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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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