UNIT-II REAL POWER – FREQUENCY CONTROL

1. What is the major control loops used in large generators?

The major control loops used in large generators are Automatic voltage regulator (AVR)

Automatic load frequency control (ALFC).

2. What is the use of secondary loop?

Secondary loop is used to maintain the fine adjustment of the frequency, and alsoby reset action maintains proper MW interchange with other pool members. This loop isinsensitive to rapid load and frequency changes but focuses instead on drift like changeswhich take place over periods of minutes.

3. What is the advantage of AVR loop over ALFC?

The advantage of AVR loop over ALFC is very fast and therefore there is atendency, for the AVR dynamics to settle down before they can make themselves felt inthe slower load frequency control channel.

4. What is the difference between large and small signal analysis?

The difference between large and small signal analysis is given below. Largesignal analysis is used where voltage and power may undergo sudden changes ofmagnitude that may approach 100 percent of operating values. Usually this type ofanalysis leads to differential equations of non-linear type. Small signal analysis is usedwhen variable excursions are relatively small, typically at most a few percent of normaloperating values.

5. What is the exciter?

The exciter is the main component in AVR loop. It delivers the DC power to thegenerator field. It must have adequate power capacity and sufficient speed of response(rise time less than 0.1 sec).

6. What is the function of AVR?

The function of the AVR is to provide constancy of the generator terminal voltageduring normal, small and slow changes in the load.

7. Explain about static AVR loop.

In a static AVR loop, the execution power is obtained directly from the generatorterminals or from the station service bus. The AC power is rectified by thyristor bridgesand fed into the main generator field via slip rings. Static exciters are very fast andcontribute to improved transient stability.

8. Write the static performance of AVR loop.

Static performance of AVR loop is to regulate the terminal |V| to within requiredstatic accuracy limit, have sufficient speed of response and be stable.

9. What is the disadvantage of high loop gain? How it is to be eliminated?

The disadvantage of high loop gain is that it causes undesirable dynamic response,possibly instability. By adding series AND/OR feedback stability compensation to theAVR loop, this conflicting situation can be resolved.

10. What are the effects of generator loading in AVR loop?

Effects of generator loading in AVR loop is given below. Added load does notchange the basic features of the AVR loop; it will however affect the values of both gainfactor Kf and the field constant. High loading will make the generator work at highermagnetic saturation levels. This means smaller changes in |E| for incremental increases inif, translating into the reduction of KF. The field time constant will likewise decrease asgenerator loading closing the armature current paths. This circumstance permits theformation of transient stator currents the existence of which yields a lower effective fieldinduction.

11. What are the functions of ALFC?

Function of ALFC’s is to maintain desired MW output of a generator unit andassist in controlling the frequency of large interconnection. The ALFC also helps to keepthe net interchange of power between pool members at predetermined values. Controlshould be applied in such a fashion that highly differing response characteristics of unitsof various types are recognized. Also unnecessary power output changes should be keptat a minimum in order to reduce wear of control valves.

12. Specify the disadvantage of ALFC loop.

The disadvantage of ALFC loop is that it will control only during normal changesin load and frequency. It is unable to provide adequate control during emergencysituations, when large MW imbalances occur.

13. How is the real power in a power system controlled?

The real power in a power system is being controlled by controlling the drivingtorque of the individual turbines of the system.

14. What is the need for large mechanical forces in speed-governing system?

Very large mechanical forces are needed to position the main valve against thehigh stream pressure and these forces are obtained via several stages of hydraulicamplifiers.

13.  Define control area.

It   is  possible  to divide an extended power system into sub­areas in which thegenerators are tightly coupled together so as to form a coherent group. Such a coherentgroup   is   called   a   control   area   in   which   the   frequency   is   assumed   to   be   the   samethroughout in static as well as dynamic conditions.

14.  What do you understand by coherent group of generators?

A control area is defined as a system to which a common generation control scheme isapplied. The electrical interconnection within each control area is very strong as compared tothe lies with the neighboring areas. All the generators in a control area swing in coherently orit is characterized by a single frequency. It is necessary to be considered as many control areaas number of coherent group.

      15.  What is meant by AFRC?∆fstat = (­1/B ∆PD)/(1 + 1/BR) = ­M/(B + 1/R) = ­M/β where β = B + 1/R

= Area frequency response co efficient or characteristics p.u MW/Hz.

16. Define electric stiffness of the interconnected system. T12 = [|V1||V2|cos(δ0

1 – δ02)]/Pr1X12

= Synchronizing power coefficient or electric stiffness.

i.e., slope of the power angle curve at the initial operating angle δ120.

It is defined as the differential power increase obtained per differential power angle increase.

17.  Define Area Control Error.

Area Control error is the change in area frequency which when used in integral control loop forced the steady state frequency error to zero.

18. What is AGC? 

Abbreviated   as   AGC,  automatic  gain  control  is   a   circuit   found   on   some   electronicdevices   that  automatically  controls   the  gain  of  a   signal.  Using AGC means   that  weakersignals receive more gain and stronger signals receive less gain or none at all. 

19. State the advantages of integral controller. 

(i) For integral controller gain in the sub­critical range i.e.,K1< Ki critical, systemis over damped and zero steady state error is achieved , after long time. 

(ii) From the response characteristics, observe that initial response with integralcontroller is just the same as without integral controller. After a small period only thecontroller becomes effective and depending on the value of integral controller gainfrequency error is minimized. Higher the gain, quicker the actions after initial drop infrequency.

20. List the advantages of Multi area operation.1. Under normal operating condition each control area should have the capacity to meetits own load from its own spinning Generator, plus the scheduled interchange betweenthe neighboring areas.2. Under emergency conditions, the energy can be drawn from the spinning reserve of allthe neighboring areas immediately due to the sudden loss of generating unit.

      21. Draw the  dynamic  response  of  change  in  frequency  for  a  step  load  change.

22. Differentiate static and dynamic response of an ALFC loop. 

Static response of an ALFC loop will inform about frequency accuracy.  The dynamic response of an ALFC loop will inform about the stability of the loop. 

      23. What is the function of load frequency control?The function of load frequency control on a power system is to change the control

valve  of   the  prime  movers   as   a   function  of   load  variations   in  order   to  hold   systemfrequency constant.

 24. How is the real power in a power system controlled?By controlling the driving torque of the individual turbines of the system.

25. What is meant by fly ball speed governor? This is purely mechanical speed-sensitive device coupled directly to the hydraulic

amplifier which adjusts the control valve opening via the linkage mechanism.

16-Mark Questions

1. Develop the block diagram model of uncontrolled two area load frequency control      system and explain the salient features under dynamic conditions. 

2. Develop the block diagram model of uncontrolled two area load frequency control      system and explain the salient features under static conditions. 

3. Draw the block diagram of LFC control of single area and derive the dynamic response.

4. Draw the block diagram of LFC control of single area and derive the steady state        frequency error. 

5. Obtain a block diagram for the representation of single control area without integral      control. 

6. What are the components of speed governor system of an alternator? Derive a transfer function and sketch a block diagram.

7. Explain the tie-line bias control of two area system.

8. Explain the importance of flat tie-line and flat frequency control.

UNIT-III REACTIVE POWER – VOLTAGE CONTROL

1. What are the sources of reactive power? How it is controlled?

The sources of reactive power are generators, capacitors, and reactors.These are controlled by field excitation.

2. Give some excitation system amplifier.

The excitation system amplifiers are, Magnetic amplifier

Rotating amplifier

Modern electronic amplifier.

3. When is feedback stability compensation used?

Feedback stability compensation is used to resolve the effect of high loop gain inAVR. High loop gain is needed for static accuracy but this causes undesirable dynamicresponse, possibly instability.

4. Give the characteristics of line compensators.

The characteristics of line compensators are,

a. Ferranti effect is minimized. b. Under excited operation of synchronous generator is not required.

5. What is known as bank of capacitors? How it is adjusted?

Bank of capacitors is the arrangement of number of capacitors connected inparallel to get the desired capacitance. These can be adjusted in steps by switching(mechanical).

6. What is the disadvantage of switched capacitors are employed for compensation? When switched capacitors are employed for compensation, these should be

disconnected immediately under light load conditions to avoid excessive voltage rise andFerro resonance in presence of transformers.

7. What are the effects of capacitor in series compensation circuit? The effects of capacitor in series compensation circuit are,

i. Voltage drop in the line reduces. ii. Prevent voltage collapse. iii. Steady state power transfer increases. iv. Transient stability limit increases.

8. Give two kinds of capacitors used in shunt compensator.

The two kinds of capacitors used in shunt compensator are,a. Static Var Compensator (SVC) b. Thyristor Switched Capacitor

10. What is synchronous condenser?

Synchronous condenser is a synchronous motor running at no-load and havingexcitation adjustable over a wide range. It feeds positive VARs into the line underoverexcited conditions and negative VARs when under excited.

11. Write about static VAR compensator (SVC).

Static VAR Compensator (SVC) comprise capacitor bank fixed or switched orfixed capacitor bank and switched reactor bank in parallel. These compensators drawreactive power from the line thereby regulating voltage, improve stability (steady stateand dynamic), control overvoltage and reduce voltage and current unbalances. In HVDCapplication these compensators provide the required reactive power and damp out subharmonic oscillations.

12. What are static VAR switches or systems?

Static VAR switches or systems are Static VAR compensators which useswitching for VAR control. It means that terminology wise SVC=SVS.

13. Give some of the static compensators schemes.

Schemes of static compensators 14. Saturated reactor 15. Thyristor- Controlled Reactor (TCR) 16. Thyristor Switched capacitor (TSC) 17. Combined TCR and TSC compensator.

14. What is tap changing transformers?

Tap changing transformers are the power transformers and many distributiontransformers which have taps in one or more windings for changing the turn’s ratio.

15. Write the types of tap changing transformers.

Types of tap changing transformers

a. Off- load tap changing transformers. b. Tap changing under load transformers.

16. What is the use of off-load tap changer and TCUL?

The off- load tap changers are used when it is expected that the ratio will need tobe changed only infrequently, because of load growth or some seasonal change. TCUL isused when changes in ratio may be frequent or when it is undesirably to de-energize thetransformer to change the tap.

17. Compare shunt and series capacitors.

1. The voltage boost due to a shunt capacitor is evenly distributed over the transmissionline whereas the change in voltage between the two ends of a series capacitor where itis connected in sudden. The voltage drop along the line is unaffected. 

2. For the same voltage, the reactive power capacity of a shunt capacitor is greater thanthat of a series capacitor. 

3. The   shunt   capacitor   improves   the   power   factor   of   the   load   whereas   the   seriescapacitor has little effect on the power factor. 

4. For   long transmission  lines  where  the total   reactance  is  high,  series capacitor  areeffective for improvement of system stability. 

18. State the functions of AVR.The  function  of  AVR excitation  control   is   to   regulate  generator  voltage  and relative

power output. As the terminal voltage varies the excitation control, it maintains the terminalvoltage to the required standard and the demand of the reactive power is also met by theexcitation control unit.

19. What are the various functions of excitation system and also mention the types.The   basic   function   of   an   excitation   system   is   to   provide   direct   current   to   the   synchronous

machine. In addition, the excitation system performs control and protective functions essentially tothe satisfactory performance of   the power  system by cooling  the  field voltage,   thereby  the  fieldcurrent.

20. List the various components in AVR loop.Exciter, comparator, amplifier, rectifier, synchronous generator.

21. What is the significance of stability compensation in AVR loop? Stability   compensation   improves   the  dynamic   response   characteristics  without

affecting the static loop again.  High loop gain is needed for static accuracy, but this causes undesirable dynamic

response i.e., possibility instability.  This conflict situation can be avoided by adding series and/or feedback stability

compensation to the AVR loop. 

22. What is the need for compensator in the AVR loop? 

Stability compensation improves the dynamic response characteristics without affectingthe static loop gain.

23. Distinguish between on­load and off­load tap changing. off­load tap changing

The   off­load   tap   changing   transformer   which   requires   the   disconnection   oftransformer when the tap settings is to be changed.on­load tap changing

In this type, the voltage is a maximum value and since the current are dividedequally and flow in opposition through the coils, the resultant flux is zero and hence theminimumimpedance.

24. What is SVC? Mention the different types of SVC.

Static VAR compensators use switching for var control. These are also called staticVAR switches or systems. It means that terminology wise SVC=SVS. And we will usethese interchangeably.

25

25. What is a synchronous condenser and also state the merits 

It is a synchronous motor running at no­load and having excitation adjustable overa  wide  range.   It   feeds  positive  VARs  into   the   line  under  overexcited  conditions  andnegative VARs when under excited.

16-Mark Questions

1. Draw the circuit diagram for a typical excitation system and derive the transferfunction model and draw the block diagram. 

2. Discuss generation and absorption of reactive power. 3. Explain different types of static VAR compensators with a phasor diagram. 4. Discuss about the various methods of voltage control. 5. Derive the relations between voltage, power and reactive power at a node for

applications in power system control 6. Explain stability compensation and effects of generator loading.7. Explain the dynamic performance of AVR loop.8. Explain typical brushless automatic voltage regulator.

UNIT-IV UNIT COMMITMENT AND ECONOMIC DISPATCH

1. Define economic dispatch problem.

Economic dispatch problem is defined as the problem which minimizes theoperating cost of active power generation.

2. Define incremental cost.

Incremental cost is defined as the rate of change of fuel cost with active powergeneration.

3. Write the load balance equation.

Load balance equation is given below.Pg-Pd-PL=0.

4. Define base point.

Base point is defined as the present operating point of the system.

5. Define participation factor.

Participation factor is defined as the change in generation required to meet powerdemand.

6. Define hydrothermal scheduling problem.

Hydrothermal scheduling problem is defined as the problem of minimizing thethermal generation cost with the constraints of water availability.

7. Define unit commitment.

Unit commitment is defined as the commitment of minimum generator to meetthe required demand.

8. Define spinning reserve.

Spinning Reserve is defined as the term that describes the total amount ofgeneration availability from all units synchronized on the system.

9. What is meant by scheduled reserve?Scheduled reserve include quick start diesel turbine units as well as most hydro

units and pumped storage hydro units that can be brought online, synchronized andbrought up to full capacity quickly.

10. What are the thermal unit constraints?

The thermal unit constraints are i. Minimum up time ii. Minimum down time iii. Crew constraints

11. What are the two approaches to treat a thermal unit to operating temperature?

The two approaches to treat a thermal unit to operating temperature are givenbelow.

a. Allows the unit boiler to cool down and then heat backup to operatingtemperature in time for a scheduled turn on. b. Requires that sufficient energy be input to the boiler to just maintain operatingtemperature.

12. What are the techniques for the solution of the unit commitment problem?

The techniques for the solution of the unit commitment problem are i. Priority list method

ii. Dynamic programming iii. Lagrange relation

13. What are the assumptions made in dynamic programming problem?

The assumptions made in dynamic programming problem are given below.

i. A state consists of an array of units with specified units operating and the rest ofthe time.

ii. The startup cost of a unit is independent of the time it has been offline.

iii. There are no costs for shutting down the units.

14. Define long range hydro scheduling problem.

Long range hydro scheduling problem is defined as the problem involves the longrange of water availability and scheduling of reservoir water releases. For an interval oftime that depends on the reservoir capacities.

15. What are the optimization techniques for long range hydro scheduling problem?

The optimization techniques for long range hydro scheduling problem are

a. Dynamic programming b. Composite hydraulic simulation methods c. Statistical production cost

16. Define short range hydro scheduling problem.

Short range hydro scheduling problem is defined as the hour by hour schedulingof all generators on a system to achieve minimum production condition for the given timeperiod.

17. Define system blackout problem.

System blackout problem is defined as the event occurs on a system that leaves itoperating with limits violated; the event may be followed by a series of further actionsthat switch other equipment out of service. If the process of cascading failures continues,the entire system of it may completely collapse.

18. What is meant by cascading outages?

Cascading outages are the process if one of the remaining lines is now too heavilyloaded, it may open due to relay action, thereby causing even more load on the remaininglines.

19. Mention the constraints in unit commitment problem. 1.Spinning reserve:

2.Thermal Constraints = Minimum up timeMinimum down timeCrew Constraint

         3.Other Constraints = Hydro ConstraintMust  run constraintFuel constraint.

20. What are base­point and participation factors?

The present operating point of the system is called base point.

The change in generation required to meet power demand is called as participation factor.

21. List the various constraints in economic dispatch problem.The equality constraint equationPD = PG1 + PG2 + …….. + PGN =     PGi …(1)

Power   demand   =   Total   Generation

Where, PGi = Output of unit i, MW.

PD = Total power received by the plant bus and transferred to the load.

Since our system is a compact one and we neglect the losses in the transmissionline. We may write equation (1) as,

H (PGi, PG2 ……….. PGN) = PD ­     PGi = 0

22. Explain penalty factor.

The exact coordination equation is given by

Lamda=(IC)i/1­(ITL)i=(IC)iLi.

Li=1/1­(ITL)i

Li is a penalty factor where ITLi incremental transmissions loss.

23. Define spinning reserve.

Spinning   reserve   is   the   total   amount   of   generation   available   from   all   unitssynchronized  on the system minus the present load and losses being supplied.

Spinning reserve = Total amount of generation – (Present load + losses)

24. Compare economic dispatch and unit commitment.

Unit CommitmentOptimum allocation of number of units to beoperated   (to  determine   the  units  of  a  plantthat   should  operate   for  a  given   load   is   theproblem of unit commitment)There are number of subsets of the completeset of n units that would satisfy the expecteddemandPurpose of units commitment is to find theoptimal   subset   among   the   subsets   whichprovide the minimum operating cost

Economic load dispatchOptimum   location   of   generation   to   eachstation (at each generating station at variousload levels)

The problem assumes that  there are n unitsalready connected to the systems

Purpose of economic dispatch problems is tofind the optimum operating policy for these nunits

25. What are the factors affecting the cost of generation?Equality constraintInequality constraintsGenerator ConstraintsVoltage ConstraintsRunning space capacity constraintsTransformers tap settings.Transmission line constraints

26. What are the advantages of using participation factor?

a) Computer implementation of economic dispatch is straight forward 

b) Execution time for economic dispatch is short 

c) It will always give consistent answers when units reach limits. 

d) It   gives   linear   incremental   cost   function   or   has   non­convex   costcurves. 

16-Mark Questions

1. Derive the coordination equation with losses neglected. 

2. Derive the coordination equation of an ‘n’ bus power system taking into account theeffect of system losses. 

3. Derive the expression for base point and participation method. 

4. State   the   unit   commitment   problem.   With   the   help   of   flowchart   explain   forwarddynamic programming solution method.

5. Explain Priority list method using full Load average production cost. State the        merits and demerits.

6. Numerical problems in economic dispatch & unit commitment.

7.   Explain about start­up and shut­down costs?

8.   Explain briefly brute­force technique.

UNIT-V COMPUTER CONTROL OF POWER SYSTEM

1. What are the functions of control center?

The functions of control center are a. System monitoring b. Contingency analysis C. Security constrained optimal power flow.

2. What is the function of system monitoring?

Function of system monitoring is to provide updated information about the powersystem.

3. What is SCADA system?

SCADA stands for supervisory control and data acquisition system. It allows afew operators to monitor the generation and high voltage transmission systems and totake action to correct overloads.

4. What are the states of power system?

The states of power system are i. Normal state ii. Contingency modeiii. Alert modeiv. Emergency mode

5. Define normal mode.

Normal mode is defined as the mode of operation if the system is in secure eventhe occurrence of all possible outages has been simulated the system remain secure.

6. Define alert mode.

Alert mode is defined as the mode of operation of the system that it does not remain in the secure for the occurrence of all possible outages.

7. What are the distribution factors?

The distribution factors are a. Line outage distribution factorb. Generation outage distribution factor.

8. Define state estimation.

State estimation is defined as the process of assigning a value to an unknownsystem state variable based on measurements from that system according to some criteria.

9. Define maximum likelihood criterion.

Maximum likelihood criterion is defined as the problem of maximizing theprobability that estimate the state variable x, is the true value of the state variable vector(i.e, to maximize the P(x) = x).

10. Define weighted least-squares criterion.

Weighted least-squares criterion is defined as the problem of minimizing the sumof the squares of the weighted deviations of the estimated measurements, from the actualmeasurement.

11. Define minimum variance criterion.

Minimum variance criterion is defined as the problem of minimizing the expectedvalue of the squares of the deviations of the estimated components of the state variablevector from the corresponding components of the true state variable vector.

12. Define must run constraint.

Must run constraint is defined as the condition that some units are given a mustrun status during certain times of the year for reason of voltage support on thetransmission network.

13. Define fuel constraint.

Fuel constrain is defined as the condition if a system in which some units havelimited fuel or else have constraints that require them to burn a specified amount of fuelin a given time.

14. What are the assumptions made in priority list method?

The assumptions made in priority list method are

i. No load cost is zero ii. Unit input-output characteristics are linear between zero output and full load

iii.There are no other restrictions startup cost are affixed amount.

15. State the advantages of forward dynamic programming approach.

Advantage of forward DP approach is given below. If the start up cost of a unit isa function of the unit is a function of the time it has been offline, then a forward dynamicprogram approach is more suitable since the previous history of the unit can be computedat each stage.

16. State the disadvantages of dynamic programming method.

The disadvantage of dynamic programming method is that it has the necessity offorcing the dynamic programming solution to search over a small number of commitmentstates to reduce the number of combinations that must be tested in each period.

17. What are the known values in short term hydro scheduling problem?

Known values in short term hydro scheduling problem are a. The load b. Hydraulic inflows c. Unit availabilities.

18. What is meant by telemetry system?

Telemetry system is the process of measurement of state of the system and transmission of these states to a control center.

19. What are the functions of security constrained optimal power flow?

The functions of security constrained optimal power flow are a. Contingency analysis is combined with an optimal power flow whichseeks to make changes to the optimal dispatch of generation

b. As well as other adjustments, so that when a securityanalysis is run, no contingency result in violations.

20. Define the state of optimal dispatch.

The state of optimal dispatch is defined as the state that the power system is inprior to any contingency. It is optimal with respect to economic operation but may not besecure.

21. Define post contingency.

Post contingency is defined as the state of the power system after a contingencyhas occurred.

22. Define secure dispatch.

Secure dispatch is defined as the state of the power system with no contingencyoutages, but with correction to the operating parameters to account for security violations.

23. What are the priorities for operation of modern power system?The priorities for operation of modern power system areA. Operating the system in such a way that power is delivered reliably B. Within the constraints placed on the system operation by reliability

considerations, the system will be operated most economically.

24. What is meant by linear sensitivity factor method?

Linear sensitivity factor method is the easiest way to provide quick calculation ofpossible overloads because many outages become very difficult to solve if it is desired topresent the results quickly.

25. What are linear sensitivity factors?

Linear sensitivity factors are i. Generation shift factors ii. Line outage distribution factors.

26. What is the use of line distribution factor?

Line distribution factor is used to apply to the testing for overloads when transmission circuits are lost.

16-Mark Questions

1. Briefly discuss the various functions of energy control centre.

2. Explain the different operating states of power system with state transition diagram.

3.   Explain the hardware components of SCADA with neat diagram and also mention           the functions of it. 

4. Explain about power system security. 

     5. What is EMS? What are its major functions in power system operation and control?      6. Explain the various controls for secure operation.

     7. Explain briefly how the system states are continuously monitored and controlled.

     8. Explain the substation control functions arranged through SCADA system.