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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.

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?

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 couples via a

common DC link.

UNIT II- SVC and its Applications

21. Write the application of SVC.

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

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?

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-Vernier) mode

45. What is the need for variable-series compensation?



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

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

The gain Kj in the transfer function decides the magnitude of left shift in the

mode of interest

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