Important Short Questions and Answers: Power Flow Analysis

1. What is power flow study?

The study of various methods of solution to power system network is reffered to as power flow or load flow study.

2. What are the informations that are obtained from a load flow study?

The information obtained from a load flow study are magnitude and phase of bus voltages, real and reactive power flowing in each line and the line losses. The load flow solution also gives the initial conditions of the system when the transient behaviour of the system is to be studied.

3. What is the need for load flow study?

The load flow study of a power system is essential to decide the best operation of existing system and for planning the future expansion of the system. Also essential for designing a new system.

Ø                 What are the works involved in a load flow study? (i)Representation of the system by single line diagram.

(ii)Determining the impedance diagram using the information in single line diagram. (iii)Formulation of network equations.

(iv)Solution of network equations.

Ø                 What are the quantities that are associated with each bus in a system?

Each bus in a system are associated with four quantities and they are real power,reactive power, magnitude of voltage and phase angle of voltage.

Ø                 What are the different types of buses? (i)Load bus or PQ bus

(ii)Generator bus or voltage controlled bus or PV bus (iii)Slack bus or Swing bus or Reference bus.

ØDefine voltage controlled bus.

A bus is called voltage controlled bus if the magnitude of voltage and real power are specified. The magnitude of voltage is not allowed to change.

8. What is PQ bus?

A bus is called PQ are load bus when real and reactive components of power are specified for the bus. In a load bus the voltage is allowed to vary within the permissible value.

9. What is swing bus?

A bus is called Swing bus when the magnitude and phase of the voltage are specified for it. Swing bus is the reference bus for load flow solution and it is required for accounting line losses.

10. What is the need for slack bus?

The slack bus is needed to account for transmission losses.

Ø   What are the iterative methods used for solution of load flow problems? Guass-Seidal (GS) method and Newton Raphson(NR) method.

Ø   Why do we go for iterative methods to solve load problems?

The load flow equations are nonlinear algebraic equations and so explicit solution is not possible. The solution of nonlinear equations can be obtained only by iterative numerical techniques.

13. When the generator bus is treated as load bus?

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

14. What are the advantages of GS method?

(i)Calculations are simple and so the programmimg task is lesser. 

(ii)The memory requirement is less.

(iii)Useful for small systems.

15.            What are the disadvantages of GS method?

(i)Requires large number of iterations to reach convergence.

(ii)             Not suitable for large systems.

(iii)           Convergence time increases with size of the system.

16.            What are the advantages of NR method?

(i)Faster, more reliable and the results are accurate.

(ii) Requires less number of iterations to reach convergence.

(iii)The numbers of iterations are independent of the size of the system.

(iv) Suitable for large systems.

17. What are the disadvantages of NR method?

(i)The programming is more complex.

(ii)Memory requirement is more.

(iii)Computation time per iteration is higher due to large number of calculations per iterations.