Thyristor-switched capacitor

Key takeaways

Thyristor-switched capacitor 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-switched capacitor to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Thyristor-switched capacitor from memory before moving on to harder problems.

Reference excerpt

A thyristor-switched capacitor (TSC) is a type of equipment used for compensating reactive power in electrical power systems. It consists of a power capacitor connected in series with a bidirectional thyristor valve and, usually, a current limiting reactor (inductor). The thyristor switched capacitor is an important component of a Static VAR Compensator (SVC), where it is often used in conjunction with a thyristor controlled reactor (TCR). Static VAR compensators are a member of the Flexible AC transmission system (FACTS) family.

Circuit diagram A TSC is usually a three-phase assembly, connected either in a delta or a star arrangement. Unlike the TCR, a TSC generates no harmonics and so requires no filtering. For this reason, some SVCs have been built with only TSCs . This can lead to a relatively cost-effective solution where the SVC only requires capacitive reactive power, although a disadvantage is that the reactive power output can only be varied in steps. Continuously variable reactive power output is only possible where the SVC contains a TCR or another variable element such as a STATCOM.

Operating principles Unlike the TCR, the TSC is only ever operated fully on or fully off. An attempt to operate a TSC in ‘’phase control’’ would result in the generation of very large amplitude resonant currents, leading to overheating of the capacitor bank and thyristor valve, and harmonic distortion in the AC system to which the SVC is connected.

Steady state current When the TSC is on, or ‘’deblocked’’, the current leads the voltage 90° (as with any capacitor). The rms current is given by:

I t s c = V s v c X t s c {\displaystyle I_{tsc}={V_{svc} \over {X_{tsc}}}}

Where:

X t s c = 1 2 π f C t s c − 2 π f L t s c {\displaystyle X_{tsc}={{1 \over {2\pi fC_{tsc}}}-2\pi fL_{tsc}}}

Vsvc is the rms value of the line-to-line busbar voltage to which the SVC is connected Ctsc is the total TSC capacitance per phase Ltsc is the total TSC inductance per phase f is the frequency of the AC system The TSC forms an inductor-capacitor (LC) resonant circuit with a characteristic frequency of :

f t s c = 1 2 π C t s c L t s c {\displaystyle f_{tsc}={1 \over {2\pi {\sqrt {C_{tsc}L_{tsc}}}}}}

The tuned frequency is usually chosen to be in the range 150-250 Hz on 60 Hz systems or 120-210 Hz on 50 Hz systems. It is an economic choice between the size of the TSC reactor (which increases with decreasing frequency) and the need to protect the thyristor valve from excessive oscillatory currents when the TSC is turned on at an incorrect point of wave (‘’misfiring’’). The TSC is usually tuned to a non-integer harmonic of the mains frequency so as to avoid the risk of the TSC being overloaded by harmonic currents flowing into it from the AC system.

Off-state voltage When the TSC is switched off, or ‘’blocked’’, no current flows and the voltage is supported by the thyristor valve. After the TSC has been switched off for a long time (hours) the capacitor will be fully discharged, and the thyristor valve will experience only the AC voltage of the SVC busbar. However, when the TSC turns off, it does so at zero current, corresponding to peak capacitor voltage. The capacitor only discharges very slowly, so the voltage experienced by the thyristor valve will reach a peak of more than twice the peak AC voltage, about half a cycle after blocking. The thyristor valve needs to contain enough thyristors in series to withstand this voltage safely.

Deblocking – normal conditions When the TSC is turned on ("deblocked") again, care must be taken to choose the correct instant in order to avoid creating very large oscillatory currents. Since the TSC is a resonant circuit, any sudden shock excitation will produce a high-frequency ringing effect which could damage the thyristor valve. The optimum time to turn on a TSC is when the capacitor is still charged to its normal peak value and the turn-on command is sent at the minimum of valve voltage. If the TSC is deblocked at this point, the transition back into the conducting state will be smooth.

Deblocking – abnormal conditions Sometimes, however, the TSC may turn on at an incorrect instant (as a result of a control or measurement fault), or the capacitor may become charged to a voltage above the normal value so that even at the minimum of valve voltage, a large transient current results. The current in the TSC will then consist of a fundamental-frequency component (50 Hz or 60 Hz) superimposed on a much larger current at the tuned frequency of the TSC. This transient current can take hundreds of milliseconds to die away, during which time the cumulative heating in the thyristors may be excessive.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Thyristor-switched capacitor

Start with the simplest possible case. Write down what Thyristor-switched capacitor 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-switched capacitor 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-switched capacitor 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-switched capacitor

In research: Thyristor-switched capacitor 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-switched capacitor 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-switched capacitor 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-switched capacitor 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-switched capacitor in 20 minutes

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

Frequently asked questions

What is Thyristor-switched capacitor in simple terms?

A thyristor-switched capacitor (TSC) is a type of equipment used for compensating reactive power in electrical power systems. It consists of a power capacitor connected in series with a bidirectional thyristor valve and, usually, a current limiting reactor (inductor).

Why does Thyristor-switched capacitor 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-switched capacitor?

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-switched capacitor.