FACTS

FACTS Technology

INTRODUCTION:

 The concepts of Flexible AC Transmission Systems ( FACTS) were introduced in 1988. The basic human need for safe, efficient and affordable “LIGHT” became the driving force which led to the development of today’s continent spanning “ Electric Energy” or “ Power System ”. This reactive power has been recognized as a significant factor in the design and operation of ac system s for a long time. In an Ideal ac power system the voltage and frequency at every supply point would be constant and free from harmonics and the power factor would be unity irrespective of consumers load. But when we go for a Non-Ideal ac power system which is due to varying loads, the terminal voltage tends to be varying and it requires the practice of generating “reactive power” as close as possible to load, which requires it rather than supplying it from a remote power station.

What is most interesting for transmission planners is that FACTS technology

opens up new opportunities for controlling power and enhancing the usable capacity

of present, as well as new and upgraded, lines. The possibility that current through a

line can be controlled at a reasonable cost enables a large potential of increasing the

capacity of existing lines with larger conductors, and use of one of the FACTS Controllers

to enable corresponding power to flow through such lines under normal and

contingency conditions.

These opportunities arise through the ability of FACTS Controllers to control

the interrelated parameters that govern the operation of transmission systems including

series impedance, shunt impedance, current, voltage, phase angle, and the damping

of oscillations at various frequencies below the rated frequency. These constraints

cannot be overcome, while maintaining the required system reliability, by mechanical

means without lowering the useable transmission capacity. By providing added flexibility,

FACTS Controllers can enable a line to carry power closer to its thermal rating.

Mechanical switching needs to be supplemented by rapid-response power electronics.

WHAT IS FACTS ? Flexible alternating current transmission systems (FACTS) technology is a collection of power electronics controllers which can be applied individually or in coordination with others to control one or more interrelated system parameters , such as voltage, current , series Impedance, shunt impedance etc., that govern the operation of transmission systems. FACTS involve conversion and/or switching power electronics in the range of few tens to a few hundred megawatts. I n short we can say FACTS –a new technology based on power electronics offering an opportunity to enhance the controllability, stability and power transfer capability of ac transmission system. Considering the opportunities in power electronics through reduction in costs, size and losses we are in early stage of power electronic revolution and there is a bright future ahead for those who are involved.

OBJECTIVES:

FACTS devices are used for the dynamic control of voltage, impedance and phase angle of high voltage ac lines. The FACTS controllers enable the routing of power in the steady state in any desired manner independent of the impedance of various other paths. It has also other potential advantages such as: Ability to damp oscillations, Improve system stability and enhances power transfer capacity of transmission network. Thus , FACTS devices provide strategic benefits for improved transmission system management through better utilization of existing transmission assets ;. The main objectives of FACTS is To increase the power transfer capability of transmission systems. To keep power flow over designated routes.

The objective is to make the best use of

the transmission asset, and to maximize the loading capability (taking into account

contingency conditions), what limits the loading capability? Basically, there are three kinds of limitations:

• Thermal

• Dielectric

• Stability

Thermal Thermal capability of an overhead line is a function of the ambient

temperature, wind conditions, condition of the conductor, and ground clearance. It

varies perhaps by a factor of 2 to 1 due to the variable environment and the loading

history. The nominal rating of a line is generally decided on a conservative basis,

envisioning a statistically worst ambient environment case scenario. Yet this scenario occurs but rarely which means that in reality, most of the time, there is a lot more

real time capacity than assumed.

Sometimes, the ambient conditions can actually be worse than assumed,

and having the means to determine actual rating of the line could be useful. The FACTS technology can help in

making an effective use of this thermal capacity.

Dielectric From an insulation point of view, many lines are designed very conservatively.

For a given nominal voltage rating, it is often possible to increase normal

operation by +10% voltage (i.e., 500 kV-550 kV) or even higher using different dielectrics. Care is then needed

to ensure that dynamic and transient overvoltages are within limits.The FACTS technology could be used to ensure acceptable over-voltage and power flow conditions.

Stability There are a number of stability issues that limit the transmission capability.

These include:

• Transient stability

• Dynamic stability

• Steady-state stability

• Frequency collapse

• Voltage collapse

• Subsynchronous resonance

The FACTS technology can certainly be used to overcome any

of the stability limits, in which case the ultimate limits would be thermal and dielectric.

BASIC TYPES OF FACTS CONTROLLERS:

·         Series controllers

·         Shunt controllers

·         Combined or unified series-series controllers

·         unified shunt-series controllers.

Series controllers: The series controllers could be variable impedance, such as capacitor, reactor, etc., power electronics based variable source of main frequency, sub synchronous and harmonic frequencies to serve the desired need . In principle, all series controllers inject voltage in series with the line . As long as the voltage is in phase quadrature with the line current, the series controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well.

TYPES: Static Synchronous Series capacitor(SSSC), Thyristor-Controlled Series capacitor, Thyristor-Switched Series capacitor

Shunt controllers: The shunt controllers may be variable impedance, variable source, or a combination of these. In principle, all shunt controllers inject current into the system at the point of connection. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes variable reactive power as shown in figure 1(c). It is important to appreciate that the series-connected Controller impacts the

driving voltage and hence the current and power flow directly. Therefore, if the purpose

of the application is to control the current/power flow and damp oscillations, the

series Controller for a given MVA size is several times more powerful than the

shunt Controller.

As mentioned, the shunt Controller, on the other hand, is like a current

source, which draws from or injects current into the line. The shunt Controller is

therefore a good way to control voltage at and around the point of connection

through injection of reactive current (leading or lagging), alone or a combination

of active and reactive current for a more effective voltage control and damping

of voltage oscillations.

Combined series-series Controllers: [Figure 1 (d)] This could be a combination

of separate series controllers, which are controlled in a coordinated manner, in a

multiline transmission system. Or it could be a unified Controller, Figure 1.4(d), in

which series Controllers provide independent series reactive compensation for each

line but also transfer real power among the lines via the power link. The real power

transfer capability of the unified series-series Controller, referred to as Interline Power

Flow Controller (IPFC), makes it possible to balance both the real and reactive power flow

in the lines and thereby maximize the utilization of the transmission system. Note that

the term "unified" here means that the dc terminals of all Controller converters are

all connected together for real power transfer.

Combined series-shunt Controllers: (Figure 1(e))

A Unified Power Flow Controller (UPFC) with series and shunt elements .In principle, combined shunt and series Controllers

inject current into the system with the shunt part of the Controller and voltage in

series in the line with the series part of the Controller. However, when the shunt and

series Controllers are unified, there can be a real power exchange between the series

and shunt Controllers via the power link.

                                        

Any of the converter-based, series, shunt, or combined shunt-series Controllers can generally

accommodate storage, such as capacitors, batteries, and superconducting magnets,

which bring an added dimension to FACTS technology [Figures 1(f), (g), and h)].

A Controller with storage is much more effective for controlling the system dynamics than the

corresponding Controller without the storage. This has to do with dynamic pumping

of real power in or out of the system

BRIEF DESCRIPTION AND DEFINITIONS OF

FACTS CONTROLLERS

Before going into a very brief description of a variety of specific FACTS Controllers,

it is worth mentioning here that for the converter-based Controllers there are

two principal types of converters with gate turn-off devices. These are the so-called

voltage-sourced converters and the current-sourced converters. As shown in

Figure 2(a), the voltage-sourced converter (VSC) is represented in symbolic

form by a box with a gate turn-off device paralleled by a reverse diode, and a dc

capacitor as its voltage source. As shown in Figure 2(b), the

current-sourced converter is represented by a box with a gate turn-off device with a

diode in series, and a dc reactor as its current source. It would

suffice to say for now that for the voltage-sourced converter, unidirectional dc voltage

of a dc capacitor is presented to the ac side as ac voltage through sequential switching

of devices. Through appropriate converter topology, it is possible to vary the ac output

voltage in magnitude and also in any phase relationship to the ac system voltage. The

power reversal involves reversal of current, not the voltage. When the storage capacity

of the dc capacitor is small, and there is no other power source connected to it, the

converter cannot supply or absorb real power for much more than a cycle. The ac

output voltage is maintained at 90 degrees with reference to the ac current, leading

or lagging, and the converter is used to absorb or supply reactive power only.

For the current-sourced converter, the dc current is presented to the ac side

through the sequential switching of devices, as ac current, variable in amplitude and

also in any phase relationship to the ac system voltage. The power reversal involves

reversal of voltage and not current. The current-sourced converter is represented

symbolically by a box with a power device, and a dc inductor as its current source.

From overall cost point of view, the voltage-sourced converters are

preferred, and will be the basis for presentations of most converter-based FACTS Controllers.

Static Synchronous Compensator (STATCOM): (SHUNT CONTROLLER):

A Static synchronous generator operated as a shunt-connected static var compensator whose capacitive or inductive output current can be controlled independent of the ac system voltage. Also known as a "static synchronous condenser" ("STATCON"), is a regulating device used on alternating current electricity transmission networks. It is based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC power to an electricity network. If connected to a source of power it can also provide active AC power.

Usually a STATCOM is installed to support electricity networks that have a poor power factor and often poor voltage regulation. There are however, other uses, the most common use is for voltage stability.

Figure 2(a) and (b) shows  simple one-line diagram

of STATCOM based on a voltage-sourced converter and a current-sourced converter.

                               

General Performance:  It is essential to balance the supply and demand of active and reactive power in an electric power system. If the balance is lost, system voltage and frequency excursions may occur , resulting in worst case, in the collapse of the power system. Appropriate voltage and reactive power control is one of the most important factor of the stable power system operation. STATCOM is one of the advanced power electronic system which provides fast and continuous capacitive and inductive reative power supply to the power system.

GCT is Gate commutated Thyristor or GTO.

When inverter o/p voltage Vc is higher than the system line voltage Vs, then STATCON acts as a capacitor and reactive VARS are generated. Otherwise, It acts as inductor and absorbs reactive VARS from the system as explained.

 Can be designed to be an Active Filter to absorb System harmonics.

Static Synchronous Generator (SSG): A chemical-based energy storage system using

shunt connected, voltage-source converters capable of rapidly adjusting the amount of

energy which is supplied to or absorbed from an ac system

SSG is a combination of STATCOM and any energy source to supply

or absorb power. The term, SSG, generalizes connecting any source of energy including

a battery, freewheel, superconducting magnet, large dc storage capacitor, another rectifier/inverter, etc. An electronic interface known as a "chopper" is generally

needed between the energy source and the converter. For a voltage-sourced converter,

the energy source serves to appropriately compensate the capacitor charge through

the electronic interface and maintain the required capacitor voltage as shown below.

 

Static Var Generator or Absorber (SVG): A static electrical device, equipment, or system

that is capable of drawing controlled capacitive and/or inductive current from an electrical

power system and thereby generating or absorbing reactive power. Generally considered

to consist of shunt-connected, thyristor-controlled reactor(s) and/or thyristor-switched capacitors.

Series Connected Controllers: Static Synchronous Series Compensator (SSSC)

SSSC is one the most important FACTS Controllers. It is like a STATCOM but STATic COMpensator (STATCOM) uses a VSC (Voltage Source Converter) interfaced in shunt to a transmission line and Static Synchronous Series Compensator (SSSC) uses a VSC interfaced in series to a transmission line as shown.

    

Fig. 3 (a) SSSC                                           (b) SSSC with storage

Combined Shunt and Series Connected Controllers

Unified Power Flow Controller (UPFC)

A combination of static synchronous compensator

(STATCOM) and a static series compensator (SSSC) which are coupled via a common

dc link, to allow bidirectional flow of real power between the series output terminals of the

SSSC and the shunt output terminals of the STATCOM. It is able

to control, concurrently or selectively, the transmission line voltage, impedance, and angle

or, alternatively, the real and reactive power flow in the line.

the active power for the series unit (SSSC) is obtained from the

line itself via the shunt unit STATCOM; the latter is also used for voltage control

with control of its reactive power. That is, the DC terminals of the two underlying VSCs are now coupled, and this creates a path for active power exchange between the converters. Hence, the active power supplied to the line by the series converter, can now be supplied by the shunt converter This is a complete Controller for controlling active

and reactive power control through the line, as well as line voltage control.

Tthe only downside of this topology is that it is entirely converter based, i.e., it uses the converters to supply both active and reactive power. For efficient operation transmission systems need distributed reactive power support. This is commonly accomplished by installing banks of capacitance at strategic locations within the system, and by switching these banks in and out as needed. The UPFC can make limited use of such hardware; by definition it uses the shunt converter to supply the active power coupled by the series converter, and once the shunt converter is in place it is also used to supply all of the needed reactive power.