Static VAR compensator

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In Electrical Engineering, a static VAR compensator (SVC) is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks. [1] [2] SVCs are part of the flexible AC transmission system [3] [4] device family, regulating voltage, power factor, harmonics and stabilizing the system. A static VAR compensator has no significant moving parts (other than internal switchgear). Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers or switched capacitor banks. [5]

Contents

The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. SVCs are used in two main situations:

In transmission applications, the SVC is used to regulate the grid voltage. If the power system's reactive load is capacitive (leading), the SVC will use thyristor controlled reactors to consume VARs from the system, lowering the system voltage. Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage. By connecting the thyristor-controlled reactor, which is continuously variable, along with a capacitor bank step, the net result is continuously variable leading or lagging power.

In industrial applications, SVCs are typically placed near high and rapidly varying loads, such as arc furnaces, where they can smooth flicker voltage. [1] [6]

Description

Principle

Typically, an SVC comprises one or more banks of fixed or switched shunt capacitors or reactors, of which at least one bank is switched by thyristors. Elements which may be used to make an SVC typically include:

One-line diagram of a typical SVC configuration; here employing a thyristor-controlled reactor, a thyristor-switched capacitor, a harmonic filter, a mechanically switched capacitor and a mechanically switched reactor Static VAR Compensator 2a.png
One-line diagram of a typical SVC configuration; here employing a thyristor-controlled reactor, a thyristor-switched capacitor, a harmonic filter, a mechanically switched capacitor and a mechanically switched reactor

By means of phase angle modulation switched by the thyristors, the reactor may be variably switched into the circuit and so provide a continuously variable VAR injection (or absorption) to the electrical network. [2] In this configuration, coarse voltage control is provided by the capacitors; the thyristor-controlled reactor is to provide smooth control. Smoother control and more flexibility can be provided with thyristor-controlled capacitor switching. [7]

Thyristor-controlled reactor (TCR), shown with delta connection Thyristor Controlled Reactor circuit.png
Thyristor-controlled reactor (TCR), shown with delta connection
Thyristor-switched capacitor (TSC), shown with delta connection Thyristor Switched Capacitor circuit.png
Thyristor-switched capacitor (TSC), shown with delta connection

The thyristors are electronically controlled. Thyristors, like all semiconductors, generate heat and deionized water is commonly used to cool them. [5] Chopping reactive load into the circuit in this manner injects undesirable odd-order harmonics and so banks of high-power filters are usually provided to smooth the waveform. Since the filters themselves are capacitive, they also export MVARs to the power system.

More complex arrangements are practical where precise voltage regulation is required. Voltage regulation is provided by means of a closed-loop controller. [7] Remote supervisory control and manual adjustment of the voltage set-point are also common.

Connection

Generally, static VAR compensation is not done at line voltage; a bank of transformers steps the transmission voltage (for example, 230 kV) down to a much lower level (for example, 9.0 kV). [5] This reduces the size and number of components needed in the SVC, although the conductors must be very large to handle the high currents associated with the lower voltage. In some static VAR compensators for industrial applications such as electric arc furnaces, where there may be an existing medium-voltage busbar present (for example at 33 kV or 34.5 kV), the static VAR compensator may be directly connected in order to save the cost of the transformer.

Another common connection point for SVC is on the delta tertiary winding of Y-connected auto-transformers used to connect one transmission voltage to another voltage.

The dynamic nature of the SVC lies in the use of thyristors connected in series and inverse-parallel, forming "thyristor valves". The disc-shaped semiconductors, usually several inches in diameter, are usually located indoors in a "valve house".

Advantages

The main advantage of SVCs over simple mechanically switched compensation schemes is their near-instantaneous response to changes in the system voltage. [7] For this reason they are often operated at close to their zero-point in order to maximize the reactive power correction they can rapidly provide when required.

They are, in general, cheaper, higher-capacity, faster and more reliable than dynamic compensation schemes such as synchronous condensers. [7] However, static VAR compensators are more expensive than mechanically switched capacitors, so many system operators use a combination of the two technologies (sometimes in the same installation), using the static VAR compensator to provide support for fast changes and the mechanically switched capacitors to provide steady-state VARs.

See also

Similar devices include the static synchronous compensator (STATCOM) and unified power flow controller (UPFC).

Related Research Articles

In electrical engineering, the power factor of an AC power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit. Real power is the average of the instantaneous product of voltage and current and represents the capacity of the electricity for performing work. Apparent power is the product of root mean square (RMS) current and voltage. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power, so more current flows in the circuit than would be required to transfer real power alone. A power factor magnitude of less than one indicates the voltage and current are not in phase, reducing the average product of the two. A negative power factor occurs when the device generates real power, which then flows back towards the source.

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<span class="mw-page-title-main">Power inverter</span> Device that changes direct current (DC) to alternating current (AC)

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<span class="mw-page-title-main">Static synchronous compensator</span> Regulating device used on transmission networks

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<span class="mw-page-title-main">GK Dürnrohr</span>

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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), 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.

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<span class="mw-page-title-main">Static synchronous series compensator</span>

A static synchronous series compensator (SSSC) is a type of flexible AC transmission system which consists of a solid-state voltage source inverter coupled with a transformer that is connected in series with a transmission line. This device can inject an almost sinusoidal voltage in series with the line. This injected voltage could be considered as an inductive or capacitive reactance, which is connected in series with the transmission line. This feature can provide controllable voltage compensation. In addition, SSSC is able to reverse the power flow by injecting a sufficiently large series reactive compensating voltage.

Voltage control and reactive power management are two facets of an ancillary service that enables reliability of the transmission networks and facilitates the electricity market on these networks. Both aspects of this activity are intertwined, so within this article the term voltage control will be primarily used to designate this essentially single activity, as suggested by Kirby & Hirst (1997). Voltage control does not include reactive power injections within one AC cycle; these are a part of a separate ancillary service, so-called system stability service. The transmission of reactive power is limited by its nature, so the voltage control is provided through pieces of equipment distributed throughout the power grid, unlike the frequency control that is based on maintaining the overall active power balance in the system.

References

  1. 1 2 De Kock, Jan; Strauss, Cobus (2004). Practical Power Distribution for Industry. Elsevier. pp. 74–75. ISBN   978-0-7506-6396-0.
  2. 1 2 Deb, Anjan K. (2000-06-29). Power Line Ampacity System. CRC Press. pp. 169–171. ISBN   978-0-8493-1306-6.
  3. Song, Y. H., Johns, A. T. Flexible AC transmission systems. IEE. ISBN   0-85296-771-3.
  4. Hingorani, N.G. & Gyugyi, L. Understanding FACTS - Concepts and Technology of Flexible AC Transmission Systems. IEEE. ISBN   0-7803-3455-8.
  5. 1 2 3 Ryan, H.M. (2001). High Voltage Engineering and Testing. IEE. pp. 160–161. ISBN   978-0-85296-775-1.
  6. Arrillaga, J.; Watson, N. R. (2003-11-21). Power System Harmonics. Wiley. p. 126. ISBN   978-0-470-85129-6.
  7. 1 2 3 4 Padiyar, K. R. (1998). Analysis of Subsynchronous Resonance in Power Systems. Springer. pp. 169–177. ISBN   978-0-7923-8319-2.

[1]

  1. "Static Var Generator Manual" (PDF). YT Electric. YT Electric.