# Power System Operation and Control Important Questions and Answers

1. Define the term “diversity factor”.

It is the ratio of sum of individual maximum demands to the maximum demand of the power station.

Diversity factor = Sum of individual maximum demands/Maximum demands of power station

2. What do you mean by spinning reserve?

The units that can be started within a short duration of time to meet the change in load when a particular unit fails in the power system is known as spinning reserve.

3. What is stability compensation?

Stability compensation improves the dynamic response characteristics without offering the static loop gain. The stability compensation will damp out the oscillations in the system.

4. List the methods of Voltage Control?

Methods of Voltage control are

• By excitation control
• Using static shunt capacitors
• Using static series capacitive
• Using static shunt reactors
• Using synchronous condensers.

5. Define state estimation.

It is the process of assigning a value to an unknown system state variable based on measurements from that system according to some criteria.

6. Mentioned the uses of SCADA in power system

• Used in control of AGC.
• Used to RAISE/LOWER command to speed changer.
• Used in monitoring the generation and HV transmission system.
• Used in small distribution systems.

7. What is a load factor?

Load factor is defined as the ratio of average load to the maximum demand during a certain period of time such as a day or a month or a year is called the load factor

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

The real power in a power system is controlled by controlling the driving torques of the individual turbines of the system.

9. What is meant by free governor operation?

It comprises of the elements which are directly responsive to speed and whose positions influence the action of other elements of speed governing system.

10. State the function of AVR.

To provide direct current to the synchronous machine.

To perform control and protective functions essentially to satisfy the performance of the power system.

11. What is synchronous condenser?

Synchronous condenser is a synchronous machine running without a prime mover or a mechanical load. By controlling the field excitation, it can made to generate or absorb reactive power.

12. What is minimum up and minimum down time in unit commitment problem?

• Minimum up time means once the unit is running, it should not be turned off immediately.
• Minimum down time means once the unit is decommitted, there is a minimum time before it can be recommitted.

13. List out the conditions for normal operations of a power system.

A system is said to be normal if both load and operating constraints are satisfied. It is one in which the total demand on the system is met by satisfying all the operating constraints.

The sum of the continuous ratings of all the electrical equipment’s connected to the supply system is know as connected load.

15. What is a “Load Curve”?

Load curve is the curve showing the variation of load on the power station with respect to time.

16. What is “AGC”?

Automatic Generation Control (AGC) is provided to hold frequency at or very close to a specified nominal value and also to maintain the correct value of interchange power between control areas. The matching of generated real power to the varying demand, while keeping the system frequency and the net power interchange constant is done by AGC.

17. What are the conditions necessary for sharing load operating in parallel between the two synchronous machines?

• The rated speed of the machines should be such as to give the same frequency.
• Both the synchronous machines must have the same output voltage rating.
• The synchronous machines should be of same type so as to generate voltages of same waveform.

18. What are the merits of synchronous compensator?

• Smooth variation of reactive VAR’s as compared to step-by-step variation in static compensators.
• Reactive power production is not affected by system voltage.

19. Define spinning reserve constraint in unit commitment problem.

Spinning reserve is the total amount of generation available from all units synchronized. (i.e., spinning) on the system minus the present load and losses being supplied.

20. Define the EMS system.

Energy management is the process of monitoring co-ordinating and controlling the generation, transmission and distribution of electrical energy. Energy management is performed at control centers called system control centers by a computer system called Energy Management Systems (EMS).

21. What is the function of load dispatch center?

1. To monitor the grid operation.
2. To be responsible for optimal scheduling and dispatch of electricity.
3. To keep accounts of the quantity of electricity transmitted through the grid.
4. To exercise supervision and control over the transmission system.
5. To collect fee charges from the generating companies engaged in transmission of electricity.

22. What is the role of computers in the operation and control of power system?

• Computer control is used in security analysis, state estimation and for solving economic dispatch problems.
• Computers are also used to solve load flow problem, transient stability analysis and fault level analysis.
• Computers are used in online monitoring and controlling the modem large scale integrated power system.

23. What are the classification of system load?

24. What is meant by “Load frequency Control”?

It senses the bus bar frequency and compares the tie-line power frequency. The difference of signal is fed to integrator and it is given to speed changer which generates the reference speed for governor. Thus tie-line frequency is maintained constant.

25. State the difference between P-f and Q-V control.

Any static change in real bus power ΔPi will affect only the bus voltage phase angles δi (p α δ) but will leave the bus voltage magnitudes almost unaffected. Static change in reactive bus power ΔQi affects essentially only the bus voltage magnitudes (Q α V2) but leave the bus voltage phase angles almost unaffected.

26. What are the methods to improve the voltage profile in the power system?

• Series capacitors and shunt capacitors.
• Static Var compensators (SVC).
• STATCOM (Static Synchronous Compensators).
• SSSC (Static Synchronous Series Compensators).

27. Write the significance of unit commitment.

• It minimizes the loss and the fuel cost.
• It Commits the enough units to supply the system load.
• It chooses the most economic unit for running to meet out the load so that unit will operate closer to its best efficiency.

SCADA (Supervisory Control and Date Acquisition) system collects the date from all the power system components and processes the data for automatic generation control and state estimation.

29. Define the term “Maximum demand”.

The greatest of all short time interval average during a given period on the power station is called the maximum demand.

30. What are types of automatic load frequency control for interconnected power system?

• Primary method (speed governing mechanism).
• Secondary method (frequency control speed governing mechanism).

31. What is automatic load dispatching?

The most efficient unit is loaded to its maximum capability, then the second most efficient unit is loaded.

32. What are the different types of reactive power compensation?

• Variable Impedance : Inductor, Capacitor
• Switching Converter : TCR, TCSC, STATCOM, SVC.

33. Define ” Booster Transformer”?

Booster transformer is a compensation device which gives as in-phase boost similar to tap changing transformer. To increase the voltage at an intermediate point in a line, the booster can be brought into the circuit by the closer of relay B and opening of A or vice versa.

34. What is incremental cost criterion?

For the unit commitment table, fuel cost curve of the plant can be determined in the form of least square fit. If transmission losses are neglected, the total system load can be optimally divided among the various generating plants using the equal incremental cost criteria

35. Name some thermal constrains in unit commitment problem.

The thermal constraints are:

• Minimum uptime,
• Minimum downtime,
• Crew Constraint.

36. Define ‘Network Topology’ in a power system.

In order to run the state estimation, we must know how the transmission lines are connected to the load and generation buses. This information is called network topology.

37. What is the need of voltage regulation in power system?

Knowledge of voltage regulation helps in maintaining the voltage at the load terminals within prescribed limits under fluctuating load conditions, by employing suitable voltage control equipment.

38. What is the effect of load factor on the cost of generation?

If the load factor is high, cost per unit generated is low.

39. What is the function of load frequency control on a power system?

In interconnected systems with two or more independently controlled areas, in addition to control of frequency, generation within each area has to be controlled to maintain scheduled power interchange.

40. Define speed droop.

It is defined as the magnitude of the change in steady-state speed, expressed in per unit of rated speed when the output of the unit is gradually reduced from 1.0pu rated power to zero.

41. Show that voltage control and reactive power control are interrelated.

• System stability in increased to maximum utilization of the transmission system. Voltage and reactive power control have a significant impact on system stability.
• The reactive power flow is minimized so as to reduce I2R and I2X losses and to operate the transmission system efficiently.

42. Define Plant capacity factor.

Plant capacity factor= Average Demand/ Installed Capacity of Power Plant

= Maximum Demand x Load Factor/Installed Capacity of Power Plant

43. Give two conditions for proper synchronizing of alternators.

• The phase sequence of incoming machine is same as that of bus-bars.
• The terminal voltage of incoming machine must be exactly equal to that of the others or of bus bars connecting them.
• The speed of incoming machine must be such that its frequency equals bus bar frequency.

44. What are the various functions of excitation system?

• To enhance the system stability,
• To provide direct current to synchronous machine field winding.
• To perform protective functions such as the capability limits of synchronous machine and excitation system are not exceeded.
• To perform control function such as control of voltages, control of reactive power flow, etc.

It is flexible to operate at all load condition.

The reactive power can be varied in a smooth manner.

They cannot be designed to work at more than 20000 V.

The cost of installation is higher.

46. What is meant by FLAC?

Full load Average production cost (FLAPC) is the product of net heat rate at full load and fuel cost.

47. What are the functions of SCADA?

• Automatic Generation Control.
• Supervisory Control.
• Data Logging.
• Trending.
• Data Acquisition.

48. Define Plant Use Factor.

Plant Use (Operating) Factor = Total kWh generated/Rated capacity of the plant * Number of operating hours

49. State any two necessities to put alternators in parallel.

• To increase the output capacity of a system beyond that of a single unit.
• To permit shutting down one machine and cutting in a standby machine without interrupting power distribution.
• To serve as additional reserve power for expected demands.

50. What are the assumptions made in dynamic response of uncontrolled case?

• Turbine dynamics are neglected.
• Action of speed changer is assumed to be instantaneous.

51. What are the different types of static VAR compensators?

• TSC (Thyristor Switched Capacitor).
• TCR (Thyristor Controlled Reactor).
• FC-TCR (Fixed Capacitor-Thyristor Controlled Reactor).
• TSC-TCR (Thyristor Switched Capacitor-Thyristor Controlled Reactor).

52. What are the objectives of Automatic Generation Control?