Power Systems MCQ Quiz - Objective Question with Answer for Power Systems - Download Free PDF

The correct answer is option 2.

The fundamental principle for electrical installations aligns directly with international safety standards such as IEC 60364 and NFPA 70 (NEC), which emphasize:

• Protection against electric shock
• Overcurrent protection (including overloads and short circuits)
• Protection against fault currents
• Proper grounding and bonding

These are foundational principles to prevent injury, fire, and equipment damage. The other options contradict safety standards and could pose serious hazards if followed.

Biomass

• Biomass is an environmentally-friendly, renewable energy source.
• It is produced when organic matter, such as food or animal waste, is broken down by microorganisms in the absence of oxygen, in a process called anaerobic digestion.
• In a biogas power plant, biomass is burned in a boiler to produce high-pressure steam. This steam flows over a series of turbine blades, causing them to rotate. The rotation of the turbine drives a generator, producing electricity.
• Bio methane is the primary component of biogas, which is produced from the anaerobic digestion of animal waste, organic waste, and agricultural residues.
• Biomass mainly consists of methane (CH4) (50-75%), carbon dioxide (CO2) (25-50%), and traces of other gases.
• Some of the most popular biomass choices are crop residues, municipal and industrial sewage, agricultural material, livestock manures, seaweed, food-processing, and paper wastes.

Explanation
 

• Biomass does not have a high calorific value compared to fossil fuels like coal or petroleum. While it provides energy, its calorific value is lower than conventional fossil fuels.
• The other statements are scientifically valid, as biomass stores solar energy, is renewable, and carbon-neutral in the long term (although combustion releases CO₂, plants absorb CO₂ during their lifecycle).

Hence, option 1 is the correct answer.

Fencing Requirements for Outdoor Substations

• The minimum height of metallic fencing around a transformer in an outdoor substation should be 1.8 meters.
• In high-security areas, the height may be increased to 2.4 meters (8 feet) or more with barbed wire on top.
• Three strands of barbed wire should be installed at a 45-degree outward angle on top of the fence.
• The IEEE Std. 80-2013 recommends grounding every 15-30 meters (50-100 feet) along the fence.

Difference between Indoor and Outdoor Substations:

Corona Effect in Transmission Lines

Corona discharge is a phenomenon in high-voltage transmission lines where ionization of air occurs around conductors, leading to a bluish glow, hissing noise, power loss, and sometimes ozone formation. It happens when the electric field intensity around the conductor exceeds the dielectric strength of air (~30 kV/cm).

The corona loss depends upon:

• Frequency of supply: Corona loss increases as the supply frequency increases.
• Air Pressure: In hilly areas, the corona effect is more dominant due to reduced pressure.
• By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.
• By increasing the spacing between conductors, the voltage at which the corona occurs is raised and hence corona effects can be eliminated.

Statement 2 is NOT true about corona.  The correct statement is: In stormy weather (high humidity, rain, etc.), corona occurs at a lower voltage because moisture increases air conductivity, reducing the breakdown voltage.

Hence, option 2 is the correct answer.

Skin effect

• The skin effect is a phenomenon that occurs in alternating current (AC) systems, where the current tends to flow primarily on the outer surface or "skin" of a conductor, rather than being uniformly distributed throughout its cross-section. This effect increases with the frequency of the AC signal.
• When an AC flows through a conductor, it generates a time-varying magnetic field around the conductor. This varying magnetic field induces eddy currents within the conductor itself, which oppose the original current due to Lenz's Law. As a result, the current is "pushed" towards the outer surface of the conductor.
• The effect increases with higher frequency.
• Induction heating works by inducing eddy currents in a conductive material using high-frequency alternating magnetic fields.
• Due to the skin effect, these eddy currents flow near the surface of the material, generating heat primarily in the outer layers.
• This makes induction heating highly effective for surface heating, hardening, and metal processing.

• Reactance relay is suitable for the protection of a short transmission line because its operation is independent of arc resistance.
• Reactance relay is used for the phase faults in a short transmission line.

Important points:

• The relay which is selected for a long transmission line should be less affected due to power swings. Hence Mho relay is preferred.
• Mho relay is used for the phase faults in a long transmission line.
• Impedance relay is used for the phase faults in the medium transmission line.

Fusing factor:

• The fusing factor is the ratio of minimum fusing current and the current rating of the fuse.
• Minimum fusing current is the minimum value of current due to which fuse melts.
• The current rating of the fuse is the maximum value of current due to which fuse does not get melted.
• The fusing factor of a fuse is always more than 1

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

Important Points:

The differences between transmission and distribution lines are given below.

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

Therefore, the correct answer is: 3) Disc

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

Ratings of circuit breaker for air conditioner of different capacity

ACSR (Aluminium Conductors Steel Reinforced ) conductor:

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

The correct answer is option 3):  Silver

Concept:

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

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

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

• A phase sequence detector protects a motor from receiving wrong voltage levels.
• The phase-sequence detector or phase-sequence checker for a three-phase supply is a device used for testing the three-phase sequence of supply in an electrical circuit or at the input of the electrical motors such as a three-phase induction motor, a  three- phase-energy meter, etc.
• The control of the motor operation happens in such a way that the protection circuit switches off the motor during under-voltage and overload conditions and corrects the phase sequence during phase reversal conditions.
• There are generally two types of the phase sequence detector

1. Static type phase sequence detector

2. Rotating type phase sequence detector

Symbols of switchgear equipment:

1.) Grounding:

2.) Earthing:

3.) Lightning arrester:

4.) ​Circuit breaker:

Oil circuit breaker:

• An oil circuit breaker is a type of circuit breaker that uses insulating oil as a dielectric medium to quench the arc and break the circuit safely. 
• In an oil circuit breaker, the contact of the circuit breaker is made to separate within an insulating oil.
• When the fault occurs in the system, the contacts of the circuit breaker are open under the insulating oil, and an arc is developed between them and the heat of the arc is evaporated in the surrounding oil.
• Due to the heat of the arc gases such as acetylene, methane and hydrogen are liberated into the atmosphere.
• Among the following options, carbon dioxide gas is not liberated during decomposition of oil.