How FACTS Controllers
Benefit
AC Transmission Systems
By
P.Narsimha reddy
1005-10-743205
Contents
Introduction
Why FACTS Controllers
Power System Constraints
Controllability of Power Systems
Examples of conventional& FACTS Controllers
Comparison of FACTS & conventional ckts
Benefits of Control of Power Systems
Overview of FACTS Controllers ckts
Introduction
To achieve both operational reliability and financial
profitability, it has become clear that more efficient
utilization and control of the existing transmission
system infrastructure is required.
In the evolving utility environment, financial and
market forces are, and will continue to, demand a
more optimal and profitable operation of the power
system with respect to generation, transmission,
and distribution
Why FACTS Controllers required
The process to permit, site, and construct new
transmission lines has become extremely difficult,
expensive, time-consuming, and controversial.
FACTS technologies provide advanced solutions as
cost-effective alternatives to new transmission line
construction
Power System Constraints
Steady-State Power Transfer Limit
Voltage Stability Limit
Dynamic Voltage Limit
Transient Stability Limit
Power System Oscillation Damping Limit
Inadvertent Loop Flow Limit
Thermal Limit
Short-Circuit Current Limit
Others
Controllability of Power Systems
Power system only has certain variables that
can be impacted by control, consider the basic
and well-known power-angle curve
Angle
Voltage
Impedance
Examples of conventional equipment
for enhancing
Series Capacitor Controls impedance
Switched Shunt-Capacitor and Reactor
-Controls voltage
Transformer LTC -Controls voltage
Phase Shifting Transformer -Controls
angle
Synchronous Condenser -Controls voltage
,
Example of FACTS controllers for
enhancing power system
control
Static Var Compensator (SVC)
Controls voltage
Unified Power Flow Controller (UPFC)
Convertible Static Compensator (CSC)
Inter-phase Power Flow Controller (IPFC)
Thyristor Controlled Series Compensator (TCSC)
-Controls impedance
Thyristor Controlled Phase Shifting Transformer
(TCPST) -Controls angle
Static Synchronous Series Controller (SSSC)
-controllers impact voltage, impedance, and/or angle
Static Synchronous Compensator (STATCOM)
-Controls voltage
Comparison of FACTS &
conventional ckts
This figure shows that the speed of mechanical
switches, can be as fast as a couple of cycles of
60 (or 50) Hz.
This speed of switching in and of itself may be
fast enough to solve many power system
constraints.
Although there is a vast improvement in
switching time from mechanical to power
electronic based solutions
The main benefit that FACTS controller solutions
provide is the “cycling/repeatability” and “smooth
control”
Benefits of Control of Power
Systems
Increased Loading and More Effective Use of
Transmission Corridors
Added Power Flow Control
Improved Power System Stability
Increased System Security
Increased System Reliability
Added Flexibility in Siting New Generation
Overview of FACTS Controllers
ckts
Static Var Compensator (SVC)
Controls voltage
Static Synchronous Compensator (STATCOM)
-Controls voltage
Unified Power Flow Controller (UPFC)
impedence, voltage magnitude and phase angle
svc
In general, the two thyristor valve
controlled/switched concepts used with SVCs
The thyristor-controlled reactor (TCR) and the
thyristor-switched capacitor (TSC).
The TSC provides a “stepped” response and the
TCR provides a “smooth” or continuously
variable susceptance
The SVC regulates voltage at its terminals by
controlling the amount of reactive power injected
into or absorbed from the power system.
When system voltageis low, the SVC generates
reactive power (SVC capacitive). When system
voltage is high, it absorbs reactive power (SVC
inductive).
The variation of reactive power is performed by
switching three-phase capacitor banks and
inductor banks connected on the secondary side
of a coupling transformer
Each capacitor bank is switched on and off by
three thyristor switches (Thyristor Switched
Capacitor or TSC).
Reactors are either switched on-off (Thyristor
Switched Reactor or TSR) or phase-controlled
(Thyristor Controlled Reactor or TCR).
STATCOM
The function of a STATCOM is similar to that ofa
synchronous condensor.
In general it provides reactive power
compensation to solve a variety of power
system, voltage fluctuation and stablity
conditions
STATCOM consists of
DC voltage sourse, gate commutated turn off
thyristors(GCT), step up transformers
Operation mode
Unified Power Flow Controller
(UPFC)
The universal and most flexible FACTS device is
the Unified Power Flow Controller (UPFC).
UPFC is the combination of three compensators’
characteristic; i.e. impedence, voltage
magnitude and phase angle.
Basic ckt diagram
UPFC construction
The UPFC consists of two voltage source
converters; series and shunt converter, which are
connected to each other with a common dc link.
Series converter or Static Synchronous Series
Compensator (SSSC) is used to add controlled
voltage magnitude and phase angle in series with
the line.
while shunt converter or Static Synchronous
Compensator (STATCOM) is used to provide
reactive power to the ac system, beside that, it will
provide the dc power required for both inverter.
The energy storing capacity of this dc capacitor
is generally small.
Therefore, active power drawn by the shunt
converter should be equal to the active power
generated by the series converter.
The reactive power in the shunt or series
converter can be chosen independently, giving
greater flexibility to the power flow control.
Control of power flow is achieved by adding the
series voltage, VS with a certain amplitude, VS
and phase shift, φ to V1.
This will gives a new line voltage V2 with
different magnitude and phase shift.
As the angle φ varies, the phase shift δ between
V2 and VR also varies.
Conclusion
In power system transmission, it is desirable to
maintain the voltage magnitude, phase angle
and line impedance. Therefore, to control the
power from one end to another end, this concept
of voltage injection is applied.
References
S. Mori, K. Matsuno, T. Hasegawa, S. Ohnishi,
“Development of a Large Static Var Generator Using
Self-Commutated Inverters for Improving Power
System Stability,” IEEE Transactions, 1993
N. Morishima, Takeda, T. Sugiyama, Y. Hamazaki, T.
Snow, A. Abed, “SDG&E Talega STATCOM Project -
System Analysis, Design, and Configuration,”
Proceedings of the IEEE PES, October 2002.
