OVERVIEW OF FACTS DEVICES
1. SVC – Static Var Compensator
Ø A SVC is an electrical device for providing fast acting reactive power on high-voltage electricity transmission networks.
Ø SVCs are part of the FACTS device family and regulating voltage and stabilizing the system.
Ø Unlike a synchronous condenser which is a rotating electrical machine a SVC has no significant moving parts and 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.
Ø The SVC is an automated impedance matching device designed to bring the system closer to unity power factor.
Ø SVCs are used in two main situations:
o Connected to the power system, to regulate the transmission voltage.
o Connected near large industrial loads, to improve power quality.
Ø In transmission applications the SVC is used to regulate the grid voltage.
Ø 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.
Ø Under inductive (lagging)conditions the capacitor banks are automatically switched on thus providing a higher system voltage and by connecting the thyristor-controlled reactor which is continuously variable along with a capacitor bank step and the net result is continuously-variable leading or lagging power.
Ø In industrial applications SVCs are typically placed near high and rapidly varying loads such as arc furnaces where they can smooth flicker voltage.
Typically an SVC comprises one or more banks of fixed or switched shunt capacitors or reactors of which atleast one bank is switched by thyristors.
The 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).
Mechanically switched capacitors or reactors.
Ø Generally SVC is not done at line voltage; a bank of transformers steps the transmission voltage down to a much lower level.
Ø This reduces the size and number of components needed in the SVC although the conductors must be very large to handle high currents associated with the lower voltage.
Ø In some SVC for industrial applications such as electric arc furnaces where there may be an existing medium-voltage bus bar present the SVC may be directly connected in order to save the cost of the transformer.
Ø The dynamic nature of the SVC lies in the use of thyristors connected in series and inverse-parallel forming “thyristor valves” and the disc-shaped semiconductors usually several inches in diameter are usually located indoors in a “valve house”.
Ø Near instantaneous response to changes in the system voltage. 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.
Ø In general, cheaper, higher-capacity, faster and more reliable than dynamic compensation schemes such as synchronous condensers.
2. Thyristor Controlled Series Capacitor (TCSC)
Ø TCSC is a power electronic based system and Thyristor Switched Capacitor is connected in series with a bidirectional thyristor valve.
Ø The TCSC can control power flow, mitigate sub-synchronous resonance, improve transient stability, damp out power system oscillations resulting increase of power transfer capability.
Ø A single diagram of TCSC shows two modules connected in series and there can be one or more module depending on the requirement to reduce the costs and TCSC may be used in conjunction with fixed series capacitors.
Ø Nowadays TCSC is being included in some of the transmission systems and the basic circuit of a TCSC in one of the phase is shown in the fig.controls the current through the reactor.
Ø The forward-looking thyristor has firing angle 900 – 1800 and firing the thyristors at this time results in a current flow through the inductor that is opposite to the capacitor current and in this loop current increases the voltage across the capacitor.
Ø Further the loop current increases as firing angle decreases from 1800.
Ø The different compensation levels are obtained by varying the firing angle of the reactor-circuit-thyristor.
3. UNIFIED POWER FLOW CONTROLLER (UPFC)
Ø The UPFC is the most versatile member of FACTS family using power electronics to control power flow on power grids.
Ø The UPFC uses a combination of a shunt controller (STATCOM) and a series controller (SSSC) interconnected through a common DC bus.
P = (V2V3 sinᵟ)/X and Q = (V2(V2 – V3 cosᵟ))/X
Ø This FACTS topology provides much more flexibility than the SSSC for controlling the line active and reactive power because active power can now be transferred from the shunt converter to the series converter through the DC bus.
4. INTEGRAL POWER FLOW CONTROLLER (IPFC)
Ø In other FACTS controllers there are two or more VSCs coupled together via a common DC bus which increases not only the controllability but also the complexity.
Ø For UPFC the connection between the shunt VSC and series VSC allows active power exchange of the two VSCs so the series VSC can control both the line active and reactive power flow.
Ø The shunt VSC regulates the bus voltage and satisfies the balance of power circulation through the DC capacitor.
Ø For IPFC two series VSCs connect to each other at the DC bus so one of them (assumed as the Master VSC) can control both line active and reactive power and the other one (assumed as Slave VSC) can only regulate line active power supporting sufficient active power to the Master VSC through the DC tie.