EE2036 Flexible AC Transmission Systems
Two Mark Question and Answers
UNIT I-Introduction
1. What is the necessity of compensation?
The reactive power through the system can significantly improve the
performance / parameters of the power system as follows
 Voltage profile
 Power angle characteristics
 Stability margin
 Damping to power oscillations
2. What are the objectives of line compensation?
 To increase the power transmission capacity of the line
 To keep the voltage profile of the line along its length within acceptable
bounds to ensure the quality of supply to the connected customer as well
as to minimize the line insulation costs
3. How is the reactive power controlled, using FACTS devices?
The SVC is a shunt device of the FACTS group, regulates voltage at its
terminals by controlling the amount of reactive power injected in to or
absorbed from the power system. When a system voltage is low, the SVC
generates reactive power (SVC Capacitive). When a system voltage is high, it
absorbs reactive power (SVC inductive)
4. How is reactive power controlled in electrical network?
Traditionally, rotating synchronous condensers and fixed or
mechanically switched capacitors or inductors have been used for reactive
power compensation. However, in recent years static VAR compensators are
used to provide or absorb the required reactive power have been developed.
5. Explain the objectives of FACTS controllers in the power system network.
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Better the control of power flow (Real and Reactive) in transmission lines.
Limits SC current
Increase the load ability of the system
Increase dynamic and transient stability of power system
Load compensation
Power quality improvement
6. What are the adv. of FACTS controllers?
 The flow of power is ordered. It may be as per the contract or as per the
requirements of the utilities
 It increases the loading capability of the lines to the thermal capability
 It improves the stability of the system and thus make the system secure
 Provides secure Tie Line connection to the neighboring utilities and regions ,
thereby decreasing overall generation reserve requirements on both sides
 Upgrade of lines
 Reduce loop flow
7. List the disadvantage of fixed series compensation.
 It is effective only during heavy loads
 Whenever an outage occurs on a line, with series compensation, the series
compensation is removed. This may cause overloading of other parallel lines
 If series compensation is added to an existing system, it is generally necessary to
have it on all the lines in parallel.
 One major drawback in the series capacitance compensation is that special
productive devices are required to protect the capacitors and bypass the high
current produced when a SC occurs
8. What is meant by thyristor switched capacitor?
TCSC is a capacitive reactance compensator, which consists of series capacitor
bank shunted by a thyristor-controlled reactor.
9. Define the term Static VAR compensator.
The SVC is a shunt device of FACTS group using power electronics to control
power flow and improve transient stability on power grids. The SVC regulates voltage at
its terminals by controlling the amount of reactive power injected into or absorbed from
the power system.
10. What are the diff types of compensation schemes?
Mainly two types of compensation are carried out,
 Load compensation
 Line compensation
11. What are the diff power electronic switching devices?
 SCR
 MOSFET
 GTO
 IGBT
 DOIDE
 BJT
12. Define the term FACTS.
 Flexible AC Transmission System
 Alternating current transmission system incorporating power electronics based and
other static controllers to enhance controllability and increase power transfer
capability
13. What is best location for SVC?
 Location of SVC strongly affects controllability of swing modes.
 In general the best location is at a point where voltage swings are greatest.
Normally, the midpoint of a transmission line between the two areas is a good
location.
14. Compare fixed series compensation and fixed shunt compensation.
 Voltage boost due to shunt compensators is uniform throughout the line.
 Power factor will be improved by the shunt capacitor whereas, series compensator
improves power system stability limit
 Protection required for the series compensator is more compared to shunt
compensator.
 Amount of voltage boost by the series capacitor is more
15. What are the main areas of application of FACTS devices?
FACTS mainly find application in following areas,
 Power transmission
 Power Quality
 Railway Grid Connection
 Wind power grid Connection
 Cable Systems
16. What is load compensation?
Load compensation is a management of reactive power to improve the quality of
supply especially the voltage and power factor levels
Three main objectives of the load compensation are
 Better voltage profile
 Power factor correction
 Load balancing
17. Define VAR compensation.
It is defined as the management of reactive power to improve the performance of
AC power systems: Maximizing stability by increasing flow of active power.
18. List the generation of facts controllers.
The following generation of FACTS controllers for the development of FACTS
controllers
First Generation of FACTS controllers:
 Static VAR Compensator (SVC)
 Thyristor Controlled Series Capacitor (TCSC)
Second Generation of FACTS controllers:
 Static Synchronous Compensator (STATCOM)
 Static Synchronous Series Compensator (SSSC)
Third Generation of FACTS Controllers:
It is designed by combining the features of previous generations Series and shunt
compensation FACTS controllers.
 Unified Power Flow Controller (UPFC)
 Interline Power Flow Controller (IPFC)
19. What are various categories of FACTS controllers?
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Series FACTS controllers
Shunt FACTS controllers
Combined series series FACTS controllers
Combined series shunt FACTS controllers
20. What is IPFC?
Interline power Flow Controller is a combination of two or more
independently controllable static synchronous series compensator (SSSC) which
are solid state voltage source converters which inject an almost sinusoidal voltage
at variable magnitude and uples via a common DC link.
UNIT II-SVC and its Applications
21. Write the application of SVC.
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SVC’s are installed to solve a variety of power system problems
Voltage regulation
Reduce voltage flicker caused by varying loads like arc furnace, etc.
Increase power transfer capacity of transmission systems.
Increase transient stability limits of a power system
Increase damping of power oscillations
22. Define the term static VAR compensator (SVC).
Static VAR Compensator is an electrical device, commonly known as SVCs, or
shunt connected devices, vary the reactive power output by controlling or switching the
reactive impedance components by means of power electronics devices. The SVC
regulates voltage at its terminals by controlling the amount of reactive power injected into
or absorb from the power system. The term “STATIC” refers to the fact that the SVC has
no moving parts. Hence it requires low maintenance.
23. What are advantages of slope in the dynamic characteristics of SVC?
 Substantially reduces the reactive power rating of the SVC for achieving nearly the
same control objectives.
 Prevents the SVC from reaching its reactive power limits too frequently
 Facilitates the sharing of reactive power among multiple compensators operating in
parallel
24. What is the best location for SVC? Justify.
 It has been proven that the midpoint of the transmission line is the optimal location
of SVC. This proof is based on the linear load which is not valid practically
 For nonlinear load model it was found that the best location for advanced Static
VAR compensator close to the receiving end where the wide range of reactive
power could be controlled.
25. What are the general characteristics of SVCs?
 The lowering of maintenance requirements due to the absence of rotating parts
 The very fast control response time
 The feasibility of individual phase control
 Reduced losses
 Highly reliable
26. List the Advantages of SVC.
 Cheaper
 Higher capacity
 Faster and more reliable
 Simple operation
 Improves steady state stability and transient stability
27. Draw VI characteristics of SVC.
28. Define voltage stability.
It is the ability of a power system to maintain steady acceptable voltages at all
buses in the system under normal operating conditions and after being subjected to a
disturbance.
29. What are the two basic modes of SVC?
 Voltage regulation mode
 VAR mode(SVC susceptance kept constant)
30. List out the prevention of voltage instability
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Placement of series and shunt capacitors
Installation of synchronous condensers
Placement of FACTS controllers
Coordination of multiple FACTS controllers
Under Voltage load Shedding
31. What are the symptoms of voltage collapse?
 Low voltage profiles
 Heavy reactive power flows
 Inadequate reactive support
 Heavily loaded system
32. What are the general characteristics of SVC?
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Lowering maintenance requirement from the absence of rotating parts
Very fast control response time
Feasibility of individual phase control
Diminished losses
High reliability
Lack of contribution to system short circuit capacity
Generation of harmonics by SVCs except thyristor switched capacitor
Variation of SVC reactive power generation as the square of terminal voltage when
it is operating outside the linear controllable range, leading to a substantial in
reactive power support at a lower voltage
33. Give the applications of synchronous condensers.
 Control of large voltage excursions
 Dynamic reactive power support at HVDC terminals
34. Give the applications of saturated reactor.
 The control of the large voltage excursions
 The alleviation of flicker
 The reactive compensation HVDC terminal
35. What are the advantages of the slope in the SVC dynamic characteristics?
 Substantially reduces the reactive power rating of the SVC for achieving nearly the
same control objectives
 Prevents the SVC from reaching its reactive power limits too frequency
 Facilitates the sharing of reactive power among multiple compensators operating in
parallel
36. Draw the power angle curve of SVC compensated SMIB system?
37. Where we can locate Svc in a transmission system?
 Buses experiencing maximum voltage excursions
 Buses where an SVC of a given rating can impart maximum electrical tamping to
the power system
 Buses where the critical voltage sensitive loads are connected
 The electrical midpoint of transmission line connecting two areas, maximum
voltage swings are likely in the absence of a SVC
UNIT III-TCSC and its Applications
38. Draw the VI Capability curves for Single Module TCSC
39. List the advantages of TCSC.
 Rapid , continuous control of transmission-line, series-compensation level
 Dynamic control of power flow in selected transmission lines within the network to
enable optimal power flow conditions and prevent the loop flow of power
 Damping of the power swings from local and inter area oscillations
 Suppression of synchronous oscillations
 Decreasing DC offset voltages.
40. What is the application of TCSC?
 Mitigation of sub synchronous resonance
 Enhancement of system damping
 Power system stability improvement
 To increase power transfer capability
41. What is meant by bypassed thyristor mode?
In this bypassed mode, the thyristor are made to fully conduct with the conduction
angle of 180 degree. The TCSC module behaves like a parallel capacitor-inductor
combination. However the net current through the module is inductive, for the
susceptance of the reactor is chosen to be greater than that of the capacitor. Also known as
the thyristor switched reactor (TSR) mode
42. What is the indication of voltage collapse points?
The Collapse points are indicative of the maximum load ability of the transmission
lines or the available transfer capability (ATC).
43. What is the effect of TCSC in SSR problem?
At sub synchronous frequency the TCSC presents an inherently resistive-inductive
reactance. The sub-synchronous oscillations cannot be sustained in the situations and
consequently get damped.
44. What are different modes of operation of TCSC?
 Bypassed thyristor mode
 Blocked thyristor mode
 Partially conducting thyristor(Capacitive-Vernier) mode
 Partially conducting thyristor(inductive-Vern
45. Enhanced base power flow and load ability of series compensator line
 Additional losses in the compensator line from the enhanced power flow
 Increased responsiveness of the power flow in the series compensated line from the
outage of other lines in the system
46. List the models of TCSC.
 Modeling for sub-synchronous resonance SSR studies
 Variable reactance model
 Transient stability model
 Long term stability model
47. How is the variation of capacitive reactance achieved in TCSC?
By varying the firing angle of the anti-parallel thyristor connected in series with the
reactor in the TCR, the fundamental frequency inductive reactance of the TCR can be
changed. This affects a change in the reactance of TCSC and it can be controlled to
produce either inductive or capacitive reactance.
48. Draw the impedance vs. delay angle characteristics of TCSC.
49. Define sub synchronous resonance (SSR)
It is an electric power system condition, where the electric network exchanges
energy with the turbine generator at one or more of the natural frequencies of the
combined system below the synchronous frequency of the system.
50. What are the causes of series compensation in long transmission lines?
 Sub-synchronous oscillations, caused by interaction between the electrical network
and the generator torsional system.
 Low frequency (1Hz-10Hz) oscillations caused by interaction between the series
capacitors and the shunt inductors, especially during line switching and faults.
These oscillations have large magnitudesand last for long periods because of high
shunt reactor Q factors.
 Switching oscillations, caused by the switching of lines.
51. What is TCSC?
TCSC is a capacitive reactance compensator, which consists of a series capacitor
bank shunted by a thyristor controlled reactor. The basic conceptual TCSC module
comprises a series capacitor, C,in parallel with a thyristor controlled reactor, Ls, in order
to provide a smoothly variable series capacitive reactance.
52. What is the basic principle of TCSC?
The basic operating principle behind the TCSC is that, it can provide a
continuously variable capacitor by means of partially cancelling the effective
compensating capacitance of the thyristor controlled reactor.
53. What are symptoms of voltage collapse?
The main symptoms of voltage collapse are low voltage profiles, heavy reactive
power flows, inadequate reactive support, and heavily loaded systems.
54. How is voltage instability identified in the power system?
Voltage instability problem is mainly because of insufficient reactive capacity of
power systems during disturbances like line outage contingencies.Voltage collapse is
mathematically indicated when the system Jacobian becomes singular.
55. What does voltage collapse means?
Voltage collapse is a loss of stability in large scale electric power systems which
causes blackout when voltages decrease terribly.
56. How is system voltage stability limit improved?
Voltage stability is primarily associated with the reactive power support. FACTS
devices can regulate the active and reactive power control as well as adaptive to voltage
magnitude control simultaneously because of their flexibility and fast control
characteristics. Placement of these devices in suitable location and proper coordination
between FACTS controllers can leads to control in line flow and maintain bus voltages in
desired level and so improve voltage stability margins and of the power systems.
57. What is Bang Bang control?
 Bang Bang control is a discrete control form in which the thyristor are either
fully switched on (α=90) or fully switched off (α=180)
 Thus, TCSC alternates between a fixed inductor and a fixed capacitor,
respectively, and it is advantageous that such control is used not only for
minimizing first swings but for damping any subsequent swings as well.
 Bang bang control is employed in face of large disturbances to improve the
transient stability.
58. What are the needs for variable series compensation?
 Enhance base power flow and load ability of the series compensated line
 Additional losses in the compensated line from the enhanced power flow
 Increase responsiveness of power flow in the series compensated line from the
outage of other lines in the system
59. What are the advantages of TCSC?
 Rapid, continuous control of the transmission line series compensation level
Dynamic control of power flow
 Damping of the power swings from local and inter area oscillations
 Suppression of synchronous oscillations
 Decreasing dc offset voltage
 Enhanced level of protection for series capacitors
 Voltage support
 Reduction of the short circuit current
60. Draw the VI capability characteristics for a two module TCSC.
61. Draw a block diagram of the variable reactance model of the TCSC.
62. What are the needs of the damping control of a TCSC?
 Stabilize both post disturbance oscillations and spontaneously growing oscillations
during normal operations
 Obviate the adverse interactions with high frequency phenomena in power system
such as network resonance
 Preclude local instabilities within the controller bandwidth
 Be robust in that it imparts the desired damping over a wide range of system
operating conditions
 Be reliable
63. What are the locations to place TCSC in a power system?
The TCSC should be located in lines that experience limiting power oscillations
 The swing of voltage on each side of the TCSC must be within acceptable limits
otherwise multiple sides may be necessary
 The control action of the TCSC in one transmission path should not cause undue
power swing in a parallel path
 Sometimes it may be advisable to distribute the control action among multiple
TCSCs rather than confining the control action to one large rating TCSC
UNIT IV-Emerging FACTS Controllers
64. What is STATCOM?
The STATCOM (or SSC) is a shunt-connected reactive power compensation device
that is capable of generating and/or absorbing reactive power and in which the output can
be varied to control the specific parameters of an electric power system.
65. State the salient features of STATCOM features.
 Compact size
 System voltage support and stabilization by smooth control over a wide range of
operating conditions
 Dynamic response following system contingencies
 High reliability with redundant parallel converter design and modular construction
 Flexibility of future reconstruction to Back to Back(BTB) power transmission or
UPFC(Unified Power Flow Control) and other configuration
66. List the application of STATCOM.
 Damping of power system oscillations
 Damping of sub synchronous oscillations
 Balanced loading of individual phases
 Reactive compensation of AC-DC converters and HVDC links
 Improvement of steady state power transfer capacity
67. Compare the V-I Characteristic of STATCOM & SVC
68. How the reactive power compensation is done using STATCOM.
A STATCOM is a controlled reactive power source. It provides the desired reactive
power generation and absorption entirely by means of electronic processing of the voltage
and current waveforms in a voltage source converter
69. List the modes of operation of STATCOM
The STATCOM can be operated in two different modes:
 Voltage regulation mode under this mode it has 3 sub divisions. There are,
 Over excited mode of operation
 Under excited mode of operation
 Normal(floating) excited mode of operation
 Var control mode
70. Draw VI characteristics of STATCOM.
71. Compare STATCOM and SVC.
 The STATCOM has the ability to provide more capacitive reactive power during
faults, or when the system voltage drops abnormally, compared to ordinary static
var compensator. This is because the maximum capacitive reactive power
generated by a STATCOM deceases linearly with system voltage, while that of the
SVC is proportional to the square of the voltage.
 Also, the STATCOM has a faster response as it has no time delay associated with
thyristor firing. Nevertheless, these advantages come at a higher price(about 20%
more)
72. What are the function of STATCOM?
 Dynamic voltage control in transmission and distribution systems.
 Power oscillation damping in power transmission systems
 Transient stability improvement
 Ability to control not only reactive power but, if needed, also active power (with a
DC energy source available)
73. Define STATCOM.
 The STATCOM has been defined as per CIGRE/IEEE with following three
operating scenarios.
 First component is static: based on solid state switching devices with no rotating
components;
 Second component is Synchronous: Analogous to an ideal synchronous machine
with 3 sinusoidal phase voltages at fundamental frequency;
 Third component is compensator: rendered with reactive compensation.
74. List the advantages/benefits of STATCOM.
The STATCOM offers following advantages:
 Superior voltage supporting capability
 Fast response
 Large reactive power generation under low system voltage condition
 Less harmonics generation
 Smaller filter capacity
 Less space requirement
75. What is UPFC?
The UPFC is a device which can control simultaneously all three parameters of line
power flow(line impedence, voltage and phase angle).Such “new” FACTS device
combines together the features of two “old” FACTS devices” the Static Synchronous
Compensator(STATCOM) and the Static Synchronous Series Compensator (SSSC). It is
proposed by Gyugyi in 1991.
76. What is role of dc link in UPFC?
The real power is supplied from, or absorbed by, the DC energy storage device called dc
link.
77. List the application of UPFC.
 Power flow control
 Power swing damping
 Voltage dips compensation
 Fault Current Limiting
78. State the salient features of UPFC.
 The UPFC is versatile and multifunction power flow controller with capabilities of
terminal voltage regulation , series line compensation and phase angle regulation
 Minimization of power losses with out generator rescheduling
 Regulating power flow through a transmission line
 More reliable
 Provides dynamic security
 Acts as harmonic isolator
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79. What are the parameters that can be improved using STATCOM in power system?
 The dynamic voltage control in transmission and distribution system
 The power oscillation damping in power transmission system
 The transient stability
 The voltage flicker control
 The control of not only reactive power but also active power in the connected line,
requiring a Dc energy source
80. What are the different constraints for operating UPFC?
 The series injected voltage magnitude
 The line current through series converter
 The shunt converter current
 The minimum line side voltage of the UPFC
 The maximum line side voltage of the UPFC
 The real power transfer between the series converter and the shunt converter
81. What are the operating modes of UPFC?
 VAR Control Mode
 Automatic Voltage Control Mode
 Direct Voltage Injection Mode
 Phase Angle Shifter Emulation Mode
 Line Impedance Emulation Mode
 Automatic Power Flow Control Mode
UNIT V-Coordination of FACTS Controllers
82. Define the term “Co-ordination”
The term coordinated implies that the controllers have been tuned simultaneously
to effect an overall positive improvement of the control scheme
83. How is coordination of FACTS controllers carried out?
Controller interactions can occur in the following combinations:
 Multiple FACTS controllers of a similar kind
 Multiple FACTS controllers of a dissimilar kind
 Multiple FACTS controllers and HVDC converter controllers
84. What is the need for coordination of different FACTS controllers?
 Need for coordination
 Adverse interaction due to fast controls
 Usually controls are tuned optimally assuming the remaining power system to be
passive
 Above parameters not optimal when dynamics of other controller are existent (
Power System Stabilizers(PSS),HVDC,FACTS)
85. What do you understand by coordination of FACTS controllers?
The term coordinated implies that the controllers have been tuned simultaneously
to effect an overall positive improvement of the control scheme
86. What is genetic algorithm (GA)?
 GA is global search technique based on mechanics of natural selection and natural
genetics. It is a general purpose optimization algorithm that is distinguished from
conventional optimization techniques by the use of concepts of population genetics
to guide the optimization search.
 These techniques provide robust, decentralized control design and are not restricted
by problems of non-differentiability, non-linearity, and non-convexity, all of which
are often limiting in optimization exercises.
87. List the advantages of genetic algorithm.
An advantage of genetic algorithm techniques is that the parameter limits can be
varied during the optimization, making the techniques computationally efficient.
88. List the possible combination of FACTS controller interactions.
Multiple FACTS controllers of a similar kind
 Multiple FACTS controllers of a dissimilar kind
 Multiple FACTS controllers and HVDC converter controllers
89. Give The frequency range of the different control interactions
 0 Hz for steady state interactions
 0-3Hz for electromechanical oscillations
 2-15Hz for small signal or control oscillations
 >15 Hz sub synchronous resonance interactions
 >15 Hz for electromagnetic transient high frequency resonance or harmonic
resonance interactions, and network resonance interactions
90. What are the basics procedures of the controller design?
 Derivation of the system model
 Enumeration of the system performance specifications
 Selection of the measurement and control signals
 Coordination of the controller design
 Validation of the design performance evaluation
91. Write the assumptions of control coordination for damping enhancement?
 All controllers in the system including FACTS have the transfer function of the
type kjGj (S)
 The component Gj(S) in the transfer function is responsible for causing the left
shift in the electromechanical mode
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The gain Kj in the transfer function decides the magnitude of left shift in the mode
of interest