100 Most Important MCQ Of Transmission and Distribution | Objective Type Question Of Transmission and Distribution

100 Most Important MCQ Of Transmission and Distribution | Electrical Engineering Multiple choices Question

Ques 1. By which of the following method electric power may be transmitted from one location to another location?

  1. UnderGround System
  2. Overhead system
  3. Both 1 and 2
  4. None of the above
Answer 3. Both 1 & 2 

Explanation: Electrical power is generally transmitted by two methods i.e underground transmission and overhead transmission

 

Ques 2. Which of the following transmission line have more initial cost?

  1. Overhead Transmission
  2. Underground transmission
  3. Both have almost the same initial cost
  4. None of the above

Answer 2. Underground Transmission line

Explanation:

  • The underground cable can be 2-3 times more expensive than the overhead cable.
  • As the voltage level increases the cost of insulation is increased therefore the underground cable is restricted to low and medium voltages.

 

Ques 3.  Name the cable or conductor which connects the distributor to the consumer terminals.

  1. Service Mains
  2. Distributor
  3. Feeders
  4. None of the above

Answer 1. Service Mains

Explanation:

  • Service Mains: The service mains conductors form connecting links between distributors and metering points at the consumer terminal.
  • The area of connection of a sub-main conductor is greater than the service mains.

 

Ques 4. Which of the following materials are not used for the transmission and distribution of electrical power?

  1. Copper
  2. Aluminum
  3. Tungsten
  4. Steel

Answer 3. Tungsten

Explanation:

  • The resistance of tungsten is very high, more the resistance more the losses, therefore, tungsten is never used for transmission and distribution purposes.

 

Ques 5.  The usual spans with R.C.C. poles are

  1. 40—50 metres
  2. 60—100 metres
  3. 200 – 300 meters
  4. 80 – 150 meters
Answer 4. 80 – 150 meters

Explanation:

Reinforced concrete pole(R.C.C Pole)

  • The Reinforced concrete pole (R.C.C) is usually called as Concrete pole and they are used for system voltage up to 33kV.
  • The minimum overall length of the R.C.C pole should be six meters.
  • The span length of R.C.C is between 80 – 200 meters because they are stronger and more durable.
  • They are free from corrosion hence poles have a longer life but these are very bulky in sizes.

 

Ques 6. Which of the following are the constants of the transmission lines?

  1. Inductance
  2. Capacitance
  3. Resistance
  4. Conductance
  5. All of the above

Answer 5. All of the Above

Explanation:

a b c d constants of the transmission line( Primary line constants)

  • The line constants are parameters that describe the characteristics of conductive transmission lines.
  • The primary line has following constants
    • R = Resistance per unit length Ω
    • L = Inductance per unit length (Henry) H 
    • C = Capacitance per unit length (Henry) H
    • G = Conductance per unit length ℧
  • All these constants are independent of frequency, therefore, they are called as primary constants and these constants are measured by considering both the wires of the transmission lines.
  • R and L elements are in series with the line (because they are properties of the conductor) and C and G are elements shunting the line (because they are properties of the dielectric material between the conductors).
  • G represents leakage current through the dielectric and in most cables is very small.

 

Ques 7. The phenomenon of rising in voltage at the receiving end of the open-circuited or lightly loaded line is called as

  1. Roman Effect
  2. Skin Effect
  3. Corona Effect
  4. Ferranti Effect

Answer 4. Ferranti Effect

Explanation:

Ferranti Effect

  • The phenomenon of the rise of voltage at the receiving end of an open-circuited or lightly loaded transmission line is called as the Ferranti effect.
  • The receiving end voltage becomes more than the sending end voltage.
  • The Ferranti effect occurs in medium and long transmission lines.
  • The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length.
  • The Ferranti effect is much more pronounced in underground cables, even in short lengths, because of their high capacitance.

 

Ques 8. Low tension cables are meant for use up to

  1. 1 kV
  2. 5 kV
  3. 10 kV
  4. 33 kV
Answer 1. 1 kV

Explanation: Low voltage cables with a range up to 1 kV are used for energy distribution from the last transformer station to the customer.

 

Ques 9. The operating voltage of high tension cables is up to

  1. 1 – 11 kV
  2. 11 – 20 kV
  3. 11 – 33 kV
  4. above 33 kV

Answer 1. 1 – 11 kV

Explanation:

  • High tension cables are used for three-phase medium. These consist of three core belted type cable with separated insulation for each core.
  • The paper insulation is provided for high-tension cables.
  • The operating voltage of high tension cable is up to 11kV. 

 

Ques 10. The operating voltage of super tension cable is

  1. 1 – 11 kV
  2. 11 – 33 kV
  3. 33 – 66 kV
  4. Above 66 kV

Answer 2. 11- 33 kV

Explanation:

Super tension cables are used upto the voltage level of 33 kV. They are further classified into 3 types

    1. H – type or screened cables
    2. SL type cables
    3. HSL types cables

Ques 11. The operating voltage of Extra high tension cable is upto

  1. 11 kV
  2. 33 kV
  3. 66 kV
  4. Above 66 kV

Answer 3. 66 kV

Explanation:

  • Three core cables are generally used for voltages upto 33 Kv but beyond that weight and size of the three core cables is not economical and nor practical.
  • Extra high tension cable can be of 2 types
    1. Oil-filled cables
    2. Gas pressure cables

 

Ques 12. Which of the following methods is used for laying of underground cables?

  1. Direct laying
  2. Solid system
  3. Draw-in-system
  4. All of the Above

Answer 4. All of the Above

Explanation:

Methods of Laying underground Cables

  • Generally, there are three methods of laying an underground cable
    1. Direct laying
    2. Solid system
    3. Draw-in-system

Direct laying Method

  • In this method, a trench of 1.5 meters is dug and it is covered with a layer of fine sand to restrict the entry of moisture from the ground.
  • The cable is laid over the sand-belt after that cable is covered with another layer of sand.
  • Then the trench is covered with bricks and stones to protect the cables from mechanical injury.
  • This method of laying underground cables is clean, safe and very cheap in comparison with the other two methods
  • The main disadvantages of this method are high initial cost and the alternation of the cable network is difficult.

Draw-in System Method

  • In congested areas where excavation is expensive and inconvenient this system of the laying of underground cables is often adopted.
  • This method of cable laying is most suitable for short length cable routes such as in workshops, railway bridge crossing, road crossing where frequent digging is costlier or impossible.

Solid system Method

  • This method of laying underground cables is rarely used because of its high cost.
  • In this system, the cable is laid in open pipes or troughs dug out in the earth along the cable route.
  • The troughing is of cast iron or treated wood and it is filled with bituminous after cables are laid.
  • The advantage of this method is that the cables are protected mechanically and from chemical reactions due to impurities in the soil.
  • The disadvantages of this method are
    1. laying and repair require more time
    2. laying and repair cannot be carried out in the rainy season.

 

Ques 13. Which of the following is the source of heat generation in the cables?

  1. Dielectric losses in cable insulation
  2. Conductor losses
  3. Sheath loses
  4. All of the above
Answer 4. All of the above Explanation:

Explanation:

Cable Losses

  • In a cable, if the rate of heat generation is more than the rate of heat dissipation than the temperature of cable increases.
  • The three main sources of heat generation in a cable are:
    1. Dielectric losses in cable insulation
    2. Conductor losses
    3. Sheath loses

Dielectric losses

  • Dielectric losses consist of losses due to leakage through cable insulation and losses caused by dielectric polarization under AC stresses.
  • These losses depend upon the voltage, frequency, and permittivity of the material.
  • The dielectric losses are more profound in the HV and EHV transmission cables.

Conductor losses

Conductor losses I2R loses depends upon the RMS current I and effective AC resistance of the cable conductor Rac

Sheath Losses

When the alternating current flows in the cable it produces a pulsating magnetic field. This pulsating field links with the lead sheath and induced current in it. This sheath current produces sheath losses.

 

Ques 14. Due to which of the following reasons the cables should not be operated too hot?

  1. The oil may lose its viscosity and it may start drawing off from higher levels
  2. Expansion of the oil may cause the sheath to burst
  3. Unequal expansion may create voids in the insulation which will lead to ionization
  4. Unequal expansion may create voids in the insulation which will lead to ionization
  5. All of the above
Answer 5. All of the above

Cables should not operate too hot for the following reasons

  1. Voids may be created in the insulation due to unequal expansion; this will lead to ionization.
  2. Owing to the rapid increase of dielectric losses with temperature, the thermal instability may arise.
  3. The sheath may burst due to the expansion of oil.
  4. The oil may lose its viscosity and may start draining off from higher levels.

 

Ques 15. Besides a method of trial and error, which of the following methods is employed for the solution of network problems in an interconnected system?

  1. Kirchhoff’s laws
  2. Superposition of currents
  3. Thevenin’s theorem
  4. All of the above

Answer 5. All of the above

  • With the increase in voltage levels and the interconnection of power grids, power systems became much more complicated. It was necessary to find an accurate way to calculate the active power and reactive power that flows in the lines
  • The Network Model of interconnected power systems in the power flow study includes the representation of generators as complex power sources, loads as complex power demands and transmission lines as a Π network consisting of series admittance and line charging admittances.
  • As the AC power system is a nonlinear system, the simple Ohm’s Law is not suitable. Using Kirchhoff’s s Law, electrical engineers wrote active power and reactive power balancing equations for all nodes of the system. They are power balancing equations, also called power flow equations. The equations are able to mathematically represent the power network.
  • The Mathematical Model for the study is a set of nonlinear simultaneous algebraic equations. Network equations in the study can be formulated by using either the Zbus or Ybus matrices. However, Ybus is preferred, as the matrix has more number of zero elements or has more ‘sparsity’. This enables fast solutions using only the non-zero entries.
  • Either the Gauss-Seidel or the Newton—Raphson iterative method is used to solve non-linear algebraic equations. While the former method is used for small-sized power systems, the latter finds application in the study of large-sized systems. Newton Raphson’s method is one of the effective ways to solve the equations. As the study is conducted under steady-state conditions of the system an only a single phase-based positive sequence network is considered and all numerical values are given as per unit values.
  • Thevenin’s and superposition’s theorems are very useful in the calculation of currents and voltages due to short-circuit faults in power system networks.

Ques 16. A booster is a

  1. Synchronous generator
  2. Shunt-wound generator
  3. Series wound generator
  4. None of the above
Answer 3. Series wound generator

Explanation:

  • A booster is a series-wound generator that is inserted into the circuit to increase the DC voltage level so that the excessive voltage drop in the feeder can be compensated.
  • A booster may be defined as a dynamo-electric machine, the armature of which is connected in series with a circuit. its generated e.m.f. being added to or subtracted from that of the circuit pending upon the polarity of its excitation.
  • Boosters may be driven by any form of a prime mover but are generally direct-connected to a motor taking current from constant-potential means.
  • The primary use of a booster is to raise the voltage of a generator or of a section of the bus bars of a central station by an amount sufficient to compensate the ohmic drop in a feeder supplying distant load. in case the load is of such character as to require the same voltage as receiving devices at or near the source of supply. Since the line drop is directly proportional to the current, the voltage of the booster should also be proportional to the current.

 

Ques 17. Which of the following D.C. distribution system is the simplest and lowest in first cost?

  1. Radial system
  2. Inter-connected system
  3. Ring system
  4. Any of the above

Answer 1. Radial system

Explanation:

Radial System

  • A radial system is an electric transmission system that supplies power from the main generating station to the various sub-station.
  • From the substation, the power line is connected to the distribution transformer where the voltage is step-down to the value required by the customers.

Advantages of Radial System

  1. The initial cost is low.
  2. Useful when the generating capacity is low.
  3. Preferred when the station is located in the center of the load.

Disadvantages of Radial System

  1. When the load on the distributor changes, the consumers at the distant end of the distributor face serious voltage fluctuations.
  2. This system is least reliable because there is no guarantee of continuous service because no backup distribution system is provided.

 

Ques 18. Which of the following faults is most likely to occur in cables?

  1. Breakdown of cable insulation
  2. Cross or short-circuit fault
  3. Open circuit fault
  4. All of the above

Answer 4. All of the above

Most of the distribution and part transmission of electrical power is nowadays carried out through underground cables because of several advantages over the over-head system. Many times locating a fault becomes a difficult task because the cable is hurried under the ground and is not accessible. The faults which are most likely to occur are :

  1. Ground fault: A breakdown of the insulation of the cable which allows current to flow from core to earth or to cable sheath.
  2. Short circuit fault: A cross or short circuit between two cables or between two cores of a multicore cable.
  3. Open circuit: Where the conductor becomes broken or a joint pulls-out. The method for locating an open circuit fault differs from those used for the other two faults. Amongst various methods used for localizing cable faults. Murray loop test is very common and is described here.

Ques 19. The voltage of the single-phase supply to residential consumers is

  1. 110 V
  2. 230 V
  3. 440 V
  4. Any of the above

Answer . 2. 230 V

Explanation:

  • In India, a single-phase supply is a 230V supply through two wires and In North America, a typical three-phase system will have 208 volts between the phases and 120 volts between phase and neutral. 

 

Ques 20. The distributors for residential areas are

  1. Three-phase four-wire
  2. Three-phase three-wire
  3. Single-phase
  4. None of the above

Answer .1. Three-phase four-wire

Explanation:

  • A four-wire system allows you to have phase voltages or line to ground voltages. In a 3 phase 4 wire system, neutral wire carries return current in case any fault occurs in the line there is more flexibility to keep the system running since it may only affect one of the phases.

Ques 21. The conductors of the overhead lines are

  1. Stranded conductors
  2. Solid conductors
  3. Both solid and stranded
  4. None of the above
Answer .1. Stranded conductors

Explanation:

  1. Stranded Conductors are widely used over solid conductors. Although there seems to be more expensive than a solid conductor but offers many advantages too, over the solid conductor.
  2. The stranded conductor offers much surface area for flow of current that results in the reduced diameter of the conductor for the same current carrying capacity.
  3. As the Stranded conductor offers much surface area so the overall resistance of the conductor is lesser than the same size solid conductor.
  4. Eddy current losses are very much lesser than the solid conductor.
  5. Stranded conductors are very suitable to an external environment and reduce the risk of loose connection.
  6. Stranded Conductors offer much flexibility hence have a longer life than the solid conductor.

 

Ques 22. High voltage transmission lines use

  1. Suspension insulators
  2. Pin insulators
  3. Any of the above
  4. None of the above

Answer .1. Suspension Insulator

Explanation:

Suspension Type Insulator

  • With an increase in the line voltage (above 33 kV) the pin insulator becomes heavy and complicated.
  • Suspension type Insulator consists of a Number of porcelain discs connected in series with the metallic links to form a chain and the line conductor is carried by the bottom-most insulator.\

Advantages of Suspension Type Insulator

  1. Each disc is designed for low voltage i.e for 11 kV. Therefore by connecting the number of an individual disc in series, the insulator can be designed for any system voltage. For e.g 33 kV working voltage will require 3 individual discs.
  2. Mechanical stress on the string is very low.
  3. Line conductors are less affected by lightning.
  4. If anyone of the insulators fails it can be replaced easily and at a very low cost.
  5. It is more economical than pin type insulator for high operating voltage.

 

Ques 23. Multicore cables generally use

  1. Square conductors
  2. Rectangular conductors
  3. Sector-shaped conductors
  4. Circular conductors
Answer .3. Sector-shaped conductors

Explanation:

Sector-shaped conductors

A sector conductor is a stranded conductor whose cross-section is approximately the shape of a sector of a circle. A multiple conductor insulated cable with sector conductors has a smaller diameter than the corresponding cable with round conductors.

 

Ques 24. The material commonly used for insulation in high voltage cables is

  1. Rubber
  2. Paper
  3. Lead
  4. Any of the above

Answer .2. Paper

Explanation:

  • Cables for power distribution of 10 kV or higher may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead sheath.
  • An extruded lead sheath over the paper cable was required to ensure that the paper remained absolutely dry.

 

Ques 25. Overhead lines generally use

  1. A.C.S.R. conductors
  2. Copper conductors
  3. Aluminum conductors
  4. Any of the above

Answer .1. A.C.S.R. conductors

Explanation:

Aluminium-conductor steel-reinforced cable(A.C.S.R. conductors)

  • Aluminum conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines.
  • The outer strands are high-purity aluminum and the center strand is the steel for additional strength to help support the weight of the conductor.
  • Steel has higher strength than aluminum which can allow increased mechanical tension to be applied on the conductor.

Ques 26. The power factor of industrial loads is generally

  1. Unity
  2. Lagging
  3. Leading
  4. Any of the above
Answer . 1. Lagging

Explanation: 

Generally industrial loads include inductive motor coil,

  • Industrial wiring systems are very inductive because they contain many inductive machines and circuits., such as electric motors, transformers, welding plants, and discharge lighting.
  • The inductive nature of the industrial load causes the current to lag behind the voltage and creates a lagging power factor.
  • The power factor is the percentage of current in an alternating current circuit that can be used as energy for the intended purpose. A power factor of say 0.7 indicates that 70% of the current supplied is usefully employed by the industrial equipment.
  • An inductive circuit, such as that produced by an electric motor, induces an electromagnetic force that opposes the applied voltage and causes the current wave to lag the voltage wave, therefore, they have lagging power factor.

Ques 27. The material generally used for the armour of high voltage cables is

  1. Copper
  2. Brass
  3. Aluminum
  4. Steel

Answer .4. Steel

Explanation:

  • Steel wire armored cable, commonly abbreviated as SWA, is a hard-wearing power cable designed for the supply of mains electricity.
  • It is one of a number of armored electrical cables – which include 11 kV Cable and 33 kV Cable – and is found in underground systems, power networks, and cable ducting.
  • Armour Steel wire armor provides mechanical protection, which means the cable can withstand higher stresses, be buried directly, and used in external or underground projects.

 

Ques 28. In transmission lines, the cross-arms are made of

  1. Steel
  2. Wood
  3. R.C.C
  4. Either Steel or Wood

Answer .4. Either Steel or Wood

Explanation:

  • Cross arms are either made up of wood or steel angle sections.
  • Cross arms are installed at the top of the pole for holding the insulator on which the conductors are fastened.
  • If the cross arms are fixed in the center of the pole then it is simply called cross arms.
  • If the cross arms are fixed on the side of the pole then it is termed as side cross arms.
  • For the three-phase line, U shaped Cross arms are used.

 

Ques 29. Transmission line insulators are made of

  1. Porcelain
  2. Glass
  3. Composite Polymer
  4. Any of the above

Answer.4. Any of the above

Explanation:

  • Insulators used for high-voltage power transmission are made from glass, porcelain or composite polymer materials.
  • Porcelain insulators are made from clay, quartz, or alumina and feldspar, and are covered with a smooth glaze to shed water.
  • Porcelain has a dielectric strength of about 4–10 kV/mm.
  • Glass has higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains.

 

Ques 30. The material commonly used for sheaths of underground cables is

  1. Lead
  2. Rubber
  3. Copper
  4. Iron

Answer.1. Lead

Explanation:

  • The insulator conductors are covered with the lead sheath. This provides additional mechanical strength and also protects the cable from moisture.
  • Note: The most common material used as the sheath in today’s cables is Poly-Vinyl-Chloride or PVC. Earlier days, the lead was used as the sheath.

Ques 31. The spacing between phase conductors of a 220 kV line is approximately equal to

  1. 2 m
  2. 3 m
  3. 6 m
  4. 10 m
Answer.3. 6 m

Explanation:

According to the Electrical safety rule, the spacing between the phase conductor of 220 kV is  6m.

 

Ques 32. The minimum clearance between the ground and a 220 kV line is about

  1. 1 m
  2. 7 m
  3. 10 m
  4. 5 m
 Answer.2. 7 m

Explanation:

According to the Electrical safety rule, the minimum clearance between the ground and a 220 kV line is about 7m.

 

Ques 33. In a D.C. 3-wire distribution system, balancer fields are cross-connected in order to

  1. Boost the generated voltage
  2. Equalize voltages on the positive and negative outers
  3. Balance loads on both sides of the neutral
  4. Make both machines run as unloaded motors

Answer.2. Equalize voltages on the positive and negative outers

Explanation:

  • The purpose of the Balancer is to redress the voltage on the positive and negative outer.
  • Let us suppose P and Q are the armatures of 2 DC machines which are connected in series across the outers and the field being cross-connected.
  • Out of balance load makes the DC machine works automatically as a motor or generator in order to compensate voltage drop or rise.
  • The balancer connected to lightly load side always work as the motor while that connected to a heavily loaded side act as the generator.
  • The energy is always transferred from the lightly loaded side to heavily loaded side.

 

Ques 34. In a D.C. 3-wire distributor using balancers and having unequal loads on the two sides

  1. Balancer connected to lightly- loaded side runs as a motor
  2. Balancer connected to heavily- loaded side runs as a motor
  3. Both balancers run as motors
  4. Both balancers run as generators
Answer.1. Balancer connected to lightly- loaded side runs as a motor
Explanation: 

Check the above question i.e question number 33.

 

Ques 35. A uniformly-loaded D.C. distributor is fed at both ends with equal voltages. As compared to a similar distributor fed at one end only, the drop at the middle point is

  1. One-half
  2. One-third
  3. One-fourth
  4. Twice
Answer.3. One-fourth
Explanation: Expression of DC distributor will be explained in the theory section.

 

Ques 36. As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only

  1. 31.25 percent of copper
  2. 66.7 percent of copper
  3. 33.3 percent of copper
  4. 125 percent of copper
 Answer.3. 33.3 percent of copper
Explanation: Expression of DC distributor will be explained in theory section.

 

Ques 37. For an overhead line, the surge impedance is taken as

  1. 20-30 ohms
  2. 400 – 600 ohms
  3. 70—80 ohms
  4. 100—200 ohms

Answer.2. 400 – 600 ohms

Explanation:

Surge Impedance

  • In power system, the characteristic impedance is sometimes referred to as surge impedance.
  • It is defined  as the √Z/Y where
    Z  = series impedance of line
    Y = shunt admittance of the line
  • For a lossless line, the surge impedance reduces to √(L/C) a pure resistance.
  • Its value is normally 400 to 600 ohms for an overhead line while for underground cable its value is typically between 40 to 60 ohms.

 

Ques 38. The presence of ozone due to corona is harmful because it

  1. Corrodes the material
  2. Transfer energy to the ground
  3. Gives odor
  4. Any of the above

Answer.1. Corrodes the material

Explanation:

  • Corona discharge is the ionization of air molecules due to high electrical potentials. The ionization process creates a plasma which we call corona discharge.
  • The high energy ions that create this effect can rapidly deteriorate power systems equipment.
  • Corona discharge typically occurs at (non-smooth) surfaces because at these points the electric field is the highest.

 

Ques 39. The power transmitted will be maximum when

  1. Corona losses are minimum
  2. Receiving end voltage is high
  3. Reactance is high
  4. Sending end voltage is high

Answer.4. Sending end voltage is high

Explanation:

  • In case if the sending end voltage is not more than the receiving end voltage then the current start flowing in reverse direction i.e from receiving end to sending end this phenomenon is called as the Ferranti effect.
  • This occurs when the line is energized, but there is a very light load or the load is disconnected. 

 

Ques 40. A 3-phase 4 wire system is commonly used on

  1. Primary transmission
  2. Secondary transmission
  3. Primary distribution
  4. Secondary distribution

Answer.4. Secondary distribution

Explanation:

  • At the local distribution centres, there are step-down distribution transformers.
  • The voltage level of 6.6 kV and 11 kV are further step-down to 400 V for an industrial region and 240 V to domestic or household region.
  • This uses 3 phase 4 wire system and the voltage between any two lines is 400 Volt and the voltage between any of the three-line and neutral is 230 V.

Ques 41. Corona discharge occurs more in

  1. Humid Weather
  2. Hot weather
  3. Cold Weather
  4. Any of the above

Answer.1. Humid Weather

Explanation:

  • In humid condition, there is more water droplet in the atmosphere this water droplet increases the electric field on the surface of the conductor hence corona is generated.
  • The high humidity decreases the breakdown strength of the air surrounding the conductor which further decreases the intensity of the discharge.
  • This electric field accelerates the free electron charge in the air surrounding the conductor.
  • This causes an avalanche which is called the corona discharge.

 

Ques 42. Which of the following relays are used on long transmission lines?

  1. Impedance Relay
  2. Mho’s Relay
  3. Differential Relay
  4. Reactance Relay

Answer.2. Mho’s Relay

Explanation:

Why is Mho’s Relay used for Long Transmission Line?

  • Mho relay comes in the category of the distance relay protection scheme.
  • The mho type relay is most suited for long lines because there are more chances of severe synchronizing power surges on the system.
  • It does not need any additional equipment to prevent tripping during these surges.
  • The mho relay occupies the least space on an R-X diagram for a given line section and is, therefore, least affected by abnormal system conditions except for the line faults.
  • Since the mho relay is most affected by arc resistance, it is used for long lines.

 

Ques 43. Which of the following relay is used for short transmission line?

  1. Impedance Relay
  2. Mho’s Relay
  3. Differential Relay
  4. Reactance Relay

Answer.4. Reactance Relay

Explanation:

Why is Reactance Relay used for Short Transmission Line?

  • In short transmission lines, the most common fault is the line to ground fault hence reactance type relay is used because more of the line can be protected at high speed.
  • The line to ground fault also causes arcing ground and the reactance relay is practically independent of the arc resistance which may be large as compared with the line impedance.

 

Ques 44. Which of the following relay is used for the protection of a medium transmission line?

  1. Impedance Relay
  2. Mho’s Relay
  3. Differential Relay
  4. Reactance Relay

Answer.1. Impedance Relay

Explanation:

Why is Impedance Relay used for Medium Transmission Line?

  • The impedance relay is less affected by synchronizing power surges as compared to reactance relay and also this relay is less affected by arc resistance as compared with the mho relay.
  • This relay only operates when the voltage/current ratio becomes less than the given value.
  • The impedance relay is, therefore, used for protecting medium length transmission lines.

 

Ques 45.  The steel used in steel cored conductors is usually

  1. Alloy steel
  2. Stainless steel
  3. Mild steel
  4. High-speed steel

Answer.3. Mild steel

Explanation:

  • The conductor used for overhead transmission systems generally used galvanized mild steel core (to provide tensile strength).
  • The price of mild steel is relatively low hence for laying the network of transmission lines around the city or state it is very cost-effective.

 

Ques 46. Which of the following characteristics should the line supports for transmission lines possess?

  1. High mechanical strength
  2. Longer life
  3. Low cost
  4. All of the above

Answer.4. All of the above

Explanation:

  • Whether the transmission line is short, medium, or long it should have high mechanical strength, long life, and low cost.

 

Ques 47. Which of the following voltage regulations is considered best?

  1. 10%
  2. 20%
  3. 100%
  4. 4%

Answer.4. 4%

Explanation:

  • In electrical engineering, particularly power engineering, voltage regulation is a measure of the change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line.
  • Voltage regulation describes the ability of a system to provide near-constant voltage over a wide range of load conditions.
  •  A 4% voltage regulation will mean that when loaded, the voltage will fluctuate 4% according to its rated voltage.
  • Means if you have a source with 240V on no-load it might drop to 236V when supplying to a load.
  • The lesser the number the better the voltage regulation since it stays close to where you want it to be.

 

Qus 48. Skin effect is proportional to

  1. Directly proportional to (Diameter of conductor)1/2
  2. Inversely proportional to (Diameter of conductor)1/2
  3. Directly proportional to  (Diameter of conductor)2
  4. Inversely proportional to  (Diameter of conductor)2

Answer.3. Directly proportional to (Diameter of conductor)2

Explanation:

Why is the skin effect proportional to the diameter of the conductor?

  • In skin effect the eddy current causes the electron to be repelled towards the outermost surface of the conductor.
  • Repelling the electron towards the outer surface of the conductor increase the conductor resistance.
  • The resistance of a conductor is defined as the resistivity of the conductor multiplied by the length of the conductor, divided by the cross-sectional area.
    R=ρl/a ………1
  • For the cross-sectional area of a conductor:
    a = πr2
    since the diameter of a conductor is twice its radius, we can write
    a = π(D/2)2
  • Putting the value of a in equation number 1 we get
    R=ρlπ/(D/2)2
  • Thus the resistance of the conductor (R) would increase with a decrease of the diameter (D).
  • Skin effect is inversely proportional to the skin depth
    skin depth = 1/√(πfµσ)
    therefore skin effect  √(πfµσ)
  • Hence skin effect is directly proportional to the (Diameter of the conductor)2

 

Ques 49. A conductor, due to sag between two supports, takes the form of

  1. Catenary
  2. Semi-circle
  3. Triangle
  4. Ellipse

Answer.1. Catenary

Explanation:

  • A catenary is a curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends.
  • The catenary curve has a U-like shape, superficially similar in appearance to a parabola, but it is not a parabola.

 

Ques 50. By which of the following methods string efficiency can be improved?

  1. Using long cross arm
  2. Grading the insulator
  3. Using a guard ring
  4. Any of the above

Answer.4. Any of the above

Explanation:

  • The ratio of voltage across the whole string to the product of the number of discs and the voltage across the disc nearest to the conductor is known as string efficiency.
  • String efficiency can be increased by the following method:
    1. By using longer cross-arms: To increase the string efficiency we can increase the arm length so that the ratio of shunt capacitance to mutual capacitance decreases thereby increasing the string efficiency.
    2. By grading the insulators: By grading the insulator the unit nearest the cross arm should have the minimum capacitance (maximum capacitive reactance) and as we go towards the power conductor the capacitance should increase. Since the voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the units in the string.
    3. By using a guard ring: Guard Ring is a metal ring that is electrically connected to the conductor and surrounding the bottom insulator. Since identical units are being used their mutual capacities are equal. Similarly, the ground capacitances are equal. The design of the ring should be such that this gives rise to the capacitances which will cancel exactly the charging current in that particular section.

Ques 51. The skin effect cause

  1. Portion of the conductor near the surface carries less current and core of the conductor carries more current
  2. Portion of the conductor near the surface carries more current and the core of the conductor carries less current
  3. Current flows through the half cross-section of the conductor
  4. None of the above
Answer.2. Portion of the conductor near the surface carries more current and the core of the conductor carries less current

Explanation:

In string effect due to eddy current, the portion of the conductor near the surface carries more current and the core of the conductor carries less current

 

Ques 52. A circuit is disconnected by isolators when

  1. Line is on full load
  2. Circuit breaker is not open
  3. There is no current in the line
  4. Line is energized

Answer.3. There is no current in the line
Explanation:

  • In electrical engineering, a disconnector, disconnect switch or isolator switch is used to ensure that an electrical circuit is completely de-energized for service or maintenance.

 

Ques 53. Current rating is not necessary in case of

  1. Circuit breaker
  2. Isolator
  3. Load break switch
  4. None of the above

Answer.2. Isolator

Explanation:

  • Isolator operates under no-load condition. They are not equipped with any arch-quenching device and don’t have any specified current breaking capacity or current making capacity.
  • Isolators are employed only for isolating the circuit when the current has already been interrupted. They ensure that the current is not switched into the circuit until the fault has been fixed.

 

Ques 54. In a substation, the following equipment is not installed

  1. Exciters
  2. Series capacitors
  3. Shunt reactors
  4. Voltage transformers

Answer.1. Exciter

Explanation:

  • To produce power electrical conductors are to be moved in a strong magnetic field. Permanent magnets have very limited strength. So there are electromagnets in the generator. This needs DC supply to produce magnetism. Supplying DC power to the magnetic coil is called excitation.
  • The exciter is used in generating station, therefore, it is not required in the substation.

 

Ques 55. The voltage drop, for constant voltage transmission, is compensated by installing

  1. Capacitors
  2. Synchronous motors
  3. Inductors
  4. All of the above

Answer.2. Synchronous motor

Explanation:

  • Synchronous motor sometimes runs on no-load with over-excitation for improving the voltage regulation of a transmission line. Such an over-excited synchronous motor is called the synchronous condenser or synchronous capacitor.

 

Ques 56. The use of strain type insulators is made where the conductors are

  1. Dead End
  2. Road Crossing
  3. Intermediate anchor towers
  4. All of the above

Answer.4. All of the above

Explanation:

Strain Type Insulator

  • Strain type insulators are used for handling the mechanical stress at the angle position of the line i.e Dead end, intermediate anchor tower, corner, sharp curve.

 

Ques 57. The current drawn by the line due to corona losses is

  1. Non-sinusoidal
  2. Triangular
  3. Square
  4. Sinusoidal

Answer.1. Non-sinusoidal

Explanation:

  • The current drawn by the line due to corona is non-sinusoidal therefore non-sinusoidal voltage occurs in the line.
  • This non-sinusoidal current and voltage cause inductive interference with the neighboring communication line.

 

Ques 58. Pin type insulators are generally not used for voltages beyond

  1. 22 kV
  2. 33 kV
  3. 11 kV
  4. 1 kV

Answer.2. 33 kV

Explanation:

  • Pin type insulators are used for transmission and distribution of electric power at voltages up to 33 kV.
  • Beyond operating voltage of 33 kV, the pin type insulators become too bulky and hence uneconomical. 

 

Ques 59. For transmission of power over a distance of 200 km, the transmission voltage should be

  1. 66 kV
  2. 132 kV
  3. 11 kV
  4. 33 kV

Answer.2. 132 kV

Explanation:

  • In this case, the line length is more than 160 Km, therefore, it is a long transmission line and the line voltage requires is greater than 100 kV.

 

Ques 60. Which of the following equipment, for regulating the voltage in the distribution feeder, will be most economical?

  1. Static condenser
  2. Tap changing transformer
  3. Synchronous condenser
  4. Shunt capacitor

Answer.4. Shunt capacitor

Explanation:

  • The synchronous condenser is most suitable for power factor correction but it is expensive therefore it is used for the high-voltage transmission line.
  • The most economical way of regulating the voltage within the permissible limit is to use shunt capacitor.
  • Shunt capacitors are installed on the feeder and substation bus to achieve the economical power factor.

Ques 61. In a tap changing transformer, the tappings are provided on

  1. Primary winding
  2. Secondary winding
  3. High voltage winding
  4. Any of the above

Answer.3. High voltage winding

Explanation:

  • Tapings are connected to the high voltage winding side, because of low current. If we connect tapings to the low voltage side, sparks will produce due to high current.
  • The number of turns in the High voltage winging is large and hence a fine voltage variation can be obtained.
  • LV winding is placed nearer to the core and HV winding is placed outside. Therefore providing taps on the HV winding is comparatively easier than that of the LV winding.

 

Ques 62. Constant voltage transmission entails the following disadvantage

  1. Short-circuit current of the system is increased
  2. Large conductor area is required for same power transmission
  3. Load power factor in heavy loads
  4. All of the above

Answer.1. Short circuit current of the system is increased

Explanation:

  • In constant voltage transmission, a constant voltage drop is maintained along the line with the help of the phase modifiers (synchronous motor running without mechanical load) at the receiving end.
  • With the change in load, the power factor of the system is changed by the synchronous motors and thus voltage drop along the line remains constant.

The main disadvantage of constant power transmission is

  1. Increased the risk of interruption of supply due to falling of synchronous motors
  2. Short-circuit current in the system is increased.

 

Ques 63. Skin effect depends upon?

  1. Frequency of the current
  2. Resistivity of the conductor material
  3. Size of the conductor
  4. Type of conductor
  5. All of the above

Answer.5. All of the above

Explanation:

Skin effect depends upon the following factor.

  • Frequency: Skin effect increases with the increase in frequency.
  • Diameter: It increases with the increase in diameter of the conductor.
  • The shape of the conductor: Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
  • Type of material: Skin effect increase with the increase in the permeability of the material
  • The resistivity of the conductor: When resistivity increases its conductivity decrease, therefore, the skin effect also increases. 

 

Ques 64. When an alternator connected to the bus-bar is shut down the bus-bar voltage will

  1. Remain unchanged
  2. Increased
  3. Decreased
  4. None of the above

Answer.1. Remain unchanged

Explanation:

  • Many alternators are connected to an infinite busbar and they are synchronized at the same frequency, so removing one of them doesn’t change the frequency.
  • It can be said that the power capabilities of the infinite bus bar are very much large as compared to an individual alternator connected to it thus when any alternator is removed no significant impact is made on the bus bar voltage or frequency and hence they don’t change. 

 

Ques 65. The angular displacement between two interconnected stations is mainly due to

  1. The synchronous reactance of both the alternators
  2. The reactance of the interconnector
  3. Armature reactance of both alternators
  4. All of the above

Answer.3. Armature reactance of both alternators

Explanation:

Interconnected station

  • In an interconnected station to transfer large load between the stations, it is required that the load on the line must be shared equally.
  • Consider the 2 generating station S-1 and S-2 supplying a Receiving station RS through line 1 and line 2.
  • To deliver equal power through line 1 and line 2 the phase and active component of line current I1 and I2 must be equal.
  • To compensate Impedance drop the regulating components (RE) are installed at the sending end of each transmission line.
  • The parallel operation and load sharing between two alternators are possible because of alternator internal reactance which limits the short circuit current and allows angular displacement between two stations.

 

Ques 66. Electro-mechanical voltage regulators are generally used in

  1. Transformer
  2. Generator
  3. Reactor
  4. All of the above

Answer.2. Generator

Explanation:

Electromechanical regulators

  • The voltage of an AC generator is controlled by the electromechanical voltage regulator.
  • The magnetic field produced by the current attracts a moving ferrous core held back under spring tension.
  • The magnetic field produced by the current attracts a moving ferrous core held back under spring tension.
  • As voltage increases, so does the current, strengthening the magnetic field produced by the coil and pulling the core towards the field.
  • When voltage decreases, so do the current, releasing spring tension or the weight of the core and causing it to retract. This closes the switch and allows the power to flow once more.
  • The magnet is physically connected to a mechanical power switch, which opens as the magnet moves into the field.

 

Ques 67. Series capacitors on transmission lines are of little use when the load VAR requirement is

  1. large
  2. Small
  3. Fluctuating
  4. Any of the above

Answer.2.Small

Explanation:

  • Series capacitor is the method of improving the system voltage and impedance by injecting the reactive power in the transmission line.
  • If the voltage drop is the limiting factor then the series capacitor is very effective. Voltage fluctuations due to the arc furnace can also be compensated by series capacitors.
  • Series capacitor is also very effective when the total line reactance is high.
  • It is mostly used in extra and ultra high voltage line but if the load vAR requirement is small series capacitor are of little use.
  • The shunt capacitor is used for lightly loaded cables or lines.

 

Ques 68. The voltage regulation in the magnetic amplifier type voltage regulator is effected by

  1. Electromagnetic induction
  2. Varying the resistance
  3. Variable transformer
  4. Varying the reactance

Answer.4.Varying the reactance

Explanation:

Magnetic amplifier

  • Magnetic amplifier voltage regulators work on the principle of the saturable magnetic core.
  • Two dc windings are arranged on the core of the opposite magnetic field which is fed through the rectifier.
  • The combination of the ampere-turns of the D.C signal coil and the series rectifying coil will produce the varying degree of reactance in the series circuit.
  • Any small change in the voltage will change the resultant ampere-turns on the control winding which is used to control the field current of the exciter of the alternator.

 

Ques 69. When a conductor carries more current on the surface as compared to core, it is due to

  1. Corona
  2. Skin Depth
  3. Skin Effect
  4. Any of the above

Answer.3. Skin Effect

Explanation:

  • In skin effect the eddy current causes the electron to be repelled towards the outermost surface of the conductor.
  • Repelling the electron towards the outer surface of the conductor increase the conductor resistance.

 

Ques 70. When a live conductor of public electric supply breaks down and touches the earth which of the following will happen?

  1. Current will flow to earth
  2. No current will flow in the conductor
  3. Either 1 or 2
  4. Supply voltage will increase

Answer.3. Either 1 or 2

Explanation:

  • The electrical current travels by means of ions dissolved in the moisture in the ground. In dry soil, this may not work as but in wet ground, current can flow easily because water is a good conductor of electricity.
  • The key point is that the current flows from one point of the circuit, through the ground, then back into the circuit. With only one connection to the ground, there is no circuit for the current to flow through.
  • If there is a second connection to ground elsewhere in the circuit (usually at the sub-station) which forms the circuit.

Ques 71. If the length of the cable is doubled, its capacitance C will become

  1. Doubled
  2. Half
  3. One-fourth
  4. Remain Unchanged

Answer.1. Doubled

Explanation:

  • Capacitance exists between two things with different electric charge and a dielectric in between.
  • The transmission line and the earth below it, have a charge difference and the air between them is dielectric. This constitutes a capacitance.
  • The longer the cable, the more part of it runs parallel to the earth and hence, more capacitance between them since the length of the cable and the capacitance of the cable are directly proportional to each other.

 

Ques 72. The advantages of high transmission voltage are

  1. Area of cross-section is reduced
  2. Reduced line losses
  3. Increase efficiency
  4. All of the above

Answer.4. All of the above

Explanation:

Advantages of High Voltage Transmission

  • The line losses are inversely proportional to the square of the voltage and the power factor (Pl∝ 1/V²P). Therefore line losses are reduced.
  • For constant current density, the line efficiency is high for the high voltage transmission line.
  • The volume of copper required is inversely proportional to the square of the voltage and the power factor. Hence the requirement of copper is less for the high voltage transmission line.
    A 1/V²
  • As the line losses are decreased the efficiency of the transmission line is increased.

 

Ques 73. The voltage regulation of flat voltage profile system is

  1. 100 %
  2. 50 %
  3. 0 %
  4. Any of the above

Answer.3. 0%

Explanation:

  • The % voltage regulation is given as (Vs-Vr)/Vr*100  where
    Vs = Supply voltage
    Vr = Receiving end Voltage
  • For flat voltage profile system all the voltage drop in the line are neglected therefore supply voltage (Vs) is equal to the Receiving end voltage (Vr) i.e Vs = Vr
  • Hence the voltage regulation for flat voltage profile system is zero.

 

Ques 74. The maximum power can be achieved in a transmission line by

  1. Reducing system transfer Reactance
  2. Increasing the Voltage level
  3. Both 1 and 2
  4. None of the above

Answer.3. Both 1 and 2

Explanation:

  • To reduce the power loss in the transmission line the source voltage is stepped up by using the transformer.
  • By increasing the voltage level the same amount of power can transfer with much smaller current thus reducing transmission line losses (I2R losses).
  • The system transfer reactance of a transmission line can be reduced by the series capacitance, bundled conductor, or by using parallel transmission line.

 

Ques 75. Ferranti effect can be reduced by which of the following method?

  1. Shunt Reactor
  2. Series Reactor
  3. Shunt Capacitor
  4. Series Capacitor

Answer.1. Shunt Reactor

Explanation:

  • A long transmission line can be considered to be composed of a high amount of capacitance and inductor distributed across the entire length of the line. Ferranti Effect occurs when current drawn by the distributed capacitance of the line is greater than the current associated with the load at the receiving end of the line which occurs during light or no load.
  • Shunt Reactor compensation at the receiving end help to reduce the effect of capacitance thus reducing the Ferranti effect.
  • Shunt Reactor absorbs the excess reactive power during no load or light load condition and thus helps in stabilizing the voltage of the Transmission Line.

 

Ques 76.  A transmission line has a reactance of 4 Pu is operating at Vs = Vr = 2 Pu. The generator is connected at source end which is delivering 0.5 Pu of active power. Find the load angle?

  1. 60°
  2. 45°
  3. 30°
  4. 35°
Answer.3.

Explanation:

Ques 77. Under excited Synchronous phase modifier works as

  1. Shunt capacitor
  2. Shunt Reactor
  3. Series capacitor
  4. Series Reactor

Answer.2. Shunt Reactor

Explanation:

  • The synchronous phase modifier is a synchronous motor which is not connected to any load.
  • When a synchronous motor is under-excited its works as an inductor or shunt Reactor.
  • The shunt reactors are used across capacitive loads or lightly loaded lines to absorb leading vars to control the voltage across the load to within certain desirable limits.

 

Ques 78. Over-excited Synchronous phase modifier work as

  1. Shunt capacitor
  2. Shunt Reactor
  3. Series capacitor
  4. Series Reactor

Answer.1. Shunt Capacitor

Explanation:

  • The synchronous phase modifier is a synchronous motor which is not connected to any load.
  • When a synchronous motor is over-excited its works as a shunt capacitor.
  • The shunt capacitors are used across an inductive load to provide leading volt-ampere-reactive vAR to reduce the line current and total kVA loading of the substation.

 

Ques 79. The specified quantities of load bus are

  1. Q and V
  2. V and δ
  3. P and Q
  4. Q and V

Answer.4. P and Q

Explanation:

All the bus in the power system is generally categorized into three categories

  1. Generation Bus
  2. Load Bus
  3. Slack Bus
  • The two of the four quantities are mentioned above are specified for each of the buses.
  • Load bus: This is also called as PQ bus and here the real power |P| and reactive power |Q| are specified.

 

Ques 80. The specified quantities of Generation bus is

  1. Q and V
  2. V and δ
  3. P and V
  4. Q and V

Answer.3. P and V

Explanation:

  • Generation Bus is also called as the voltage controlled bus.
  • The specified quantities of Generation bus is P and V and here the real power |P| and voltage magnitude |V| are specified.

Ques 81. The specified quantities of Slack bus is

  1. Q and V
  2. V and δ
  3. P and V
  4. Q and V

 Answer.2. V and δ

Explanation:

  • The slack bus is also known as the swing bus or reference bus.
  • The voltage magnitude |V| and phase angle |δ| are specified here.
  • This bus is selected to provide additional real and reactive power to supply the transmission losses.

 

Ques 82. The range of accelerating factor is

  1. 10 to 20
  2. 20 -30
  3. 15.5 -20.2
  4. 1.6 -1.8

Answer.4. 1.6 to 1.8

Explanation:

  • For Gauss-seidel load flow method, an acceleration factor normally used to lead fast convergence as it reduces the number of iterations.
  • After doing repeated load flow it has been observed that normally it work well for the acceleration factor of 1.6 to 1.8.

 

Ques 83. The advantage of the Newton Raphson method is

  1. Less number of iteration
  2. Less computation Time
  3. Applicable for large power system network
  4. All of the above

Answer.4. All of the above

Explanation:

Newton Raphson’s method is perhaps the best-known method for finding successively better approximations to the zeroes (or roots) of a real-valued function.

The advantages of the Newton-Raphson method are

  • Newton Raphson method needs less number of iterations to reach convergence, hence take less computation time.
  • The accuracy of the N-R method is more and convergence is not affected by the choice of slack bus.
  • The accelerating factor is not required in the Newton Raphson method.
  • The N-R method is best suited for a large power network.

 

Ques 84. The disadvantages of the Newton-Raphson method is

  1. Large computer memory required
  2. Computer programming is difficult
  3. Per iteration time is large
  4. All of the above

Answer.4.All of the above

Explanation:

  • The disadvantages of the Newton-Raphson method are
    1. It requires more memory space.
    2. It requires more number of calculation, therefore, programming is complex.
    3. The time requires per iteration is more in the Newton-Raphson method.

 

Ques 85. The advantages of the Gauss-Seidel method is

  1. The less computational time required
  2. Simplicity in technique
  3. Less computer memory required
  4. All of the above

Answer.4. All of the above

Explanation:

The advantages of the Gauss-Seidel method are:-

  1.  A simple algebraic equation is used hence it requires less number of arithmetic operation to complete iteration and therefore the time required for each iteration is less
  2. The need for computer memory is less.
  3. This method is most suitable for small size networks.

 

Ques 86.  The disadvantages of Gauss-seidel method are

  1. A large number of iteration required
  2. Slow convergence rate
  3. Inefficient for large system network
  4. All of the above

 Answer.4. All of the above

Explanation:

The disadvantages of the Gauss-Seidel method are:-

  1. This method is not applicable to the large power system.
  2. The convergence is affected by the choice of slack bus.
  3. It requires more number of iteration to obtain the solution.
  4. The rate of convergence is slow.
  5. It required an accelerating factor for convergence.
  6. Increase in the number of iteration with an increase in the number of buses.

 

Ques 87. Z bus matrix is a

  1. Full matrix
  2. Null Matrix
  3. Unit matrix
  4. None of the above

Answer.1. Full Matrix

Explanation:

  • Z Matrix or bus impedance matrix is an important tool in power system analysis.
  • It is an important tool in power system studies like short circuit analysis or fault study.
  • Z Matrix can be formed by either inverting the Ybus matrix or by using Z bus building algorithm.
  • Z matrix is a full matrix i.e zero elements in Ybus become non-zero elements in the corresponding Zbus.

 

Ques 88.  Which of the following matrix is used for load flow studies?

  1. Unit matrix
  2. Null matrix
  3. Y bus matrix
  4. Z bus matrix
Answer.3. Y bus matrix

Explanation:

  • Y bus is bus admittance matrix which is used (generally) in Load flow analysis.
  • This matrix helps in finding values of P, Q, |V| & δ.
  • With any type of changes in the network, the elements can be changed without actually changing the whole matrix in Y bus matrix.
  • We know that I= V/Z and Z= 1/Y.
    Hence I=Y*V.
    All these are matrices and hence the division by matrices is very hard we use multiplication using Y instead of dividing with Z.
  • Y is additive is parallel and we know that in power systems mostly loads are connected in parallel, that’s why power system calculation is done in admittance matrix form Y rather than Z because it helps to simplify calculations.

 

Ques 89.  For n number of buses the power system size of Y, bus matrix is

  1. (n-1)2
  2. (n-2)2
  3. n×n
  4. None of the above
Answer.3. (n x n)

Explanation:

  • In power engineering, the nodal admittance matrix (or just admittance matrix) or Y Matrix or Ybus is the square matrix of an order of N x N matrix describing a power system with N buses.
  • Y matrix is symmetrical and there are many zeros in it. Hence it requires less memory as load flow analysis is done in computers.

 

Ques 90.  Find the number of strands of ACSR conductor for 3 layer transmission line?

  1. 10
  2. 19
  3. 19
  4. 27
Answer.2. 19

Explanation:

The total number of strands in ACSR conductor is given as
N = 3x² – 3x + 1  where
x = number of layer

So the total number of strands
N = 3×3² – 3×3 + 1 = 19

Ques 91.  Find the total diameter of the ACSR conductor with 2 layers and the diameter of each strand is 3?

  1. 9
  2. 10
  3. 5
  4. 15
Answer.1. 9

Explanation:

  • The diameter of the ACSR  conductor can be calculated by
    D = (2x – 1)xd where
    x = number of layer
    d = diameter of each strand
  • Therefore, Total diameter of ACSR conductor
    D = (2×2 -1)3
    D = 9

 

Ques 92. Telecommunication lines are transposed to reduce the

  1. Voltage level
  2. Short circuit current
  3. Radio interference in communication line
  4. All of the above
Answer.3. Radio interference in communication lines

Explanation:

  • Usually, telecommunication lines and power lines travel close to each other, then the current flowing through power line produces magnetic flux linkage between telecommunication lines so that voltage is induced in the telecommunication line which causes the disturbance in communication.
  • To reduce this effect t the conductors are transposed. Transposition can be made either to the power conductors or telephone (communication ) lines at regular intervals.
  • When conductors are not transposed at regular intervals, the inductance and capacitance of the conductors will not be equal.
  • When conductors such as power lines are run in parallel to transmission lines, there is a possibility of high voltages induced in the lines.  This can result in acoustic shock or noise.

 

Ques 93.  Which of the following produces the radio interference in communication lines?

  1. Electromagnetic induction
  2. Electrostatic induction
  3. Both 1 & 2
  4. None of the above
Answer.3. Both 1 & 2

Explanation:

  • High voltage transmission lines and substations can cause radio interference.
  • Since radio noise is associated with corona, it mainly depends on the potential gradients at the conductors.
  • Radio interference due to corona is of the importance for lines above 200 kV.
  • In electromagnetic effect, the current flowing in the power line produces magnetic flux linkage with the communication line conductor which induces an emf in the telecommunication line conductor.
  • An electrostatic effect the electric field is produced by the charges of the earth induces a voltage in between the conductors of the telecommunication lines.

 

Ques 94. Bundled conductors in EHV transmission lines help in

  1. Decrease capacitance
  2. Decrease Inductance
  3. Increase capacitance
  4. Increase inductance

Answer.2. Decrease inductance

Explanation:

  • Total inductance is given as L = 2*10-7 * ln(d/r’)
    A bundle conductor is a conductor made up of two or more sub-conductors and If we use bundled conductors, the effective radius will increase hence increase in radius will decrease the inductance.
  • Bundled conductor Primarily used to reduce the potential gradient on the surface of the conductors so that occurrence of Corona can be reduced.
  • It helps in obtaining better voltage regulation and efficiency by reducing the inductance and skin effect present in the power lines.

 

Ques 95.  If the frequency is increased, then the skin effect will

  1. Decrease
  2. Remain the same
  3. Increase
  4. None of the above

Answer.3. Increase

Explanation:

  • The skin effect is inversely proportional to the skin depth.
  • The skin depth is given as
    skin depth = 1/√(πfµσ) where
    f = frequency
    µ = permeability

    σ = conductivity
  • So by increasing the frequency, the skin depth decreases, and skin effect increases.

 

Ques 96. Proximity effect is more in case of

  1. Overhead line
  2. Power cable
  3. DC transmission
  4. None of the above

Answer.2. Power cable

Explanation:

Proximity effect

  • The alternating magnetic flux produced in the conductor due to the current flowing through the neighboring conductor gives rise to the circulating current which increases the resistance of the conductor. This phenomenon is called the proximity effect.
  • The proximity is more in the case of power cable and underground line where the distance between the conductor is small.
  • This effect is negligible in the case of overhead transmission lines because the distance between the conductors is large.
  • The proximity effect is absent in the case of DC transmission because the frequency of DC transmission is zero.
  • The proximity effect depends upon the size of the conductor, spacing between the conductor, frequency and permeability of the conductor material.

 

Ques 97. The distribution constant of the transmission line is

  1. Resistance
  2. Inductance
  3. Capacitance
  4. Resistance, Inductance, Capacitance, Conductance

Answer.4. Resistance, Inductance, Capacitance, Conductance

Explanation:

Transmission line constant

  • Transmission lines are the circuit with the distributed constant such as Resistance, Inductance, Capacitance, Conductance which is distributed along the length of the transmission line.
  • These constant are not concentrate at any point. There value expressed as per kilometer length of the line.

 

Ques 98.  In which of the following transmission lines capacitance effect is negligible?

  1. Long transmission lines
  2. Short transmission line
  3. Medium transmission line
  4. Any of the above

Answer.2. Short transmission line

Explanation:

  • Generally, the length of the short transmission line is less than 80 km, therefore, charging current and leakage current is very less.
  • Hence the capacitance and conductance effects are negligible for short transmission lines only.

 

Ques 99.  The fact that current density is higher at the surface when compared to the centre is known as

  1. Skin effect
  2. Proximity effect
  3. Corona effect
  4. None of the above

Answer.1. Skin effect

Explanation:

  • In skin effect the eddy current causes the electron to be repelled towards the outermost surface of the conductor.
  • Repelling the electron towards the outer surface of the conductor increase the conductor resistance.
  • Due to skin effect, the area of the current flowing path is reduced.

 

Ques 100. The charging current drawn by the cable is

  1. Lags voltage by 90o
  2. Lead voltage by 90o
  3. Lead voltage by 45o
  4. None of the above

Answer.2. Lead voltage by 90o

Explanation:

  • The charging current drawn by the cable is capacitive in nature, therefore, it leads the voltage by 90o

FOR TRANSMISSION AND DISTRIBUTION SYSTEM MCQ CLICK HERE

FOR DC MOTOR MCQ CLICK HERE

FOR 1-Φ INDUCTION MOTOR MCQ CLICK HERE

FOR 3-Φ INDUCTION MOTOR MCQ CLICK HERE

FOR SYNCHRONOUS GENERATOR OR ALTERNATOR MCQ CLICK HERE

FOR SYNCHRONOUS MOTOR MCQ CLICK HERE

FOR DC MOTOR MCQ CLICK HERE

FOR DC GENERATOR MCQ CLICK HERE

FOR POWER SYSTEM MCQ CLICK HERE

FOR TRANSFORMER MCQ CLICK HERE

FOR BASIC ELECTRICAL MCQ CLICK HERE

FOR CONTROL SYSTEM MCQ CLICK HERE

 

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