FLEXIBLE AC TRANSMISSION
FLEXIBLE AC TRANSMISSION CONTROLLERSBY:
S. Sivaraman, G.A Jagannath,
Final B.E. (EEE) Final B.E. (EEE)
K.S.R. College of Technology K.S.R. College of Technology
Tiruchengode. Tiruchengode.
E-mail ID: [email protected]
[email protected]
Abstract:
Energy is the basic necessity for the economic development of
the country. The rate of growth of electricity usage thought out
the world is increasingly high in the last ten years. in recent
years ecological concerns and high installation costs have put
constraints over construction of new plants and overhead lines in
many countries; thereby forcing existing system to be used more
efficiently. AC transmission systems are thought of as being
'inflexible. Power flow in AC networks simply follows ohms law and
ordinarily cannot be made to flow from the source to load is in
inverse proportion to the relative impedance path take the largest
fraction of flow; but all lines in the interconnection are a part
of the flow path.
With these number of operating constraints in terms of voltage
profile, power flow, stability and sub synchronic resonance (SSR)
are figured. Conventional means such as circuit breakers,
isolators, shunt and series compensators along with synchronous
condensers help a lot to deal with these problems. But the overall
control is slow and at times may result in loss of synchronism and
serious over voltages on load buses.
Development of flexible A.C transmission systems (FACTS)
technologies has made it possible the control of power flow on AC
transmission lines and utilizes the existing facilities all the way
to its thermal limit without degrading reliability. FACTS include
the use of high power electronics, advanced control centers and
communications links.
This paper covers the modern trends in flexible A.C transmission
system, envisaging the FACTs controllers with present and future
applications.
1. INTRODUCTION
The need for more efficient electricity management has given
rise to innovative technologies in power generation and
transmission. The combined cycle power station is good example of a
new development in power generation and flexible A.C transmission
systems Facts; as they are generally known are new devices that
improve transmission systems. Worldwide transmission system are
undergoing continues changes and restricting. They are becoming
more heavily loaded and are being operated in ways not originally
envisioned. Transmission system must be flexible to react more
diverse generation and load patterns. In addition to the economical
utilization of transmission systems is of vital importance to
enable utilities in industrialized countries to remain competitive
and survive. In developing countries; the optimized use of
transmission system investments is also important to supply
industries, create employment and utilize efficiently scarce
economic resources.
FACTs is a technology that responds to these needs. It
significantly alters the way transmission systems are developed and
controlled together with improvements in assets utilization, system
flexibility and system performance.
FACTs devices consists of thruster controlled series and shunt
compensation and make is possible to operate transmission lines
stably at power levels well beyond those normally used.FACTs is in
the development stages from quite a few time and considerable
innovation have been reported recently in FACTs.the latest in the
series is the unified power flow controller (UPFC).and the
generalized unified power flow controller (GUPFC).the evolution of
UPFC not only allows the combined application of phase angle
control but also real time transition from one selected
compensation mode into another; to handle particular system
contingencies more effectively
FACTs use controllable series and shunt compensation located at
intermediate substation with the following objectives.
Rapid control of reactive power and voltage profile flow using
series and shunt connected controllers.
Secure loading of lines close to their thermal limits.
Improve power tansferability.transcient stability and dynamic
stability of line during fault switching etc..
2.GENERATION OF FACTS CONTROLLERS:
FACTs devices are used for the dynamic control of
voltage,impedence and phase angle of high voltage A.C transmission
lines.FACTs devices can be divided into number of generations
depending upon their development stages. These
are2.1First-generation FACTs controllers
Static VAR compensator (SVC)
Thyristor controlled reactor -fixed capacitor (TCR -FC)
system
Thyristor switched capacitor (TSC) systemTCR-TSR system
Thyristor controlled phase angle regulator (TCPAR) System or
static phase shifter (SPS)2.2SECOND GENERATION FACTS
CONTROLLERS:
Thyristor controlled series compensator (TCSC)
Static condensers (STATCONS)2.3TODAY'S FACTS CONTROLLERS
Unified power flow controllers (UPFC)
Generalized unified power flow controllers (GUPFC)
3.DEVELOPMENT OF FACTS CONTROLLERS:
Static VAR compensator (SVC)
An SVC is based on thyristor controlled reactors (TCR),
thyristor controlled capacitor (TSC) and /or fixed capacitors (FC)
tuned filters. A TCR consists of a fixed reactor in series with a
bidirectional thyristor valve. A TSC consists of a capacitor bank
in series with a bidirectional thyristor valve and a damping
reactor, which also serves to de-tune the circuit to avoid parallel
resonance with the network. two very common design types, both
having each their specific merits
3.1Few improvements obtained by use of SVC :
The transient stability and steady state power handling capacity
can be improved by using SVC at intermediate buses of long
lines.
Dynamic stability is increased due to the increased damping
provided.
Steady state and temporary over voltages can be controlled
Load power factor is improved and hence system efficiency is
improved.
SVC is in faster in response than synchronous condensers and
provides damping for SSR for oscillations
3.2TCPAR SYSTEM
This is also known as static phase shifter (SPS).The phase shift
with respect to bus voltage is achieved by adding or subtracting
variable voltage component in quadrature with the bus voltage. The
variable quadrature voltage component in one phase is obtained by
thyristor switches and by exciting transformer, which is connected
between the two other phases.
The quadrature voltage is injected in series with the
transmission line by a boosting transformer.
4.THYRISTOR CONTROLLED SERIES COMPENSATION:
TCSC is used to control the effective line reactance by
connecting a TCR combination with mechanically switched capacitors
sections in series. Series compensation is used with long lines for
providing compensation of reactive power and giving higher power
transferability. It is preferred to the shunt
Compensation as the rating required for the series compensation
is comparatively smaller. However the series compensated lines
suffer from the drawback of producing series resonance at
frequencies lower than power frequencies, which is called Sub
Synchronous Resonance. Sub Synchronous Resonance results in damage
to rotor shafts of turbo alternators producing torsional
torque.5.STATCON (static condensers)
STATCON is actually a shut compensation device. The major
differences between a SVC and STATCON are
Use of gate turn off switch(GTO)in STATCON compared to use of
conventional thyristors in SVC
SVC is a voltage regulator and a variable susceptance controller
whereas STATCON is based on voltage source converter(VSC)
The operating principle is like a synchronous condensers is
coupled to circuit through a transformer, which provides the safe
operating voltage and small reactance. An inverter generates the
three phase voltages in phase with the ac system voltages. The
current lags if the inverter voltage is greater than the system
voltage. The reactive power delivered by STATCON is a function of
voltage and current. This device can deliver reactive power under
reduced voltage condition and has a better performance than a
static var compensator.
Expect a very small loss-taking place in the VSC, no real energy
exchange taking place.The STATCON is capable of supplying required
reactive power even at small values of bus voltages where reactive
power supply capability gets limited to its susceptence limit. The
susceptence decreases linearly with decrease in bus voltage
Due to susceptence limit., SVC cannot have a short time overload
capacity whereas STATCON can have the same.
STATCON can serve as a real power exchanger if it has an energy
source at DC bus, conversely supply DC power.6.UNIFIED POWER FLOW
CONTROLLER (UPFC):
This is the latest in the series of FACTs controllers. The
principle is based upon injecting a variable voltage in series with
the transmission line in order to control real and reactive power
flow through the line. The device has two branches:
A series branch
A shunt branch
Each branch is a GTO based inverter branch. The series branch
has a voltage source inverter, which injects a voltage in series
through a transformer. In doing so it can exchange real power with
the transmission line by injecting a voltage in series through a
transformer. But this exchange of power is only possible if it has
an energy source at its DC terminal. The UPFC with only series
branch functioning but exchanging real power is called Static
Synchronous Series Compensation (SSSC)
The shunt branch exchanges the reactive power independently with
the system.6.1GENERALIZED UNIFIED POWER FLOW CONTROLLER: The
Generalized unified power flow controller (GUPFC) is multilane
UPFC, which can control bus voltage and power flows of more than
one line or even a sub network. The GUPFC with combing three or
more converters working together extends the concepts of voltage
and power flow control beyond what is achievable with the known
two-converter UPFC Facts controller.
The simplest GUPFC consists of three converters, one connected
in shunt and other two in series with two in series with two
transmission lines in a substation. It can control total five power
system quantities such as a bus voltage and independent active and
reactive power flows of two lines. With more series converters
included in GUPFC, more degree of freedom can be introduced and
hence more control objective can be achieved. Real power can be
exchanged among shunt and series converters via the common DC
link
The GUPFC can construct a multi terminal sub network, which can
control active and reactive power flows for a group of lines and
selected bus voltage within a substation to their specified
objectives. This has significantly extended the voltage and power
flow control capability that was achieved by the independent
STATCON or SSSC or UPFC.7.FACTS IMPLEMENTATIONSteps for the
identification of FACTS Projects
Operation of FACTS Devices
Maintenance of FACTS Devices
Testing of FACTS Devices7.1WORLD WIDE IMPLEMENTATION OF FACTS
CONTROLLERS:
TCSC Schemes:
-345kv Kanawha river substation, west Virginia,USA
-230kv kayenta substation,Arizona
-500kv slatt substation in Oregon,USA STATCON Schemes:
A 100 MVA at Sullivan substation in TVA power system
UPFC schemes:
By American Electric power Co. at Inez station in Kentucky,
USA in two phases.
Phase I with the installation of (+-) 160 MVAr shunt inverter
for voltage support
Phase II with (+-) 160 MVAr series inverter is being
installed
The UPFC is located at mead to control the west wing-Mead power
flow and is rated for 1060 MVA (series injection) with 475 MVAr
shunt var compensation capability.
Mead phoenix project involves the construction of a 500kv line
from the phoenix Area (West wing) to the las vegas area (mead and
Marketplace) and on to los angeles
A 100 MVA,48 pulse inverter UPFC, installed at the Sullivan
substation of Tennessee valley authority transmission network to
provide (+-) MVAr compensation.8.IMPACT OF FACTS IN INTERCONNECTED
NETWORKS:
It enables the participating to share the benefits of large
power systems such as optimization of power generation, utilization
of difference in load profiles and polling of reversed
capacity.
Environmental benefits are achieved when e.g. surplus of clean
hydro resources from one origin can help to replace polluting
fossil fuelled generation in another.
FACTS help to enable mutually beneficial trade of electric
energy between the countries.8.1FACTS for minimizing of grid
investments:
It has been mentioned that an improvement incentive for
considering of FACTS for grid planning is its being an economically
as well as politically and environmentally attractive to larger,
more costly and more time consuming investments in extended
transmission networks i.e. basically more lines.
Thus for instance it can be shown that the cost of installing
series capacitors as means for improving the power transmission
capacity of existing line amounts to only a friction of the cost
for installing one or more several lines. This is valid for all
existing transmission voltages and for all transmission distances
where series compensation from the very beginning, power
transmission between regions can be planned with a minimum of
transmission circuits ,there by minimizing of transmission
circuits, there by minimizing costs as well as environmental impact
from the start.9.FUTURE DEVELOPMENTS IN FACTS
Future developments will include the combination of existing
devices, e.g. combining a STATCON with a TSC (thyristor switched
capacitor) to extend the operational range. In addition, more
sophisticated control systems will improve the operation of FACTS
devices. Improvements in semiconductor technology (e.g. higher
current carrying capability, higher blocking voltages) could
replace the costs of FACTS devices and extend their operation
ranges. Finally developments in superconductor technology open the
door to new devices like SCCL (super conducting current limiter)
and SMES (Super conducting magnetic energy storage). There is a
version for as high voltage transmission system around the world to
generate electrical energy economically and environmental friendly
and provide electrical energy where it is needed. FACTS are the key
to make this vision live. 9.1BENEFITS OF UTILIZING FACTS
DEVICES
Better Utilization of existing transmission system assets
Increased transmission system reliability and availability
Increased dynamic and transient grid stability
Increased quality of supply for sensitive industries
Environmental Benefits10.CONCLUSION
The advent of FACTs controller has provided fast, reliable
control of bus voltages, stability and SSR. With the advent of
FACTs the AC system will become more and more flexible using the
thyristor as base component. It can be foreseen that the world is
heading for a power system, which is electronically controlled than
mechanically controlled providing better flexibility and
reliability to the existing AC system. It is strongly believed that
FACTs controllers will be able to solve the problems related to
voltage profile, stability and SSR in a big way in the coming
decades.
11.REFERENCES:Basic Control of Unified Power Controller, by I
Papic, P. Zuko, IEEE transaction power system, vol. 12 No.4,
November 1997
Hingorani NG 1993, Flexible A.C. Transmission System, IEEE
Spectrum.
Padiyar K.R. and Kulkarni A.M.,
G. A. Jagannath,
Final B.E. (EEE)
K.S.R. College of Technology
Tiruchengode.
