Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 Define a substation.

Ans. : A substation is defined as the assembly of apparatus which is used to change the characteristics of supply system such as voltage, frequency, a.c. to d.c. or power factor.

 

Q.2 How will you select an ideal location for a distribution substation ?

Ans. : Following factors are to be taken into consideration while selecting an ideal location for a substation.

1. Availability of suitable and sufficient land

2. Facility for communication

3. Avoiding area causing atmospheric pollution

4. Easy operation and maintenance

5. Adequate drainage facility

 

Q.3 State the functions of a circuit breaker.

Ans. : Following are functions of a circuit breaker,

1. Carrying full load current continuously.

2. Opening and closing the circuit on no load.

3. Making and breaking normal operating current.

4. Making and breaking fault currents of magnitudes upto which it is designed for.

 

Q.4 Name the factors that should be taken care of while designing and erecting a substation.

Ans. : The design and erection of a substation involves civil work after proper site selection and electrical work. The electrical design of substation involves choice of bus bar arrangement , transformer selection, selection of various protective and measuring devices such as isolators, circuit breakers, relays and lightning arrestors etc. The provision of facilities such as earthing, cabling, oil handling, illumination, communication etc.

Following factors are to be taken into consideration while erecting a substation :

1. Type of substation

2. Availability of suitable and sufficient land

3. Facility for communication

4. Avoiding area causing atmospheric pollution

5. Easy operation and maintenance

6. Drainage facility

 

Q.5 State the advantages of outdoor substation over indoor substation.

Ans. : Following are the advantages of outdoor substation over indoor substation :

1. Time required for erection is less

2. Future expansion is easier

3. The capital cost required is less

4. All equipments are within view and the fault location is easier

5. The cost of switchgear installation is less and amount of building material required is small.

 

Q.6 What is function of isolators ?

Ans. : The function of isolators is to disconnect a part of power system for repair and maintenance and operated after switching off the load by means of a circuit breaker.

 

Q.7 State the conditions for laying out a substation.

Ans. : Following are the conditions for laying out a substation :

1. The arrangement provided by a substation must be safe and reliable.

2. The site should be properly located with good atmospheric conditions.

3. Capital cost requirement should be small.

4. Operation and maintenance must be easier.

 

Q.8 Mention the significance of neutral grounding.

Ans. : The earthing or grounding is nothing but the connection of neutral point of the supply system to the general mass of the earth in such a way that immediate discharge of electricity can take place without danger. It ensures safety of personnel or human beings against electrical shocks (including animals and plants) and equipments. It also avoids accidents which otherwise may take place. The equipment is also protected against lightning and voltage surges. The voltage stress on the lines is also reduced along with that on the equipment with respect to earth under abnormal conditions.

 

Q.9 What are objectives of earthing ?

Ans. : Following are the objectives of earthing

i. It should provide adequate safety of operating personnel/human being (including animals and plants) against electrical shocks /hazards and avoid accidents.

ii. It should provide an alternative path for the fault current to flow so as to reduce danger of the user.

iii. It should ensure that all the exposed conductive parts do not reach a dangerously high potential and should reduce the stress on the lines along with that on the equipment with respect to earth under abnormal conditions.

iv. It should be able to maintain the voltage at any part of an electrical system at a known value so as to avoid excessive voltage on the appliances or equipment or over current situation.

 

Q.10 What are various methods of earthing in substations ?

Ans. : The earthing in substations is carried out with the help of equipments such as electrodes, driven rods, risers and grounding grid or mat consisting of number of meshes and connected to several earth electrodes driven at intervals. The various methods adopted for earthing are

i. Solid or effective grounding

ii. Resistance grounding

iii. Reactance grounding

iv. Resonant grounding

 

Q.11 Write are advantages of neutral grounding ?

Ans. : Following are advantages of neutral grounding

i. The faulty part can be isolated from the remaining system with the help of earth fault relays.

ii. The transient voltage in the system with grounded neutral is small.

iii. The arcing grounds are prevented from occurring by using suitable switchgears.

iv. The voltage of healthy phases during faulty condition remains nearly constant as the neutral point is not shifted.

v. The static charges which gets induced are grounded immediately. This avoids disturbance in the system.

vi. Reliability of the system is improved.

vii. The discriminative type of fault indicator can be installed on such systems.

 

Q.12 What is need of an earthing system ?

Ans. : In order to provide safety of personnel or human beings against electrical shocks (including animals and plants), to reduce the possibility of getting electrical shocks and avoiding accidents, to protect the equipments, buildings, machinery/appliances against lightning and voltage surges, to reduce stress on the lines along with that on the equipment with respect to earth under abnormal conditions earthing is required.

 

Q.13 State the disadvantages of ungrounded neutral.

Ans. : Following are the disadvantages of ungrounded neutral

i. Under faulty condition, large transient voltages with magnitude several times that of normal value is produced causing the breakdown of insulation resulting in damage of the concerned equipment.

ii. System cannot be protected from earth fault and arcing ground may occur.

iii. There is no path to earth for voltages due to lightning surges.

 

Q.14 What are terminal and through substations ?

Ans. : In terminal substation, a line which is supplying to that substation ends or terminates. Its location is either at the end of the main line or may be at a point away from main line route.

In through substation, the incoming line is not terminated but it passes through at the same voltage and the tapping is taken which feeds the transformer.

 

Q.15 What are the materials mainly used in bus bars ?

Ans. : The bus bars are either rigid type or strain type. For rigid type bus bars, copper or aluminium bars are used. Such bus bars are used for low and medium voltage levels. For strain type bus bars mainly stranded aluminium (ACSR) conductors are used which are supported by strain insulators. The strain type bus bars are used for high voltage levels.

 

Q.16 What are the classifications of substation according to service ?

Ans. : According to service, the substations are classified as :

1. Transformer substations : In these substations, power is transformed from one voltage level to other.

These are further classified as, transmission or primary substation, subtransmission or secondary substation and step down or distribution substation.

2. Industrial substations : For industries demanding huge power, a seperate substation is installed.

3. Switching substations : These are used for switching operations of power lines.

4. Synchronous substations : These are used for synchronous phase modifiers used for the improvement of power factor.

5. Frequency change substations : These are used for converting normal frequency to other useful frequency.

6. Converting substations : These are used for converting a.c. to d.c. required for electric welding, battery charging etc.

 

Q.17 What are the major equipments of a substation ?

Ans. : Refer section 8.4.

 

Q.18 What is gas insulated substation 7

Ans. : Gas insulated substations make use of sulfur hexafluoride (SF6) gas. SF6 gas has excellent dielectric properties when used at moderate pressure for phase to phase and phase to ground insulation. The high voltage conductors, circuit breaker interrupters, current transformers, voltage transformers, switches and lightning arresters are enclosed in SF6 gas inside grounded metal enclosures.