Classification Scheme for FACTS Controllers

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    A Technical Paper on

    Classification

    Scheme for FACTS

    Controllers

    Authorised By

    SANTOSH BHARADWAJ REDDY

    Email: [email protected]

    Engineeringpapers.blogspot.com

    More Papers and Presentations available on above site

    mailto:[email protected]:[email protected]
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    ABSTRACT

    There currently exists no formal classification scheme for

    flexible alternating-current transmission system (FACTS)

    controllers based on control parameters and attributes. This

    paper aims to remedy this.

    Recently, classification has been done mostly on physical

    parameters, like connection, commutation, energy storage

    and DC port. But this proposed classification is based on

    control parameters and attributes of the controllers viz.,

    voltage, impedance, transmission angle, real power, reactive

    power and stability.

    In this paper only six FACTS controllers are examined,

    and only these controllers are tabulated and classified.

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    1. INTRODUCTION

    Flexible alternating-current transmission systems (FACTS) are defined by the

    IEEE as "ac transmission systems incorporating power electronics-based and other staticcontrollers to enhance controllability and increase power transfer capability" Similarly, a FACTS

    controller is defined as "a power electronics-based system or other static equipmentthat provides control of one or more ac transmission parameters" .In recent years, many different

    FACTS controllers have been proposed, performing a wide variety of functions.

    The appearance of FACTS controllers designed for the direct control of transmission

    lines is completely changing, the way of transmission systems are controlled and operated.Most FACTS controllers are basically based on variable shunt or series compensation of

    transmission systems.

    The major benefits of utilizing the FACTS controllers are:

    Better utilization of existing system assets.

    Increased transmission reliability and availability.

    Increased dynamic and transient grid stability and reduction of loop flows

    Increased quality of supply for sensitive industries.

    Environmental benefitsThere are several reasons why this situation is unsatisfactory, from both academic and

    practical viewpoints. It would be advantageous to organize the existing FACTS

    controllers into family groups, rather than regarding them as a collection of disparateitems.

    Fig.1 (a) SMIB System

    2. CONTROLLABILITY OF POWER SYSTEMS

    To illustrate that the power system has certain variables that can be impacted by

    control, consider the basic well known power angle curve, shown in fig 1. Although thisis a steady state curve and the implementation of FACTS is primarily for dynamic issues

    this illustration demonstrates the point that there are primarily three main variables that

    can be controlled in the power system to impact its performance. These are voltage,Impedance, Transmission angle. One could also make the point that direct control ofpower is a fourth variable of controllability in power systems.

    Examples of FACTS controllers for enhancing power system control.

    Static Synchronous Compensator (STATCOM)- Controls Voltage.

    Static Var Compensator (SVC) - Controls Impedance.

    Thyristor Control Series Capacitor(TCSC) - Controls Impedance

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    Thyristor Control Phase Shifting Transformer(TCPST)- Controls Transmissionangle.

    Unified Power Flow Controller (UPFC) - Controls voltage.Impedance,

    angle & power.

    Static Synchronous Series Controllers (SSSC) - Controls voltage.Impedance, angle & power.

    Fig. 1(b). Power angle curve

    Further the benefits of FACTS controllers are important to achieve inthe overall planning and operation of power systems. Such benefits canusually be tied back to an area or region at a defined dispatch, whilemeeting the following criteria:

    Voltage stability criteria.

    Dynamic voltage criteria.

    Transient Stability criteria.

    3. PROPERTIES OF IDEAL CLASSIFICATION SCHEME

    An ideal classification scheme should have these properties:it should be simple to apply and understandit should be objective and quantitativeit should be unambiguous and clear, it should be useful to those interested inthe field; it should beextendible, to cope with further advances inthe field of FACTS. which we believe goes a considerable way towards meeting these goals.

    Our proposed classification scheme is multidimensional, in that itclassifies FACTS controllers according control parameters. A drawback withany multi-dimensional classification scheme is that certain combinations ofcharacteristics may be impossible or impractical, resulting in gaps. Forexample, the combination of "high frequency switching" and "naturalcommutation" is inapplicable, because fast SCRs are not currently available.This is a practical limitation, which might change with the introduction of yetunforeseen devices. A positive aspect of these "missing combinations" is that

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    they can stimulate new ideas which might be developed to improve powersystem operation. Therefore, we believe our classification scheme for FACTScontrollers has value not only in organizing existing technology into acoherent body of knowledge, but also in providing a starting point forresearchers who wish to develop new techniques.

    4. PROPOSED CLASSIFICATION SCHEME

    In our proposal, FACTS controllers are classified by consideringdifferent controlParameters and control attributes viz.

    Voltage Impedance Phase angle Real power

    Reactive power Stability

    Voltage controlThe power / current can also be controlled by regulating the

    magnitude of voltage of sending end or receiving end. But theregulation of the magnitude of the voltage of sending end or receivingend is much more influenced over the reactive power flow than theactive power flow. It was observed that varying the amplitude of theinjected voltage in series, both the active and reactive current flow canbe influenced. Voltage injection methods form the most important

    portfolio of voltage controllers. Impedance control

    FACTS controllers modify the series and parallel impedancesof transmission lines. The way a FACTS controller is connected to theac power system has a direct effect on the transfer of active andreactive power within the system. Series connected controllers areusually employed in active power control and to improve the transientstability of power systems. Shunt connected controllers governreactive power and improve the dynamic stability.

    Transmission angle controlControl of transmission angle, which in turn controls the

    driving voltage, provides a powerful means of controlling thecurrent flow and hence active power flow when the angle is notlarge. For relatively small angular adjustments, the resultantangular change is approximately proportional to the injectedvoltage, while the voltage magnitude remains almost constant.

    Real power control

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    In many ways real power can be controlled. Control of lineimpedance X can provide a powerful means of current control which inturn gives active power control. Control of transmission angle , whichin turn controls the driving voltage, provides a powerful means ofcontrolling the current flow and hence active power flow when the

    angle is not large.

    Reactive power controlInjecting voltage in series with the line and with any phase angle

    with respect to the driving voltage can control the magnitude and phase ofthe line current. This means that injecting a voltage phasor with variablephase angle can provide a powerful means of precisely controlling thereactive power(as well as active power) flow. Combination of the lineimpedance control with a series controller and voltage regulation with shunt controller provides a cost effective means to control both the activeand reactive power flow between the two systems.

    StabilityThere are number of stability issues that limit the

    transmission capability. These includes

    Transient stability

    Dynamic stability

    Voltage stability

    The FACTS technology can certainly be used to overcome any of thestability limits, in which case the ultimate limits would be thermal anddielectric. Power transfer in most integrated power systems is

    constrained by dynamic stability, transient stability and voltagestability. These constraints limit a full utilization of availabletransmissionCorridors. As FACTS technology is based on the use of reliable high speed power electronics, advanced control technology, advancedmicrocomputers, and powerful analytical tools and also the availablepower electronic switching devices

    5. CLASSIFICATION EXAMPLES

    In principle, the proposed scheme allows more classifications;in practice the number will be smaller, as certain combinations ofcharacteristics are unlikely or impossible. Here we examine andclassify sixFACTS controllers. These examples have been chosen tocover the main applications and different approaches.

    Static VAR Compensator (SVC)

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    It is a shunt connected static VAR generator or absorber as shown in fig.2, whose outputis adjusted to exchange capacitive or inductive current so as to maintain or control

    specific parameters of the electrical system (typically bus voltage). SVC is based on

    thyristors with out gate turn off capability. They are also employed for transient anddynamic stability improvement. The controllable parameter of the SVC is Impedance. It

    has a single port with a parallel connection to the power system. The thyristors arenaturally commutated. They switch at the main (low) frequency; it contains insignificant

    energy storage elements; the SVC has no dc port.

    Fig. 2: SVC

    Thyristor Controlled Series Capacitor (TCSC)

    It is a capacitive reactance compensator, which consists ofa series capacitor bank shunted by a thyristor - controlledreactor in order to provide a

    smoothly variable capacitive reactance TCSC is based on thyristorswithout gate turn off ability. The variable series capacitivecompensation is useful in steady state control of power flow, transientstability improvement, power oscillations damping and balancingpower flow in parallel lines. The controllable parameter is impedance.The TCSC as shown in fig. 3.

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    Static Synchronous Compensator (STATCOM)

    Static synchronous generator is static self commutated switchingpower converter supplied from an appropriate electric energy sourceand operated to produce a set of adjustable multi phase output. A

    STATCOM is a static Synchronous generator as a shunt connectedstatic VAR.Compensator whose capacitive or inductive output current can becontrolled independent of system voltage. It can be a voltage source orcurrent source inverter. The ability of the STATCOM to produce thecurrent at low system voltage makes it more effective than SVC inimproving transient stability. The ability of the STATCOM to generateand absorb reactive power make it suitable for power oscillationdamping. Its controllable parameter is Voltage. Excluding the dc port,the STATCOM (shown in fig. 4) is a one-port circuit shunted across abus bar; it uses forced commutation; its switching frequency is high; its

    energy storage element is a dc capacitor; and this implies a dc port.

    Fig. 4: STATCOM

    Thyristor Controlled Phase Angle Regulator

    It is a phase shifting transformer, adjusted by Thyristorswitches to provide a rapidly variable phase angle. In general, thephase shifting is obtained by adding a perpendicular voltage vector inseries with a phase. This vector is derived from other twoPhases using shunt connected transformers. The perpendicular series

    voltage is made variable with a variety of power electronics topologies.The TCPAR can be used to regulate the transmission angle to maintainbalanced power flow in multiple transmission paths or to control so asto increase the transient and dynamic stability of the power system.The controllable parameter of TCPAR (shown in fig. 5) is transmission Itis a two port circuit; it uses natural commutation; its switchingfrequency is low; it has angle .no significant energy storage elements;and it has a dc port.

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    Vnc Znc - c -Pc Qnc - 3Sc

    Fig. 5: TCPAR

    Static Series Synchronous Series Compensator ( SSSC )

    SSSC is one of the most important FACTS controllers. It is a staticsynchronous generator operated without an external energy source asa series compensator whose output voltage is in quadrature with theline current. Its purpose is to increase or decrease the overall reactivevoltage drop across the line and thereby control the transmittedpower. Fig. 6 shows the SSSC model. The controllable parameter ofSSSC is voltage (reactive). It is a one port circuit in series with atransmission line;

    it uses forced commutation; its switching frequency is high; its energystorage element is a capacitor; and it has a dc port

    Fig. 6: SSSC

    Unified Power Flow Controller (UPFC)

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    It is a combination of STATCOM and a SSSC which are coupledvia a common dc link, to allow bi directional flow of real powerbetween the series output terminals of the SSSC and the shunt outputterminals of the STATCOM control both the active and reactive power

    flow in the line. It can also provide independently controllable shuntreactive compensation. In other words, the UPFC can providesimultaneous control ofall the basic transmission line parameters concurrently or selectively.The controllable parameters of UPFC (shown in fig. 8) are voltage,impedance and phase angle. It is a two port circuit (in series withtransmission line and parallel with a bus bar); it uses forcedcommutation; its switching frequency high; its energy storageelement is a capacitor; and it has a dc port.

    Vc Zc c - Pc Qc - 3Sc

    Fig. 8: UPFC

    CONCLUSIONAll existing FACTS controllers may be classified according to the

    proposed scheme, which allows more number of possibilities (includingall possible and impossible cases). It is extendible in future by addingfurther choices to the existing categories andby adding new characteristics if these should become relevant. Wehope that this proposal will be adopted as the basis of a universalclassification scheme for FACTS controllers.

    REFERENCES

    1. N.G.Hingorani and Laszlo Gyugui Understanding FACTS, IEEEPress.2. N.G... Hingorani, "Flexible ac transmission", IEEE Spectrum, pp. 40-45, Apr. 1993

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