Power System Analysis Important Questions and Answers

1. What is one line diagram?


It is the diagrammatic representation of power system in which the components are represented by their symbols and the interconnection between them are shown by a single straight line. The ratings and the impedances of the components are also marked on the one line diagram.

2. What are the models used to represent generators in short circuit analysis?


  1. Zero Sequence,
  2. Negative Sequence,
  3. Positive Sequence.

3. Define steady state stability.


It is defined as the ability of a power system to remain stable (i.e., without loosing synchronism) for small disturbances.

4. What are the causes of oscillatory and non-oscillatory instabilities in power systems?


Oscillatory instability –  Lack of sufficient damping torque.

Non-Oscillatory Instability – Insufficient synchronizing torque.

5. What are the advantages of pu representation?


  • The per-unit values of impedance, voltage and current of a transformer are the same regardless of whether they are referred to the primary or the secondary side. This a great advantage since the different voltage levels disappear and the entire system reduces to a system of simple impedance.
  • The per-unit systems are ideal for the computerized analysis and simulation of complex power system problems.

6. What condition onbase and rated voltage should be satisfied to give equal pu impedances viewed from either side of a two-winding transformer?


Base Voltage =  Rated Voltage

7. What is Jacobian matrix?


Jacobian matrix is the matrix formed from the first derivatives of load flow equations and it is denoted by J. In each interaction the elements of the jacobian matrix are obtained by partially differentiating the load flow equations with respect to a unknown variable and then evaluating the first derivative using the solution of previous interaction.

8. What is the need for fault analysis in power system?


  • To obtain the rating of the protective switchgears
  • To determine bus voltages and line currents during various types of faults.
  • To obtain proper relay setting and coordination
  • The three-phase balanced fault information is used to select and set phase relays.

9. What is meant by fault level at a bus and give the expression for the same in pu?


SCC = √3 VLk Ik(F) x 10-3 MVA.

10. Define voltage stability.


Voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to s disturbance.

11. State two techniques to improve stability of power systems.


  • Using of Automatic Voltage Regulator (AVR)
  • Use of high speed transfer reactance
  • Use of high speed reclosing breakers
  • Using FACTS controllers and power system stabilizers (PSSP.

12. What are the advantages of per unit system?


  • Per unit date representation yields valuable relative magnitude information.
  • Circuit analysis of systems containing transformers of various transformation ratio is greatly simplified.
  • Circuit parameters tend to fall in relatively narrow numerical ranges making erroneous data easy to spot.
  • The P.U systems are ideal for the computerized analysis and simulation of complex power system problems.

13. What is slack bus?


In load flow study single line diagram the bus specification is mentioned as voltage magnitude and phase angle for any one of the Bus. On other buses real power and reactive power are mentioned. That specific bus is called slack bus.

14. Mentioned the objectives of short circuit analysis?


The system must be protected against heavy flow of short circuit currents by disconnecting the faulty section from the healthy section by means of circuit breaker. To estimate the magnitude of fault current for the proper choice of circuit breaker and protective relays, short circuit study is essential. The short circuit study is more important in design of protective scheme for various parts of the system.

15. Write down the balanced and unbalanced faults occurring in a power system.


Symmetrical or balanced faults:

  • Triple line fault

Un-symmetrical or un-balanced faults

  • Line to ground fault
  • Line to line fault
  • Double line to ground fault

16. What is sequence network?


In three phase system, the phase currents are resolved into three symmetrical components they are positive, negative and zero sequence. They are mentioned by I+a, Ia and I0a respectively. When all the sequence representations are connected together in a phase is called sequence network.

17. Write the symmetrical components of a three phase systems?


Symmetrical components of 3Φ system.

  1. Positive sequence components.
  2. Negative sequence components.
  3. Zero sequence components.

18. Define critical clearing angle.


For any given initial load in the case of a fault clearances on a synchronous machine connected to an infinite bus bar. there is a critical clearing angle. If the actual clearing angle is greater than the critical clearing angle value, the system is unstable, otherwise the system is stable.

19. What is single line diagram?


The entire power system is represented for simplicity by a single phase or single line diagram, which is a most practical way to represent a power system by means of a simple symbols for each components.

20. How are the loads represented in reactance or impedance diagram?


The loads are represented by impedance which is the combination of resistance and inductive reactance in series.

21. How are the disadvantages of Newton-Raphson method overcome?


The solution involves the inversion of J and J matrices whose dimensions are approximately one forth the full size jacobina matrix which saves 35-50% of storage when compared to original Newton method. It reduces the computation time for each iteration that is 10% – 20% less that original Newton method.

22. What is the need for short circuit studies?


It is essential in the economic planning and design of power system. It provide the engineer with information by which he can design to assure the disconnection of faulted equipment with a minimum of damage and a minimum of disturbance to the operation of the remaining systems.

23. List various type of shunt and serious faults.


Shunt Fault:

  • 3 phase faults
  • Line to line fault
  • Line to ground fault
  • Double-line to ground fault.

Series Fault:

  • One open conductor fault
  • Two open conductor fault

24. Define negative sequence impedance.


When only negative sequence current are present, the impedance is called the impedance to negative sequence current.

25. Name the faults which do not have zero sequence currents flowing.


Double line faults does not have zero sequence currents flowing in it.

26. State Equal Area Criterion.


The equal area for stability states that the system is stable if the area under P – δ curve reduces to zero at some value of δ.

If the possible area under P – δ curve us equal to the negative area under P – δ curve for a finite change in δ. Hence stability criterion is called Equal Area Criterion.

27. What are the components of power system?


The components of power systems are:

  • Generator
  • Power transformers
  • Transmission lines
  • Substation transformers
  • Distribution transformers
  • Loads.

28. What are the information that are obtained from a power flow study?


The information obtained from power flow study are magnitude and phase of bus voltages, real and reactive power flowing is each line and the line losses.

It also gives initial condition of the system when transient behavior of system to be studied.

29. What is meant by a fault?


A fault in a circuit is any failure which interferes with the normal flow of current, the faults are associated with abnormal change in currents, volts and frequency of the power system.

30. List the symmetrical and unsymmetrical faults that occur in a power system.


Symmetrical fault:

  • Three phase fault.

Unsymmetrical fault:

  • Line to ground fault
  • Line-line fault
  • Double line to ground fault
  • One to two open conductors fault.

31. Name the faults in which all the three sequence component currents are equal and in which positive and negative sequence currents together is equal to zero sequence current.


Double line to ground fault.

32. Define transient stability and transient stability limit.


Transient stability is defined as the ability of a power system to remain stable for large disturbances.

Transient stability limit is the maximum power that can be transmitted by a machine to a fault or a receiving system during a transient state without loss of synchronism.

33. What is critical clearing angle and critical clearing time?


The critical clearing angle (sec) is the maximum allowable change in power (s) before clearing the fault, without loss of synchronism.

The critical clearing time, tcc can be defined as the maximum time delay that can ne allowed to clear a fault without loss of synchronism.

34. What do you mean by treatment and steady state condition?


Transient state is the state changes from one state to another state in which voltage or current changes. Steady state is the state having constant source.

35. What is meant by acceleration factor in load  flow studies?


The process of convergence in Guass-Seidal method can be speeded up by using the factor called acceleration factor. It can be obtained by running trial load flow studies.

36. What are the date required for load flow study?


The date required for load flow studies are:

  • Magnitude of voltage (i.e) | E |.
  • Phase angle of the voltage (i.e) δ.
  • Real power (i.e) P.
  • Reactive power (i.e) Q.

37. What is the need for short circuit studies?


The short circuit studies are essential in order to design or develop the protective schemes for various parts of system.

38. What is an infinite bus?


The bus is which many components are connected with many parallel lines are called as infinite bus.

39. What is meant by critical clearing angle?


The critical clearing angle, δa is the maximum allowable change in the power angle σ before learning the fault, without loss of synchronism.

40. What is meant by percentage reactance?


A reactance of X% will have a voltage drop of X% of normal voltage when carrying full load current corresponding to normal rating.

41. What is the necessity for slack bus?


The slack bus is needed to account for transmission line losses.

42. What is meant by acceleration factor?


The multiplication of the constant will increase the amount of correction to being the voltage closer to the value it is approaching. These multipliers are called acceleration factors.

43. Distinguish symmetrical and unsymmetrical faults.


Symmetrical faults are the faults in which fault current is equal in all the phases. The unsymmetrical faults are the fault in which fault current are unequal in all the phases.

44. What is meant by fault level?


Fault level at a bus is defined as the product of the magnitude of the pre fault bus voltage and the post fault current.

45. What is meant by power angle curve?


The curve plotted between displacement angle (δ) and real power (P) is called power angle curve.

46. What are the functions of modern power systems?


Functions of modern power system:

  • To monitor the voltages at various components,
  • To design the circuit breaker,
  • To plan future expansion of existing system.

47. Name the diagonal and off-diagonal elements of bus impedance matrix.


The diagonal elements of bus admittance matrix are called self-admittance of the buses and off diagonal elements are call mutual admittances of the buses.

48. Why do Ybus used in load flow study instead of Zbus?


The Ybus is used in load flow study instead of Zbus because it is symmetrical along the leading diagonal. Many of the off diagonal elements are zero. Therefore it is easy to analyze.

49. When will the generator bus be treated as load bus?


If the reactive power of a generator bus violated the specified limits then the generator bus is treated as load bus.

50. What is the order of severity and occurrence of different types of fault?


The various faults in order of decreasing severity are given below:

  • 3-phase fault
  • double line-to-ground fault
  • line-to-line fault
  • single line-to-general fault
  • open conductor fault.

51. What are the characteristics of shunt and series faults?


  • Series fault will increase the voltage and frequency and decrease in current.
  • Shunt fault will decrease the voltage and frequency but increases the current flow.

52. What are the observations made from the analysis of various faults?


  • To develop the protection schemes for carious parts.
  • To select the appropriate devices (relay, CBs).

53. Differentiate between voltage stability and rotor angle stability.


Rotor angle stability: It is the ability of interconnected synchronous machines of a power system.

Voltage Stability: It is the ability of power system to maintain steady acceptance voltages at all buses.

54. Define swing curve? What is the use of this curve.


The swing curve is the plot or graph between the power angle δ and time t.