Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 Why three phase supply system is used for transmission and distribution purposes ?

Ans. : A three phase supply system has following advantages :

1. For a given size and voltage a three phase alternator occupies less space and has less cost compared to single phase alternator of same rating.

2. A three phase system needs less copper for transmission lines and hence transmission becomes very much economical.

3. Single phase supply can be obtained from three phase but three phase cannot be obtained from single phase.

4. The instantaneous power in symmetrical three phase system is constant hence gives steady output. In single phase system instantaneous power is function of time which can cause stability problem.

 

Q.2 Why transmission lines are 3 phase 3 wire while distribution lines are 3 phase 4 wire circuits ?

Ans. : The transmission is at very high voltage level and such a balanced three phase system does not require neutral conductor. Hence the transmission line circuits are 3 phase 3 wire circuits. While distribution, it is necessary to supply single phase loads along with the three phase loads. For single phase distribution a neutral conductor is must. Hence distribution lines are 3 phase 4 wire circuits.

 

Q.3 State the meaning of an electrical grid.

Ans. : An electrical grid is a network in which the various generating, transmission and distribution systems are interconnected with each other to supply electricity to the consumers.

 

Q.4 List the advantages of an electrical grid.

Ans. : 1. It maintains reliability of an electric supply to the consumers.

2. Excess of electricity in one area can be shared with shortage of electricity in another area through the grid system.

3. The reserved capacity of generating stations is reduced.

4. The overall efficiency and reliability improves.

 

Q.5 State any four advantages of a.c. systems.

Ans. :

1. It is possible to build up high a.c. voltage levels, using high speed a.c. generators of large capacities.

2. The a.c. voltages can be raised or lowered as per the requirement with the help of transformers. 

3. The a.c. supply can be converted to d.c. wherever required.

4. The maintenance of a.c. substations is cheaper and easy.

5. The cost of a.c. generators is low.

 

Q.6 State any four disadvantages of a.c. systems.

Ans. :          1. The resistance of a.c. transmission line is higher due to skin effect causing more voltage drop.

2. There is voltage drop due to inductance of a.c. lines.

3. For an open line, the charging current flows due to capacitance of a.c. lines which causes an additional power loss.

4. The a.c. lines are more sensitive to corona effect.

 

Q.7 State any four advantages of d.c. systems.

Ans. :          1. As frequency of d.c. is zero, there is no inductance or capacitance associated with the lines hence power losses and voltage drops are much less.

2. Due to reduced voltage drops the regulation is better.

3. Absence of skin effect makes use of entire cross-section of conductor.

4. For the same voltage level, the voltage stress on the insulation is less hence the insulation requirement is less.

5. The stability and synchronising difficulties are absent in d.c. systems.

 

Q.8 State any four disadvantages of d.c. systems.

Ans. :          1. The power generation at very high voltage levels is not possible due to commutation problems.

2. The voltages cannot be raised or lowered by transformers.

3. Obtaining a.c. from d.c. is not practically easy.

4. The d.c. generators and motors need a lot of maintenance and their cost is higher than a.c. machines of same capacity.

 

Q.9 State the advantages of high voltage in transmission system.

AU : Dec.-07, 11, 15, May-15, 16

Ans. :          1. The volume of copper required is less at high voltage levels.

2. The line efficiency is higher at high voltage levels.

3. The line drop becomes less at high voltage levels.

4. As the line drop is less the voltage regulation becomes better at high voltage levels. 

5. The power handling capacity of the line increases at high voltage levels.

6. The number of circuits and the land requirement reduces at high voltage levels.

7. The total line cost per MW per km decreases considerably at high voltage levels.

 

Q.10 State the disadvantages of high voltage in transmission system.

Ans. : 1. The corona loss and radio interference increases at higher voltage levels.

2. Extra amount of material is required to provide line supports to high voltage lines which increases the cost.

3. As the voltage level increases the insulation requirement of line conductors also increases which increases the cost.

4. There are many difficulties in erecting high voltage lines.

5. The cost of equipments such as transformers, switchgear and protective equipments increases with the increase in voltage level.

 

Q.11 List the various types of systems used for the power transmission.

Ans. : Refer sections 7.1, 7.2 and 7.3.

 

Q.12 Explain the necessity for EHV AC transmission.

Ans. : Refer section 7.24.

 

Q.13 State the advantages of HVDC transmission.

Ans. : 1. Economical for bulk transmission of power for long distances as d.c. system requires only two conductors or even one if ground is used as return conductor.

2. No stability problems.

3. The line length is not the limitation as there is no charging current in d.c. systems.

4. The skin effect is low and the cables do not suffer from high dielectric losses.

5. No serious problems of voltage regulation as there is no reactance drop.

6. The corona and interference problems are less.

7. With HVDC link there is easy reversibility and controllability of power flow.

8. Shunt compensation is not required. 

9. During fault with HVDC system, the grid control of the converter reduces the fault current significantly.

 

Q.14 State the disadvantages of HVDC transmission.

Ans. : 1. Not economical if length of transmission is less than 500 km as HVDC system additionally requires converters, inverters and filters.

2. With multi-terminal d.c. the circuit breaking is difficult and expensive.

3. Considerable reactive power is required by converter stations.

4. Harmonics are generated hence Alteration is necessary.

5. Overload capacity of HVDC converters is low.

6. The maintenance of insulators in HVDC system is more.

 

Q.15 Mention the types of HVDC links.

Ans. : 1. Monopolar HVDC

2. Bipolar HVDC

3. Homopolar HVDC

4. Back to back HVDC coupling 5. Multi-terminal HVDC

 

Q.16 State the advantages of FACTS.

Ans. : 1. It controls line impedance angle and voltage which helps in controlling the power flow in transmission lines.

2. The power flow in transmission lines can be made optimum.

3. It helps in damping out the oscillations and prevent the damage of equipments.

4. It supports the power system security by increasing the transient stability limit.

5. It limits the overloads and short circuit currents.

6. The reserve requirements for generators are considerable reduced.

7. The loading capacity of the line is greatly increased upto their thermal capabilities.

 

Q.17 State the main objectives of FACTS controllers.

Ans. : 1. The power transfer capability of transmission system is to be increased.

2. The power flow is to be kept over the designated routes.

 

Q.18 List out the basic types of FACTS controllers.

Ans. : 1. Series controllers

2. Shunt controllers

3. Combined series-series controllers

4. Combined series-shunt controllers

 

Q.19 List out the various FACTS devices. 

Ans. : 1. Static Synchronous Compensator (STATCOM)

2. Static Synchronous Generator (SSG)

3. Static VAR Compensator (SVC)

4. Thyristorised witched or controlled reactor (TSR / TCR)

5. Static VAR Generator ( SVG )     6. Static VAR System (SVS)

7. Thyristor Controlled Braking Resistor (TCBR)

8. Static Synchronous Series Compensator (SSSC)

9. Interline Power Flow Controller (IPFC)

10. Unified Power Flow Controller (UPFC)

11. Inter Phase Power Controller (IPC)

12. Thyristor Controlled Voltage Limiter (TCVL)

 

Q.20 What is the highest a.c. transmission voltage we have in India ?

AU : Dec.-03, May-12

Ans. : The highest a.c. transmission voltage we have in India is now 1200 kV in Madhya pradesh.

 

Q.21 Compare STATCOM and SVC.

Ans. : Refer section 7.29.2.1.

 

Q.22 What is SCADA ?

Ans. : The SCADA is Supervisory Control And Data Acquisition system which is used in Accelerated Power Development Reform (APDRP) schemes to improve the reliability and quality of power.

 

Q.23 Give any two HVDC lines in India.

Ans. :          The HVDC lines in India are,

i) Rihand and Dadri ii) Talcher - Kolar iii) Kanpur

 

Q.24 State the advantages of EHVAC transmission system.

Ans. :

1. The volume of copper required is less at high voltage levels.

2. The line efficiency is higher at high voltage levels.

3. The line drop becomes less at high voltage levels.

4. As the line drop is less the voltage regulation becomes better at high voltage levels.

5. The power handling capacity of the line increases at high voltage levels.

6. The operation with EHVAC voltage is simple and can be adopted easily and naturally to the synchronously operating a.c. systems.

7. The total line cost per MW per km decreases considerably at high voltage levels. 

 

Q.25 What is breakeven distance in transmission system ?

Ans. : For the long distance the saving in cost is important. In d.c. systems only two conductors are required and in a.c. three conductors are rquired. While the cost of terminal equipments in d.c. system is more than the a.c. system. The distance of the line at which the cost of a.c. and d.c. systems is same is called breakeven distance in transmission system.

 

Q.26 What is STATCOM ?

Ans. : Refer section 7.29.1.

 

Q.27 Explain the term regional grid.

Ans. : In order to achieve economy, reliability and continuity in the supply, individual power systems generating electrical power are arranged in the form of electrically connected areas called regional grid.

 

Q.28 Mention the terminal equipments in HVDC system.

Ans. : 1. Inductor and harmonic filters on d.c. side

2. Converter transformers

3. Reactive power source

4. Harmonic filters on a.c. side

5. Ground electrode

6. Communication link

 

Q.29 List the various systems of power transmission in D.C. system.

Ans. : The various systems of power transmission in D.C. system are :

i. D.C. two wire system ii. D.C. two wire with midpoint earthed, iii. D.C. three wire.

 

Q.30 List the various systems of power transmission in A.C. system.

Ans. : 1. Single phase A.C. system

1. Single phase two wire ii. Single phase two wire with midpoint earthed iii. Single phase three wire.

2. Two phase A.C. system

i. Two phase four wire ii. Two phase three wire

3. Three phase A.C. system

i. Three phase three wire ii. Three phase four wire

 

Q.31 State the various environmental aspects of EHVAC transmission.

(Refer section 7.24.5)

 

Q.32 State the requirements of good distribution system.

Ans. :          1. The continuity in the power supply must be ensured.

2. The specific consumer voltage must not vary more than the specified prescribed limits. 

3. The efficiency of the lines must be as high as possible.

4. The system should be free from leakage and must be safe from the consumer point of view.

5. The lines should not be overloaded.

6. The layout should not affect the appearance of the locality.

7. The system should be economical.

 

Q.33 What is radial distribution system ?

Ans. : When the distributor is connected to the substation on one end only with the help of feeder then the system is called radial distribution system. The feeders, distributors and service mains are radiating away from the substation hence it is called radial system.

 

Q.34 State the advantages of radial distribution system.

Ans. :         

1. Simple as is fed at only one end.

2. The initial cost is low.

3. Useful when the generation is at low voltage.

4. Useful when the generation is located at the centre of the load.

 

Q.35 State the disadvantages of radial distribution system.

Ans. : 1. The end of distributor near to the substation gets heavily loaded.

2. When load on the distributor changes, the consumers at the distant end of the distributor face serious voltage fluctuations.

3. When a fault on a feeder or a distributor is developed, it causes interruption in the supply.

 

Q.36 What is ring main distribution system ?

Ans. : In this system, the feeder covers the whole area of the supply in the ring fashion and finally terminates at the substation from where it is started. The feeder is in closed loop form and looks like a ring hence the name given to the system is ring main distribution system.

 

Q.37 State the advantages of ring main distribution system.

Ans. : 1. The feeders get equally loaded.

2. If fault develops on one of the feeders then consumer gets continuous supply from the other part of the feeder.

3. It eliminates the possibility of the voltage fluctuations. 

4. Easy from the maintenance and repair point of view without interrupting the supply to the consumers.

5. Great saving in copper required.

 

Q.38 List out the types of loading in distribution system.

Ans. : The types of loading in distribution system are :

i) Uniform or distributed loading ii) Concentrated loading and iii) Combination of distributed loading and concentrated loading.

 

Q.39 What is the importance of voltage control ?

Ans. :          For satisfactory operation of various loads at the consumer end, it is necessary to keep voltage constant. Because of change in the load, voltage at the user end changes. With increase in load, drop in alternator impedance, transmission line, feeders etc. increases while with decrease in load, such drop decreases. Such variations in voltage are undesirable and must be kept within proper limits. Hence voltage control is important.

 

Q.40 State the applications of HVDC transmission.

Ans. :          1. For long distance, bulk power transmission by overhead lines.

2. For long underwater cable crossings.

3. For damping system oscillations and provide the stability using HVDC link.

4. For back to back asynchronous interconnections.

 

Q.41 Mention the various methods of voltage control in transmission lines.

AU : Dec.-16

Ans. :          1. Reactive power injection

2. Static VAR generator

3. Rotating VAR generator.

4. By tap changing transformer

 

Q.42 Mention the transmission voltages that are followed in Tamil Nadu.

Ans. : The Tamilnadu Transmisssion Corporation Limited maintains all the substations which fall under one of the following voltage levels,

1. 400/230 kV substations

2. 230/110 kV substations

3. 110 kV substations

4. 33 kV substations

These are the transmission voltages followed in Tamilnadu. 

 

Q.43 How does a.c. distribution differs from d.c. distribution ?

Ans. : The a.c. distribution differs from d.c. distribution in following respects :

1. In case of d.c. system, the voltage drop is due to resistance only which in a.c. system it is due to combined effect of resistance, inductance and capacitance.

2. The voltages or currents are added or subtracted arithmetically in case of d.c. system whereas they are added or subtracted vectorially in case of a.c. system.

3. It is required to take into account the power factor while making calculations in a.c. system which is absent in d.c. system. The distributors are normally tapped at different points with the loads having different power factors.

4. In d.c. distribution, the voltage level can not be changed easily while in a.c. distribution the voltage level can be easily changed using transformers hence it is economical.

 

Q.44 What is interconnected system ?

Ans. : When a ring main system is supplied by two or more than two generating stations then it is called interconnected system.

 

Q.45 What are the advantages of an interconnected system ?

Ans. : 1. Reliability of supply increases. In case of fault on one source, supply can be continued with the help of other sources.

2. Additional load demand in one area can be fed from other source where load demand is less. This reduces the reserve power capacity and improves the efficiency of the distribution system.