FACTS Technology – State of the Art, Current
Challenges and the Future Prospects
Narain G. Hingorani, Life Fellow, IEEE
Abstract – This contribution to the Panel on “Intelligent
Techniques Applied to Transmission Systems” the FACTS
technology, its principle power electronics Controllers,
state of the art and planning issues
Index Terms – FACTS, AC Transmission, Power
Electronics, STATCOM, TCSC, SVC, UPFC
I INTRDUCTION
Flexible AC Transmission Systems Technology (FACTS) was
first proposed by the author in 1985. Since then through EPRI
and others R&D funding, several FACTS Controllers, also
referred to as FACTS Devices, have been demonstrated. IEEE
and CIGRE Working Groups have produced guides and
documents on various aspects of FACTS technology and
applications including planning guide.
Numerous
Universities throughout the world have undertaken research
and published many papers on their ideas of FACTS
Controllers, their potential applications and benefits. Many
Universities have introduced Power Electronics including
HVDC, FACTS and Custom Power in to their Power System
curriculum and producing new generation of power system
engineers. While the author believes that routine application
of FACTS technology as inevitable, application of FACTS
technology has lagged behind expectations, largely due to
market uncertainties created by regulatory environment, and
to some extent consolidation among transmission businesses
during long period of downturn in transmission businesses. In
general, power electronics provides an opportunity for
enhanced value of transmission in terms of loading capability,
reliability and availability and flexibility of ac transmission.
Given the proven commercially available state-of-the-art,
FACTS technology can provide stability, voltage control and
network loading control. It can also enhance available
transmission capacity for given transmission facilities or
upgraded and new transmission facilities. Potentially for the
future, if economically viable short-time storage becomes
available, it can also serve the functions of rapid frequency
control and may be even system restarts.
II FACTS CONTROLLERS
Flexible AC Transmission System (FACTS) is defined by an
IEEE Working Group as: "Alternating current transmission
systems incorporating power electronic-based and other static
Controllers to enhance controllability and increase power
transfer capability.”
Dr. Narain Hingorani is aretred EPRI Vice President of Electrical Systems
and is now an Independent Consultant.
The significance of the power electronics and other static
Controllers is that they have high-speed response and there is
no limit to the number of operations. Like a transistor leads to
a wide variety of processors, power devices such as Thyristor,
GTO, and IGBT lead to a variety of FACTS Controllers as
well as HVDC converters. These Controllers can dynamically
control line impedance, line voltage, and active and reactive
power flow. They can absorb or supply reactive power and
with storage they can supply and absorb active power as well.
All this can be done at high speed and hence control dynamics
and steady state conditions.
Figure 1 show, that there are three kinds of FACTS
Controllers, all with high speed control. One kind can be
characterized as injection of voltage in series with the line; the
second kind as injection of current in shunt and the third kind
are a combination of voltage injection in series and current
injection in shunt. These Controllers will of course have
constraint according to the specific type of Controller, its
characteristics and rating.
Figure 1 FACTS Concepts
Line
Line
Line
• May be active static switch or impedance
converter or a combination thereof.
• When in shunt, cause current injection into the
line, and when in series, causes voltage
injection in series with the line.
Then there are Controllers that are based on conventional
thyristors (without turn off capability) and those that are based
on turn-off Thyristor (GTO, SGTO, IGCT) and transistor
(IGBT).
Figure 2 shows two types of shunt Controllers for injection of
reactive current, the conventional-thyristor based Static VAR
Compensator (SVC), and the turn-off Thyristor or IGBT
based Static Compensator (STATCOM). Their primary
function is dynamic voltage control. Also shown alongside are
the characteristics of the two types. In case of SVC, the
current is a function of the line voltage and hence its reactive
power is function of square of the line voltage. Thus when the
dynamic voltage is say 80%, the injected reactive power is
reduced to 64%, just when more is needed. The converter
based STATCOM on the other hand injects the required
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current of either polarity and hence injects appropriate
reactive power as needed. For similar performance
STATCOM size would be much smaller and should be the
more cost effective of the two in many applications.
Numerous SVCs and many STATCOMs have been installed
throughout the world.
Figure 2. Thyristor based SVC and Turn-off thyristor based STATCOM
There are also some other Controllers that are described in the
book from Wiley, "Understanding FACTS" by Narain G.
Hingorani and Laszlo Gyugyi, published in 1999.
Given many FACTS Controllers, TCSC, SVC and
STATCOM can effectively and economically serve the
needed system dynamic control.
Figure 3 Thyristor based TCSC and Turn-off thyristor based SSSC
Figure 3 shows two types of series Controllers for injection of
reactive voltage in series, the Thyristor Controlled Series
Capacitor (TCSC) and Static Synchronous Series
Compensator (SSSC), whose primary function is current flow
control. TCSC is variable capacitive impedance; hence its
injected voltage is proportional to the current. SSSC on the
other hand directly injects variable voltage of either polarity
in quadrature with the current. TCSC should be the more cost
effective of the two. There is also a turn-off device based
controlled series capacitor, called GTO Controlled Series
Capacitor (GCSC.) It is interesting to note that replacing
thyristors with turn-off devices gives a greater control of
capacitive impedance and is to be highly recommended.
Figure 4 shows two types of Shunt-Series Controllers, the
Thyristor Controlled Phase Angle Controller (TCPR) and
Unified Power Flow Controller (UPFC). There are a variety of
TCPRs circuits and it injects variable voltage in quadrature
with the line to ground voltage. UPFC on the other hand is a
combination of SSSC and STATCOM. It injects variable
voltage in series, with variable angle with respect to the
current. It can therefore control both real and reactive power
flow Furthermore, the shunt converter not only supplies
required active power for the series element, it can also supply
variable reactive power in shunt; thus UPFC can dynamically
control active and reactive power flows as well as the line
voltage. The characteristics and performance range of UPFC
and TCPR are significantly different and one must evaluate
the Controllers on the cost-for-performance basis and not on
MVA size basis. Other than EPRI’s demonstration there are
no other UPFC installed as yet. On the other hand, several
Dynamic Voltage Restorers, (DVRs) which are similar to the
STATCOMs, but with function of providing high quality
power to the customer, have been installed up to 40 MVA
size.
Figure 4 Thyristor based TCPAR and Turn-off thyristor based UPFC
III SYSTEM PLANNERS PERSPECTIVE
System Planner has to consider a variety of options and they
have to make decisions not only based on technical and cost
considerations, but also based on return on investment. In
many parts of the world, deregulation/re-regulation,
restructuring and continued uncertainties of what is yet to
come has led utilities to differ investment. Many utilities that
were vertically integrated and were involved in long term
planning, have to now focus on unbundling with consequent
uncertainties of potential rewards from investment in
transmission. In general the transmission systems must now
be economical in a market as well as meet the traditional
requirements for system security, reliability and sufficient
capacity to meet the needs of customers. In transmission
system, benefits of investment often occur on system wide
basis and it is not clear who should make the investment.
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IV STATE OF THE ART AND FUTURE PROSPECTS
It is important to recognize that inclusion of FACTS
Controllers in the planning process does not change the
planning procedures. This process will include engineering
studies including load flow, transient and dynamic stability,
voltage stability, transient analysis etc. FACTS simply
provides additional means to enhance the value of investment.
FACTS technology is also an enabling technology in terms of
continuous control of active and reactive power flows. Of
course one needs transmission lines in the first place to
consider use of FACTS to enhance the value of transmission
lines. FACTS should be considered as part of planning
scenario for a new line but also enhancing existing lines.
FACTS offers solutions to overcome constraints on useable
transmission capacity.
These constraints may be due to:
Dynamic conditions of:
• Transient and Dynamic Stability
• Subsynchronous Oscillations
• Dynamic Over Voltages and Under Voltages
• Voltage Collapse
Or Steady State conditions of:
• Undesirable Power Flow
• Excess Reactive Power Flows
• Steady State Voltage
• Thermal Limits
Many steady state conditions can be overcome by
mechanically switched inductors and capacitors, unless the
frequent switching is necessary.
It should be noted that a FACTS Controller can be used for
multiple purposes. For example a TCSC or SSSC can be used
for multiple functions including current control, damping
oscillations, transient and dynamic stability, voltage stability,
fault current limiting, even though primary reason for
selecting such a Controller may be say, dynamic stability.
A characteristic of FACTS controllers is their ability to have
control algorithms structured to achieve multiple objectives.
Since FACTS Controllers have control systems with
embedded digital processors, it is possible to switch between
control algorithms and to include different types of nonlinear
limiting functions.
A characteristic of FACTS controllers is their ability to have
control algorithms structured to achieve multiple objectives.
Since FACTS Controllers have control systems with
embedded digital processors, it is possible to switch between
control algorithms and to include different types of nonlinear
limiting functions.
For FACTS Controllers to be included in transmission system
plans, there must be appropriate models for all the analyses
that are normally performed.
There are three FACTS Controllers that are well established
and widely used, Thyristor-based TCSC and turn-off device
(GTO, IGCT and IGBT) based STATCOM, Long established
Thyristor-based SVC. These three are the most cost effective
Controllers and serve the need.
The TCSC serves the need for stability and damping subsynchronous oscillations. To my knowledge it has not been
effectively used for power flow control, but in time it will be
used for power flow control as well. Hopefully turn-off device
based GCSC will also be used, because it offers more
effective control.
SVC and STATCOM serve the need for dynamic voltage
control, to compensate for frequent voltage fluctuation and
reduce dynamic over-voltages. STATCOM is relatively new
and more expensive and has superior performance.
While TCSC, STATCOM and other FACTS Controllers have
been demonstrated by EPRI, exit of the manufacturer who
designed and built them and retirement of the technical team
and depressing regulatory environment has been a serious
setback for commercialization of these Controllers. In
transmission business it often takes a generation change to
introduce new technologies. Never the less, the three FACTS
Controllers mentioned above SVC, STATCOM and TCSC are
well proven and do serve the need.
V REFERENCES
1. Hingorani, N.G., "Power Electronics in Electric Utilities:
Role of Power Electronics in Future Power Systems,"
Proceedings of the IEEE Special Issue Vol. 76, no. 4, April
1988.
2. Hingorani, N.G., "High Power Electronics and Flexible AC
Transmission System IEEE Power Engineering Review, vol. 8
no 7, July 1988. Reprint of Joint American Power
Conference/IEEE Luncheon Speech, Chicago, Illinois, April
1988.
3. IEEE FACTS Terms and Definitions Task Force of the
FACTS Working Group of the DC and FACTS
Subcommittee, A. Edris, Chair, “Proposed Terms and
Definitions for Flexible AC Transmission Systems (FACTS),
IEEE Transactions on Power Delivery, October 1997, pp.
1848-1853.
4. Narain G. Hingorani and Laszlo Gyugyi Understanding
FACTS Concepts and Technology of Flexible AC
Transmission Systems, IEEE Press, New York, NY, ISBN 07803-3455-8, 2000, Available from Wiley.
5. Task Force of the IEEE FACTS Working Group on FACTS
Applications, Fred Sener, Chairman, “FACTS Applications,”
IEEE Special Publication 96-TP-116-0, 1996.
6. Transmission System Application Requirements for
FACTS Controllers, A Special Publication for System
Planners. IEEE WG 15.05.13
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IV BIOGRAPHY
Narain Hingorani: In 1995, following a twenty year career at EPRI, Dr.
Hingorani retired from EPRI as Vice President of Electrical Systems and
started consulting in Application of Power Electronics in Power Systems.
Prior to joining EPRI, Dr. Hingorani spent 6 years at Bonneville Power
Administration; his responsibilities included commissioning of the Pacific
DC Intertie and Series Capacitor compensation for AC Interties. Dr.
Hingorani is credited with originating concepts of Flexible AC Transmission
System (FACTS) and Custom Power. He has authored over 150 papers and
co-authored two books, one on HVDC power transmission (1960) and the
other on Flexible AC Power Transmission (1999). Dr. Hingorani received his
B.Sc. Electrical Engineering from Baroda University, India, and M.Sc., Ph.D.
and D.Sc from University of Manchester Institute of Science and Technology
in England. In 1985, Dr. Hingorani was presented the Uno Lamm Medal by
the IEEE PES for outstanding contributions in High Voltage Direct Current
Technology, and later received the 1995 IEEE Lamme Gold Medal for
leadership and pioneering contributions to the transmission and distribution of
electric power.
In 1906 Dr. Hingorani received the prestigious Franklin Institute’s Bower
Medal and Prize for Science. In 1988, Dr. Hingorani was elected to the US
National Academy of Engineering. From 1988 to 1996, he was Chairman of
CIGRE Study Committee 14: DC Links and Power Electronics.
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