Editing Static VAR compensator

{{Short description|Alternatively Static VAr Generator}}
In [[electrical engineering]], a '''static VAR compensator''' ('''SVC''') is a set of electrical devices for providing fast-acting [[reactive power]] on [[high-voltage]] [[electric power transmission|electricity transmission]] networks.<ref name="PPDI">{{cite book|last=De Kock|first=Jan|author2=Strauss, Cobus |title=Practical Power Distribution for Industry|publisher=[[Elsevier]]|year=2004|pages=74&ndash;75|isbn=978-0-7506-6396-0|url=https://books.google.com/books?id=N8bJpt1wSd4C&pg=PA74}}</ref><ref name="Deb">{{cite book|last=Deb|first=Anjan K.|title=Power Line Ampacity System|publisher=[[CRC Press]]|pages=169&ndash;171|isbn=978-0-8493-1306-6|url=https://books.google.com/books?id=ebZHT8gzpksC&pg=PA169|date=2000-06-29}}</ref> SVCs are part of the [[flexible AC transmission system]] (FACTS)<ref>Song, Y. H., Johns, A. T.  
Flexible AC transmission systems. IEE. {{ISBN|0-85296-771-3}}.</ref><ref>Hingorani, N.G. & Gyugyi, L. Understanding FACTS - Concepts and Technology of Flexible AC Transmission Systems. IEEE. {{ISBN|0-7803-3455-8}}.</ref> 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 condenser]]s or switched capacitor banks.<ref name="Ryan">{{cite book|last=Ryan|first=H.M.|title=High Voltage Engineering and Testing|publisher=IEE|year=2001|pages=160&ndash;161|isbn=978-0-85296-775-1|url=https://books.google.com/books?id=Jg1xA65n56oC&pg=PA160}}</ref>

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:
* Connected to the power system, to regulate the transmission voltage ("transmission SVC")
* Connected near large industrial loads, to improve power quality ("industrial SVC")

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 reactor]]s to consume [[Volt-ampere reactive|VARs]] from the system, lowering the system voltage. Under [[inductance|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 furnace]]s, where they can smooth [[power quality|flicker voltage]].<ref name="PPDI" /><ref>{{cite book|last=Arrillaga|first=J.|author2=Watson, N. R. |title=Power System Harmonics|publisher=Wiley|pages=126|isbn=978-0-470-85129-6|url=https://books.google.com/books?id=1h9aqRj4o8EC&pg=PA126|date=2003-11-21}}</ref>

== Description ==

===Principle===
Typically, an SVC comprises one or more banks of fixed or switched shunt [[capacitor]]s or [[inductor|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)
*[[LC_circuit#Series_circuit|Harmonic filter(s)]]
*Mechanically switched capacitors or reactors (switched by a [[circuit breaker]])

[[File:Static VAR Compensator 2a.png|center|thumb|560px|[[One-line diagram]] of a typical SVC configuration; here employing a [[thyristor-controlled reactor]], a [[thyristor-switched capacitor]], a [[LC_circuit#Series_circuit|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 [[AC power|VAR]] injection (or absorption) to the electrical network.<ref name="Deb" /> In this configuration, coarse [[Potential difference|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.<ref name="Padiyar">{{cite book|last=Padiyar|first=K. R.|title=Analysis of Subsynchronous Resonance in Power Systems|publisher=Springer|year=1998|pages=169&ndash;177 |isbn=978-0-7923-8319-2|url=https://books.google.com/books?id=QMSELoMjsg0C&pg=PA169}}</ref>

[[File:Thyristor Controlled Reactor circuit.png|thumb|Thyristor-controlled reactor (TCR), shown with delta connection]]

[[File:Thyristor Switched Capacitor circuit.png|thumb|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.<ref name="Ryan"/> Chopping reactive load into the circuit in this manner injects undesirable odd-order [[harmonic]]s and so banks of high-power [[Electronic filter|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 [[Feedback|closed-loop]] controller.<ref name="Padiyar" /> Remote [[SCADA|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 [[transformer]]s steps the transmission voltage (for example, 230 kV) down to a much lower level (for example, 9.0 kV).<ref name="Ryan" /> 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 furnace]]s, 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 [[thyristor]]s 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.<ref name="Padiyar" /> 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.<ref name="Padiyar" />  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 [[STATCOM|static synchronous compensator]] (STATCOM) and [[unified power flow controller]] (UPFC).

== References ==
{{Reflist}}

[[Category:Electric power]]
[[Category:Electric power systems components]]
[[Category:Power (physics)]]
<ref>{{cite web |title=Static Var Generator Manual |url=https://www.ytelect.com/uploadfile/downloads/YTPQC-SVG%20Static%20Var%20Generator%20V4005.pdf |website=YT Electric |publisher=YT Electric}}</ref>