Variable-frequency transformer: Difference between revisions

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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.

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, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator. 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.

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 MVA 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’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). 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 [http://www.ettexas.com/projects/vft.asp The Laredo VFT Project].)

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]].

==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 Transmission Rights (FTRs) are a financial instrument used to balance energy congestion and demand costs.

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==References==

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==External links==

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

[[Category:Electric power infrastructure]]