TCSC FOR PROTECTION OF TRANSMISSION LINE
TCSC FOR PROTECTION OF TRANSMISSION LINE P.S.Chaudhari#i, #,,3,4
P. P.Kulkarni#2, R.M.Holmukhe#3, Mrs.P.A.Kulkarni #4
#iScientist, DRDO, Pune, India, #2DRDO, Pune, India Bharati Vidyapeeth University College of Engineering, Pune, India. #3
[email protected],
Abstract: A grid of transmission
lines operating at high or extra high voltages is required to transmit power from generating stations to load. In addition to transmission lines that carry power from source to load, modern power systems are highly interconnected for economic reasons. The large interconnected transmission networks are prone to faults due to the lightning discharges and reduce insulation strength. Changing loads and atmospheric conditions are unpredictable factors. This may cause overloading of lines due to which voltage collapse takes place. All the above said things are undesirable for secure and economic operation of a line. These problems can be eased by providing sufficient margin of working parameters and power transfer, but it is not possible due to expansion of transmission network. Still the required margin is reduced by introduction of fast dynamic control over reactive and active power by high power electronic controllers. Here my seminar explains the discussion on the effect of this device on protection of transmission line. The TCSC is considered as a dynamical device and its transient process is modeled in order to have the response to disturbances based on its own control strategy. It is shown that not only the TCSC affects the protection of its line, but also the protection of adjacent lines would experience problems. The study is done first analytically by using simple models, then the simllated results of power system and the protective relays in Real Time Digital Simulator (RTDS) are used. Finally, the simulation results are varified by using a commercial relay. The results are reviewed for the study of the impact of TSCS on protection of transmission line
Keywords- TCSC, Transmission
line
I.INTRODUCTION
Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages. Overhead transmission lines are uninsulated wire, so
design of these lines requires minimum clearances to be observed to maintain safety. Engineers design transmission networks to transport the energy as efficiently as feasible, while at the same time taking into account economic factors, network safety and redundancy. These networks use components such as power lines, cables, circuit breakers, switches and transformers. Failure protection Under excess load conditions, the system can be designed to fail gracefully rather than all at once. Brownouts occur when the supply power drops below the demand. Blackouts occur when the supply fails completely. Rolling blackouts,or load shedding, are intentionally-engineered electrical power outages, used to distribute insufficient power when the demand for electricity outstrips the supply. Communications Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protection relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored. Protection of the transmission line from short circuits and other faults is usually so critical that common carrier telecommunications are insufficiently reliable. In remote areas a common carrier may not be available at all. Communication systems associated with a transmission project may use: Microwaves or Power Line Communication. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical or thermal limit of the line. Deregulation of electricity companies in many countries has led to renewed interest in reliable economic design of transmission networks. Transmission line protection depends upon a core group of protective elements. These elements must be dependable and secure for all power system conditions as any single weakness can cause problems with a relaying scheme. Some common concerns related to a line distance relay are operating speed, load and fault impedance induced element under- and overreach, out-of-step (OOS) conditions, and Capacitive Voltage Transformer (CVT) transients.
A major program to develop new technologies for enhancing power transmission has been initiated. This program, named "FACTS' for "Flexible AC Transmission Systems," is intended to provide new systems and methods of operation to help electric utilities get the most from their investments in transmission networks. Scoping studies were conducted in 1989-1990 to identify the benefits of FACTS concepts and to provide direction for the development of hardware systems. One outcome of those studies was to show that a ThyristorControlled Series Compensation (TCSC) system should be developed, as a number of benefits could be achieved in a cost-effective manner [1,3]. What is FACTS? The FACTS technology is a collection of controllers, which can be applied individually or in coordination with others to control one or more of the interrelated system parameters, such as series impedance, shunt impedance, current, voltage, and damping of oscillations.
Fig 1. FACTs Controller FACTS controller is defmed as power electronic based system and other static equipment that provide control of one or more AC transmission system parameters. Out of no. of FACTS devices the one which is very efficient is the TCSC.
roles in the operation and control of power scheduling power flow; decreasing components; reducing net loss; providing limiting short-circuit currents; mitigating resonance (SSR); damping the power enhancing transient stability.
systems, such as unsymmetrical voltage support; sub synchronous oscillation; and
Advances in high-power, high-efficiency power electronics have led to the development of thyristor-controlled series compensators in power systems. In contrast to capacitors switched by circuit breakers, TCSC will be more effective because thyristors can offer flexible adjustment, and more advanced control theories can be easily applied. Series capacitor is used in a long distance EHV lines for increasing power transfer. Use of series capacitor is for the most economic enhancing power flow though series have a problem of SSR. Series is only is used for power transfer as compared to shunt. Shunt has the main problem of location not of SSR. To provide variable series compensation thyristor control is used due to which the major problem of SSR is been reduced to much lesser extent. The first demo project of TCSC was done at west Virginia USA. In 1991.in October 1992 l" TCSC was installed at Arizona substation. and then in1993 in Oregon. Series compensation can be achieved in two ways discrete and continuos .Discrete by using TCSC and continuos by using TCSC or GTO. TCSC- thyristor
controlled
series capacitor
Classification of FACTS controllers a) shunt connected b) series connected c) combined series-series d) combined series - shunt Classification of FACTS controllers by use of power devices A) variable impedence type B) voltage source converter based which includes VCS includes i. SVC- static var compensator ii. TCSC- thyristor controlled series capacitor iii. TCPSFthyristor controlled phase shift transformer iv. STATCOM- static synchronous compensator v. SSSC - static synchronous series compensator vi. IPFC- interline power flow controllers vii. UPFC- unified power flow controllers Why TCSC only? There have been significant activities and achievement in the research and application of flexible AC transmission systems (FACTS). Thyristor-controlled series compensation (TCSC) is an important device in the FACTS family. It can have various
Fig 2.
Basic TCSC diagram
Fig 3. TCSC with sries inductor for current control The above two figures shows that TCR is used in parallel with a fixed capacitor to enable continuos control over series compensation. Harmonics are present in thyristor switches can be mitigated by TCSC along with SSR. GCSC is same as
shown above only SCR is replaced by a GTO. In between two SCR is more advantageous as it has lower cost.
Equivalent circuit of TCSC
Xc
~rJ Xtcr
~
< Iter capacitive
operation
Xc
~r~ Xtc:r
~ Iter ;;.
Inductive operation
Fig 4. Equivalent circuit ofTCSC
Operation of TCSC Ld
circuit breaker
B
capacitor, MOV and bypass switches are used for its protection, while for the TCSC, co-operative operation of MOV, thyristors and bypass switches are applied for a reliable protection. This is the main difference in the behaviour of the' distance relays in the two compensation methods. The apparent impedance seen by the distance relay depends largely on the TCSC mode of operation. The TCSC mode of operation during a fault is not unique and it may transit from a mode to others sequentially; meanwhile the distance relay experiences various apparent impedances in the fault period. In normal operating mode, called capacitive-boost mode or vernier mode, the thyristor are firing properly and the bypass switch is open. From the system point of view, this mode inserts capacitors into the line up to nearly three times the fixed capacitor. TCSC controls fire pulse to change thermistor's fire angle and make TCSC to operate in thyristor blocked mode, thyristor bypassed mode and vernier mode with partial thyristor conduction. The vernier mode let a inductive loop current to flow through capacitor produce a capacitive reactance which is larger than that of fused capacitor. Normally, TCSC's stability fundamental frequency impedance is relative only to fire angle a. When a is between 145 degree and 180 degree ,the equivalent reactance is capacitive, larger a is corresponding to less capacitor reactance. When a is between 90deg and l40deg , the equivalent reactance is inductive, larger a is corresponding to larger inductive reactance. When a fault occurs, TCSC's control system would react swiftly to take some protection ensures. If short circuit current is large enough, MOV would be fired, TCSC's control system would rapidly send its commands to bypass thyristor and make TCSC become inductive. So TCSC's capacitive reactance would decrease and gradually change into inductive reactance. This characteristic makes it possible for distance relay with memory polarizing voltage and proper setting value to be used in TCSC line. If short circuit current is not large enough, TCSC's control system will not send its commands to bypass the thyristors. x A
capacitive
C
B Une cul'tent
Fig. 1
Simple power system compensated by TCSC
Figure shows a TCSC module with different protective elements [6] in the middle of a simple power system. Basically, it comprises a series capacitor C in parallel with a thyristor-controlled reactor (TCR), Ls. A metal-oxide varistor MOV is connected across the series capacitor to prevent the occurrence of high capacitor over voltages. A circuit breaker is also installed across the TCSC module to bypass it if a severe fault or equipment malfunction occurs. A currentlimiting inductor Ld is incorporated in the circuit to restrict both the magnitude and the frequency of the capacitor current during the capacitor-bypass operation. There are different modes of operation of TCSC in the normal and fault conditions. During a fault, an overvoltage appears across the TCSC due to the fault current. For a conventional series
o
_--~G
Inductive
E
Fig.2
OIK'>"
P. P.Kulkarni#2, R.M.Holmukhe#3, Mrs.P.A.Kulkarni #4
#iScientist, DRDO, Pune, India, #2DRDO, Pune, India Bharati Vidyapeeth University College of Engineering, Pune, India. #3
[email protected],
Abstract: A grid of transmission
lines operating at high or extra high voltages is required to transmit power from generating stations to load. In addition to transmission lines that carry power from source to load, modern power systems are highly interconnected for economic reasons. The large interconnected transmission networks are prone to faults due to the lightning discharges and reduce insulation strength. Changing loads and atmospheric conditions are unpredictable factors. This may cause overloading of lines due to which voltage collapse takes place. All the above said things are undesirable for secure and economic operation of a line. These problems can be eased by providing sufficient margin of working parameters and power transfer, but it is not possible due to expansion of transmission network. Still the required margin is reduced by introduction of fast dynamic control over reactive and active power by high power electronic controllers. Here my seminar explains the discussion on the effect of this device on protection of transmission line. The TCSC is considered as a dynamical device and its transient process is modeled in order to have the response to disturbances based on its own control strategy. It is shown that not only the TCSC affects the protection of its line, but also the protection of adjacent lines would experience problems. The study is done first analytically by using simple models, then the simllated results of power system and the protective relays in Real Time Digital Simulator (RTDS) are used. Finally, the simulation results are varified by using a commercial relay. The results are reviewed for the study of the impact of TSCS on protection of transmission line
Keywords- TCSC, Transmission
line
I.INTRODUCTION
Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages. Overhead transmission lines are uninsulated wire, so
design of these lines requires minimum clearances to be observed to maintain safety. Engineers design transmission networks to transport the energy as efficiently as feasible, while at the same time taking into account economic factors, network safety and redundancy. These networks use components such as power lines, cables, circuit breakers, switches and transformers. Failure protection Under excess load conditions, the system can be designed to fail gracefully rather than all at once. Brownouts occur when the supply power drops below the demand. Blackouts occur when the supply fails completely. Rolling blackouts,or load shedding, are intentionally-engineered electrical power outages, used to distribute insufficient power when the demand for electricity outstrips the supply. Communications Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protection relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored. Protection of the transmission line from short circuits and other faults is usually so critical that common carrier telecommunications are insufficiently reliable. In remote areas a common carrier may not be available at all. Communication systems associated with a transmission project may use: Microwaves or Power Line Communication. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical or thermal limit of the line. Deregulation of electricity companies in many countries has led to renewed interest in reliable economic design of transmission networks. Transmission line protection depends upon a core group of protective elements. These elements must be dependable and secure for all power system conditions as any single weakness can cause problems with a relaying scheme. Some common concerns related to a line distance relay are operating speed, load and fault impedance induced element under- and overreach, out-of-step (OOS) conditions, and Capacitive Voltage Transformer (CVT) transients.
A major program to develop new technologies for enhancing power transmission has been initiated. This program, named "FACTS' for "Flexible AC Transmission Systems," is intended to provide new systems and methods of operation to help electric utilities get the most from their investments in transmission networks. Scoping studies were conducted in 1989-1990 to identify the benefits of FACTS concepts and to provide direction for the development of hardware systems. One outcome of those studies was to show that a ThyristorControlled Series Compensation (TCSC) system should be developed, as a number of benefits could be achieved in a cost-effective manner [1,3]. What is FACTS? The FACTS technology is a collection of controllers, which can be applied individually or in coordination with others to control one or more of the interrelated system parameters, such as series impedance, shunt impedance, current, voltage, and damping of oscillations.
Fig 1. FACTs Controller FACTS controller is defmed as power electronic based system and other static equipment that provide control of one or more AC transmission system parameters. Out of no. of FACTS devices the one which is very efficient is the TCSC.
roles in the operation and control of power scheduling power flow; decreasing components; reducing net loss; providing limiting short-circuit currents; mitigating resonance (SSR); damping the power enhancing transient stability.
systems, such as unsymmetrical voltage support; sub synchronous oscillation; and
Advances in high-power, high-efficiency power electronics have led to the development of thyristor-controlled series compensators in power systems. In contrast to capacitors switched by circuit breakers, TCSC will be more effective because thyristors can offer flexible adjustment, and more advanced control theories can be easily applied. Series capacitor is used in a long distance EHV lines for increasing power transfer. Use of series capacitor is for the most economic enhancing power flow though series have a problem of SSR. Series is only is used for power transfer as compared to shunt. Shunt has the main problem of location not of SSR. To provide variable series compensation thyristor control is used due to which the major problem of SSR is been reduced to much lesser extent. The first demo project of TCSC was done at west Virginia USA. In 1991.in October 1992 l" TCSC was installed at Arizona substation. and then in1993 in Oregon. Series compensation can be achieved in two ways discrete and continuos .Discrete by using TCSC and continuos by using TCSC or GTO. TCSC- thyristor
controlled
series capacitor
Classification of FACTS controllers a) shunt connected b) series connected c) combined series-series d) combined series - shunt Classification of FACTS controllers by use of power devices A) variable impedence type B) voltage source converter based which includes VCS includes i. SVC- static var compensator ii. TCSC- thyristor controlled series capacitor iii. TCPSFthyristor controlled phase shift transformer iv. STATCOM- static synchronous compensator v. SSSC - static synchronous series compensator vi. IPFC- interline power flow controllers vii. UPFC- unified power flow controllers Why TCSC only? There have been significant activities and achievement in the research and application of flexible AC transmission systems (FACTS). Thyristor-controlled series compensation (TCSC) is an important device in the FACTS family. It can have various
Fig 2.
Basic TCSC diagram
Fig 3. TCSC with sries inductor for current control The above two figures shows that TCR is used in parallel with a fixed capacitor to enable continuos control over series compensation. Harmonics are present in thyristor switches can be mitigated by TCSC along with SSR. GCSC is same as
shown above only SCR is replaced by a GTO. In between two SCR is more advantageous as it has lower cost.
Equivalent circuit of TCSC
Xc
~rJ Xtcr
~
< Iter capacitive
operation
Xc
~r~ Xtc:r
~ Iter ;;.
Inductive operation
Fig 4. Equivalent circuit ofTCSC
Operation of TCSC Ld
circuit breaker
B
capacitor, MOV and bypass switches are used for its protection, while for the TCSC, co-operative operation of MOV, thyristors and bypass switches are applied for a reliable protection. This is the main difference in the behaviour of the' distance relays in the two compensation methods. The apparent impedance seen by the distance relay depends largely on the TCSC mode of operation. The TCSC mode of operation during a fault is not unique and it may transit from a mode to others sequentially; meanwhile the distance relay experiences various apparent impedances in the fault period. In normal operating mode, called capacitive-boost mode or vernier mode, the thyristor are firing properly and the bypass switch is open. From the system point of view, this mode inserts capacitors into the line up to nearly three times the fixed capacitor. TCSC controls fire pulse to change thermistor's fire angle and make TCSC to operate in thyristor blocked mode, thyristor bypassed mode and vernier mode with partial thyristor conduction. The vernier mode let a inductive loop current to flow through capacitor produce a capacitive reactance which is larger than that of fused capacitor. Normally, TCSC's stability fundamental frequency impedance is relative only to fire angle a. When a is between 145 degree and 180 degree ,the equivalent reactance is capacitive, larger a is corresponding to less capacitor reactance. When a is between 90deg and l40deg , the equivalent reactance is inductive, larger a is corresponding to larger inductive reactance. When a fault occurs, TCSC's control system would react swiftly to take some protection ensures. If short circuit current is large enough, MOV would be fired, TCSC's control system would rapidly send its commands to bypass thyristor and make TCSC become inductive. So TCSC's capacitive reactance would decrease and gradually change into inductive reactance. This characteristic makes it possible for distance relay with memory polarizing voltage and proper setting value to be used in TCSC line. If short circuit current is not large enough, TCSC's control system will not send its commands to bypass the thyristors. x A
capacitive
C
B Une cul'tent
Fig. 1
Simple power system compensated by TCSC
Figure shows a TCSC module with different protective elements [6] in the middle of a simple power system. Basically, it comprises a series capacitor C in parallel with a thyristor-controlled reactor (TCR), Ls. A metal-oxide varistor MOV is connected across the series capacitor to prevent the occurrence of high capacitor over voltages. A circuit breaker is also installed across the TCSC module to bypass it if a severe fault or equipment malfunction occurs. A currentlimiting inductor Ld is incorporated in the circuit to restrict both the magnitude and the frequency of the capacitor current during the capacitor-bypass operation. There are different modes of operation of TCSC in the normal and fault conditions. During a fault, an overvoltage appears across the TCSC due to the fault current. For a conventional series
o
_--~G
Inductive
E
Fig.2
OIK'>"
