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FLEXIBLE AC TRANSMISSION SYSTEMS(FACTS)

Prof. M.V. AwareElectrical Engg. Dept.VNIT , NAGPUR

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Simple illustration of the power transmission system

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Power system structure

= PGenerator + PLoad + PCompensation

= QGenerator + QLoad + QCompensation

Pi

QiS = P + jQ

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FLOW OF POWER IN AN AC SYSTEM

Many transmission facilities confronts one or more limiting

network parameters plus inability to direct flow at will

Electrical systems are self regulating: If generation is less than load----

the voltage and frequency drop

The load goes down to equal the generation minus the transmission losses.

There is only a few percent margin for such a self regulation

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Apparent Complex Power: S = P + jQ

Real Power:

Reactive Power: )cos1()2/sin(2

X

VIVQ

sin2

X

VP

V voltage

X reactancephase angleI current

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Power Transfer Capacity Limiting Factors

:Thermal Limit

Steady State Stability Limit

System Damping

Steady-state-stability Limit (MW)

Electrical Damping Limit (MW)

Thermal Limit (MW)

Transient-stability Limit (MW)

Different limits on power flow in transmission systems

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Power flow in parallel paths

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Applying Flexibility to the Electric Power System

The power industry term

FACTS

(Flexible AC Transmission Systems)covers a number of technologies that enhance the

security, capacity and flexibility of power

transmission systems.

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FACTS are defined as

Alternating current transmission systems incorporating

power-electronic based and other static controllersto enhance controllability and increase power transfer capability

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FACTS are utilised for:

Increase/control of power transmission capacity in a line andfor preventing loop flows

Improvement of system transient stability limits

Enhancement of system damping

Mitigation of sub-synchronous resonanceAlleviation of voltage stability

Limiting short circuit currents

Improvement of HVDC converter terminal performance

Grid Integration of Wind Power Generation Systems

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FACTS solutions enable power grid owners toincrease existing transmission network capacitywhile maintaining or improving the operatingmargins necessary for grid stability. As a result,more power can reach consumers with a minimumimpact on the environment, after substantiallyshorter project implementation times, and at lower

investment costs - all compared to the alternative ofbuilding new transmission lines or power generationfacilities.

The two main reasons for incorporating FACTS devices inelectric power systems are:

- Raising dynamic stability limits- Provide better power flow control

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Possibilities of power flow control:

Control of line impedance X

Angle controls the active power

Injecting the voltage in series with line

Combination of the line impedance control with a series controllerand voltage regulation with a shunt controller

Can also provide a cost effective means to control boththe active and reactive power flow between TWO systems.

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BASIC TYPES OF FACTSCONTROLLERS

Series Controllers

Shunt Controllers

Combined series-series Controllers

Combined series-shunt controllers

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TYPES OF FACTS DEVICES

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FACTS controllers are classified into two types:

1) Thyristor based FACTS controllers

-Static Var Compensator (SVC)-Thyristor controlled Series Compensator (TCSC)

2) Voltage Source Converters (VSC) Based Controllers

-Static Synchronous Compensator (STATCOM)-Static Synchronous Series Compensator (SSSC)- Unified Power Flow Controllers (UPFC)

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STATIC VAR COMPENSATOR (SVC)

It is a shunt-connected static var generator or absorber

It adjust the exchange of capacitive or inductive current to maintain

or control specific parameters of the electrical power system(Typically bus voltage)

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SVC Configurations:

Thyristor controlled Reactor (TCR)orThyristor Switched Capacitor (TSC)OrCombination of both.

Other combination of SVC

Fixed Capacitor-TCR (FC-TCR)OrTCR-Mechanically Switched Capacitor (TCR-MSC)

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The dynamic V-I characteristics of the SVC

Linear range of control over which SVC terminalVoltage varies linearly with SVC current (Capacitive to inductive range)

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THYRISTOR CONTROLLED SERIES CAPACITORS (TCSC)

Control of capacitive reactance of the line

Provides continuous control of power on ac line over a wide range

The basic principle of variable series compensation from the systemviewpoint is to simply increase the fundamental frequency voltage across

a fixed capacitor in a series compensated line throughappropriate variation of the firing angle

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STATIC SYNCHRONOUS COMPENSATOR (STATCOM)

It is a controlled reactive power source

It provides desired reactive power generation as well as absorption

By processing voltage and current waveforms in a --

Voltage Source Converter (VSC)

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STATCOM CONTROL

Reactive power exchange between the converter and the ac systemcan be controlled by varying the amplitude of the converter

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V-I Characteristics of a STATCOM

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STATIC SYNCHRONOUS SERIES COMPENSATOR (SSSC)

Series connected synchronous voltage source

Effective impedance variation by injecting a voltagewith appropriate phase angle in relation to the linecurrent power

Capable of real and reactive power exchange with thetransmission system

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UNIFIED POWER FLOW CONTROLLER (UPFC)

-Voltage regulation-Series compensation-Phase shifting

It can independently control both the real and reactive power flow inTransmission line with extremely rapid speed.

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