Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 What is meant by stranded conductors ?

Ans. : A stranded conductor is defined as the conductor which is composed of a group of wires or a combination of groups of wires which are electrically parallel, usually twisted together possessing greater flexibility and providing higher mechanical strength.

 

Q.2 Define transposition in transmission line.

Ans. : Transposition is the periodic swapping of positions of the conductors of a transmission line, so that each conductor occupies the original position of every other conductor over an equal distance so as to achieve balance in the three phases.

 

Q.3 What is skin effect and what is its consequence ?

Ans. :          Skin effect is defined as the tendency for the Alternating Current (AC) to flow mostly near the outer surface of a conductor which causes non-uniform distribution of current. Thus the current density is largest near the surface of the conductor and decreases with greater depths inside the conductor. The effect becomes more and more apparent as the frequency increases.

Due to reduction in effective area of cross section offered to the flow of current through the conductor, the resistance of conductor increases.

 

Q.4 State two advantages of bundled conductors ?

Ans. : The advantages of bundled conductors are

i) Reduction in corona loss.

ii) Minimization of interference with communication circuit.

iii) Reduction in voltage drop which increases circuit capacity and boosting of operating voltage.

iv) Low reactance, increase in capacitance and surge impedance loading. 

 

Q.5 Differentiate between bundled conductors and stranded conductors. Ans. :

 

Q.6 Define the term "Interference with neighboring circuits".

Ans. : The power lines and communication lines run along the same path or even run on same support s along the same route. The transmission lines transmit bulk power with relatively high voltage and produce electromagnetic and electrostatic fields of appreciable magnitudes. Because of these fields, voltages and currents are induced in the neighboring communication line causing distortion in the line which is known as interference of power lines with neighboring communication circuits.

 

Q.7 Write the expression for capacitance of the 3 phase unsymmetrically spaced transmission lines.

Ans. : The capacitance of the 3 phase unsymmetrically spaced transmission lines is given by,

 

Q.8 On which factors does proximity effect depends ?

Ans. : The proximity effect depends on following factors

i) Frequency of the current.     ii) Resistivity of the conductor material,

iii) Shape of the conductor.     iv) Diameter of the conductor.

 

Q.9 State the advantages of ACSR conductors when used for overhead lines.

Ans. : The advantages of ACSR conductors when used for overhead lines are,

i) Due to high mechanical and tensile strength, the line span can be increased. Since the sag is small, shorter supports are required for line. 

ii) Low corona loss.

iii) Less skin effect.

iv) The conductors are inexpensive as compared to copper conductors.

 

Q.10 What is the effect of bundled conductors on the transmission line inductance?

Ans : The inductance of a bundled conductor is given by,

where x is number of subconductors in each bundle.

Thus with increase in number of subconductors in each bundle, there is corresponding increase in D_s which in effect reduces the inductance of the transmission line due to which inductive reactance is also lowered.

 

Q.11 Briefly explain ACSR.

Ans. :          ACSR stands for Aluminium Conductor with Steel Reinforcement where the mechanical strength of the conductor is increased by adding steel core to the conductor. The central core is made up of steel which is surrounded by aluminium conductors in suitable layers. For a given resistance, conductors of different strengths can be made by taking different properties of steel and aluminium areas. The steel core does not contribute to conduction of current practically. The current capacity and resistance of this conductor is dependent on conductivity of aluminium. The advantages of ACSR conductors are high mechanical strength, low corona loss, lower skin effect and inexpensive as compared with copper conductors.

 

Q.12 State the expression for loop inductance of a single phase line.

Ans. : The expression for the loop inductance of a conductor with spacing D between the conductors and r' as the effective radius (GMR) is given by

4 × 10^-7 ln D / r’

 

Q.13 What is proximity effect ? What is its consequence ?

Ans. : Due to non uniform distribution of current, the current density is non uniform which increases the effective resistance of the conductor. As the distance between the conductors goes on reducing, the distribution of current becomes more and more non uniform. This is known as proximity effect. Thus in a conductor carrying alternating current, if currents are flowing through one or more other nearby conductors, the distribution of current within the first conductor will be constrained  to smaller regions which results in current crowding. This effect of current crowding is known as proximity effect which increases the effective resistance of the circuit and increases with frequency.

 

Q.14 On which factors does skin effect depends ?

Ans. : The skin effect depends on following factors

i) Frequency of the current.     ii) Resistivity of the conductor material,

iii) Shape of the conductor.     iv) Diameter of the conductor.

 

Q.15 What is bundling of conductors ?

Ans. : The bundling of conductors is nothing but the formation of two or more subconductors in specific geometric configuration with certain constant distance within them for transmission of bulk power so as to reduce corona loss, line reactance and radio interference.

 

Q.16 Write the expression for capacitance of a single phase overhead transmission line.

Ans. : The capacitance of single phase overhead transmission line between the conductors having radius r and separated by a distance D is given by,

whereas capacitance of single phase overhead transmission line between one of the conductors and the neutral is given by,

 

Q.17 Why is skin effect absent in dc system 7

Ans. :  When dc current flows in the conductor, it distributes equally and uniformly inner and outer whereas in the case of ac, higher current flows in outer surface and lower in inner surface due to which the resistance increases in inner surface. Thus due to uniform distribution of current in dc system through the cross section of conductor, skin effect is absent.

 

Q.18 Why transmission lines are 3 phase 3 wire circuits while distribution lines 3 phase 4 wire circuits ?

Ans. : For distribution of electrical power, 3 phase 4 wire circuits are used so as to supply single phase and three phase loads depending on the requirement. While for transmission, power is not supplied to load, but only carried from generating stations towards distribution. So 4th wire i.e. neutral wire is not required so 3 phase 3 wire circuits are used.

 

Q.19 A 3 phase transmission line has its conductor at the corners of an equilateral triangle with side 3 m. The diameter of each conductor is 1.63 cm. Find the inductance per km per phase of the line.

Ans. : The expression for the inductance of a conductor A in a three phase line with spacing D between the conductors and r' as the effective radius (GMR) is given by

 

Q.20 What are the advantages of double circuit lines.

Ans. : The advantages of double circuit lines are

i) Continuity of supply is possible.

ii) More reliable.

iii) Less spacing of conductors required.

iv) Less inductance and reactance. Efficiency is enhanced as compared with single circuit lines.

 

Q.21 State the factors on which the inductance of a line is dependent.

Ans. : The factors on which the inductance of a line is dependent are

i) Radius of the conductor.

ii) Spacing between the conductors.

 

Q.22 What do you mean by neutral plane.

Ans. : A neutral plane is defined as the plane that exists at midway between the two long, parallel and oppositely charged conductors where the electric potential and electric field intensity is zero.

 

Q.23 What are the parameters associated with a transmission line.

Ans. : The parameters associated with a transmission line are

i) Resistance

ii) Inductance

iii) Capacitance

iv) Conductance between the conductors or between conductors and ground which is very small and often neglected.

 

Q.24 What are the various types of aluminium conductors.

Ans. : The various types of aluminium conductors are

i) AAC All Aluminium Conductor.

ii) AAAC All Aluminium Alloy Conductor.

iii) ACSR Aluminium Conductor with Steel Reinforcement.

iv) ACAR Aluminium Conductor with Alloy Reinforcement.

 

Q.25 What do you mean by inductance of a line.

Ans. : Inductance of a line is defined as the property of a conductors in the line by which a change in current flowing through it induces a voltage in both the conductors as well as in any nearby conductors. Its unit is Henry (H).

 

Q.26 What do you mean by symmetrical spacing of a line.

Ans. : When the distance between the various conductors of the three phase line is same then the line is said to have symmetrical spacing.

 

Q.27 State the advantages of aluminium conductors when used for overhead lines.

Ans. : The advantages of aluminium conductors when used for overhead lines are,

i) Low cost  ii) Less resistance

iii) Low corona loss iv) Less weight

 

Q.28 What do you mean by unsymmetrical spacing of a line.

Ans. : When the distance between the various conductors of the three phase line is different then the line is said to have symmetrical spacing.

 

Q.29 What are the factors to be considered while designing a transmission line.

Ans. : The various factors to be considered while designing a transmission line are

i) Type and size of conductor.

ii) Voltage level.

iii) Power flow capability, stability and efficiency of transmission.

iv) Line regulation and control of voltage.

v) Power system structure.

vi) Economic aspects.

 

Q.30 What are the various factors influencing telephone interference ?

Ans. : The various factors influencing telephone interference are

i) Harmonics in power circuits, their frequency and magnitudes.

ii) Electromagnetic coupling between power and telephone conductor.

iii) Due to imbalance in power and telephone circuits.

iv) Type of return telephone circuit i.e. metallic or ground return.

v) Screening effects.

 

Q.31 What are the various ways for reducing telephone interference ?

Ans. : The steps for reducing telephone interference are

i) The harmonics at the source can be reduced with AC and DC harmonic filters and smoothening reactors.

ii) Greater spacing between power and telephone lines.

iii) Parallel run between power and telephone line is avoided.

iv) Underground lines for telephones can be used.

v) If telephone circuit is ground return then replace it with metallic return.

 

Q.32 State the factors on which electrostatic and electromagnetic effects depend.

Ans. : The main factors on which electrostatic and electromagnetic effects depend are the distance between power and communication circuits and the length of the route over which they are parallel.

 

Q.33 What do you mean by symmetrical and unsymmetrical spacing ?

Ans. : If the conductors are placed equidistant from each other they are said to be symmetrically spaced e.g. conductors placed at the comers of an equilateral triangle.

If the conductors are not placed equidistant from each other but the distance between the conductors is different from each other they are said to be unsymmetrically spaced e.g. conductors placed in a horizontal or vertical plane.

 

Q.34 Compare between single and double circuit arrangement.

Ans. : The comparison between single and double circuit arrangement is given below.

 

Q.35 Define electrical supply system.

Ans. : The flow of electrical power from the generating station to the consumer is called an electrical supply system. It consists of following components :

i) Generating station ii) Transmission network and iii) Distribution network

 

Q.36 State the practical transmission and distribution voltage levels commonly used.

Ans. : Generating stations : 6.6 kV, 11 kV or 22 kV.

Primary transmission : 66 kV / 132 kV / 220 kV / 400 kV.

Secondary transmission : 11 kV / 22 kV / 33 kV.

Primary distribution : 6.6 kV / 3.3 kV / 11 kV.

Secondary distribution : 400 V / 230 V.

 

Q.37 What is single line diagram ?

Ans. : The single line diagram is the simplified representation of power system components with each other, with each component represented by its symbol.

 

Q.38 List the various elements of power system.

Ans. : The various elements of power system are generators, transformers, transmission lines, bus bars, circuit breakers, isolating switches, feeders, distributors, service mains etc.

 

Q.39 What is bulk power supply system ?

Ans. : The generating stations at which power is generated at 11 kV and the transmission lines used to transfer power by stepping up the voltage level to very high level are often called bulk power supply system.

 

Q.40 What is feeder ?

Ans. : The feeders are the conductors which are of large current carrying capacity. The feeders connect the substation to the area where power is to be finally distributed to the consumers.

 

Q.41 What is service mains ?

Ans. : The small cables used to connect the distributors and the actual consumer premises are called service mains.

 

Q.42 Draw a single line diagram of a modern electric power system.

Ans. :

 

Q.43 What is a distributor ?

Ans. : A distributor is a conductor which is used to distribute an electrical power from the substations to the consumers. 

 

Q.44 Why the concept of self GMD is not applicable for capacitance calculation ?

AU : Dec.-17

Ans. : The concept of self GMD depends on the size and shape of the conductor i.e. radius of conductor and not on the spacing between the conductors. While capacitance depends on the spacing between the conductors. Hence the concept of self GMD is not applicable for capacitance calculations.

 

Q.45 What is transposition ? Why are transmission line transposed ?

AU : Dec.-17

Ans. : The positions of the conductors are exchanged at regular interval along the line so that each conductor occupies the original position of every other conductor over an equal distance. This exchange of conductor positions is called transposition

The advantages of transposition are,

i) The balance in three phases is restored.

ii) The inductance and capacitance of conductors are equal.

iii) It reduces the possibility of acoustic shock or noise.

iv) It stabilizes the voltage unbalance.

v) It reduces the effect of inductive coupling and imbalanced currents.

vi) Reduces cross-talks and improves the overall transmission.

 

Q.46 Distinguish between a feeder and a distributor 7

Ans. : The feeders connect the substation to the distribution centre while the

distributors are the conductors, which are used to transfer power from distribution centre to the consumers. The feeders have large current carrying capacity than distributors. The tappings are taken for the supply to the consumers from distributors but no tappings are taken from the feeders.

 

Q.47 Distinguish between self and mutual GMD.

Ans. : The distance of filament of conductor from itself is called self Geometrical Mean Distance i.e. self GMD. It is also called Geometric Mean Radius (GMR). It depends on the size and shape of he conductor and not on the spacing between the conductors. The mutual GMD is defined as the geometrical mean of the distances from one conductor to the others. It represents equivalent geometrical spacing.