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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
**T**he
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 n^{th} row and column in a n*n matrix.

Yjk =
Yjk-(Y_{jn}Y_{nk} / Y_{nn})

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 Z_{b} from a new bus P to the reference
bus. Case 2:Adding a branch impedance Z_{b} from a new bus P to the
existing bus q. Case 3:Adding a branch impedance Z_{b} from a existing
bus q to the reference bus.

Case 4:Adding a branch impedance Z_{b}
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 |

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