Transcript
Prof. M.V. Aware Electrical Engg. Dept. VNIT , NAGPUR
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Simple illustration of the power transmission system
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Simple illustration of the power transmission system
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Power system structure
PCompensation Pi = PGenerator + PLoad + PCompensation S = P + jQ
Qi
= QGenerator + QLoad + QCompensation
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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:
Real Power:
P
V
S = P + jQ
2
X
sin
Reactive Power: Q V I sin( / 2)
V
2
X
(1 cos )
V voltage X reactance phase angle I current
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Power Transfer Capacity Limiting Factors :
Thermal Limit
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Steady State Stability Limit
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System Damping
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Steady-state-stability Limit (MW) Thermal Limit (MW) Transient-stability Limit (MW) Electrical Damping Limit (MW)
Different limits on power flow in transmission systems MVA-VNIT NAGPUR
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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 controllers to enhance controllability and increase power transfer capability”
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FACTS are utilised for: Increase/control of power transmission capacity in a line and for preventing loop flows Improvement of system transient stability limits Enhancement of system damping Mitigation of sub-synchronous resonance Alleviation 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 to increase existing transmission network capacity while maintaining or improving the operating margins necessary for grid stability. As a result, more power can reach consumers with a minimum impact on the environment, after substantially shorter project implementation times, and at lower investment costs - all compared to the alternative of building new transmission lines or power generation facilities. The two main reasons for incorporating FACTS devices in electric 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
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Angle controls the active power
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Injecting the voltage in series with line
Combination of the line impedance control with a series controller and voltage regulation with a shunt controller
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Can also provide a cost effective means to control both the active and reactive power flow between TWO systems.
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BASIC TYPES OF FACTS CONTROLLERS Series Controllers
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Shunt Controllers
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Combined series-series Controllers
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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) or Thyristor Switched Capacitor (TSC) Or Combination of both. Other combination of SVC Fixed Capacitor-TCR (FC-TCR) Or TCR-Mechanically Switched Capacitor (TCR-MSC)
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The dynamic V-I characteristics of the SVC Linear range of control over which SVC terminal Voltage 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 system viewpoint is to simply increase the fundamental frequency voltage across a fixed capacitor in a series compensated line through appropriate 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 system can 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 voltage with appropriate phase angle in relation to the line current power Capable of real and reactive power exchange with the transmission 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 in Transmission line with extremely rapid speed.
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