Thyristor switched capacitor

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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), [1] [2] 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.

Capacitor electrical component used to store energy for a short period of time

A capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuit, a capacitor is a component designed to add capacitance to a circuit. The capacitor was originally known as a condenser or condensator. The original name is still widely used in many languages, but not commonly in English.

Thyristor semiconductor device with three or more p-n junctions, having two steady states: off (non-conducting) and on (conducting)

A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed. A three-lead thyristor is designed to control the larger current of the Anode to Cathode path by controlling that current with the smaller current of its other lead, known as its Gate. In contrast, a two-lead thyristor is designed to switch on if the potential difference between its leads is sufficiently large.

Current limiting reactor

In electrical engineering, current limiting reactors can reduce short-circuit currents, which result from plant expansions and power source additions, to levels that can be adequately handled by existing distribution equipment. They can also be used in high voltage electric power transmission grids for a similar purpose. In the control of electric motors, current limiting reactors can be used to restrict starting current or as part of a speed control system.


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 . [3] 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.


A harmonic is any member of the harmonic series. The term is employed in various disciplines, including music, physics, acoustics, electronic power transmission, radio technology, and other fields. It is typically applied to repeating signals, such as sinusoidal waves. A harmonic of such a wave is a wave with a frequency that is a positive integer multiple of the frequency of the original wave, known as the fundamental frequency. The original wave is also called the 1st harmonic, the following harmonics are known as higher harmonics. As all harmonics are periodic at the fundamental frequency, the sum of harmonics is also periodic at that frequency. For example, if the fundamental frequency is 50 Hz, a common AC power supply frequency, the frequencies of the first three higher harmonics are 100 Hz, 150 Hz, 200 Hz and any addition of waves with these frequencies is periodic at 50 Hz.

An nth characteristic mode, for n > 1, will have nodes that are not vibrating. For example, the 3rd characteristic mode will have nodes at L and L, where L is the length of the string. In fact, each nth characteristic mode, for n not a multiple of 3, will not have nodes at these points. These other characteristic modes will be vibrating at the positions L and L. If the player gently touches one of these positions, then these other characteristic modes will be suppressed. The tonal harmonics from these other characteristic modes will then also be suppressed. Consequently, the tonal harmonics from the nth characteristic modes, where n is a multiple of 3, will be made relatively more prominent.

Thyristor Switched Capacitor circuit.png

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:


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 :

The tuned frequency is usually chosen to be in the range 150-250Hz on 60Hz systems or 120-210Hz on 50Hz systems [4] . 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.

Thyristor switched capacitor waveforms 1.png

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.

Thyristor switched capacitor waveforms 2.png

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.

Thyristor switched capacitor waveforms 3.png

Main equipment

A TSC normally comprises three main items of equipment: the main capacitor bank, the thyristor valve and a current-limiting reactor, which is usually air-cored.

Capacitor bank

The largest item of equipment in a TSC, the capacitor bank, is constructed from rack-mounted outdoor capacitor units, each unit typically having a rating in the range 500 – 1000 kilovars (kVAr).

TSC reactor

The function of the TSC reactor is to limit the peak current and rate of rise of current (di/dt) when the TSC turns on at an incorrect time. The reactor is usually an air-cored reactor, similar to that of a TCR, but smaller. The size and cost of the TSC reactor is heavily influenced by the tuning frequency of the TSC, lower frequencies requiring larger reactors.

The TSC reactor is usually located outside, close to the main capacitor bank.

Thyristor valve

The thyristor valve typically consists of 10-30 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.

Typical TSC valve Thyristor switched capacitor valve.png
Typical TSC valve

In addition to the thyristors themselves, each inverse-parallel pair of thyristors has a resistor–capacitor ‘’snubber’ 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.

Resistor Passive electrical component providing electrical resistance

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements, or as sensing devices for heat, light, humidity, force, or chemical activity.

The thyristor valve for a TSC is very similar to that of a TCR, but (for a given AC voltage) generally has between 1.5 and 2 times as many thyristors connected in series because of the need to withstand both the AC voltage and the trapped capacitor voltage after blocking.

The thyristor valve is usually installed in a purpose-built, ventilated building, or a modified shipping container. Cooling for the thyristors and snubber resistors is usually provided by deionised water.

Special types of TSC

Some TSCs have been built with the capacitor and inductor arranged not as a simple tuned LC circuit but rather as a damped filter. This type of arrangement is useful when the power system to which the TSC is connected contains significant levels of background harmonic distortion, or where there is a risk of resonance between the power system and the TSC.

In several “Relocatable SVCs” built for National Grid (Great Britain), [3] three TSCs of unequal size were provided, in each case with the capacitor and inductor arranged as a “C-type” damped filter. In a C-type filter, the capacitor is split into two series-connected sections. A damping resistor is connected across one of the two capacitor sections and the inductor, the tuned frequency of this section being equal to the grid frequency. In this way, damping is provided for harmonic frequencies but the circuit incurs no power loss at grid frequency.

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  1. Song, Y.H., Johns, A.T. Flexible ac transmission systems. IEE. ISBN   0-85296-771-3
  2. Hingorani, N.G. & Gyugyi, L. Understanding FACTS - Concepts and Technology of Flexible AC Transmission Systems. IEEE. ISBN   0-7803-3455-8.
  3. 1 2 [Horwill, C., Young, D.J., Wong, K.T.G. A design for a relocatable tertiary connected SVC. IEE Conference on AC and DC power transmission, London, 1994.
  4. R. Mohan Mathur; Rajiv K. Varma (27 February 2002). Thyristor-Based FACTS Controllers for Electrical Transmission Systems. John Wiley & Sons. pp. 73–75. ISBN   978-0-471-20643-9.