All questions of Power Systems for Electrical Engineering (EE) Exam

Unit of heat rate curve is
  • a)
    Rs-hr
  • b)
    Rs/MWh
  • c)
    Rs/hr
  • d)
    Million kcal/hr
Correct answer is option 'D'. Can you explain this answer?

Samarth Khanna answered
Unit of Heat Rate Curve is Million kcal/hr because it represents the rate at which heat is being generated or consumed in a power plant. The heat rate is a measure of the thermal efficiency of a power plant and is usually expressed in units of energy per unit of time.

- Heat Rate Curve:
The heat rate curve is a graphical representation of the relationship between the heat input to a power plant and the power output. It shows how the heat rate varies with the power output of the plant. The heat rate curve is typically plotted with heat rate on the y-axis and power output on the x-axis.

- Importance of Heat Rate:
The heat rate of a power plant is an important parameter as it indicates how efficiently the plant converts fuel into electricity. A lower heat rate means that the plant is more efficient and requires less fuel to generate a given amount of electricity. On the other hand, a higher heat rate indicates that the plant is less efficient and requires more fuel for the same power output.

- Million kcal/hr:
The unit of heat rate in the heat rate curve is Million kcal/hr. This unit represents the amount of heat energy generated or consumed by the power plant in one hour. It is a large unit of energy and is commonly used in the power generation industry.

- Conversion to Other Units:
The heat rate can also be expressed in other units such as BTU/kWh or MMBtu/MWh. These units represent the amount of heat energy required to generate one kilowatt-hour or one megawatt-hour of electricity, respectively. However, the unit of Million kcal/hr is widely used in the industry and is convenient for comparing the performance of different power plants.

- Conclusion:
In conclusion, the unit of heat rate curve is Million kcal/hr. This unit represents the rate at which heat energy is being generated or consumed by a power plant. The heat rate curve is an important tool for assessing the efficiency of a power plant and comparing the performance of different plants.

Which is the conventional source of energy?
  • a)
    Solar
  • b)
    Radio-active substances
  • c)
    Geothermal
  • d)
    Wind
Correct answer is option 'B'. Can you explain this answer?

Juhi Joshi answered
Conventional source of energy are: Water, Coal, Gas, radioactive substances. Non-conventional source of energy are: Wind, Solar energy, Geothermal.

Series capacitive compensation on EHV transmission lines is used to
  • a)
    reduce the line loading
  • b)
    improve the protection of the line
  • c)
    reduce the voltage profile
  • d)
    improve the stability of the system
Correct answer is option 'D'. Can you explain this answer?

With series capacitive compensation, the net transfer reactance of the line will be X = (XL - XC) due to which the power transmitted from sending to the receiving end is increased. Thus, it will improve the steady-state stability of the system.

Shunt capacitance in an EHV line is restored to
  • a)
    improve the stability
  • b)
    reduce fault level
  • c)
    improve the voltage
  • d)
    none of the above
Correct answer is option 'C'. Can you explain this answer?

Srestha Gupta answered
Shunt Capacitance in EHV Line

Shunt capacitance is an important parameter of an EHV transmission line. It is the capacitance between the line conductors and the ground.

Importance of Shunt Capacitance

- Shunt capacitance helps in maintaining the voltage level of the transmission line.
- It also helps in reducing the corona losses.
- Shunt capacitance is responsible for the charging current of the transmission line.
- It affects the overall transmission line impedance.

Restoring Shunt Capacitance to Improve Voltage

The shunt capacitance in an EHV line is restored to improve the voltage level of the transmission line. When the shunt capacitance is low, the voltage level of the transmission line decreases. This can lead to power losses and also affect the stability of the system.

By restoring the shunt capacitance, the voltage level can be improved. This is particularly important in long transmission lines where the voltage drop is significant. The shunt capacitance helps in maintaining the voltage level within acceptable limits.

Conclusion

Shunt capacitance is an important parameter of an EHV transmission line. It helps in maintaining the voltage level and reducing corona losses. Restoring shunt capacitance can improve the voltage level of the transmission line and ensure stable operation.

Unit of λ is
  • a)
    Rs/hr
  • b)
    Rs/MW
  • c)
    Rs/MWh
  • d)
    MW/Rs
Correct answer is option 'C'. Can you explain this answer?

Alok Khanna answered
Measurement: A standardized quantity used to express the magnitude or amount of something.

Consider the following statements:
1. AAC (All Aluminium conductors) are universaly employed for feeders as well as distributors.
2. The conductor size for a feeder is mainly governed by the permissible voltage drop in the line.
Which of the above statement(s) is/are true?
  • a)
    1 only
  • b)
    1 and 2
  • c)
    none
  • d)
    2 only
Correct answer is option 'C'. Can you explain this answer?

Isha Singh answered
ACSR conductors are universally employed for distribution systems (feeders as well as distributors), AAC can be used for distribution purpose provided the spans are small.
Hence, statement-1 is false.
The conductor size for a feeder is mainly governed by current carrying capacity and overall economy. Hence, 2 is also a false statement.

HVDC transmission is preferred to EHV AC because
  • a)
    HVDC terminal equipment are inexpensive
  • b)
    VAR compensation is not required in HVDC system
  • c)
    system reliability can be improved
  • d)
    harmonic problem is avoided
Correct answer is option 'C'. Can you explain this answer?

HVDC transmission is more reliable than EHV AC because it uses ground or sea return and therefore, in case of fault on a line it can still be used to supply power with the help of healthy lines.

For a synchronous phase modifier the load angle is
  • a)
    00
  • b)
    250
  • c)
    300
  • d)
    None of these
Correct answer is option 'A'. Can you explain this answer?

Arya Mukherjee answered
Synchronous phase modifier is an overexcited synchronous motor operating on no-load under wide variation of excitation. For no-load operation, δ = 0.

Three synchronous generators, each of which is rated to 100 MVA, 11 KV, have an impedance per unit of 0.15. If all three generators are replaced by a single equivalent generator, what will be the effective per unit impedance of each generator?
  • a)
    0.45
  • b)
    0.15
  • c)
    0.025
  • d)
    0.05
Correct answer is option 'D'. Can you explain this answer?

Mahesh Singh answered
Calculation:

Given data:
- Number of generators: 3
- Rating of each generator: 100 MVA
- Voltage rating: 11 kV
- Impedance per unit: 0.15

To find:
- Effective per unit impedance of each generator when replaced by a single equivalent generator

Step 1: Calculation of equivalent impedance

The total impedance of the three generators connected in parallel can be calculated as follows:

Zeq = Zg / N

Where,
Zeq = Equivalent impedance
Zg = Impedance of a single generator
N = Number of generators

Given that Zg = 0.15, and N = 3, we can substitute these values into the equation:

Zeq = 0.15 / 3
Zeq = 0.05

Therefore, the equivalent impedance of the three generators connected in parallel is 0.05 per unit.

Step 2: Calculation of effective per unit impedance

To find the effective per unit impedance of each generator, we need to divide the equivalent impedance by the number of generators:

Effective per unit impedance = Zeq / N

Given that Zeq = 0.05 and N = 3, we can substitute these values into the equation:

Effective per unit impedance = 0.05 / 3
Effective per unit impedance = 0.0167

Therefore, the effective per unit impedance of each generator when replaced by a single equivalent generator is 0.0167.

Step 3: Comparing the options

a) 0.45 - This is not the correct answer.
b) 0.15 - This is not the correct answer.
c) 0.025 - This is not the correct answer.
d) 0.05 - This is the correct answer.

Therefore, option D, 0.05, is the correct answer.

Conclusion:

When three synchronous generators with an impedance per unit of 0.15 are replaced by a single equivalent generator, the effective per unit impedance of each generator becomes 0.05.

The series impedance matrix of a short three-phase transmission line in phase coordinates is . If the positive sequence impedance is (1 + j 10) Ω, and the zero sequence is (4 + j 31) Ω, then the imaginary part of Zm ­(in Ω) is _______ (up to 2 decimal places).
    Correct answer is '7'. Can you explain this answer?

    Given that, positive sequence impedance (Z1) = (1 + j10) Ω
    Zero sequence impedance (Z0) = (4 + j31) Ω
    We know that, Z1 = Zs – Zm
    and Z0 = Zs + 2Zm
    Z1 = 1 + j10 = Zs – Zm → (1)
    and Z0 = 4 + j31 = Zs + 2Zm → (2)
    from equations (1) and (2)
    ⇒ Zm = 1 + j7
    Imaginary part of Zm = 7 Ω.

    Galvanised steel is generally used as:
    • a)
      stray wire
    • b)
      earth wire
    • c)
      A, B, D
    • d)
      structural components
    Correct answer is option 'C'. Can you explain this answer?

    Sanya Agarwal answered
    • If the wires are made up with some other ordinary steels, rust and corrosion attacked the wire after some time due to the environmental wet conditions.
    • A layer of zinc oxide can be provided on the steel wire by some chemical process.
    • This zinc oxide layer protects the steel wire from the rust and corrosion effects. That’s why galvanized steel is used in many places.

    A station operates with a load factor and plant capacity factor of 0.75, with a maximum demand of 100 MW. The reserve capacity is
    • a)
      100 MW
    • b)
      75 MW
    • c)
      0 MW
    • d)
      133.33 MW
    Correct answer is option 'C'. Can you explain this answer?

    Load Factor and Plant Capacity Factor

    Load factor is the ratio of average load to the maximum demand, and plant capacity factor is the ratio of actual output to the maximum capacity.

    Load factor = (Average load) / (Maximum demand)
    Plant capacity factor = (Actual output) / (Maximum capacity)

    Given load factor and plant capacity factor are 0.75, the maximum demand is 100 MW.

    Reserve Capacity

    Reserve capacity is the amount of extra capacity available to meet sudden increases in demand or unexpected outages. It is calculated by subtracting the maximum demand from the maximum capacity.

    Reserve capacity = Maximum capacity - Maximum demand

    Since the load factor and plant capacity factor are both 0.75, it means that the actual output is 75 MW, which is 75% of the maximum capacity. Therefore, the maximum capacity is:

    Maximum capacity = Actual output / Plant capacity factor
    Maximum capacity = 75 MW / 0.75
    Maximum capacity = 100 MW

    Thus, the reserve capacity is:

    Reserve capacity = Maximum capacity - Maximum demand
    Reserve capacity = 100 MW - 100 MW
    Reserve capacity = 0 MW

    Therefore, the correct answer is option C, which is 0 MW.

    The line current in a three phase unbalanced load are Ia = 4 + j6, Ib = 2 - j2, Ic = -3 + j2, then zero sequence component of current will be
    • a)
      3 + j6
    • b)
      9 + j10
    • c)
      1 + j2
    • d)
      3 - j6
    Correct answer is option 'C'. Can you explain this answer?

    Pooja Patel answered
    Concept:
    The relation between the line currents in terms of the symmetrical components of currents is given below.
    Ia0 = Zero Sequence Component of Current
    Ia1 = Positive Sequence Component of Current
    Ia2 = Negative Sequence Component of Current
    a = 1∠120°, which represents the rotation of 120° in the clockwise direction.
    a2 = 1∠-120° or 1∠240° in anticlockwise direction or clockwise direction, respectively.
    1 + a + a2 = 0
    Calculation:
    Given that
    Ia = 4 + j6, Ib = 2 - j2, Ic = -3 + j2
    Zero sequence component of current,

    ∴ I0 = 1 + j2 A

    There is a problem of high transient over voltages while interrupting
    • a)
      capacitive currents
    • b)
      inductive currents
    • c)
      heavy short circuit current
    • d)
      (a) or (c)
    Correct answer is option 'A'. Can you explain this answer?

    Palak Verma answered
    High transient over voltages occurs in power system due to formation of capacitance across the breaker poles. These are also called switching over voltages.

    For a given power system, its zero and maximum regulation will occur at the impedance angle of
    • a)
      45
    • b)
      90
    • c)
      0
    • d)
      60
    Correct answer is option 'A'. Can you explain this answer?

    Aditya Basu answered
    Zero and maximum regulation in a power system occur at the impedance angle of 45 degrees. This can be explained by considering the nature of reactive power flow in a power system and the relationship between the voltage and current.

    1. Reactive Power Flow:
    Reactive power is the power that oscillates between the generation and consumption points in a power system. It is responsible for maintaining the voltage levels and stability in the system. Reactive power flow depends on the phase difference between the voltage and current.

    2. Voltage and Current Relationship:
    The voltage and current in a power system are related by the impedance angle, which is the phase difference between them. The impedance angle determines the power factor of the system and affects the flow of reactive power.

    3. Regulation:
    Regulation refers to the ability of a power system to maintain a steady voltage level at different load conditions. It is measured as the percentage change in voltage from no load to full load.

    4. Zero Regulation:
    Zero regulation occurs when the impedance angle is 45 degrees. At this angle, the reactive power flow is balanced, and the system is able to maintain a constant voltage level regardless of the load variations. This means that the voltage at the load remains unchanged from no load to full load.

    5. Maximum Regulation:
    Maximum regulation occurs when the impedance angle is different from 45 degrees. At angles other than 45 degrees, the reactive power flow becomes unbalanced, leading to a change in voltage at the load with varying load conditions. This results in a decrease in regulation, as the voltage at the load deviates from the desired level.

    In conclusion, the zero and maximum regulation in a power system occur at the impedance angle of 45 degrees. At this angle, the reactive power flow is balanced, and the system is able to maintain a constant voltage level regardless of the load variations. At angles other than 45 degrees, the reactive power flow becomes unbalanced, leading to a change in voltage at the load and a decrease in regulation.

    Which of the following rotor is used in thermal power plants? 
    • a)
      squirrel cage rotor
    • b)
      salient pole rotor
    • c)
      either of these
    • d)
      cylindrical rotor
    Correct answer is option 'D'. Can you explain this answer?

    Rotor in Thermal Power Plants

    In thermal power plants, the rotor used is the cylindrical rotor.

    Explanation:

    - The cylindrical rotor is a type of rotor used in synchronous generators.
    - It consists of a solid cylinder of magnetic material with slots on its surface to accommodate the field winding.
    - The rotor is mounted on a shaft and rotates inside the stator winding.
    - The stator winding in thermal power plants is made up of a series of coils arranged in a circular pattern around the rotor.
    - When the rotor rotates, the magnetic field created by the field winding on the rotor interacts with the stator winding, inducing an electrical current in the stator winding.
    - This current is then transmitted to the grid through a transformer and power lines.
    - The cylindrical rotor is preferred in thermal power plants because it can handle a large amount of power and is highly efficient.
    - It is also more durable than other types of rotors and can operate at high temperatures without any significant loss of efficiency.

    Conclusion:

    Thus, the cylindrical rotor is the most suitable rotor for use in thermal power plants due to its high efficiency, durability, and ability to handle large amounts of power.

     Which of the following statement is true?
    • a)
      At higher voltage, cost of transmission is increased
    • b)
      At higher voltage, cost of transmission is decreased
    • c)
      efficiency decreased
    • d)
      all of the above
    Correct answer is option 'B'. Can you explain this answer?

    Athul Banerjee answered
    At higher voltage, cost of transmission is decreased.

    Explanation:
    Higher voltage transmission has several advantages over lower voltage transmission. Let's discuss them in detail:

    1. Reduced Current:
    - When the voltage is increased, the current required for the same power transmission decreases.
    - According to Ohm's Law (V = I × R), for the same power (P), if voltage (V) increases, the current (I) decreases.
    - As a result, lower current reduces resistive losses in the transmission lines.

    2. Reduced Resistive Losses:
    - The resistive losses in a transmission line are given by the formula P_loss = I^2 × R, where I is the current and R is the resistance of the line.
    - As discussed earlier, higher voltage transmission reduces the current, which in turn reduces the resistive losses.
    - Lower resistive losses mean that a greater amount of power reaches its destination, resulting in increased efficiency.

    3. Reduced Size of Conductors:
    - As the current decreases with higher voltage transmission, it allows for the use of smaller conductors.
    - Smaller conductors are cheaper and require less material, reducing the overall cost of transmission line construction.

    4. Increased Efficiency:
    - Higher voltage transmission reduces resistive losses and allows for the transmission of more power with the same amount of current.
    - This increased efficiency means that less power is wasted during transmission, resulting in cost savings.

    5. Improved Voltage Regulation:
    - Higher voltage transmission helps to maintain voltage levels within acceptable limits.
    - When power is transmitted over long distances, voltage drops occur due to resistive losses. With higher voltage, the initial voltage at the source is higher, compensating for the voltage drop, and ensuring that the desired voltage is maintained at the load end.

    Conclusion:
    Based on the above explanations, it is clear that at higher voltage, the cost of transmission is decreased. This is mainly due to the reduced current, lower resistive losses, reduced size of conductors, increased efficiency, and improved voltage regulation.

    Which of the following is not the advantage of higher transmission voltage:
    • a)
      Power transfer capability of the transmission line is reduced
    • b)
      Transmission line losses are reduced
    • c)
      Area of cross section and volume of the conductor is reduced
    • d)
      Power transfer capability of the transmission line is increased
    Correct answer is option 'A'. Can you explain this answer?

    Introduction:
    Higher transmission voltage is advantageous in electrical power systems for various reasons. It offers several benefits such as reduced transmission line losses, reduced conductor size, and increased power transfer capability. However, one of the options mentioned, "Power transfer capability of the transmission line is reduced," is not an advantage of higher transmission voltage.

    Explanation:
    Let's analyze each option to understand why option 'A' is the correct answer:

    a) Power transfer capability of the transmission line is reduced:
    This option is not an advantage of higher transmission voltage. In fact, higher transmission voltage leads to increased power transfer capability. It allows for the transmission of more power over longer distances without significant voltage drops. Higher voltage reduces the line losses, enabling the system to deliver more power efficiently.

    b) Transmission line losses are reduced:
    Higher transmission voltage significantly reduces the losses in the transmission lines. According to Ohm's law, power losses in a transmission line are directly proportional to the square of the current passing through it. By increasing the transmission voltage, the current can be reduced for the same power transfer, resulting in lower losses.

    c) Area of cross section and volume of the conductor is reduced:
    With higher transmission voltage, the current required for a given power transfer is reduced. This reduction in current allows for smaller conductor sizes and reduces the cross-sectional area and volume of the conductor. Smaller conductors are cost-effective and easier to install and maintain.

    d) Power transfer capability of the transmission line is increased:
    This option correctly states the advantage of higher transmission voltage. Increased voltage allows for the transmission of more power through the same transmission line. By reducing the current, higher voltages enable the system to transfer more power efficiently, reducing line losses.

    Conclusion:
    In summary, higher transmission voltage offers several advantages in electrical power systems, including reduced transmission line losses, reduced conductor size, and increased power transfer capability. The only option that does not align with the advantages of higher transmission voltage is "Power transfer capability of the transmission line is reduced" (option 'A').

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