Thyristor-controlled reactor

Key takeaways

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

Reference excerpt

In an electric power transmission system, a thyristor-controlled reactor (TCR) is a reactance connected in series with a bidirectional thyristor valve. The thyristor valve is phase-controlled, which allows the value of delivered reactive power to be adjusted to meet varying system conditions. Thyristor-controlled reactors can be used for limiting voltage rises on lightly loaded transmission lines. Another device which used to be used for this purpose is a magnetically controlled reactor (MCR), a type of magnetic amplifier otherwise known as a transductor. In parallel with series connected reactance and thyristor valve, there may also be a capacitor bank, which may be permanently connected or which may use mechanical or thyristor switching. The combination is called a static VAR compensator.

Circuit diagram A thyristor controlled reactor is usually a three-phase assembly, normally connected in a delta arrangement to provide partial cancellation of harmonics. Often the main TCR reactor is split into two halves, with the thyristor valve connected between the two halves. This protects the vulnerable thyristor valve from damage due to flashovers, lightning strikes etc.

Operating principles The current in the TCR is varied from maximum (determined by the connection voltage and the inductance of the reactor) to almost zero by varying the "Firing Delay Angle", α. α is defined as the delay angle from the point at which the voltage becomes positive to the point at which the thyristor valve is turned on and current starts to flow.

Maximum current is obtained when α is 90°, at which point the TCR is said to be in "full conduction" and the rms current is given by:

I t c r − m a x = V s v c 2 π f L t c r {\displaystyle I_{tcr-max}={V_{svc} \over {2\pi fL_{tcr}}}}

Where: Vsvc is the rms value of the line-to-line busbar voltage to which the SVC is connected Ltcr is the total TCR inductance per phase The current lags 90° behind the voltage in accordance with classical AC circuit theory. As α increases above 90°, up to a maximum of 180°, the current decreases and becomes discontinuous and non-sinusoidal. The TCR current, as a function of time, is then given by:

ω t < π − α : I ( ω t ) = I t c r − m a x 2 [ − c o s ( α ) − c o s ( ω t ) ] {\displaystyle \omega t<{\pi -\alpha }:I(\omega t)=I_{tcr-max}{\sqrt {2}}[-cos(\alpha )-cos(\omega t)]}

α < ω t < 2 π − α : I ( ω t ) = I t c r − m a x 2 [ c o s ( α ) − c o s ( ω t ) ] {\displaystyle \alpha <\omega t<2\pi -\alpha :I(\omega t)=I_{tcr-max}{\sqrt {2}}[cos(\alpha )-cos(\omega t)]}

ω t > π + α : I ( ω t ) = I t c r − m a x 2 [ − c o s ( α ) − c o s ( ω t ) ] {\displaystyle {\omega t>{\pi +\alpha }}:I(\omega t)=I_{tcr-max}{\sqrt {2}}[-cos(\alpha )-cos(\omega t)]}

Otherwise, zero.

Main equipment A TCR comprises two main items of equipment: the reactor itself, which is usually air-cored (although iron-cored reactors are possible) and the thyristor valve. Depending on the system voltage, an intermediate power transformer may be required to step up from the voltage handled by the thyristors to the transmission system voltage.

Thyristor valve The thyristor valve typically consists of 5-20 inverse-parallel-connected pairs of thyristors connected in series. The inverse-parallel connection is needed because most commercially available thyristors can conduct current in only one direction. The series connection is needed because the maximum voltage rating of commercially available thyristors (up to approximately 8.5 kV) is insufficient for the voltage at which the TCR is connected. For some low-voltage applications, it may be possible to avoid the series-connection of thyristors; in such cases the thyristor valve is simply an inverse-parallel connection of two thyristors.

In addition to the thyristors themselves, each inverse-parallel pair of thyristors has a resistor - capacitor circuit connected across it, to force the voltage across the valve to divide uniformly amongst the thyristors and to damp the "commutation overshoot" which occurs when the valve turns off.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Thyristor-controlled reactor

Start with the simplest possible case. Write down what Thyristor-controlled reactor claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Thyristor-controlled reactor 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 Thyristor-controlled reactor 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 Thyristor-controlled reactor

In research: Thyristor-controlled reactor appears in science 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 Thyristor-controlled reactor 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: Thyristor-controlled reactor is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric power, Electric power systems components, so understanding it makes those chapters shorter.
In everyday life: Look for Thyristor-controlled reactor 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.

How to study Thyristor-controlled reactor in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Thyristor-controlled reactor 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 Thyristor-controlled reactor out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Thyristor-controlled reactor in simple terms?

In an electric power transmission system, a thyristor-controlled reactor (TCR) is a reactance connected in series with a bidirectional thyristor valve. The thyristor valve is phase-controlled, which allows the value of delivered reactive power to be adjusted to meet varying system conditions.

Why does Thyristor-controlled reactor matter?

Because it connects several science 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 Thyristor-controlled reactor?

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 Thyristor-controlled reactor.