Q.B. FACTS

Subject code/Name: EE 2306-Flexible AC Transmission Systems

QUESTION BANKUNIT I INTRODUCTION

PART – A 1. What are the applications of FACTS devices? (Dec ’14)

FACTS are used to set in order the flow of power. It may be as per the contract or as per the requirements of the utilities. It is used to increase 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.

2. Explain the need for FACTS controllers. (Dec ’12)In the evolving utility environment, financial and market forces are demand a more optimal and profitable operation of the power system with respect to generation, transmission, and distribution. So, FACTS controllers are required to achieve both operational reliability and financial profitability, it has become clear that more efficient utilization and control of the existing transmission system.

3. Define the term FACTS. (June ’12)Flexible AC Transmission System is defined as the Alternating current transmission system incorporating power electronics based and other static controllers to enhance controllability and increase power transfer capability

4. State the objectives of FACTS controllers.FACTS controllers are required to achieve both operational reliability and financial profitability. They control the power flow (Real and Reactive) in transmission lines by• Limiting the SC current• Increasing the loadability of the system• Increasing the dynamic and transient stability of the power system• Improving the Load compensation and works for Power quality improvement

5. What is meant by passive compensation? The methods of supplying or absorbing reactive power in transmission line in order to

achieve effective power transmission in line is called principle of reactive power compensation.

The passive devices such as shunt capacitors shunt reactors, series capacitors and synchronous condenser used for this reactive power compensation purpose is called passive compensation.

6. Why need of reactive power compensation in transmission lines?The increase in electrical energy demand has presented higher requirements from the power industry. However the most commonly used devices in present power grid are the mechanically controlled circuit breakers. The long switching periods and discrete operation make them difficult to handle the frequently changed loads smoothly and damp out the transient oscillations quickly. In order to compensate these drawbacks, reactive power compensation is required for large operational margins and redundancies are maintained to protect the system from dynamic variation and recover faults will increase the stability and the security of the power systems.

FACTS 8 KCE/EEE/QB/IVYR/FACTS


7. What are the principles of shunt and series reactive power compensation? The methods of supplying or absorbing reactive power in transmission line in order to

achieve effective power transmission in line is called principle of reactive power compensation.

In principle, series controllers inject voltage in series with the line and the shunt controllers inject current into the system at the point of connection. The combined series shunt controllers inject current into the system with the shunt part of the controllers and voltage in series in the line with the series part of the controllers.

8. What is the use of shunt compensation? Regulate the voltage magnitude Improve the voltage quality Enhance the system stability

9. What are the objectives of FACTS? FACTS technologies allow for improved transmission system operation with minimal

infrastructure investment, environmental impact and implementation time compared to the construction of new transmission lines

Provide advanced solutions to cost effective alternatives to new transmission line construction.

10.What are the uses of shunt capacitors in transmission system?Shunt capacitors used in transmission line increases the power transfer capacity and to compensate for the reactive voltage drop in the line. Shunt capacitors banks are always connected to the bus rather than to the line. They are connected either directly to the high voltage bus or to the tertiary winding of the main transformer. Shunt capacitor banks are breaker switched either automatically by a voltage relays of manually.

11.What is the purpose of shunt compensation in transmission system near load side?The primary purpose of transmission system shunt compensation near load areas is voltage control and load stabilization. In other words, shunt capacitors are used to compensate for XI 2

losses in transmission system and to ensure satisfactory voltage levels during heavy load conditions.

12.What are the tasks for dynamic series compensation? Reduction of transmission angle Increase of system stability Reduction of load dependent voltage drops Reduction of system transfer impedance Load flow control for specified power paths Damping of active power oscillations.

13.What are the disadvantages of synchronous condensers? They require substantial foundations and a significant amount of starting and protective

equipment. They also contribute to the short circuit current and they cannot be controlled fast enough to

compensate for rapid load changes. Moreover their losses are much higher than those associated with static compensators, and the

cost is much higher compared with static compensators.


Subject code/Name: EE 2306-Flexible AC Transmission Systems14.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

15.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 i) Better voltage profile, ii)Power factor correction and iii) Load balancing

16.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.

17.Define Reactive Power. (Dec ’14) Reactive power is the component of power that oscillates back and forth through the lines,

being exchanged between electric and magnetic fields and not getting dissipated. It is denoted by the symbol Q and reactive power is measured in VAR (also written Var or VAr),

for volt-ampere reactive. Its magnitude is given below and the angle is the phase differenceᵩ between voltage and current.

Q = I RMS V RMS Sin Φ

18.Distinguish between reactive power absorbers and reactive power suppliers.It is common, that devices which consume the reactive inductive current are called reactive power absorbers or receivers, while devices supplying reactive capacitive current are referred to as reactive power suppliers. Most of the industrial equipment consumes reactive power as they are inductive in nature viz.. electric motors, transformers, conductors, choke, converters, arc furnaces and power electronic devices etc.. The devices such as capacitor banks and synchronous motors are sources of reactive power.

19. What are the advantages of FACTS controllers? (May ’13)-2FACTS technologies allow for improved transmission system operation with minimal infrastructure investment, environmental impact, and implementation time compared to the construction of new transmission lines and as well as provide advanced solutions to cost-effective alternatives to new transmission line construction.

20.State the features of Interline Power Flow Controller

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.


Subject code/Name: EE 2306-Flexible AC Transmission Systems21.Define the term IPFC

The interline power flow controller (IPFC) addresses the problem of compensation a number of transmission lines at a given substation. With IPFC active power can be transferred between deferent lines. Therefore, it is possible to:

Equalize both active and reactive power flows between lines Reduce the burden of overloaded lines by active power transfer Compensate against resistive line voltage drops and the corresponding reactive power

demand, And increase the electiveness of the overall compensating system for dynamic

disturbances.

22.State the features of Interline Power Flow Controller (IPFC). (Apr ‘15)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 Electro mechanical mode• The gain Kj in the transfer function decides the magnitude of left shift in the mode of interest

23.How is reactive power controlled in an electrical network? The reactive power can be compensated using VAR generators, avoiding its circulation

between the load (inductive or capacitive) and the source, and therefore improving stability of the power system.

In principle, series controllers inject voltage in series with the line and the shunt controllers inject current into the system at the point of connection. The combined series shunt controllers inject current into the system with the shunt part of the controllers and voltage in series in the line with the series part of the controllers.

PART – B1. Describe briefly the load and system compensation schemes.2. Explain in detail about series and shunt compensation in transmission lines. (June ’12)-23. Explain the operation of UPFC with diagram.4. Explain the reactive power compensation at the sending, mid-point and receiving ends of the transmission lines.5. Explain the principle, working and characteristics of static VAR compensator with a neat circuit diagram.6. Explain the working and characteristics of Thyristor switched series capacitor with a neat diagram.7. Discuss briefly about the variation of the TCSC reactance with firing angle ‘alpha’.8. Explain the V-I capability characteristics of single module TCSC.9. Explain with neat circuit diagram about fixed capacitor-Thyristor controlled reactor.10. (i) Explain briefly about load compensation (4) (Apr ‘15)

(ii) What are the objectives of line compensation? Explain the effect of shunt and series compensation on power transmission capacity of a short symmetrical transmission line. (12)

11. Explain in detail about that classification of different FACTS controller.12. (a) (i) Explain the concept and need for reactive power. (ii) Discuss the possible and control actions to maintain the voltage at rated value in transmission line.13. Explain the effect of shunt and series compensation power on power transmission capacity.14. (i) Give the complete analysis of lossless distributed parameter transmission lines and drive


Subject code/Name: EE 2306-Flexible AC Transmission Systems power equations for symmetrical case15. What is the objectives of line compensation? Explain the effect of shunt and series compensation on power transmission capacity of a short symmetrical transmission line.

UNIT II - STATIC VAR COMPENSATOR (SVC) AND APPLICATIONSPART – A

1. Compare the Performance of STATCOM and SVC. (May ’13)S.No STATCOM SVC

1 It operates as a shunt connected synchronous voltage source

It operates as a shunt connected controlled reactive admittance

2 STATCOM is more effective than the SVC in providing voltage support under large system disturbances

SVC is not more effective than the STATCOM in providing voltage support under large system disturbances

3 In non linear operating range, the STATCOM is able to control its output current over the rated maximum capacitive or inductive range independently of AC system voltage.

The maximum attainable compensating current of the SVC decreases linearly with AC voltage.

2. 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.

3. Define the droop in VI characteristics of SVCThe slope are droop of the VI characteristics of SVC is defined as the ratio of voltage magnitude change to current magnitude change over the linear controlled the range of the compensator. It is expressed in Ohms. Slope = Change in voltage/ Change in Current

4. Write the application of SVC.SVC’s are installed to solve a variety of power system problems like,• 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

5. 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.

6. Draw the control characteristics of SVC.



7. Draw the Power angle curve of SVC. (Dec ’14)

8. Write the advantages of slope in SVC dynamics characteristics (May ’13)-4 Significance reduction of the reactive power rating of the SVC for achieving same control

objectives Prevention of reaching reactive power too frequently Sharing of reactive power during operation of multiple compensators in parallel.

9. What are the advantages of SVC? Used to achieve improved transient stability of system. Used to change the susceptance of passive devices to control reactive power Used to improve the power transfer capacity of a transmission line, which is also

characterized as the steady state power transfer limit. During failure of the system, SVC inject reactive power to the system, to raise systems voltage,

thereby increase the active power absorbed by load to achieve the inhibition of power oscillation.

10.Compare fixed series compensation and fixed shunt compensation. (Dec ’12)• 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

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


Subject code/Name: EE 2306-Flexible AC Transmission Systems• 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

12.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.

13. 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.

14.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.

15.List the applications of SVC. Improvement of the system stability limit Enhancement of transient stability Prevention of voltage collapse Power system oscillation damping enhancement.

16.How will you consider influence of SVC on system voltage without considering coupling transformer? The efficient use of SVC in regulating the system voltage is dependent on the relative

strength of the connected AC system. The system strength or equivalent system impedance mainly determines the magnitude of voltage variation caused by the change in the SVC reactive current.

In this representation, the effect of the coupling transformer is not considered and the SVC is modeled as a controlled susceptance at the high voltage bus.

17. How will you consider influence of SVC on system voltage with considering coupling transformer?

The representation of the SVC including coupling transformer creates a low voltage bus Connected to the SVC and the transformer reactance XT is separated from source side XS.

18.How the SVC is employed for prevention of voltage instability?


Subject code/Name: EE 2306-Flexible AC Transmission Systems The Static VAR Compensator is frequently used to regulate the voltage at dynamic loads. But

also it is used to provide a voltage support inside of a power system when it takes place small gradual system changes such as natural increase in system load, or large sudden disturbances such as loss of a generating unit or a heavily loaded line.

These events can alter the pattern of the voltage waveform in such a manner that it can damage or lead to mal-function of the protection devices.

19.What are the functional requirements of SVC used for transient stability improvement band power oscillation damping?The functional requirements of static VAR compensators, used for transient stability improvements, power oscillation damping can be stated simply as follows.

They must be able to stay in synchronism with the voltage under the all conditions, including major disturbances.

They must be able to regulate or control rapidly the terminal voltage by generating reactive power, or absorbing it from, the ac power system.

20. Define SVC dynamic interactions. The conventional static VAR compensators (SVC’s) which are composed of thyristor controlled reactors (TCR’s) and either fixed capacitors (FC’s). it is to be needed that to investigate contributions of SVCs and their associated voltage control loops to the phenomenon of small signal dynamics of power systems in the high frequency range (above 4 Hz).

PART – B

1. Explain the design of SVC voltage regulator. Also discuss the influence of SVC on system voltage2. Discuss in detail the effect of SVC for the enhancement of transient stability. (June ’12)3. Explain the basic construction, working and characteristic of any one type of SVC. (June ’12)4. Using a general schematic diagram, explain the three basic modes of SVC control in detail.5. Derive the Voltage and Power expression in SVC (Dec ’14)6. Explain the application of SVC for prevention of voltage instability.7. How do you enhance the damping in power system using SVC8. Explain the design of SVC voltage regulator and discuss the voltage control capability of it.(Jun ’12)9. What are the advantages of slope in dynamic characteristics of SVC?10. Discuss in detail about the role of SVC in improving the stability limit and enhancing the power system damping.11. Describe the construction and operating characteristics of synchronous condensers.12. Derive and explain the series & shunt compensation of symmetrical transmission lines. (Jun ’12)13. An SVC is connected to 765 kV system has a reactive power range of 550 MVAR production to 250 MVAR absorption. If the per unit proportional gain of voltage regular is to be 0.-65 determine the short-circuit level of the system. The SVC has a slope of 3%. (Dec ’12)14. Explain about IEEE basic models that are available for the SVC to represent in the stability studies? (Dec ’12)15. Consider a 765kV symmetrical lossless transmission line with i=0.965mH/km, c=11.6nf/km, and a line length of 800km.calculate the MVAR that is required to maintain the midpoint voltage at 1.04 p.u. when the power flow through the line is in range c>f 750-850MW. (Dec ’12)16. Consider a SMI8 system in which the synchronous machine is generating 0.9 p.u. MW and 0.3 p.u. MVAR. The voltage of Infinite bus is 0.995+j0.0 p.u. The machine transient reactance is 0.3p.u. and


Subject code/Name: EE 2306-Flexible AC Transmission Systemsthe transmission •line reactance is 0.650 p.u. -(a) Calculate what should be the net susceptance of SVC to maintain Vm at 1 p.u. 4b) Calculate synchronizing torque co-efficient and with and without SVC at midpoint of the line. (Dec ’12)17. (i) Consider a 765 kV symmetrical transmission line with i=0.965mH/km, c=11.6nf/km, and a line length of 1000km. Calculate the voltage at a distance of 550 km from the sending end when the power flow through the line is 860MW. (4) (ii) Derive from the fundamentals the expressions for the voltage and current at any distance X from the sending end of lossless distributed parameter line (12) (May ’13)18. Explain how the SVC will be able to regulate the HVAC bus voltage. Derive the V-I characteristics of power system and SVC(FC+TCR)19. Explain uncompensated Transmission line. (Dec ’14)20. Describe the working principle of the two types of Static VAR Compensator with neat schematic diagrams. (Apr ‘15)

UNIT III - THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC) AND APPLICATIONS

PART – A

1. 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.

2. What is meant by bypassed Thyristor mode? (June ’12)In bypass mode the thyristor valve is triggered continuously and therefore the valve stays

conducting all the time. In this mode, the thyristors are made to fully conduct with a conduction angle of 1800. Gate pulses are applied as soon as the voltage across the thyristors reaches zero and becomes positive, resulting in a continuous flow of current through the thyristor valves. The TCSC behaves like a parallel connection of the series capacitor and the inductor which gives the equivalent reactance.

3. What is meant by thyristor switched series capacitor (TCSC)?A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor switched reactor to provide a step wise control of series capacitive reactance.

4. What is the indication of voltage collapse points? (June ’12)The Collapse points are indicative of the maximum load ability of the transmission lines or the available transfer capability (ATC). The TCSC’s can significantly enhance the load ability of transmission networks, thus obviating voltage collapse at existing power transfer levels.

5. 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.

6. State the advantages of TCSC. (Apr ‘15)


Subject code/Name: EE 2306-Flexible AC Transmission Systems Cheaper • Higher capacity• Faster and more reliable• Simple operation• Improves steady state stability and transient stability 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.

7. What is the application of TCSC?• Mitigation of sub synchronous resonance• Enhancement of system damping• Power system stability improvement• To increase power transfer capability

8. What is TCSC? (Apr ’14)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.

9. 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

10.What is blocked Thyristor mode in TCSC operation? In this mode the thyristor value is not triggered and the thyristors are kept in non conducting

state. If the thyristor are conducting and a blocking command is given, the thyristors turn off as soon as the current through them reaches a zero crossing.

The line current passes only through the capacitors bank (XTCSC = XC). In this mode the TCSC performs like a fixed series capacitor and the net TCSC reactance is capacitive.

11.State the need for variable series compensation Enhanced base-power and load ability of the series-compensated line Minimize additional losses in the compensated line from the enhanced power flow Increased awareness of power flow in the series-compensated line from the outage of other

lines in the system.


Subject code/Name: EE 2306-Flexible AC Transmission Systems12.What are the main advantages of mechanically switched capacitor banks compared to

static VAR compensators?Compared to static VAR compensators, mechanically switched capacitor banks have the advantage of much lower cost and switching speeds can be quite fast. Following a transmission line outage, capacitor bank energization should be delayed to allow time for line reclosing. However capacitor switching should be before significant amounts of load are restored by transformer tap changer or distribution voltage regulators.

13.What is Bang-Bang control in TCSC?Bang Bang control is a discrete control form in which the thyristors are either fully switched on ( =90) or fully switched off ( =180). Thus, TCSC alternates between a fixed inductor and a fixedα α 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.

14.What are the functions of a damping control of a TCSC? During normal operation stabilize both post disturbance oscillations and impulsively

increasing oscillations. Prevent the high frequency resonance network interaction Prevent local instabilities within the controller bandwidth.

15.What is the difference between Thyristors Switched Series capacitor and TCSC? (June ’12)-2

The difference is based on the operating modes of thyristor valve following variants of the TCSC Occur: Thyristor switched series capacitor (TSSC), which follows a discrete control of the capacitive reactance. Thyristor controlled series capacitor (TCSC), which allows a continuous control of capacitive or inductive reactance.

16.Define blocking mode operation mode of TCSC. In this mode the thyristor valve is not triggered and the thyristors are kept in non-conducting

state. If the thyristors are conducting and a blocking command is given, the thyristors turn off as soon as the current through them reaches a zero crossing.

The line current passes only through the capacitor bank(XTCSC = XC). in this mode the TCSC performs like a fixed series capacitor and the set TCSC reactance is capacitive.

17.Define Vernier control mode operation of TCSC. This mode is also called partially conducting thyristor mode which allows the TCSC to behave

either as a continuously controllable capacitive reactance or as a continuously controllable reactance.

This mode is achieved by changing the thyristor pair firing angle in an appropriate range. Due to resonant region between the two modes, a smooth transition from the capacitive to

inductive mode is not permitted.

18.Define capacitive boost mode operation of TCSC.


Subject code/Name: EE 2306-Flexible AC Transmission SystemsIn capacitive boost mode a trigger pulse is supplied to the thyristor having forward voltage just before the capacitor voltage crosses the zero line, so a capacitor discharge current pulse will circulate through the parallel inductive branch. This discharge current pulse adds to the line current through the capacitor and causes a capacitor voltage that adds to the voltage caused by the line current. The capacitor peak voltage thus will be increased in proportion to the charge that passes through the thyristor branch.

19.Define inductive boost mode operation of TCSC.In inductive boost mode the circulating in the TCSC thyristor branch is bigger than the line

current. In this mode, larger thyristor currents result and further the capacitor voltage waveform is very much distorted from its sinusoidal shape. The peak voltage appears close to the turn on. The poor waveform and the high valve stress make the inductive boost mode less attractive for steady state operation.

20.What is the need of voltage limits of the TCSCThe maximum voltage limits of operating equipment are determined by the equipment’s insulation level. The voltage constraint equipments depend on the duration of voltage application. The overvoltage limit of the MOV is more critical than that of the even for short durations (normally less than 2s).

21.What is the need of current limits of the TCSC?Current limits may need to be imposed on the currents in the thyristor valve fixed capacitor and surge inductor to prevent overheating. Generated harmonics can also causes heating Effects and therefore have a constraining effect on the TCSC operation.

22.What is the role of Static Synchronous Compensator?The static synchronous compensator is based on a shunt connected solid state reactive compensation device, implemented with voltage source converter that generates the voltage wave comparing it to the one of the electric system to realize the exchange of reactive power and connected in parallel to the power system through a coupling transformer or reactor.

23.What are the two basic approaches for controllable series compensation? Thyristor switched series capacitor, which allows a discrete control of the capacitive

reactance. Thyristor controlled series capacitor (TCSC.), which allows a continuous control of capacitive

or inductive reactance.

24.What is the method of including finite delay associated with firing control in TCSC modeling? A desired magnitude of TCSC reactance in TCSC modeling is obtained that is implemented

after a finite delay caused by the firing controls and the natural response of the TCSC. This delay is modeled by a lag circuit having a time constant around 15-20 ms.

The output of the lag block is subject to variable limits based on the TCSC reactance-capability curve.

Such modulation controller is used for damping enhancement.

PART – B



1. Explain the principle of operation of TCSC. Also discuss the different modes of TCSC. (Dec ’14)2. Explain the effect of TCSC for the enhancement of system damping. (June ’12)3. Explain the need for variable and fixed series compensation schemes.4. Describe the variable reactance model of TCSC. (June ’12)5. (i) Explain the different modes of operation of TCSC. (Dec ’12)

(ii) Draw V-l and X-l characteristics curves for single modules TCSC and Two module TCSC6. Describe the modeling of TCSC for load flow study.7. Explain the working, characteristics and operating modes of variable reactance model of

thyristor controlled series capacitor.8. Explain in detail the applications of thyristor controlled series capacitor. 9. How TCSC is used for the improvement of the stability of a system. 10. Consider the SMIB system in which the synchronous machine is generating 0.8 pu MW and

0.25 MVAR. The infinite bus voltage is 1 at angle of 0. The machine transient reactance is 0.32 p.u and the transmission line reactance is 0.65p.u.Calculate the value of net reactance offered by the TCSC and the voltage that has to be injected by the TCSC to enhance the power flow to 1.0 p.u (Dec ’12)

11. (i) Explain various operating modes of TCSC. (6) (Apr ‘ 14) (ii) From the fundamental derive the expression for steady state thyristor current when the

TCSC is operating in the Vernier mode? (10)12. (i) Explain the constant power control model of TCSC with the block-diagram. (8)(Apr ’14)

(ii)What are the important limits that are considered in the capability characteristics of TCSC. (8)

13. Draw the basic and practical modules of TCSC. Explain the basic principle and different modes of operation of TCSC. (Apr ’15)

14. Draw and explain the block diagram of the variable reactance model of TCSC and hence derive transient stability and long term stability models. (Apr ’15)

15. Explain the variable reactance modeling of TCSC. (May ’13)

UNIT IV EMERGING FACTS CONTROLLERS

PART – A

1. State the capabilities of 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

2. 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.

3. State the salient features of STATCOM features.


Subject code/Name: EE 2306-Flexible AC Transmission Systems• 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

4. 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

5. Write the applications of UPFC. (Apr ‘15)Power flow control• Power swing damping• Voltage dips compensation• Fault Current Limiting

6. Draw the UPFC model for power flow studies.The load flow model assumes that the UPFC is operated to keep

Real and reactive power flows at the receiving bus and sending bus voltage magnitude at their pre-specified values.

7. Define UPFC. (Dec ’14) The UPFC is a representative of the last generation of FACTS devices which can control

simultaneously all three parameters of line power flow (line impedance, voltage and phase angle).

The UPFC combines together the features of the static synchronous compensator (STATCOM and the static synchronous compensator (SSSC).

8. State the function of converter 1 in UPFC The two convertors can work independently of each other by separating the dc side. So in

that case, the convertors 1 (shunt convertors) is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point.

9. What is the role of dc link in UPFC? The series inverter can be used to control the real and reactive line power flow inserting an

appropriate voltage with controllable magnitude and phase angle in series with the transmission line. So, this inverter will exchange active and reactive power with the line.

The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) form the line keeping the voltage across the storage capacitor Edc constant. So the net real power absorbed from the line by the UPFC is equal only to the losses of the two inverters and their transformers.

10.Distinguish between UPFC and IPFC.


Subject code/Name: EE 2306-Flexible AC Transmission Systems UPFC is a typical shunt and series controller: It is able to control the active , reactive power

flow and nodal voltage with its unique combination of shunt and series compensation. The UPFC is implemented by two back-to-back converter-based synchronous voltage sources

(SVSs) which can generate or absorb reactive power independently. IPFC – Series Capacitive Compensators are used to increase the transmittable active power

over a given line but they are unable to control the reactive power flows in, and thus the proper load balancing of the line. With IPFC active power can be transferred between lines. Therefore it’s possible to:

o Equalize both active and reactive power flow between the lines,o Reduce the burden of overloaded lines by active power transfer,o Compensate against resistive line voltage drops and the corresponding reactive power

demand

11.State the optimization problem of control coordination. In techniques that explicitly consider the robustness requirements, any changes in the

system operating points caused by load variations, line switching or contingencies are included as changes in the matrix elements of the linearized systems.

The robust control design with state feedback matrix can maintain stability of the closed loop system for any permissible uncertainty.

12.State the salient features of UPFC. (June ’12)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 without generator rescheduling• Regulating power flow through a transmission line• More reliable• Provides dynamic security• Acts as harmonic isolator

13.Differentiate between STATCOM and SVC. (June ’12)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)

14.List the parameters that can be improved by 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

15.What is the responsibility of STATCOM controller?FACTS 22 KCE/EEE/QB/IVYR/FACTS

Subject code/Name: EE 2306-Flexible AC Transmission SystemsThus the STATCOM controller provides voltage support by generating or absorbing reactive power at the point of common coupling without the need of large external reactors or capacitor tanks.

16.What is the main advantage of the multi-level inverter circuits used in STATCOM?The main advantage of the multi level inverter circuits is their ability to produce quasi harmonic neutralized output voltage waveforms without magnetic waveform summation circuits.

17.How real power exchanges made during operation of STATCOM? With the provision of DC source or energy storage device on its DC side, there can be a little

active power exchange between the STATCOM and the electric power system. When the phase angle of the AC power system leads the inverter phase angle, the STATCOM

absorbs real power from the AC system; if the phase angle of the AC power system lags the inverter phase angle, the STATCOM supplies real power to AC system.

18.How the response of STATCOM obtained at low system voltage?The ability of the STATCOM to produce full capacitive output current at low system voltage also makes it more effective than the SVC in improving the transient (first swing) stability. This is because the STATCOM is able to maintain full compensating current at depressed line voltage occurring during the first swing as a result of sharply increasing electric power transmission.

19.What are the control parameters of UPFC?UPFC can control simultaneously all three parameters of line power flow as Line impedance Voltage and Phase angle.

20.How power flow can be controlled using UPFC?The UPFC can be used to control the flow of active and reactive power through the line and to control the amount of reactive power supplied to the line at the point of installation. While operating both inverters as a UPFC, the exchanged power at the terminals of each inverter can be real as well as reactive.

21.What is the function of shunt converter and series converter in UPFC? The two VSI’s can work independently of each other by separating the DC side. So in that case,

the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point.

Instead, the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power flows on the transmission line.

PART - B



1. With a neat sketch, explain the implementation of UPFC.2. Explain the principle and operation of UPFC with the aid of block diagram. (Dec ’12)3. Explain the working of STATCOM with a neat sketch. In what way it differs from SVC?4. Explain the construction, operation of STATCOM with its V-I characteristics. (June ’12)5. Describe the modeling of UPFC for power flow and transient stability studies. (Apr ’15)6. Explain the basic principle and control capability of unified power flow controller.7. Explain the power transfer capability of UPFC and compare its capabilities with other FACTS controllers.8. Describe the construction of UPFC with a block diagram and its characteristics with phasor diagrams.9. Derive the expression of UPFC connected at the midpoint. (Dec ‘14)10. Explain the power flow control and oscillation damping in the two area system using UPFC.11. With phasor diagram explain the different modes of operation of UPFC. (June ’12)-212. Consider a transmission line a STATCOM is connected at midpoint of the line. Assume that-both end voltages are regulated at 1 p.u, the transmission line reactance is 0.8 p.u. calculate the current that must be injected by STATCOM to maintain the midpoint voltage at 1.01 p.u. When the load at receiving end is varied from 0 to 0.9 p.u. (Dec ’12)13. (i) Discuss about the operation of SSSC and compare the performance of SSSC with that of fixed capacitors. (8) (May ’13)-2 (ii) Consider a SMI8 system in which the synchronous machine is generating 0.95 p.u. MW and 0.35 p.u. MVAR at a terminal voltage of 1Ļ360. The machine transient reactance is 0.25p.u. and the transmission •line reactance is 0.60 p.u. If the damping ratio has to be 0.12 calculate how the voltage injected by the SSSC has to be modulated. (8)14. Explain the operation of UPFC with relevant diagrams . (May ’13)15. (i) Explain the principle of operation of STATCOM. (8) (Apr ’14) (ii) What are the various ways of controlling the output voltage of STATCOM. Explain them with necessary block-diagrams. (8)16. (i) Consider a 765 kV symmetrical lossless transmission line with l=0.98mH/km, c=14nF/km, and length of line is 800km.Calculate the midpoint voltage at 56% of the surge impedance loading.(4) (Apr ’13) (ii). Derive the steady state space model of STATCOM from the fundamentals. (12)17. (i) Explain the operation of UPFC with necessary diagrams. (10) (Apr ’14) (ii) Compare the fixed series compensation and fixed shunt compensation. (6)18. Explain the protection of UPFC. (Dec ‘14)19. Explain the performance of VSC based STATCOM.

UNIT V - CO-ORDINATION OF FACTS CONTROLLERSPART – A

1. What is the need for coordination of different FACTS controllers? (Dec ’12)-2• 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)

2. Classify FACTS controller interactions.


Subject code/Name: EE 2306-Flexible AC Transmission SystemsThe frequency ranges of the different control interactions have been classified as follows: Steady state interactions (frequency range : 0 HZ) Electromechanical oscillations (frequency range : 0 – 3/5HZ) Small signal or control oscillations (frequency range : 2-5 HZ) Sub synchronous resonance (SSR) interactions (frequency range : 10 – 50/60 HZ) Electromagnetic transients, high frequency resonance or harmonic resonance interactions.

And network- resonance interactions (frequency range : Higher than 15 HZ)

3. What is meant by coordination of FACTS controllers? (June ’12)The term coordinated implies that the controllers have been tuned simultaneously to effect an overall positive improvement of the control scheme. 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

4. What way STATCOM differs in synchronous machine? In analog with a synchronous machine, generating balanced set of three sinusoidal voltages at

the fundamental frequency, with controllable amplitude and phase shift angle. This equipment, however, has no inertia and no overload capability.

5. Define steady state interactions. Steady state interactions between different controllers (FACTS-FACTS or FACTS-HVDC) occur between their systems related controls. They are steady state in nature and do not involve any controller dynamics. These interactions are related to issues such as steady state voltage stability limit and steady state power limit. Such control co ordination may be occurs between the steady state voltage control of FACTS equipment and the HVDC supplementary control for ac Voltage regulations.

6. What is meant by Electromechanical Oscillation Interaction? The Electromechanical oscillation comprises local mode oscillations, normally in the range of 0.8 to 2 Hz, and inters area mode oscillations, normally in the range of 0.2 to 0.8Hz. Electromechanical oscillation interactions between FACTS controllers also involve synchronous generators, compensators machines, and associated power system stabilizer controls. The local mode oscillation is contributed by synchronous generators in a plant level and the inter area mode oscillations results from the power exchange between tightly coupled generators in two areas connected by weak transmission lines.

7. What are the possible interactions occur in Electromechanical: small disturbance? Interaction between PSSs Interaction between PSSs and FACTS Devices Interaction between PSSs and Governors Interaction between PSSs and torsional modes.

8. What are the possible interactions occur in Electromechanical: large disturbance?At the same power plants:


Subject code/Name: EE 2306-Flexible AC Transmission Systems Interaction between PSS and AVR during fault and in immediate post fault period. Interactions between under excitation limiters and AVRs Interactions with load controls

Among power plants: Interaction among PSS other controls.

9. Define small signal oscillations. The oscillations in the range of 2 to 15 Hz occurring between individual FACTS controllers and the network or between FACTS controllers and HVDC links are called small signal oscillations. The emergence of these oscillations significantly influences the tuning of controller gains.

10.Define Sub Synchronous Resonance (SSR) interactions. Resonant frequencies below the fundamental frequency are called sub synchronous frequencies. Sub synchronous (SSR) occurs due to interaction between series capacitors and nearby turbine generators. Application of series capacitors in long electric power transmission lines is a cost effective method to increase power transfer but use of series capacitors has sometimes been limited because of the concerns for sub synchronous resonance (SSR).

11.Define high frequency interactions. Harmonic instabilities may also occur from synchronous of ac systems, transformer energization, or transformer saturation caused by geo-magnetically induced currents. High frequency oscillations in excess of 15 Hz are caused by large nonlinear disturbances, such as the switching of capacitors, reactors, or transformers, for which reason they are classified as electromagnetic transients.

12.Define PSS power oscillation damping controllers. PSSs are designed for damping local electromechanical oscillations. However in large power

systems these PSSs may not provide enough damping for inter-area modes. PSS acts through the excitation system of generator to increase the damping of

electromechanical oscillations by generating a component of electrical torque proportional to speed change.

13.What is the design steps followed in controller design? System model derivation System performance specifications Selection of the measurement and control signals Co ordination of the controller design Justification of the design and performance evaluation.

14.What are the criteria to be satisfied by damping controller? It should help the system survive the first few oscillations


Subject code/Name: EE 2306-Flexible AC Transmission Systems After occurrence of a severe disturbance the system still live with sufficient safety margin and

safety margin in terms of bus voltage levels should not be ensured. In steady state after a disturbance minimum level of damping must be ensured. Harmful interactions with other controls should be avoided. The controller should be robust.

15.Define Linear Quadratic Regulator (LQR). The linear quadratic regulator (LQR) is one of optimal controller having well known design

technique that provides practical feedback gains. This can be used to co ordinate the controllers with the overall objective of damping low-

frequency inter area oscillation with highly stressed power system environments.

16.What is meant by Genetic Algorithm (GA)? GA’s are search procedures based on the mechanics of natural selection and natural genetics. They were developed to allow computers to evolve solutions to difficult problems such as function Optimization and artificial intelligence.

17.What are the three basic operations of genetic algorithms? Maintain a population of solutions to a problem Select the better solutions for recombination with each other, Use their offspring to replace poorer solutions.

18.What is the procedure for constraint optimization technique? In techniques that explicitly consider the robustness requirements, any changes in the system

operating points caused by load variations, line switching or contingencies are included as changes in the matrix elements of the linearized systems.

The robust control design with state feedback matrix can maintain stability of the closed loop system for any permissible uncertainty.

This method is somewhat difficult in apply large power systems as it applicable on a linearized system model.

Pole placement is one technique in which the robustness requirement is not explicitly considered. In this method the critical electromechanical modes are assigned priori new locations that are placed deeper into the left half of the s plane.

19.Why the shunt compensation is attempted always at midpoint? The magnitude of the midpoint voltage depends on the power transfer. This voltage influences

the line insulation and therefore needs to be well understood For a symmetrical line where the end voltages are held at nominal values the midpoint voltage

shows the highest magnitude variation. So shunt compensation is attempted always at midpoint.

20.State the use of frequency response curve in the interaction analysis. Instabilities are likely to occur in power systems because of the amplification of lower order

harmonics in FACTS controller loops. Harmonic instabilities may also occur from


Subject code/Name: EE 2306-Flexible AC Transmission Systemssynchronization of ac systems, transformer energization, or transformer saturation caused by geo-magnetically induced currents.

High frequency oscillations in excess of 15 Hz are caused by large nonlinear disturbance, such as the switching of capacitors, reactors, or transformer, for which reason they are classified as electromagnetic transients.

Frequency response curve or used to analyze and minimize or negate such interactions.

21.Mention the possible combinations 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.

22.What are the two main reasons for incorporating FACTS devices in electric power systems?

Increase the power transmission capacity of the line to keep the voltage profile at the acceptable limit.

To minimize line drop and enhance the system stability.

PART – B

1. Using linear control techniques, explain the co-ordination of multiple controllers.2. Explain the controller interactions between multiple SVCs (SVC-SVC) in a large power system. (June ’12)3. Explain the FACTS controller interactions with similar, dissimilar HVDC controller links.4. Describe the genetic algorithm based control co-ordination of FACTS controllers.5. Explain the co-ordination of multiple controllers using genetic algorithm. 6. Explain in detail about different control interactions. (June ’12)7. Explain about the effect of electrical coupling and short circuit level on the controller interaction between multiple SVCs that are located in a power system. (Dec ’12)-29. (i) Write about the basic procedure that has to be followed for the controller design. (8) (ii) Explain the steps that are to be followed for the co-ordinated tuning of FACTS controllers. (8)10. Explain in Linear coordination technique. (Dec‘14)11. Explain Quantitative treatment in FACTS controllers. (Dec ‘14)12. What is the need for coordination of different FACTS controllers? Explain the different control interactions that are occurring in multiple FACTS controllers.13. Describe the following control techniques used for coordination of multiple FACTS controllers.

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Q.B. FACTS

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