facts intro

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CHAPTER 1

INTRODUCTION

The Ability to control power flow in an electric power system without generation

rescheduling or topology changes can improve the power system performance using controllable

components, the line flows can be changed in such a way that thermal limits are not exceed,

losses are minimized, stability margins are increased and contractual requirements are fulfilled

without violating the economic generation dispatch. Flexible AC Transmission systems

(FACTS) technology is the ultimate tool for getting the most out of existing equipment via faster

control action and new capabilities. The most striking feature is the ability to directly control

transmission line flows by structurally changing parameters of the grid and to implement high-

gain type controllers based on fast switching. The application of FACTS devices to powersystem security has been an attractive ongoing area of research. In most of the reported studies,

attention has been focused on the ability of these devices to improve the power system security

by damping system oscillations and minimal attempts have been made to investigate the effect of

these devices on power system reliability. The opportunities arise through the ability of FACTS

controllers to control the interrelated parameters that governs the operation of transmission

systems including series impedance and shunt impedance, current, phase angle and damping of

oscillations at various frequencies below the rated frequency. These constraints cannot be

overcome otherwise, while maintaining the required system stability, by mechanical means

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

controller can enable a line to carry power closer to its thermal rating. Mechanical switching

needs to be supplemented by rapid-response power electronics. The facts technology can

certainly be used to overcome any to the stability limits, in which case the ultimate limits would

be thermal and dielectric.

Static VAR controllers control only one of three important parameters (voltage,

impedance, phase angle) determining the power flow in the AC power system viz. the amplitude

of voltage at selected terminals of transmission line . It has long been realized that an all solid-

state or advanced, static VAR compensator, which is true equivalent of ideal synchronous

condenser, is technically feasible with the use of Gate Turn-Off (GTO) Thyristor. The UPFC is


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recently introduced FACTS controller which has the capability to control all the four

transmission parameters. The UPFC not only performs the functions of STATCOM, TCSC, and

the phase angle regulator but also provides additional flexibility by combining some of the

functions of these controllers.

FACTS DEVICES

1.1 Introduction to FACTS Devices:

Most of the worlds electric supply systems are widely interconnected. This is done for

economic reasons, to reduce the cost of electricity and to improve its reliability, it must however

be kept in mind that these inter connections are very complex and they emerged gradually based

upon the requirements of various power utilities. These interconnections apart from delivering

the power pool power plants and load centers in order to pool power generation and reduce fuel

cost. Thus they reduce the overall number of generating sources, but as the saying goes a coin

has two sides, like wise as the power transfer grows. The power system becomes increasingly

complex to operate and system can become less secure for riding through major outages. It may

lead to large power flows with inadequate control, excessive reactive power, and large dynamic

swings between different parts of the system. Thus the full potential of a transmission

connection cannot be utilized. It is very difficult to control such transmission of power in suchsystems. Most of the controllers designed in the past were mechanical in nature. But mechanical

controllers have numerous intrinsic problems. Many power electronics controllers have been

designed to supplement the potentially faulty mechanical controllers. These power electronic

controllers are all grouped in a category called flexible AC transmission controller or FACTS

controllers.

Facts technology opens up new opportunities for controlling power and enhancing the

usable capacities of present, as well as new and upgraded lines, the possibility that current

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

capacity of existing lines with large conductors. Also, the use of one of the FACTS controllers to

enables corresponding power flow through such lines under normal and contingency conditions.

These opportunities arise through the ability of FACTS controllers to control the interrelated


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parameters that govern the operation of transmission system. Series Impedance, Shunt

Impedance, Current, Voltage, Phase angle etc., are some of the interrelated parameters that are

controlled. These constrains cannot be overcome while maintaining the system reliability by

mechanical means without lowering the usable transmission capacity. By providing added

flexibility FACTS controllers can enable a line to carry power closer to it thermal rating. It must

however be emphasized that FACTS is an enabling technology, and not a one to one substitute.

The FACTS technology is not a single high power controller but rather a collection of

controllers, which can be applied individually or in co-ordination with others to control one or

more of the interrelated system parameters mentioned above. A Well-chosen FACTS controller

can overcome specific limitations of designated transmission line on a corridor. But all FACTS

controller represent applications of some basic technology, their production can eventually take

advantage of technologies of scale. Just as the transistor is the basic element for whole variety of

microelectronic chips and circuits, the thyristor or high power transistor is the basic element for a

variety of high power electronic controllers. FACTS technology also lends itself to extending

transmission limits in a step-by-step manner with an incrementing investment as and when

required. A planner could force a progressive scenario of mechanical switching means and

enabling FACTS controllers such that the transmission lines will involve a combination of

mechanical and FACTS controller to achieve the objective in an appropriate, stage investment

scenario. It is also worth nothing that in implementation of the FACTS technology, we are

dealing with base technology, proven through HVDC and high power industrial drives.

Nevertheless, as power semiconductor devices continue to improve, particularly the devices with

turn off capability cost of FACTS controller tend to decrease.

FIRST GENERATION OF FACTS

Static VAR Compensator (SVC)


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It is known that the SVCs with an auxiliary injection of a suitable signal can considerably

improve the dynamic stability performance of a power system]. In the literature, SVCs have been

applied successfully to improve the transient stability of a synchronous machine .Hammad

presented a fundamental analysis of the application of SVC for enhancing the power systems

stability. Then, the low frequency oscillation damping enhancement via SVC has been analyzed

.It is shown that the SVC enhances the system damping of local as well as inter area oscillation

modes.

Thyristor-Controlled Series Capacitor (TCSC)

Many different techniques have been reported in the literature pertaining to investigating the

effect of TCSC on power system stability .Several approaches based on modern control theory

have been applied to TCSC controller design.

A fuzzy logic controller for a TCSC was proposed in .The impedance of the TCSC was adjusted

based on machine rotor angle and the magnitude of the speed deviation. In addition, different

control schemes for a TCSC were proposed such as variable structure controlle ,bilinear

generalized predictive controller ,andH-based controller. The neural networks have been

proposed for TCSC-based stabilizer design. The parameters of the stabilizers are determined by

genetic algorithm (GA) technique. Lee and Moon presented a hybrid linearization method in

which the algebraic and the numerical linearization technique were combined.


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Thyristor-Controlled Phase Shifter (TCPS)

A considerable attention has been directed to realization of various TCPS schemes .However, a

relatively little work in TCPS control aspects has been reported in the literature. Baker developed

a control

algorithm for TCPS using stochastic optimal control theory.

In real-life power system with a large number of generators, the rotor angle of a single generator

measured with respect to the system reference will not be very meaningful. Tan and Wang

proposed a direct feedback linearization technique to linearize and decouple the power system

model to design the excitation and TCPS controllers.

SECOND GENERATION OF FACTS

Static Compensator (STATCOM)

The emergence of FACTS devices and in particular GTO thyristor-based STATCOM has

enabled such technology to be proposed as serious competitive alternatives to conventional SVC.

From the power system

dynamic stability viewpoint, the STATCOM provides better damping characteristics than the

SVC as it is able to transiently exchange active power with the system.

The effectiveness of the STATCOM to control the power system voltage was presented in

.However, the effectiveness of the STATCOM to enhance the angle stability has not been

addressed. Abido presented a

singular value decomposition (SVD) based approach to assess and measure the controllability of

the poorly damped electromechanical modes by STATCOM different control channels. It was


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also concluded that the STATCOM-based damping stabilizers extend the critical clearing time

and enhance greatly the power system transient stability. Haque demonstrated by the use of

energy function the capability of the STATCOM to provide additional damping to the low

frequency oscillations.

The STATCOM damping characteristics have been also analyzed and addressed where different

approaches to STATCOM-based damping controller design have been adopted such as loop-

shaping poleplacement

,multivariable feedback linearization ,H control, and intelligent contol .

Static Synchronous Series Compensator (SSSC)

The SSSC has been applied to different power system studies to improve the system performance

.There has been some work done to utilize the characteristics of the SSSC to enhance power

system stability .Wang investigated the damping control function of an SSSC installed in power

systems. The linearized model of the SSSC integrated into power systems was established and

methods to design the SSSC damping controller were proposed A control strategy of an SSSC to

enlarge the stability region has been derived using the direct method. The effectiveness of the

SSSC to extend the critical clearing time has been confirmed though simulation results on a

single machine infinite bus system.

Unified Power Flow Controller (UPFC)

A unified power flow controller (UPFC) is the most promising device in the FACTS concept. It

has the ability to adjust the three control parameters, A UPFC performs this through the control

of the in-phase voltage, quadrature voltage, and shunt compensation .It was shown that a

significant reduction in the transient swing can be obtained by using a simple proportional


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feedback of machine rotor angle deviation. Fujita et al, investigated the high frequency power

fluctuations induced by a UPFC. Several trials have been reported in the literature to model a

UPFC for steady-state and transient studies. Under the assumption that the power system is

symmetrical and operates under three-phase balanced conditions,

1.2 Basic Types of FACTS Controllers:

Basically the FACTS controllers are four types:-

1. Series controllers2. Shunt controllers3. Combined Series-Series Controllers4. Combined Series -Shunt controllers

1.Series controller-.By means of controlling impedance or phase angle or series injection of

voltage a series FACTS control can control the flow of current. Hence, the series controller could

be variable impedance, such as capacitor, reactor or power electronics based variable source to

serve the desired need. But generally all series controllers inject variable voltage in series with

line. Even variable impedance multiplied by current flow through it represents an injected series

voltage. As long as voltage is in quadrature with the line current, the series controller only

supplies or consumes variable reactive power. Any other phase relationship will involve real

power as well.

2.Shunt Controllers: - As in the case of series controllers, shunt controllers may be variable

impedance, variable source or a combination of these. In principle all shunt controller inject

current into the system. Even variable shunt impedance causes a variable current injection into

the line. As long as injected current is in phase quadrature with the line voltage it supplies or

consumes variable reactive power. Any other phase relationship will involve real power

exchange also.


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3. Combined series-series controller: - This could be a combination of separate series

controllers, which are controlled in a coordinated manner, or it could be a unified controller. The

series controllers could provide independent series reactive compensation but also could transfer

real power among the lines via the power link (D.C link). The real power transfer capability of

the unified series-series controller, referred to as interline power flow controller, makes it

possible to balance both the real and reactive power flow in the lines. And there by 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.

4.Combined series-shunt controller: - This is a combination of series and shunt controllers

which are controlled in a coordinated manner or a unified power flow controller with series and

shunt elements. In principle combined shunt and series controller inject current in to 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.

Inferring from one and two of above, the shunt converter of the UPFC injects current into

the line while the series converter injects voltage in series with the line. The power link enables

real power exchange between the two.

From the above discussion it can be inferred that the word unified emphasizes or refers

to the existence of a power link via which the combination of controllers i.e. series series and

series - shunt exchange real power, also the power link if provided with a storage system such as

d.c. capacitors, batteries etc, is much more effective for controlling the system dynamics. This

has to do with dynamic pumping of real power in and out of the system as against only

influencing transferring of real power within the systems as in the case of power links lacking

storage.

After all the above discussion FACTS can be defined as:-

Alternating current transmission systems incorporating power electronic based and other

controllers to enhance controllability and increase power transfer capability.


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FACTS controller can be defined as:-

A power electronic based system and other static equipment that provide control of one

or more AC transmission system parameters

Below a list of FACTS controllers that fall into the four categories discussed has been

given. The working of each has not been discussed as their general principal of working has

already been discussed.

Shunt Controllers:-

1. Static Synchronous Compensator (STATCOM)2. Static Synchronous Generator (SSG)3. Battery Energy Storage System (BESS)4. Super Conducting Magnetic Energy Storage (SMES)5. Static VAR Compensator (SVC)6. Thyristor Controlled Reactor (TCR)7. Thyristor Switched Reactor (TSR)8. Thyristor Switched Capacitor (TSC)9. Static VAR Generator or Absorber.

Series controllers:-

1. Static Synchronous Series Comparator (SSSC)2. Inter line Power Flow Controller (IPFC)3. Thyristor Controlled Series Capacitor (TCSC)4. Thyristor Switched Series Capacitor (TSSC)5. Thyristor Controlled Series Reactor (TCSR)6. Thyristor Switch Series Reactor (TSSR)


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Combined shunt and series connected controllers:-

1.Unified Power Flow Controller (UPFC)

2 Thyristor Controlled Phase Shifting Transformer (TCPST)

1.3 Advantages of FACTS

The following are the benefits that are principally derived by using the FACTS

controllers.

1. The flow of power is ordered. It may be as per the contract or as per therequirements of utilities.

2. It increases the loading capability of the lines to their thermal capability.Overcoming their limitations and sharing of power among lines can accomplish

this.

3. It improves the stability of the system and thus makes the system secure.4. Provides secure tie line connections to neighboring utilities and regions, thereby

decreasing over all generation reserve requirements on both sides.

5. Provides greater flexibility in sitting new generation.6. Upgrade of lines7. Reduce loop flows8. Minimizes the cost of transmission and hence the overall cost of generation.9. FACTS devices improve the speed of operation of the overall system.

OR

ADVANTAGES OF FACTS CONTROLLERS:

1.Control of power flow in transmission network by controlling:

Line Impedence,


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Angle and

Voltage.

2. Secure loading of transmission lines to levels nearer their thermal limits.

3. Increase the system security through rising:

The transient stability limit

Limiting short circuit current

Overloads

.

4. Provide secure and controllable tie line connections to neighboring utilities and regions.

5. Damp out of power system oscillations.

6. Provide greater flexibilities in sting new generation.

.

7. Reduce reactive power flow, thus, allowing the lines to carry more lines to carry more active

power.

7. Increase utilization of the lowest cost generation.


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10.Optimum power flow for certain objectives.

COMPARISON OF FACTS CONTROLLERS:

NAME OF

CONTROLLER

LOAD

FLOW

CONTROL

VOLTAGE

CONTROL

TRANSIENT

STABILITY

OSCILLATION

ON DAMPING

SVC/STATCOM Small Strong Small Medium

TCSC Medium Small Strong Medium

SSSC Strong Small Strong Medium

TCPAR Strong Medium Small Medium

UPFC Strong Strong Strong Strong


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