engineering

Variable-frequency transformer

Variable-frequency transformer is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Variable-frequency transformer rather than just read about it. In short: A variable-frequency transformer (VFT) is used to transmit electricity between two (asynchronous or synchronous) alternating current frequency domains. The VFT is a relatively recent development.

Key takeaways

Variable-frequency transformer belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Variable-frequency transformer to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Variable-frequency transformer from memory before moving on to harder problems.

Reference excerpt

A variable-frequency transformer (VFT) is used to transmit electricity between two (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, but without the ability to control power flow between the systems, or with phase-shifting transformers 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 wound rotor, connected by slip rings to one external power circuit. The stator is connected to the other circuit. 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 phase-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 a rotary converter, syncing two networks of different frequency through a mechanical system locked to the relative frequency. 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.

Projects Five small variable-frequency transformer 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 (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. The operating experience since April 2004 has demonstrated the VFT's inherent compatibility with the nearby generators 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). Smaller VFTs are used in large land-based wind turbines, so that the turbine rotation speed can vary while connected to an electric power distribution grid. In this application, they may also be known as Doubly Fed Induction Generators 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 & New York Independent System Operator (NYISO). This installation is in parallel with three existing phase-shifting transformers to regulate synchronous power flow.

Economics of energy trading VFTs provide the technical feasibility to flow power in both directions between two grids, permitting 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.

See also HVDC Induction regulator Quadrature booster Doubly fed electric machine

References

Worked examples

Example 1 — a first encounter with Variable-frequency transformer

Start with the simplest possible case. Write down what Variable-frequency transformer claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Variable-frequency transformer before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Variable-frequency transformer ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Variable-frequency transformer

In research: Variable-frequency transformer appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Variable-frequency transformer in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Variable-frequency transformer is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric power infrastructure, Electric transformers, Frequency changers, so understanding it makes those chapters shorter.
In everyday life: Look for Variable-frequency transformer outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Variable-frequency transformer in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Variable-frequency transformer means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Variable-frequency transformer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Variable-frequency transformer in simple terms?

A variable-frequency transformer (VFT) is used to transmit electricity between two (asynchronous or synchronous) alternating current frequency domains. The VFT is a relatively recent development.

Why does Variable-frequency transformer matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Variable-frequency transformer?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Variable-frequency transformer.