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# Important Short Questions and Answers: Power System Analysis - Introduction

Power System Analysis - Introduction - Important Short Questions and Answers: Power System Analysis - Introduction

1. What is single line diagram?

A single line diagram is a 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.

2. What are the components of a power system?

The components of a power system are Generators, Power transformers, Transmission lines, Substation transformers, Distribution transformers and Loads.

3. Define per unit value.

The per unit value of any quantity is defined as the ratio of actual value of the quantity to base value of the quantity.

Per unit = Actual value / Base value

4. What is the need for base value?

The components or various sections of power system may operate at different voltage and power levels. It will be convenient for analysis of power system if the voltage, power, current and impedance ratings of components of power system are expressed with a common value called base value. Hence for analysis purpose a base value is chosen for voltage, power, current and impedance.

5. Write the equation for converting the p.u. impedance expressed in one base to another.

Zp.u,new = Zp.u,old) * (kVb,old / kVb,old) * (MVAb,new  / MVAb,old)

6. What are the advantages of per unit computations?

·    manufacturers usually specify the impedance of a device or machine in per unit on the basis of the name plate details.

·       The p.u. values of widely different rating machines lie within a narrow range eventhough the ohmic values has a very large range.

·     The p.u. impedance of circuit element connected by a transformer expressed on a proper base will be same if it is referred to either side of a transformer.

·      The p.u. impedance of a 3-phase transformer is independent of the type winding connection.

7. What are the approximations made in impedance diagram?

·        The neutral reactances are neglected.

·        The shunt branches in equivalent circuit of induction motor are neglected.

8. What are the approximations made in reactance diagram?

§  The neutral reactance are neglected.

o   The shunt branches in equivalent circuit of induction motor are neglected.

o   The resistances are neglected.

o   All static loads and induction motor are neglected.

o   The capacitances of the transmission lines are neglected.

9. Give the equations for transforming base kV on LV side to HV side and viceversa. Base kV on HT side = Base kV on LT side * (HT voltage rating / LT voltage rating) Base kV on LT side = Base kV on HT side * (LT voltage rating / HT voltage rating)

MODELLING OF VARIOUS COMPONENTS/ACCESSORIES

1. What is a bus?

The meeting point of various components in a power system is called a bus.

2. What is bus admittance matrix?

The matrix consisting of the self and mutual admittance of the network of a power system is called bus admittance matrix.

3. Name the diagonal and off-diagonal elements of bus admittance matrix.

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

4. Write the equation to find the elements of new bus admittance matrix after eliminating nth row and column in a n*n matrix.

Yjk = Yjk-(YjnYnk / Ynn)

5. What is bus impedance matrix?

The matrix consisting of driving point impedances and transfer impedances of the network of a power system is called bus impedance matrix.

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

The diagonal elements of bus impedance matrix are called driving point impedances of the buses and off-diagonal elements are called transfer impedances of the buses.

7. What are the methods available for forming bus impedances matrix?

(1)Form the bus admittances matrix and then take its inverse to get bus impedance matrix.

(2)Directly form the bus impedance matrix from the reactance diagram. This method utilizes the techniques of modification of existing bus impedance matrix due to addition of new bus.

8. Write the four ways of adding an impedance to an existing system so as to modify bus impedance matrix.

Case 1:Adding a branch impedance Zb from a new bus P to the reference bus. Case 2:Adding a branch impedance Zb from a new bus P to the existing bus q. Case 3:Adding a branch impedance Zb from a existing bus q to the reference bus.

Case 4:Adding a branch impedance Zb between two  existing bus h and q.

9. What is off-nominal transformer ratio?

When the voltage or turns ratio of a transformer is not used to decide the ratio of base kV then its voltage or turns ratio is called off-nominal turns ratio.

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Power System Analysis : Introduction : Important Short Questions and Answers: Power System Analysis - Introduction |