Variable-frequency transformer: Difference between revisions

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Revision as of 10:02, 14 August 2014

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A '''variable-frequency transformer''' ('''VFT''') is used to transmit [[electric power|electricity]] between two ([[asynchronous]] or synchronous) [[alternating current]] [[frequency]] 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 and "ordinary" transformers, but without the ability to control power flow between the systems.

==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 [[Rotor (electric)|rotor]], connected by [[slip ring]]s to one external power circuit. A The [[direct-currentstator]] torque motor is mountedconnected onto the same shaftother. Changing the direction of torque applied to the shaft changes the direction of power flow; withWith no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator. A [[statordirect-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==

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==Economics of energy trading==

VFTs provide the technical feasibility to flow power in both directions between two grids, permitting [[electricity exchange|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.

==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 [[Rotor (electric)|rotor]], connected by [[slip ring]]s 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 [[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.

==See also==