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Chapter: Transmission and Distribution : Structure of Power System

Important Short Questions and Answers: Transmission and Distribution - Structure of Power System

Transmission and Distribution - Structure of Power System - Important Short Questions and Answers: Transmission and Distribution - Structure of Power System

Transmission and Distribution



1. Why all transmission and distribution systems are 3 phase systems?


A 3 phase a.c circuit using the same size conductors as the single phase circuit can carry three times the power which can be carried by a 1 phase circuit and uses 3 conductors for the 2 phases and one conductor for the neutral. Thus a 3 phase circuit is more economical than a 1 phase circuit in terms of initial cost as well as the losses. Therefore all transmission and distribution systems are 3 phase systems.


2. Why the transmission systems are mostly overhead systems?

Because of the cost consideration, the transmission systems are mostly overhead systems.


3. Why all overhead lines use ACSR conductors?


ACSR conductors comprises of hard drawn aluminium wires stranded around a core of single or multiple strand galvanized steel wire. They provides the necessary conductivity while the steel provides the necessary mechanical strength. Has less corona loss. The breaking load is high and has less weight.


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


A Balanced 3 phase circuit does not require the neutral conductor, as the instantaneous sum of the 3 line currents are zero. Therefore the transmission lines and feeders are 3 phase 3 wire circuits. The distributors are 3 phase 4 wire circuits because a neutral wire is necessary to supply the 1 phase loads of domestic and commercial consumers.


5. Why overhead line conductors are invariably stranded?

They are stranded to make them flexible during erection and while in service.


6. State the advantages of interconnected systems.


Any area fed from one generating station during overload hours can be fed from another power station and thus reserved capacity required is reduced, reliability of supply is increased and efficiency is increased.


7. What is a ring distributor?


A ring distributor is a distributor which is arranged to form a closed circuit and is fed at one or more than one point.


8. State any two advantages of ring main system.


Less voltage fluctuations at consumer’s terminals.Less copper is required as each part of the ring carries less current than in radial system.


9. Mention the disadvantages of a 3 wire system



In 3 wire system a third wire is required .The safety is partially reduced .A balancer is required and therefore cost is increased.


10.            What are the advantages of a 3 wire dc distribution system over a 2 wire dc distribution system?


If 3 wire system is used to transmit the same amount of power over the same distance with same efficiency with same consumer voltage we require 0.3125 times copper as required in 2 wire system.


11.            Mention the differences between 3 wire and 3 phase 4 wire distribution system?


i)3 phase 3 wire is employed for balanced loads, but 3 phase 4 wire is employed for unbalanced loads.


ii)3 phase 3 wire is used for transmission but 3 phase 4 wire is used for distribution of power to consumers.


12. State kelvin’s law.


The annual expenditure on the variable part of the transmission system should be equal to the annual cost of energy wasted in the conductor used in that system.


13. State any two limitations of kelvin’s law.


It is difficult to estimate accurately the annual charge on the capital outlay. It does not give the exact economical size of the conductor.


14. Define resistance of the transmission line.


It is defined as the loop resistance per unit length of the line in a single phase system. In 3 phase system it is defined as the resistance per phase.


15.            Mention the demerits of HVDC transmission.

i. Electric power cannot be generated at high dc voltages.

ii. The dc voltages cannot be stepped up for transmission of power at high voltages.

iii. The dc switches and circuit breakers have their own limitations.


16.            What are the advantages of high voltage ac transmission.


The po wer can be generated at high voltages. The maintenance of ac substation is easy and cheaper.


17. Mention the disadvantages of high voltage ac transmission.


An ac line requires more copper than a dc line. The construction of an ac line is more complicated than a dc transmission line. Due to skin effect in the ac system the effective resistance of the line is increased.


18 Mention the limitations of using very high transmission voltage.


The increased cost of insulating the conductor. The increased cost of transformers ,switch gears and other terminal apparatus.


19.Mention the terminal equipments necessary in HVDC system.



Converters, mercury arc valves and thyristors.


20.Why HVDC line do not require any reactive power compensation ?

Due to absence of charging currents .


21.Mention the equipments that supply reactive power in HVDC converter stations ?

AC filters Static shunt capacitors Synchronous condensers StaticVAR compensators .


22.Why dc transmission is economical and preferable over ac transmission for large distances only ?


Because with larger distances ,the saving in cost of dc overhead lines become greater than the additional expenditure on terminal equipment .


23.Why is voltage regulation better in case of dc transmission ?

Because of absence of inductance in dc systems .


24.What are the advantages of adopting EHV/UHV for transmission of ac electric power?


Reduced line losses


High transmission efficency


Improved voltage regulation


Reduced conductor material requirement


Flexibility for future system growth


increase in transmission capacity of the line


increase of SIL .


25.Mention the problems associated with an EHV transmission ?


The problems associated with EHV transmission are corona loss and radio interference , requirements of heavy supporting structures erection difficulties and insulation requirements .


26.What for series and shunt compensation provided in EHV lines ?


Series compensation is provided to reduce the series reactance of the line so as to improve stability, voltage regulation and transmission efficiency. Shunt compensation is provided to reduce the line susceptance so as to improve the voltage regulation under light load condition .


27.What is the voltage that has been selected for HVDC transmission ?

± 500 KV .


28.Define an electric power supply system?


The system of generation, transmission and distribution of electrical power is called the electrical power system .


29.What are service mains?

Service mains are conductors which connect the consumers terminals to the distributor.


30.What is the usable voltage for secondary distribution ?



415 40 V (415 volts for 3-phase loads and 240 volts for 1-phase loads)


31.Why galvanized steel wire is not suitable for EHT lines for the purpose of transmitting large amounts of power over long distance?


because of Poor conductivity


High internal reactance & eddy current & hysterisis


31.Mention the transmission voltages that are used in India?

33/66/110/132/220/400/765 KV. 765KV is EHV.


32.            On what basis the conductor size is determined for transmission lines upto 220 KV?

is determined on the basis of its current carrying capacity.


33.            On what basis the conductor size of EHV line is determined?

is decided on the basis of corona


34.            Mention the sources of audible noise generation in EHV transmission systems?



Humming of transformers Cooking Systems

Mechanical and Electrical auxillaries


35.            Mention the material universally employed for overhead transmission line?

ASCR (Aluminium Conductor Steel Reinforced )


36.            What is the economical value of span for 400kv transmission line ?



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