UNIT-II POWER FLOW ANALYSIS

EE 1352 POWER SYSTEM ANALYSIS

Questions and Answers

UNIT-II POWER FLOW ANALYSIS

1. What is a bus?
The meeting point of various components in a power system is called a bus. The bus is a conductor made of copper or aluminium having negligible resistance .At some of the buses power is being injected into the network, whereas at other buses it is being tapped by the system loads.
2. What is bus admittance matrix?
The matrix consisting of the self and mutual admittance of the network of the power system is called bus admittance matrix (Ybus).
3. What are the methods available for forming bus admittance matrix?
• Direct inspection method.
• Singular transformation method.(Primitive network)
4. What is power flow study or load flow study?
The study of various methods of solution to power system network is referred to as load flow study. The solution provides the voltages at various buses, power flowing in various lines and line losses.
5. What are the informations that are obtained from a load flow study?
The information obtained from a load flow study is magnitude and phase angle of 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 behavior of the system is to be studied.
6. 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. It is also essential foe designing a new power system.
7. What are the quantities associated with each bus in a system?
Each bus in a power system is associated with four quantities and they are real power (P), reactive power (Q), magnitude of voltage (V), and phase angle of voltage (δ).

8. What are the different types of buses in a power system? Or how the buses are classified and what are its types?
Types of bus Known or specified quantities Unknown quantities or quantities to be determined.
Slack or Swing or Reference bus V, δ P,Q
Generator or Voltage control or PV bus P, V Q, δ
Load or PQ bus P, Q V, δ




9. What is the need for slack bus?
The slack bus is needed to account for transmission line losses. In a power system the total power generated will be equal to sum of power consumed by loads and losses. In a power system only the generated power and load power are specified for buses. The slack bus is assumed to generate the power required for losses. Since the losses are unknown the real and reactive power are not specified for slack bus.
10. Why do we go for iterative methods to solve load flow problems?
The load flow equations are non linear algebraic equations and so explicit solution as not possible. The solution of non linear equations can be obtained only by iterative numerical techniques.
11. What are the methods mainly used for solution of load flow study?
The Gauss seidal method, Newton Raphson method and Fast decouple methods.
12. What do you mean by a flat voltage start?
In iterative method of load flow solution, the initial voltages of all buses except slack bus assumed as 1+j0 p.u. This is refereed to as flat voltage start
13. Discuss the effect of acceleration factor in load flow study.
Acceleration factor is used in gauss seidal method of load flow solution to increase the rate of convergence. Best value of A.F=1.6
14. When the generator buses are treated as load bus.
If the reactive power constraints of a generator bus violates the specified limits then the generator is treated as load bus.
15. What are the advantages and disadvantages of Gauss seidal method?
Advantages: Calculations are simple and so the programming task is lessees. The memory requirement is less. Useful for small systems; Disadvantages: Requires large no. of iterations to reach converge .Not suitable for large systems. Convergence time increases with size of the system
16. What are the advantages and disadvantages of N.R method?
Advantages: Faster, more reliable and results are accurate, require less number of iterations; Disadvantages: Program is more complex, memory is more complex.
17. Compare the Gauss seidel and Newton raphson methods of load flow study.

S.No G.S N.R FDLF
1 Require large number of iterations to reach convergence. Require less number of iterations to reach convergence. Require more number of iterations than N.R method.
2 Computation time per iteration is less Computation time per iteration is more Computation time per iteration is less
3 It has linear convergence characteristics It has quadratic convergence characteristics ------
4 The number of iterations required for convergence increases with size of the system The number of iterations are independent of the size of the system The number of iterations are does not dependent of the size of the system
5 Less memory requirements. More memory requirements. Less memory requirements than N.R.method.