50 Important mcq of Power System with Explanation

Ques 11. An alternator has a phase sequence of RYB. In case the field current is reversed, the phase sequence will become

  1. RBY
  2. RYB
  3. YRB
  4. YBR
Answer.2. RYB

Explanation:

  • The direction of rotation of the synchronous generator is determined by its starting direction, as initiated by induction motor action.
  • Thus, to reverse the direction of a 3 phase synchronous generator, it is necessary to first stop the motor and then reverse the phase sequence of the 3 phase connections at the stator.
  • REVERSING THE CURRENT TO FIELD WINDINGS WILL NOT AFFECT THE DIRECTION OF ROTATION.

 

Ques 12. Gauss-Seidel iterative method can be used for solving a set of

  1. Linear differential equations only
  2. The linear algebraic equation only
  3. Both linear and nonlinear algebraic equations
  4. Both linear and nonlinear differential equations
Answer.2. The linear algebraic equation only

Explanation:

The advantages of Gauss-Seidel method are:-

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

 

Ques 13. The pure inductive circuit takes power (reactive) from the a.c. line when

  1. Both applied voltage and current rise
  2. Applied voltage decreases but current Increases
  3. Both applied voltage and current decrease
  4. None of these

 

Ques 13. A square matrix is called singular if its

  1. Determinant is zero
  2. The determinant is infinity
  3. Rank is unity
  4. None of the above
Answer.1. Determinant is zero

Explanation:

  • In linear algebra, an n-by-n square matrix A is called invertible (also nonsingular or nondegenerate) if there exists an n-by-n square matrix B such that
    AB = BA = In
  • A square matrix that is not invertible is called singular or degenerate. A square matrix is singular if and only if its determinant is 0. Singular matrices are rare in the sense that a square matrix randomly selected from a continuous uniform distribution on its entries will almost never be singular.

 

Ques 14. In load flow analysis, the load connected to a bus is represented as

  1. Constant impedance connected to the bus
  2. Voltage and frequency dependent source at the bus
  3. Constant real and reactive drawn from the bus
  4. Constant current is drawn from the bus
Answer.3. Constant current is drawn from the bus

Explanation:

  • Load flow studies determine if system voltages remain within specified limits under normal or emergency operating conditions and whether equipment such as transformers and conductors are overloaded.
  • Load flow study is the steady-state analysis of power system network.
  • To solve non-linear algebraic equations it is important to have fast, efficient and accurate numerical algorithms.
  • The output of the load flow analysis is the voltage and phase angle, real and reactive power (both sides in each line), line losses and slack bus power.

 

Ques 15. If transformer frequency is changed from 50 Hz to 60 Hz the ratio of eddy current loss 50 Hz to 60 Hz, at constant voltage will be

  1. 1
  2. 2
  3. 5/6
  4. 25/36
Answer.4. 25/36

Explanation:

  • Eddy current losses are given as
    Pc ∝ Ke B2m f2

 If transformer frequency is changed from 50 Hz to 60 Hz the ratio of eddy current loss 50 Hz to 60 Hz, at constant voltage will be

 

Ques16. When a d.c source is switched is purely inductive, the current response is

  1. Increase exponentially and reach a constant value
  2. Exponentially decaying curve
  3. A straight line passing through the origin
  4. A straight line offset from the origin
Answer.1.Increase exponentially and reach a constant value

Explanation:

  • When an inductor is connected to the circuit and the supply is turned on, initially, for a very short time, there will be an open circuit. Inductor will develop magnetic field during this period. After that, the current will increase exponentially       (1 – e(-t)) and reach a constant value.
  • Current will be maximum and inductor will behave like a short circuit.

 

Ques 17. A 100 km long transmission line is loaded at 110 kV. If the loss of line is 5 MW and the load is 150 MVA, the resistance of the line is

  1. 8.06 ohms per phase
  2. 0.806 ohms per phase
  3. 0.0806 ohms per phase
  4. 80.6 ohms per phase
Answer.1. 8.06 ohms per phase

Explanation:

The power loss in the line is given as
PL =IL2R

A 100 km long transmission line is loaded at 110 kV. If the loss of line is 5 MW and the load is 150 MVA, the resistance of the line is

 

Ques 18. At a particular unbalanced node, the real powers specified are
leaving the node 20 MW, 25 MW
entering the node 60 MW, 30 MW
then the balanced power will be

  1. 30 MW leaving the node
  2. 45 MW leaving the node
  3. 45 MW entering the node
  4. 22.5 MW entering and 22.5 MW leaving the node
Answer.3.  45 MW entering the node

Explanation:

According to Kirchhoff’s current law (KCL) at any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node.

Sum of power entering the node 60 + 30 =90 MW
Sum of power leaving the node 20 + 25 = 45

To make the power flow balanced sum of power entering the node must be equal to the sum of power leaving the node. Hence the sum of power entering the node must be equal to 45 MW.

 

Ques 19. Series capacitive compensation on EHV transmission lines is used to

  1. Reduce the line loading
  2. Improve the stability of the system
  3. Reduce the voltage profile
  4. Improve the protection of the line
Answer.2. Improve the stability of the system

Explanation:

The series capacitive compensation decrease the overall effective series transmission impedance from the sending end to the receiving end. Hence it increases the power system stability.

Synchronising power Psyn is given as


Series capacitive compensation on EHV transmission lines is used to

The advantages of series compensation are

  1. Increase transmission capability
  2. Increase power corridor angular stability
  3. Damp oscillation
  4. Enhance power-sharing between parallel line

 

Ques 20. The transmission line feeding power on either side of the main transmission line is called as

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

 

Answer.4.Secondary transmission

Explanation:

Secondary Transmission

  • The primary transmission line continues via transmission towers till the receiving stations.
  • At the receiving stations, the voltage level is reduced to 22 kV or 33 kV using via the step-down transformer.
  • Then at the reduced voltage level of 22 kV or 33 kV, the power is then transmitted to various substations using overhead 3 phase 3 wire system.
  • The conductors used for the Secondary transmission are called feeders.
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