Editing Static VAR compensator (section)

== 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".