Static VAR compensator

Last updated December 21, 2023

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]

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:

Thyristor-controlled reactor (TCR), where the reactor may be air- or iron-cored
Thyristor-switched capacitor (TSC)
Harmonic filter(s)
Mechanically switched capacitors or reactors (switched by a circuit breaker)

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]

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.

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 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.

A high-voltage direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems.

<span class="mw-page-title-main">Rectifier</span> Electrical device that converts AC to DC

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation is performed by an inverter.

A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.

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 capability of the network. It is generally a power electronics-based system.

An HVDC converter station is a specialised type of substation which forms the terminal equipment for a high-voltage direct current (HVDC) transmission line. It converts direct current to alternating current or the reverse. In addition to the converter, the station usually contains:

In electrical engineering, a synchronous condenser is a DC-excited synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage, or to improve power factor. The condenser’s installation and operation are identical to large electric motors and generators.

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

A static synchronous compensator (STATCOM), is a shunt-connected, reactive compensation device used on transmission networks. It uses power electronics to form a voltage-source converter that can act as either a source or sink of reactive AC power to an electricity network. It is a member of the FACTS family of devices.

An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The electrical grid can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centers to the load centers, and the distribution system that feeds the power to nearby homes and industries.

Dynamic voltage restoration (DVR) is a method of overcoming voltage sags and swells that occur in electrical power distribution. These are a problem because spikes consume power and sags reduce efficiency of some devices. DVR saves energy through voltage injections that can affect the phase and wave-shape of the power being supplied.

McNeill HVDC Back-to-back station is an HVDC back-to-back station at 50°35'56"N 110°1'25"W, which interconnects the power grids of the Canadian provinces Alberta and Saskatchewan and went in service in 1989. McNeill HVDC back-to-back station is the most northerly of a series of HVDC interconnectors between the unsynchronised eastern and western AC systems of the United States and Canada. The station, which was built by GEC-Alstom, can transfer a maximum power of 150 MW at a DC voltage of 42 kV. The station is unusual in many respects and contained several firsts for HVDC.

<span class="mw-page-title-main">Unified power flow controller</span> Electrical device for reactive power compensation on high-voltage electricity transmission networks

A unified power flow controller (UPFC) is an electrical device for providing fast-acting reactive power compensation on high-voltage electricity transmission networks. It uses a pair of three-phase controllable bridges to produce current that is injected into a transmission line using a series transformer. The controller can control active and reactive power flows in a transmission line.

The Levis De-Icer is a High voltage direct current (HVDC) system, aimed at de-icing multiple AC power lines in Quebec, Canada. It is the only HVDC system not used for power transmission.

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.

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.

An HVDC converter converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies. HVDC converters capable of converting up to two gigawatts (GW) and with voltage ratings of up to 900 kilovolts (kV) have been built, and even higher ratings are technically feasible. A complete converter station may contain several such converters in series and/or parallel to achieve total system DC voltage ratings of up to 1,100 kV.

In an electric power transmission grid system, switchyard reactors are large inductors installed at substations to help stabilize the power system.

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

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[PPDI-1] 1 2 De Kock, Jan; Strauss, Cobus (2004). Practical Power Distribution for Industry. Elsevier. pp. 74–75. ISBN 978-0-7506-6396-0.

[Deb-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.

[Ryan-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.

[Padiyar-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.

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