DMRC JE Electrical 10th- April- 2018 Paper With Solution and Explanation

Ques 51. In busbar protection what is the method of providing an earthed metal barrier surrounding a bus throughout its length called?

  1. Fault bus protection
  2. Time graded Overcurrent protection
  3. Distance protection
  4. Differential protection

Fault Bus Protection

The design of such a station in which the faults that develop are mostly earth faults is feasible by providing an earthed metal barrier (known as a fault bus) surrounding each conductor throughout its length in the bus structure. In such an arrangement every fault that might occur must involve a connection between a conductor and an earthed metal. The faults can be detected and located by directing the flow of the earth-fault current. Such a scheme of protection is called fault bus protection.

It is one of the most simple forms of protection and is applicable to small size metal-clad switchgear. It is more favored for indoor and outdoor installation.

Frame leakage protection

This is applicable to metal-clad type switchgear installations, The framework is insulated from the ground. The insulation is light, anything over 10 ohms is acceptable. This scheme is most effective in the case of isolated-phase construction type switchgear installation in which all faults involve ground. To avoid the undesired operation of the relay due to spurious currents, a check relay energized from a C.T. connected in the neutral of the system is employed. An instantaneous overcurrent relay is used in the frame leakage protection scheme if a neutral check relay is incorporated. If a neutral check relay is not employed, an inverse time delay relay should be used.

 

Ques 52. According to the Tellegen theorem, the sum of instantaneous power for the n branches in a network is always?

  1. In phase with the current
  2. Equal to zero
  3. A constant
  4. Alternating

Tellegen’s theorem is one of the most powerful theorems in network theory. The physical interpretation of Te|legen‘s theorem is the conservation of power. As per the theorem, the sum of powers delivered to or absorbed by all branches of a given lumped network is equal to 0 i.e. the power delivered by the active elements of a network is completely absorbed by the passive elements at each instant of time.

Tellegen’s theorem depends on KCL and KVL but not on the type of the elements. Tellegen theorem can be applied to any network linear or non-linear, active or passive, time-variant or time-invariant.

 

Ques 53. The voltage and efficiency of four transformers A, B, C, and D are as given below

A-  5% regulation & 94% efficiency

B- 2% regulation & 96% efficiency

C- 5% regulation & 97% efficiency

D- 2% regulation & 97% efficiency

The transformer that is in good working condition is

  1. D
  2. B
  3. A
  4. C

Voltage regulation of a transformer is defined as the change or difference in the secondary voltage from no load to full load at a given power factor with the same primary voltage for both conditions, i.e., rated load and no load. It is generally expressed as a percentage of the full load value.

${\text{Voltage Regulation = }}\dfrac{{{E_{{\text{2 no – load}}}} – {V_{{\text{2 full – load}}}}}}{{{V_{{\text{2 full – load}}}}}}$

Voltage regulation depends on the voltage drop and the impedance of the transformer, load current, and load power factor.

Suppose a 10% regulation means if you have a source with 100V on no-load it might drop to 90V when supplying to a load. The greater the number is, the higher the difference is between no load and full load voltage.

Less voltage regulation is considered beneficial because if voltage regulation is of a larger value, the voltage will fluctuate more when you connect a load. If voltage regulation has a smaller value, this fluctuation will be less.

However, low voltage regulation is not always preferable. High impedance and high voltage regulation transformers are used to reduce the fault current in a circuit.

  • The efficiency of a transformer depends on its design and is equal to the power output divided by the power input.
  • Efficiency = output / input .
  • The difference between these two quantities is the power loss, which comes out in the form of heat.
  • Efficiency = output / output +losses
  • It is necessary to use higher efficiency at the higher power levels because the amount of energy wasted is significant.

Hence from the theory of voltage regulation and the efficiency. The transformer that is in good working condition is 2% regulation & 97% efficiency

 

Ques 54. If during a no-load test on an induction Motor it takes 10 A current and 300 watts of power at a line voltage of 200 V, the stator core loss will be (assume stator resistance per phase as 0.3Ω)

  1. 165 W
  2. 300 W
  3. 210 W
  4. 192 W

Given

Motor current per phase Io = 10 A
No-load power = 300 watts
Stator resistor per phase

Stator copper loss = 3I2oR

3 × 102 × 0.3 = 90 W

Stator core loss = No load power – Stator copper loss

= 300 − 90 = 210 W

 

Ques 55. For unity power factor loads, the effect of an armature reaction in an alternator is:

  1. De-magnetising
  2. Magnetizing
  3. Distortional
  4. Cross magnetizing

Unity Power Factor Load (Cross-Magnetizing)

Consider a purely resistive load connected to the alternator, having a unity power factor. As induced e.m.f Eph drives a current Iph and load power factor is unity, Ephand Iph is in phase with each other.

If φf is the main flux produced by the field winding responsible for producing Ephthen Eph lags φf by 90°.

Now current through armature Ia produces the armature flux say to,, So flux φa, and la are always in the same direction.

Unity Power factor

It can be seen from the phasor diagram that there exists a phase difference of 90°between the armature flux and the main flux. From the waveforms, it can be seen that the two fluxes oppose each other on the left half of each pole while assisting each other on the right half of each pole. Hence average flux in the air gap remains constant but its distribution gets distorted. Hence such distorting effect of armature reaction under unity p.f. the condition of the load is called the cross magnetizing effect of armature reaction. Due to such distortion of the flux, there is a small drop in the terminal.

Zero Lagging Power Factor Load (Demagnetizing)(Over-excited)

Consider a purely inductive load connected to the alternator having zero lagging power factor. This indicates that Iph driven by Eph lags Eph by 90° which is the power factor angled.

Induced e.m.f. Eph lags main fluxes φf by 90° while φa is in the same direction as that of Ia.

It can be seen from the phasor diagram that the armature flux and the main flux art exactly in the opposite directions to each other. So armature flux tries to cancel the main flux Such an effect of armature reaction is called the demagnetizing effect of the armature reaction.

img.1

As this effect causes the reduction in the main flux, the terminal voltage drops. This drop in the terminal voltage is more Man the drop corresponding to the unity p.f. load.

Zero Leading Power Factor Load (Magnetizing) Under-excited

Consider a purely capacitive load connected to the alternator having zero leading power factor. This means that armature current Iaph driven by Eph leads Eph by 90° which is the power factor angle φ.

Induced e.m.f. Eph lags φf by 90° while Iaph and φa, are always in the same direction.

img.2

The armature flux and the main field flux are in the same direction i.e. they are helping each other. This results in the addition of the main flux. Such an effect of each armature reaction due to which armature flux assists field flux is called the magnetizing effect of the armature reaction

As this effect adds the flux to the main flux, greater e.m.f. gets induced in the armature Hence there is an increase in the terminal voltage for leading power factor loads.

For intermediate power factor loads i.e. between zero lagging and zero leading the armature reaction is partly cross magnetizing and partly demagnetizing for lagging power factor loads or partly magnetizing for leading power factor loads.

 

Ques 56. The function of brushes in a DC generator is

  1. Hold the armature winding
  2. Provide a low reluctance path for the magnetic flux
  3. Convert AC to DC
  4. Collect current from the commutator

 

construction of DC generator

Armature core

  • The armature is the rotating part of the DC machine.
  • The purpose of the armature core is to hold the armature winding and provide a low reluctance path for the flux through the armature from N pole to S pole.

Commutator

  • The commutator is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
  • The commutator converts the alternating current generated in the armature into the unidirectional current.

Brushes

  • We know that commutator is connected to the armature so as the armature is rotating (to cut the flux in order to induce the EMF ) obviously commutator will also rotate with the armature.
  • So in order to collect the current, we should have something which is stationarily and can be fit into the commutator, and that is brushes.
  • The number of brushes depends on how much current we need to tap from the commutator.
  • Brushes are made up of materials like carbon, copper, and graphite.
  • Copper brushes are used for the machine designed for large current at low voltage.
  • Graphite and Carbon Graphite are self-lubricated and therefore widely used.

 

Ques 57. The input power provide during the short circuit test of the transformer equals

  1. Output power
  2. Iron Loss
  3. Copper Loss
  4. Total Loss

Short circuit or Impedance test

SC test

  • A short circuit test or Impedance test is performed to determine
    ⇒Copper loss at full load
    ⇒Equivalent impedance (Zo1 or Zo2)
    ⇒Leakage reactance (Xo1 or Xo2)
  • In this test low voltage winding is short-circuited by a thick conductor.
  • A short circuit test is performed on the HV side of the transformer.
  • Low voltage (5 to 10%) is applied to the primary and slowly increases till full load current is flowing both in primary and secondary.
  • The ammeter reading gives full load current IL.
  • Since applied voltage is small so flux (Φ) is also small therefore core loss is small hence core loss can be neglected. Therefore the wattmeter reading can be taken as equal to copper losses in the transformer.

 

Ques 58. Which of the following motors can be used as part of the control circuit in the robotic application?

  1. AC series Motor
  2. Universal Motor
  3. Servo Motor
  4. Schrage Motor

A servomotor is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity, and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.

Servo motors are motors combined with a sensor so that the position/velocity/torque of the motor at a given instance can be measured and corrected using the fundamentals of control systems.
In servomotors, the desired value of position/velocity/torque is provided as an input and the actual value of the same is taken as the output.

The voltage/current is varied in such a way that the difference between the output and input is minimized. The device which varies the voltage/current of the motor is called the controller of the motor. One of the most famous and widely used controllers is PID controller.

Robots are designed in such a way that they are able to perform a wide variety of tasks. Move from one point to another at different speeds. Pick objects from any random point and place it at any other point. So, while doing this the speed and position of the must be precisely controlled. This can be achieved using servo motors. There are other popular motors types, that are used in robots, stepper motors are one example. However, due to torque and speed considerations servo motors are preferred.

 

Ques 59. Which is Not the static compensation instrument for the transmission line?

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

METHODS OF INCREASING TRANSMISSION CAPABILITY OF EHV LINES

The power-transfer capability of EHV transmission lines can be increased by maintaining the loaded voltage within specified limits. The following methods are used for maintaining the voltage within limits:

  1. Shunt capacitor bank
  2. Shunt reactors
  3. Synchronous condensers
  4. Static VAR compensators at heavy loads
  5. Series compensation

Shunt capacitor banks: Shunt capacitor banks are operated to inject the reactive power for maintaining the voltage within limits at heavy loads. These are installed near the load terminals i.e., factory substations and switching substations.

Shunt reactors: Shunt reactors are used to absorb the reactive power from the line to control the voltage under lightly-loaded and no-load conditions. These are provided at both ends of all transmission lines.

Synchronous condensers: Synchronous condenser is the synchronous motor running under no load. It generates and absorbs the reactive power depending upon the excitation. Under low loads, it works as a VAR absorber. Under heavy loads, it works as a VAR generator. So, by adjusting the excitation of the synchronous machine, the receiving-end voltage can be maintained within the limits. The synchronous motor not only is an active power source but also a main reactive power source.

Static VAR compensators: The functioning of a static VAR compensator is the same as that of a synchronous condenser. The only difference is that there are no moving parts. Static VAR compensators (SVCs) use static power control devices such as SCRs or IGBTs and switch a bank of capacitors and inductors to generate reactive currents of the required makeup. The important features of this compensator are higher speed of response, better reliability factor, low maintenance cost, and reduced power system oscillation. It also improves the power transfer capacity.

Series compensation: in series compensation, a capacitor is connected in series with the line. So, the net reactance of the line will be reduced, and due to the reduced reactance, the net voltage drop is also reduced. The performance of the transmission line is thus improved.

Synchronous compensators have several advantages over static compensators. Synchronous compensators contribute to system short-circuit capacity. Their reactive power production is not affected by the system voltage. During power swings (electromechanical oscillations) there is an exchange of kinetic energy between a synchronous condenser and the power system. During such power swings, the synchronous condenser can supply a large amount of reactive power, perhaps twice its continuous rating. It has about 10 to 20% overload capability for up to 30 minutes Unlike other forms of shunt compensation, it has an internal voltage source and it is better able to cope with low System voltage conditions.

 

Ques 60. Intrinsic semiconductors at room temperature have

  1. Number of holes > Number of free electrons
  2. The number of holes do not depend on the free electrons
  3. Number of holes < Number of free electrons
  4. The equal number of holes and free electrons

The semiconductor is divided into two types. One is an Intrinsic Semiconductor and the other is an Extrinsic semiconductor.

Intrinsic Semiconductors

A sample of the semiconductor in its purest form is called an intrinsic semiconductor. The impurity content in the intrinsic semiconductor is very very small, of the order of one part in 100 million parts of the semiconductor.

Extrinsic Semiconductor

In order to change the properties of intrinsic semiconductors a small amount of some other material is added to it. The process of adding other material to the crystal of intrinsic semiconductors to improve its conductivity is called doping. The impurity added is called dopant. The doped semiconductor material is called extrinsic semiconductors. The doping increases the conductivity of the basic intrinsic semiconductors hence the extrinsic semiconductors are used in practice for the manufacturing of various electronic devices such as diodes, transistors etc.

Electrons and Holes in intrinsic Semiconductors

Let us see what happens at room temperature. At room temperature, the thermal energy of the atoms may allow a small number of electrons to participate in the conduction process. For semiconductors, as the temperature increases, the thermal energy of the valence electrons increases, and When an electron gains enough energy to escape the electrostatic attraction of its parent atom, it leaves behind a vacancy that may be filled with another electron.

img.30

The vacancy produced can be considered as the second carrier of positive charge. It Is known as a hole. As electrons flow through the semiconductor, holes flow in the opposite direction. If there are n free electrons in an intrinsic semiconductor, then there must also be n holes. Holes and electrons created in this way are known as intrinsic charge carriers. The carrier concentration or charge density defines the number of charge carriers per unit volume. This relationship can be expressed as n = p where n is the number of electrons and p is the number of holes per unit volume. The variation in the energy gap between different semiconductor materials means that the intrinsic carrier concentration at a given temperature also varies.

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