchronous)alternating currentfrequency domains. The VFT is a relatively recent development. Most asynchronous grid inter-ties usehigh-voltage direct current converters, while synchronous grid inter-ties are connected by lines and "ordinary"transformers, but without the ability to control power flow between the systems, or withphase-shifting transformers with some flow control.
It can be thought of as a very high powersynchro, or arotary converter acting as afrequency changer, which is more efficient than amotor–generator of the same rating.
A variable-frequency transformer is adoubly fed electric machine resembling a vertical shafthydroelectric generator with athree-phase woundrotor, connected byslip rings to one external power circuit. Thestator is connected to the other circuit.[1]
The physical orientation of the shaft affects the phase angle developed across the rotor windings, similar to the behavior of asynchro orinduction regulator. For two networks of the same frequency, adirect-current torque motor mounted on the same shaft can hold the shaft at a fixed position, effecting aphase-shifting transformer. Changing the direction of torque applied to the shaft changes the direction of power flow.
Alternatively, the shaft can be freed to orient naturally. If the two connected networks have different frequencies, the shaft will rotate at the difference in line frequency. It thus acts as arotary converter, syncing two networks of different frequency through a mechanical system locked to the relative frequency.
Unlike power electronics solutions such as back-to-backHVDC, the variable frequency transformer does not demand harmonic filters and reactive power compensation. Limitations of the concept are thecurrent-carrying capacity of the slip rings for the rotor winding.
Five small variable-frequency transformer with a total power rate of 25 MVA were in use atNeuhof Substation, Bad Sachsa, Germany for coupling power grids of former East andWest Germany between 1985 and 1990.
Langlois Substation inQuébec,Canada (45°17′13.76″N74°0′56.07″W / 45.2871556°N 74.0155750°W /45.2871556; -74.0155750) installed a 100 MW variable-frequency transformer in 2004 to connect the asynchronous grids in Québec and thenortheastern United States. This was the first large scale, commercial variable frequency transformer, and was installed atHydro-Québec Langlois substation and is located electrically near sixteen hydro generators atLes Cèdres, Quebec and thirty-six more hydro generators atBeauharnois, Quebec. The operating experience since April 2004 has demonstrated the VFT's inherent compatibility with the nearby generators[2]
AEP Texas installed a 100 MW VFT substation inLaredo, Texas,United States (27°34′13.64″N99°30′34.98″W / 27.5704556°N 99.5097167°W /27.5704556; -99.5097167) in early 2007. It connects the power systems ofERCOT (in the United States) toCFE (in Mexico).[3]
Smaller VFTs are used in large land-basedwind turbines, so that the turbine rotation speed can vary while connected to anelectric power distributiongrid. In this application, they may also be known asDoubly Fed Induction Generators[citation needed]
General Electric installed a 3 × 100 MW VFT substation inLinden, New Jersey, in the United States in 2009. It connects the power systems ofPJM &New York Independent System Operator (NYISO). This installation is in parallel with three existing phase-shifting transformers to regulate synchronous power flow.[4]
VFTs provide the technical feasibility to flow power in both directions between two grids, permittingpower 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 balanceenergy congestion and demand costs.
