Transmission and Distribution MCQ Quiz - Objective Question with Answer for Transmission and Distribution - Download Free PDF

Classification of cable based on construction

Belted cable:

• In such cables, the conductors (usually three) are bunched together and then bound with an insulating paper ‘belt’.
• In such cables, each conductor is insulated using paper impregnated with a suitable dielectric. The gaps between the conductors and the insulating paper belt are filled with a fibrous dielectric material such as Jute or Hessian. 
• There are some limitations of such construction. Since the electric field is tangential, the insulation provided is stressed. As a result, the dielectric strength falls over time. Hence, such construction isn’t preferred for voltage levels above 11 kV.

Screened cable:

It is further divided as H type and S.L. type cables:

H-Type Cable:

• H-type cables are high-voltage paper-insulated cables designed for underground power transmission. They are mainly used for voltages up to 66 kV.
• The three cores are individually insulated with paper and then covered by a metallic screen/cover. These metallic covers are perforated. As a result, such construction allows the three metallic screens to touch each other.

S. L. Type Cable:

• The full form of SL-type cable is Separate Lead type cable. Such construction is limited for voltages up to 66kV only.
• SL-type cables are an improved version of H-type cables, used for underground high-voltage power transmission. They feature separate lead sheaths over each core, providing better insulation and fault protection compared to H-type cables.

Pressure cables:

• For voltages beyond 66 kV, the electrostatic stresses in the cables exceed the acceptable values and solid cables become unreliable.
• This occurs mainly because voids are created when voltages exceed 66 kV. Hence, instead of solid cables, we use Pressure cables.
• Typically, such cables are either oil filled or gas filled.

Construction of the underground cable

Cores or Conductors: 

• A cable may have one or more than one core (conductor) depending upon the type of service for which it is intended.
• The conductors are made of tinned copper or aluminum and are usually stranded to provide flexibility to the cable.
   

Insulation

• Each core or conductor is provided with a suitable thickness of insulation, the thickness of the layer depending upon the voltage to be withstood by the cable.
• The commonly used materials for insulation are impregnated paper, varnished cambric, or rubber mineral compounds.
   

Metallic sheath

• To protect the cable from moisture, gases, or other damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation.
   

Bedding

• Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and mechanical injury due to armoring.
   

Armouring

• Over the bedding, armoring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and during handling. 
   

Serving

• To protect armoring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armoring. This is known as serving

• That part of power system which distributes electric power for local use is known as distribution system. In general, the distribution system is the electrical system between the substation fed by the Transmission system and the consumer’s meters.
• It generally consists of feeders, distributors, and service mains.

• Feeder: A feeder is a conductor which connects the sub-station (or localized generating station) to the area where power is to be distributed. Feeders feed power to distributors. Generally, no tapings are taken from the feeder so that current in it remains the same throughout. The main consideration in the design of a feeder is the current carrying capacity.
• Distributors: A distributor is a conductor from which tapings are taken for supply to the consumers. In Fig. AB, BC, CD and DA are the distributors. The current through a distributor is not constant because tapings are taken at various places along its length. While designing a distributor, voltage drop along its length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the consumers’ terminals.
• Service mains: A service mains is generally a small cable which connects the distributor to the consumers’ terminals.

The correct answer is "option 4"

Concept:

• Porcelain is the most commonly used material for overhead insulators in the present day.
• The porcelain is aluminum silicate. The aluminum silicate is mixed with plastic kaolin, feldspar, and quartz to obtain final hard and glazed porcelain insulator material.
• The surface of the insulator should be glazed enough so that water should not be traced on it.
• Porcelain also should be free from porosity since porosity is the main cause of deterioration of its dielectric property.
• It must also be free from any impurity and air bubble inside the material which may affect the insulator properties.
• Porcelain has a dielectric strength of 60kV/cm.

Additional Information

• The electrical strength of glass generally remains unchanged during the entire working period, and its aging process is much slower than that of porcelain.
• Under the effect of standard shock wave, the electrical breakdown strength of glass medium is 1350～1700 kV/cm, which is about 3.8 times that of porcelain.
• Glass is made through a process called annealing and it has a higher dielectric strength as compared to porcelain.
• While the resistivity of glass is very high, the coefficient of thermal expansion is low..
• The glass insulators have higher resistant to breaking as compared to the ceramics and its mechanical compressive strength is 1.5 times higher than ceramics.
• The electrical resistance is much higher than that of ceramic insulators (between 500 kV/cm and 1000 kV/cm).

In India, household appliances generally operate on 230 V, 50 Hz single-phase supply

Transmission lines and distribution lines can be discriminated by their operating voltage. The voltage levels of transmission and distribution are given below.

Important Points:

The differences between transmission and distribution lines are given below.

For an 11 kV line at a dead end, the type of insulator typically employed is a disc insulator. Disc insulators are commonly used in high voltage applications, especially where the line tension is considerable, as is the case at dead ends. They are designed to withstand the mechanical load and provide the necessary electrical insulation.

Therefore, the correct answer is: 3) Disc

Important Points Other options such as shackle insulators are mainly used for low-voltage distribution systems, and pin insulators are commonly used for lower voltage levels (up to 33 kV), but disc insulators are most suitable for the given scenario.

ACSR (Aluminium Conductors Steel Reinforced ) conductor:

• It is a type of high-capacity, high-strength stranded conductor having a steel core and outer strands of aluminum.
• It is chosen for its good conductivity, low weight, low cost, resistance to corrosion, and decent mechanical stress resistance.
• The center strand is galvanized steel for additional strength to help support the weight of the conductor and provide additional mechanical strength.
• Steel has higher strength than aluminum which allows for increased mechanical tension to be applied to the conductor.

The correct answer is option 3):  Silver

Concept:

• A fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit.
• The fuse wire  will have low melting point 
• The material used for fuse elements must have the following properties:

1. Low melting point
2. Low ohmic loss
3. High conductivity (or low resistivity)
4.  Free from detraction 

• The materials used for fuse: The material mainly used as fuse elements are tin, lead, silver, copper, zinc, aluminium, and an alloy of lead and tin An alloy of lead and tin is used for small current rating fuses
• Silver provides an excellent balance of mechanical strength, moderate melting and evaporation temperatures, high electrical conductivity to oxidation during prolonged operation at elevated temperatures.
• Silver fuse elements provide stable and reliable performance under normal load conditions, predictable operation under circuit overloads, and current limiting and rapid circuit interruption under fault conditions.
• While expulsion fuses are typically limited to interruption currents below 20 kA, HRC fuses are available to interrupt fault currents of 50 - 200 k
• melting point of silver is 961.8 c , melting point of copper is 1085 , melting point of bronze is 913 C. Though melting bronze is minimum it will react with oxygen easily.So usage of bronze for fuse is not advisible

Transformers used in conjunction with measuring instruments for measurement purposes are called instrument transformers. The actual measurements are done by the measuring instruments only.

Where the current and voltage are very high, direct measurements are not possible as these current and voltage are too large for reasonably sized instruments and the cost of the meter will be high.

The solution is to step-down the current and voltage with instrument transformers, so that, they could be metered with instruments of moderate size.

Instrument transformers are two types: potential transformer and current transformer.

The potential transformer is used to step down the high voltages and current transformers are used to step down the high currents.

Classification of a switchgear according to constructional feature:

1.) Outdoor switchgear:

• The switchgear equipment is mounted outside for a voltage higher than 66 kV.
•  It is because, for such voltages, the clearances between conductors and the space required for switches, circuit breakers, and other equipment become so great that it is not economical to install the equipment indoors.

2.) Indoor switchgear:

• For voltages up to 11 kV, equipment is invariably installed inside of the substation.
• When the atmosphere is contaminated with impurities, this switchgear can be erected for a voltage of up to 66 kV.

3.) Underground switchgear:

• In thickly populated, the space available is less and more expensive. Under such conditions, the substation is created underground.

4.) Pole-mounted switchgear:

• It is an outdoor substation with a pole mounted on a 4 pole structure.
• It is the cheapest form of substation for voltage up to 11 kV.

• In armored cable, Steel Wire Armour (SWA) used for higher mechanical protection. So that the cable can withstand higher stresses or impact.
• It can be buried directly and used in external or underground projects.
• The armored cable can withstand extreme temperature conditions up to 15000°C.
• Using armored cables helps in insulating the transmission components from emitting the interference and also from being affected by interferences that are being emitted from neighboring cables and also appliances.

• Cable lugs are also known as Cable terminal ends or cable shoes.
• Cable lugs in electrical supplies utilized to securely connect or terminate cables to electrical devices, power or control panels, junction boxes, equipment, and machinery. These lugs are often used to join power cables together as well.
• Cable lugs are the devices used for connecting cables to electrical appliances, other cables, surfaces, or mechanisms.
• The clamps that connect wires to an automotive battery, the ends of battery jumper cables are common examples of cable lugs.

Additional InformationThe terms cable ends and cable shoes are often used interchangeably, but they refer to different components in electrical systems. Here's the difference between the two:

• Cable Ends :
  Definition: Cable ends refer to the exposed tips of electrical cables where the insulation has been removed to allow the wire to be connected to another device, terminal, or conductor.
  Purpose: The purpose of exposing cable ends is to create a point of electrical contact or connection, either by inserting the exposed wires into a terminal block or by attaching them to another wire or device.
  Appearance: Cable ends consist of bare wires, sometimes twisted or prepared to be inserted into terminals or connected with a connector like a lug or a shoe.
   
• Cable Shoes(Cable Lugs) :
  Definition: Cable shoes, also called cable lugs, are metal connectors attached to the cable ends. These lugs are designed to securely connect cables to terminals, switches, or other electrical devices.
  Purpose: The purpose of a cable shoe is to provide a stable and secure connection between the cable and the electrical device. Cable shoes ensure a firm and reliable connection, reducing the risk of loose connections or arcing.
  Appearance: Cable shoes are metal pieces with a hole at one end (for bolting or screwing) and a barrel at the other end where the exposed cable wire is inserted and crimped.
  
  Key differences: 
• Cable ends are simply the exposed ends of wires, whereas cable shoes are metal fittings or connectors that are attached to the cable ends to make secure electrical connections.
• Cable ends need to be connected directly to terminals, while cable shoes offer a more permanent and durable solution, allowing the cables to be bolted or screwed into place.
  
  Summary : Both components are part of electrical connections, but cable shoes enhance the functionality of cable ends by ensuring secure and reliable attachments.
• Cable Ends: Exposed tips of wires.
• Cable Shoes: Metal connectors (lugs) attached to cable ends for secure electrical connections.

Primary transmission :

The electric supply (132 kV, 220 kV, 500 kV or greater) is transmitted to load center by three-phase three-wire (3 phase - 3 wires) overhead transmission system.

Secondary transmission :

At the receiving station, the level of voltage reduced by step-down transformers up to 132 kV, 66 or 33 kV and electric power is transmitted by three-phase three-wire (3 phase - 3 wires) overhead system to different substations.

Primary distribution :

At a substation, the level of secondary transmission voltage (132KV, 66 or 33KV) is reduced to 11 kV (in a three-phase three-wire overhead system) by step down transformers.

Secondary distribution :

• Electric power is given to (from primary distribution line (i.e.) 11 kV) distribution substation.
• This substation is located nearby consumers area where the level of voltage reduced by step down transformers is 415 V.
• In a 3 phase four wire system (3 phase - 4 wires), there are 415 volts (Three-phase supply system) between any two phases and 230 volts (single-phase supply) between neutral and any one of the phases (lives) wire.
• Residential load (i.e. Fans, light, and TV, etc) may be connected between any one phase and neutral wires, while three-phase loads may be connected directly to the three-phase lines.
• A three-phase 415V, supply is used for supplying small industrial and commercial loads such as garages, schools, and blocks of flats. A single-phase 230 V supply is usually provided for individual domestic consumers.

The correct answer is option 2):(Conductor connecting consumer’s terminal to the distributor )

Concept:

Service Mains:

• The conductor connecting the consumer’s terminals to the distributor is called the service main
• Service mains of the consumers may be either connected to the distributors or sub-distributors