16-08-2012, 10:59 AM
Shunt Connected Facts Device in a Series Compensated Long Transmission Lines
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1. INTRODUCTION
The flexible AC transmission system (FACTS) has received much attention in the last 2 decades. It uses high current power electronic devices to control the voltage, power flow, stability, etc. of a transmission system. FACTS technologies can essentially be defined as highly engineered power-electronics-based systems, integrating the control and operation of advanced power semiconductor- based converters (or valves) with software based information and control systems, which produce a compensated response to the transmission network that is interconnected via conventional switchgear and transformation equipment.
FACTS devices can be connected to a transmission line in various ways, such as in series with the power system (series compensation), in shunt with the power system (shunt compensation), or both in series and shunt. For example, the static VAR compensator (SVC) and static synchronous compensator (STATCOM) are connected in shunt; static synchronous series compensator (SSSC) and thyristor controlled series capacitor (TCSC) are connected in series; thyristor controlled phase shifting transformer (TCPST) and unified power flow controller (UPFC) are connected in a series and shunt combination. In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source. Series inductance occurs in long transmission lines, and when a large current flow causes a large voltage drop. To compensate, series capacitors are connected.
In shunt compensation, power system is connected in shunt with the and definition of various FACTS devices are described. The pressure associated with economical and environmental constraints has forced the power utilities to meet the future demand by fully utilizing the existing resources of transmission facilities without building new lines. FACTS devices are very effective and capable of increasing the power transfer capability of a line, as thermal limits permit, while maintaining the same degree of stability. Numerous recent applications of FACTS have proven to be cost-effective, long-term solutions. With the improvements in current and voltage handling capabilities of the power electronic devices that have allowed for the development of Flexible AC Transmission System (FACTS), the possibility has arisen in using different types of controllers for efficient shunt and series compensation. Applying FACTS on a broad-scale basis for both local and. Shunt FACTS devices are used for controlling transmission voltage, power flow, reducing reactive losses, and damping of power system oscillations for high power transfer levels.With the widespread and active consideration of the installation of FACTS controllers for better controllability.
1.1 FACTS Concepts Similar To HVDC
While some of the relevant technology i.e Static VAR Compensation is already in
Wide use, the FACTS concept has brought to the table a tremendous potential for
thyristor basedcontrollers which will surely revolutionize the power system. The
technology offers the utilities the ability to:
1. Control power flows on their transmission routes;
2. Allow secure loading of transmission lines to their full thermal capacity.
FACTS technology, while allowing use of transmissiorr to its thermal capacity, does not do away with the need for additional transmission lines or the upgrading of existing line
where thermal limits have been reached or when evaluation of losses added to the cost
of FACTS technology shows that new lines or upgrading of existing lines is the most optimum answer. Often, ac transmission systems are thought of as being "inflexible".
Power flow in ac networks simply follows Ohm's law and ordinarily cannot be made to flow along specific desired paths. As a result, ac networks suffer from parallelpath, or
"loop" flows. The power flows from source to load in inverse proportion to the relative impedances of the transmission paths. Low impedance paths take the largest fraction of
flow, but all lines in the interconnection are a part of the flow path. Thus, utilities not
involved in an interchange power transaction can be affected. A fundamental notion behind FACTS is that it is possible to contirluously vary the apparent impedance of specific transmission lines so as to force power to flow along a "contract path". This is a brand new concept for many system planners. As illustrated in Figure1.1 with precise control
of the impedance of transmission lines using FACTS devices, it is possible to maintain
constant power flow along a desired path in the presence of continuous changes of load
levels in the external ac network, and tlo react in a planned way to contingencies. Just as
in HVDC applications, FACTS controls could be designed to enhance the behavior of
the uncontrolled systems.
FIG 1.1: FACTS can direct flow along a Desired Path
The flexible system owes its tighter transmission control to its ability to manage
the interrelated parameters that constrain today's systems, including series impedance,
shunt impedance, phase angle, and the occurence of oscillations at various frequencies
below the rated frequency. By adding to flexibility in this way, the controllers enable
In a transmission line to function nearer its thermal rating. For example, a 500kV line
may have a loading limit of 1000MW to 2000 MW for safe operation, but a thermal limit of 3000 MW. It is often not possible
both to overcome these constraints and maintain the required system reliability by
conventional mechanical means alone, such as tap changers, phase shifters, and
switched capacitors and reactors (inductors). Granted, mechanical controllers are
on the whole less expensive, but they increasingly need to be supplemented by rapidly
responding power electronics controllers.
'The new technology is not a single, high-power electronic controller, but rather a
collection of controllers, which can be applied
individually or col1ectively in a specific power system to control the five interrelated
functions already mentioned. The thyristor is their basic element, just as the transistor is the basic element for a whole variety of nlicroelectronic circuit. Because all controllers
for the flexible transmission system are applications of similar technology, their use will eventually benefit from volume production and further development of high-power
electronics. Electric power networks integrate generation and load centers within
each utility system and through interconnections among neighboring systems, share
power with vast regional grids.The purpose of this is to take advantage of the diversity
of loads, changes in peak demand due to weather and time differences, the availability
of different generation reserves in various geographic regions, power sharing
arrangements among utilites, shifts in fuel prices, regulatory changes, and other
discrepancies. The rapid advancements in power electronics and microprocessors over
the last decade has had and will have a significant impact on the way utillities operate
their power systems. The use of power electronics and microprocessors will help to
decrease the transmission and distribution of electricity more reliable, controllable,
and efficient. The term FACTS is an acronym for Flexible AC Transmission System.
FACTS is the word used to describe the use of high speed power electronic controllers
to control the electric power system. The use of FACTS technology will allow greater
control of power flows, increase the loading on power lines so as the lines operate more closely to their thermal limits , enhance the system's ability to transfer power between
controlled areas, and damp power system oscillations.
1.2 Control of Power Flow
Power flow through a system depends on transmission line impedance.
Unfoltunately, it does not depend on thermal limits or ownership. Consider the example in Figure
Refering to Figure, lines AB, BC, and AC have ratings of 1000 MW,1250 MW and 2000 MW respectively. With the given impedances the three lines should Carry
600MW, 1600 MW, and 1400 MW, respectively. Notice that line BC is overloaded. By
inserting a capacitor whose reactance is 5 ohms, the impedance c)f line AC goes from
10 ohms to 5 ohms. The new power flows will be 250 MW, 1250 MVV, and 1750 MW, respectively. This system could be mechanically controlled, but such a breaker would
eventually fail due to the stress that many switchings would place on the breakers
mechanical components . Using a series capacitor that is thyristor controlled would
allow switchings as neede'd. On the other hand, the series capacitor may lead to a
subsynchronous frequency that could cause damage to thegenerator's shaft. The
switchings could bernodulated so as to damp any subsynchronous resonance conditions. A thyristor controlled series capacitor can greatly improve the stability of the network.
[attachment=31600]
1. INTRODUCTION
The flexible AC transmission system (FACTS) has received much attention in the last 2 decades. It uses high current power electronic devices to control the voltage, power flow, stability, etc. of a transmission system. FACTS technologies can essentially be defined as highly engineered power-electronics-based systems, integrating the control and operation of advanced power semiconductor- based converters (or valves) with software based information and control systems, which produce a compensated response to the transmission network that is interconnected via conventional switchgear and transformation equipment.
FACTS devices can be connected to a transmission line in various ways, such as in series with the power system (series compensation), in shunt with the power system (shunt compensation), or both in series and shunt. For example, the static VAR compensator (SVC) and static synchronous compensator (STATCOM) are connected in shunt; static synchronous series compensator (SSSC) and thyristor controlled series capacitor (TCSC) are connected in series; thyristor controlled phase shifting transformer (TCPST) and unified power flow controller (UPFC) are connected in a series and shunt combination. In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source. Series inductance occurs in long transmission lines, and when a large current flow causes a large voltage drop. To compensate, series capacitors are connected.
In shunt compensation, power system is connected in shunt with the and definition of various FACTS devices are described. The pressure associated with economical and environmental constraints has forced the power utilities to meet the future demand by fully utilizing the existing resources of transmission facilities without building new lines. FACTS devices are very effective and capable of increasing the power transfer capability of a line, as thermal limits permit, while maintaining the same degree of stability. Numerous recent applications of FACTS have proven to be cost-effective, long-term solutions. With the improvements in current and voltage handling capabilities of the power electronic devices that have allowed for the development of Flexible AC Transmission System (FACTS), the possibility has arisen in using different types of controllers for efficient shunt and series compensation. Applying FACTS on a broad-scale basis for both local and. Shunt FACTS devices are used for controlling transmission voltage, power flow, reducing reactive losses, and damping of power system oscillations for high power transfer levels.With the widespread and active consideration of the installation of FACTS controllers for better controllability.
1.1 FACTS Concepts Similar To HVDC
While some of the relevant technology i.e Static VAR Compensation is already in
Wide use, the FACTS concept has brought to the table a tremendous potential for
thyristor basedcontrollers which will surely revolutionize the power system. The
technology offers the utilities the ability to:
1. Control power flows on their transmission routes;
2. Allow secure loading of transmission lines to their full thermal capacity.
FACTS technology, while allowing use of transmissiorr to its thermal capacity, does not do away with the need for additional transmission lines or the upgrading of existing line
where thermal limits have been reached or when evaluation of losses added to the cost
of FACTS technology shows that new lines or upgrading of existing lines is the most optimum answer. Often, ac transmission systems are thought of as being "inflexible".
Power flow in ac networks simply follows Ohm's law and ordinarily cannot be made to flow along specific desired paths. As a result, ac networks suffer from parallelpath, or
"loop" flows. The power flows from source to load in inverse proportion to the relative impedances of the transmission paths. Low impedance paths take the largest fraction of
flow, but all lines in the interconnection are a part of the flow path. Thus, utilities not
involved in an interchange power transaction can be affected. A fundamental notion behind FACTS is that it is possible to contirluously vary the apparent impedance of specific transmission lines so as to force power to flow along a "contract path". This is a brand new concept for many system planners. As illustrated in Figure1.1 with precise control
of the impedance of transmission lines using FACTS devices, it is possible to maintain
constant power flow along a desired path in the presence of continuous changes of load
levels in the external ac network, and tlo react in a planned way to contingencies. Just as
in HVDC applications, FACTS controls could be designed to enhance the behavior of
the uncontrolled systems.
FIG 1.1: FACTS can direct flow along a Desired Path
The flexible system owes its tighter transmission control to its ability to manage
the interrelated parameters that constrain today's systems, including series impedance,
shunt impedance, phase angle, and the occurence of oscillations at various frequencies
below the rated frequency. By adding to flexibility in this way, the controllers enable
In a transmission line to function nearer its thermal rating. For example, a 500kV line
may have a loading limit of 1000MW to 2000 MW for safe operation, but a thermal limit of 3000 MW. It is often not possible
both to overcome these constraints and maintain the required system reliability by
conventional mechanical means alone, such as tap changers, phase shifters, and
switched capacitors and reactors (inductors). Granted, mechanical controllers are
on the whole less expensive, but they increasingly need to be supplemented by rapidly
responding power electronics controllers.
'The new technology is not a single, high-power electronic controller, but rather a
collection of controllers, which can be applied
individually or col1ectively in a specific power system to control the five interrelated
functions already mentioned. The thyristor is their basic element, just as the transistor is the basic element for a whole variety of nlicroelectronic circuit. Because all controllers
for the flexible transmission system are applications of similar technology, their use will eventually benefit from volume production and further development of high-power
electronics. Electric power networks integrate generation and load centers within
each utility system and through interconnections among neighboring systems, share
power with vast regional grids.The purpose of this is to take advantage of the diversity
of loads, changes in peak demand due to weather and time differences, the availability
of different generation reserves in various geographic regions, power sharing
arrangements among utilites, shifts in fuel prices, regulatory changes, and other
discrepancies. The rapid advancements in power electronics and microprocessors over
the last decade has had and will have a significant impact on the way utillities operate
their power systems. The use of power electronics and microprocessors will help to
decrease the transmission and distribution of electricity more reliable, controllable,
and efficient. The term FACTS is an acronym for Flexible AC Transmission System.
FACTS is the word used to describe the use of high speed power electronic controllers
to control the electric power system. The use of FACTS technology will allow greater
control of power flows, increase the loading on power lines so as the lines operate more closely to their thermal limits , enhance the system's ability to transfer power between
controlled areas, and damp power system oscillations.
1.2 Control of Power Flow
Power flow through a system depends on transmission line impedance.
Unfoltunately, it does not depend on thermal limits or ownership. Consider the example in Figure
Refering to Figure, lines AB, BC, and AC have ratings of 1000 MW,1250 MW and 2000 MW respectively. With the given impedances the three lines should Carry
600MW, 1600 MW, and 1400 MW, respectively. Notice that line BC is overloaded. By
inserting a capacitor whose reactance is 5 ohms, the impedance c)f line AC goes from
10 ohms to 5 ohms. The new power flows will be 250 MW, 1250 MVV, and 1750 MW, respectively. This system could be mechanically controlled, but such a breaker would
eventually fail due to the stress that many switchings would place on the breakers
mechanical components . Using a series capacitor that is thyristor controlled would
allow switchings as neede'd. On the other hand, the series capacitor may lead to a
subsynchronous frequency that could cause damage to thegenerator's shaft. The
switchings could bernodulated so as to damp any subsynchronous resonance conditions. A thyristor controlled series capacitor can greatly improve the stability of the network.