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
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
connected in series with a bidirectional
thyristor
A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...
valve and, usually, a
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 use ...
(
inductor
An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
). The thyristor switched capacitor is an important component of a
Static VAR Compensator
A static VAR compensator (SVC) is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the flexible AC transmission system device family, regulating voltage, power ...
(SVC),
[Hingorani, N.G. & Gyugyi, L. Understanding FACTS - Concepts and Technology of Flexible AC Transmission Systems. IEEE. .] where it is often used in conjunction with a
thyristor controlled reactor
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 t ...
(TCR). Static VAR compensators are a member of the
Flexible AC transmission system
A flexible alternating current transmission system (FACTS) is a system composed of static equipment used for the alternating current (AC) transmission of electrical energy. It is meant to enhance controllability and increase power transfer capabi ...
(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
harmonic
A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
s and so requires no filtering. For this reason, some SVCs have been built with only TSCs .
[[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.] 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:
Where:
V
svc is the rms value of the line-to-line busbar voltage to which the SVC is connected
C
tsc is the total TSC capacitance per phase
L
tsc is the total TSC inductance per phase
f is the frequency of the AC system
The TSC forms an inductor-capacitor (LC)
resonant circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
with a characteristic frequency of :
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.
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
thyristor
A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...
s 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.
![Thyristor switched capacitor valve](https://upload.wikimedia.org/wikipedia/commons/2/2e/Thyristor_switched_capacitor_valve.png)
In addition to the thyristors themselves, each inverse-parallel pair of thyristors has a
resistor
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 el ...
–capacitor ‘’
snubber
A snubber is a device used to suppress ("snub") a phenomenon such as voltage transients in electrical systems, pressure transients in fluid systems (caused by for example water hammer) or excess force or rapid movement in mechanical systems.
Ele ...
’ 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.
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
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
between the power system and the TSC.
In several “Relocatable SVCs” built for
National Grid (Great Britain)
In the electricity sector in the United Kingdom, the National Grid is the high-voltage electric power transmission network serving Great Britain, connecting power stations and major substations and ensuring that electricity generated anywhere on ...
,
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.
See also
*
Switched capacitor A switched capacitor (SC) is an electronic circuit that implements a function by moving charges into and out of capacitors when electronic switches are opened and closed. Usually, non-overlapping clock signals are used to control the switches, so ...
(SC)
References
{{Reflist
External links
ABB FACTS Alstom Grid FACTS SolutionsAlstom Grid homepage
Siemens Flexible AC Transmission Systems (FACTS) Siemens, Energy Sector homepage
*https://web.archive.org/web/20090614120113/http://www.amsc.com/products/transmissiongrid/static-VAR-compensators-SVC.html
*https://web.archive.org/web/20111008175713/http://www.amsc.com/products/transmissiongrid/reactive-power-AC-transmission.html
Electric power
Electric power systems components