Variable-frequency transformer: Difference between revisions

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{{Short description|Transmits electricity between two alternating current frequency domains}}

{{refimproveMore citations needed|date=May 2016}}

A '''variable-frequency transformer''' ('''VFT''') is used to transmit [[electric power|electricity]] between two ([[wikt:asynchronous|asynchronous]] or synchronous) [[alternating current]] [[frequency]] domains. The VFT is a relatively recent development. Most asynchronous grid inter-ties use [[high-voltage direct current]] converters, while synchronous grid inter-ties are connected by lines and "ordinary" transformers[[electrical transformer|transformer]]s, but without the ability to control power flow between the systems, or with [[quadrature booster|phase-shifting transformer]]s with some flow control.

It can be thought of as a very high power [[synchro]], or a [[rotary converter]] acting as a [[frequency changer]], which is more efficient than a [[motor–generator]] of the same rating.

==Construction and operation==

A variable-frequency transformer is a [[doubly fed electric machine]] resembling a vertical shaft [[hydroelectric]] generator with a [[three-phase power|three-phase]] wound [[Rotorrotor (electric)|rotor]], connected by [[slip ring]]s to one external power circuit. The [[stator]] is connected to the other. With no applied torque<ref>Fairley, thePeter shaft(November rotates2007). due to"[https://web.archive.org/web/20080220120342/https://www.spectrum.ieee.org/nov07/5714 thePower differencetransmission in frequency betweenwithout the networkspower connectedelectronics]", to the rotor and stator. Ain ''[[direct-currentIEEE Spectrum]]''.</ref> torque motor is mounted on the same shaft; changing the direction of torque applied to the shaft changes the direction of power flow.

The physical orientation of the shaft affects the phase angle developed across the rotor windings, similar to the behavior of a [[synchro]] or [[induction regulator]]. For two networks of the same frequency, a [[direct-current]] torque motor mounted on the same shaft can hold the shaft at a fixed position, effecting a [[quadrature booster|phase-shifting transformer]]. Changing the direction of torque applied to the shaft changes the direction of power flow.

The variable-frequency transformer behaves as a continuously adjustable [[quadrature booster|phase-shifting transformer]]. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back [[high-voltage direct current|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.

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 a [[rotary converter]], syncing two networks of different frequency through a mechanical system locked to the relative frequency.

The variable-frequency transformer behaves as a continuously adjustable [[quadrature booster|phase-shifting transformer]]. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back [[high-voltage direct current|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.

==Projects==

Five small variable-frequency [[electrical transformer|transformer]]s with a total power rate of 25 MWMVA were in use at [[Neuhof Substation]], Bad Sachsa, Germany for coupling power grids of former East and [[West Germany]] between 1985 and 1990.

Langlois Substation in [[Québec]], [[Canada]] ({{Coord|45|17|13.76|N|74|0|56.07|W|}}) installed a 100 MW variable-frequency transformer in 2004 to connect the asynchronous grids in Québec and the [[northeastern United States]]. This was the first large scale, commercial variable frequency transformer, and was installed at [[Hydro-Québec]] Langlois substation and is located electrically near sixteen hydro generators at [[Les Cèdres, Quebec]] and thirty-six more hydro generators at [[Beauharnois,_Quebec Quebec]]. The operating experience since April 2004 has demonstrated the VFT’sVFT's inherent compatibility with the nearby generators<ref> Marchen, P.E, Marczewski, J.J, D'Aquila, R, Hassink, P., Roedel, J.H. and Bodo, R.L. (2014). [https://www.academia.edu/10527124/VFT_A_Smart_Transmission_Technology_That_Is_Compatible_With_the_Existing_and_Future_Grid?auto=download&campaign=weekly_digest VFT – A Smart Transmission Technology That Is Compatible With the Existing and Future Grid]. Power Systems Conference and Exposition, 2009, 1-7. doi:10.1109/PSCE.2009.4840083 Retrieved from https://www.academia.edu/10527124/VFT_A_Smart_Transmission_Technology_That_Is_Compatible_With_the_Existing_and_Future_Grid?auto=download&campaign=weekly_digest </ref>

[[AEP Texas]] installed a 100 MW VFT substation in [[Laredo, Texas|Laredo]], Texas, [[United States]] ({{Coord|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 [[Comisión Federal de Electricidad|CFE]] (in Mexico). (See <ref>[https://web.archive.org/web/20150621212836/http://www.ettexas.com/projects/vft.asp The Laredo VFT Project], a 2015 press release.)</ref>

Smaller VFTs are used in large land-based [[wind turbine]]s, so that the turbine rotation speed can vary while connected to an [[electricalelectric power distribution]] [[electrical grid|grid]].{{cn|date=August 2025}}

[[General Electric]] installed a 3 × 100 MW VFT substation in [[Linden, New Jersey]], in the United States in 2009. It connects the power systems of [[PJM Interconnection|PJM]] & [[New York Independent System Operator]] (NYISO). This installation is in parallel with three existing phase-shifting transformers to regulate synchronous power flow.<ref>{{cite web |url=http://geenergyfinancialservices.com/press_releases/view/140 |title=GE to Offer Additional Electric Transmission Capacity from its Smart Grid Transformers at New Jersey Power Plant |author=<!--Staff writer(s); no by-line.--> |date=11 Dec 2012 |website=GE Energy Financial Services Press Release |accessdateaccess-date=7 Apr 2014}}</ref>

==Linden VFT==

[[General Electric]] installed a 3 × 100 MW VFT substation in [[Linden, New Jersey]], in the United States in 2009. It connects the power systems of [[PJM Interconnection|PJM]] & New York Independent System Operator (NYISO). This installation is in parallel with three existing phase-shifting transformers to regulate synchronous power flow.<ref>{{cite web |url=http://geenergyfinancialservices.com/press_releases/view/140 |title=GE to Offer Additional Electric Transmission Capacity from its Smart Grid Transformers at New Jersey Power Plant |author=<!--Staff writer(s); no by-line.--> |date=11 Dec 2012 |website=GE Energy Financial Services Press Release |accessdate=7 Apr 2014}}</ref>

==Economics of energy trading==

VFTs provide the technical feasibility to flow power in both directions between two grids, permitting [[electricity exchangemarket|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 Transmissiontransmission Rightsright]]s (FTRs) are a financial instrument used to balance [[energy congestion]] and demand costs.

==See also==

*[[HVDC]]

*[[Induction regulator]]

*[[Quadrature booster]]

==References==

{{reflistReflist}}

{{Electric transformers}}

==External links==

* [http://www.spectrum.ieee.org/nov07/5714 Power Transmission Without the Power Electronics]

[[Category:Electric power infrastructure]]

[[Category:TransformersFrequency (electrical)changers]]

[[Category:Electric transformers]]