Transmission And Distribution — Part 1 MCQs with Explanations

A bus-bar is rated based on current, voltage, frequency, and short time current . Current: The maximum amount of current the bus-bar can carry without overheating. Voltage: The highest voltage the bus-bar can handle safely without breakdown. Frequency: The frequency of the alternating current (AC) the bus-bar can operate with. Short time current: The maximum short-circuit current the bus-bar can withstand for a brief period without damage. These ratings ensure the bus-bar operates safely and efficiently within its designed parameters.

Voltage regulation in transmission and distribution systems refers to the change in voltage from no-load to full-load conditions. It's usually expressed as a percentage. A lower percentage voltage regulation means there's less voltage drop from no load to full load. This is desirable because it ensures that the voltage at the customer's end remains relatively constant, regardless of the load. Consistent voltage is important for the proper functioning of electrical equipment. Therefore, a lower voltage regulation is considered better. Out of the given options, 2% represents the smallest change in voltage and is thus the best.

For a D.C. 2-wire feeder, the transmitted power is given by P = VI , where V is the supply voltage and I is the current. Since the transmitted power remains constant, if the supply voltage is increased by 100% , the new voltage becomes 2V . Therefore, the current becomes: I' = P / (2V) = I / 2 The weight of copper required in a feeder is proportional to the conductor cross-sectional area, and for a given voltage drop, it is proportional to I² . Hence, when the current is reduced to half, the copper required becomes: (I/2)² = I²/4 This means the new copper requirement is only 25% of the original amount. Therefore, the saving in copper is: 100% − 25% = 75% Thus, by doubling the supply voltage while transmitting the same power, the saving in copper is 75% . Hence, Option C (75 percent) is the correct answer.

This question asks about the voltage drop at the middle of a uniformly loaded DC distributor when it's fed from both ends compared to when it's fed from one end only . Let's break it down: Imagine a wire supplying power to multiple loads evenly spread along its length. One-End Fed: If the power comes from just one end, the farthest point (the other end) will have the lowest voltage. The voltage drop increases as you move away from the feeding point. The maximum voltage drop occurs at the far end , and the voltage drop at the midpoint will be significant. Both-Ends Fed: Now, imagine feeding the same wire from both ends with the same voltage. Each end supplies power to half of the loads. The point with the lowest voltage will now be the middle point, because it is the farthest point from each supply end. However, the voltage drop to the middle point will be much smaller because it's only fed with half of the total current. Here's the key idea for the mathematically inclined: For a distributor fed at one end, the voltage drop is proportional to the square of the distance. For a distributor fed at both ends, with uniform loading, the maximum voltage drop (at the midpoint) is one-fourth of what it would be if it were fed from only one end. Therefore, the correct answer is Option A: One-fourth

The correct answer is B: Circular conductors Here's why: Multi-core cables contain multiple insulated conductors within a single outer sheath. Shape of Conductors : The shape of each individual conductor plays a vital role in the cable’s flexibility and efficiency. Circular Conductors : Generally chosen for multi-core cables because: Flexibility : Circular conductors allow the cable to be bent and flexed more easily without damaging the insulation or conductors. This is essential for applications where the cable needs to be routed around corners or through tight spaces. Ease of Manufacturing : It's easier and more cost-effective to manufacture circular conductors compared to square or rectangular ones. Better Insulation : Circular conductors allow for more uniform insulation around the conductor, reducing the risk of electrical breakdown. Square or Rectangular Conductors : While these shapes might offer slightly better space utilization in some applications, their sharp edges can create stress points in the insulation, leading to premature failure, and they are much less flexible. So, for practicality, ease of manufacturing, flexibility, and insulation reliability, circular conductors are the standard choice in multi-core cables.

The correct answer is Option B: stranded Here's why: Solid conductors are generally too rigid and inflexible for overhead lines. They would be prone to breaking due to vibrations, wind, and temperature changes. Stranded conductors are made up of multiple wires twisted together. This construction provides flexibility, making them less susceptible to damage from external forces. The flexibility also makes them easier to handle and install. Therefore, overhead lines use stranded conductors for better durability and ease of use.

Definition: A substation is a part of the power system used to transform voltage levels, control power flow, and provide protection and switching operations. Equipment in substations: Common equipment includes series capacitors for voltage support, shunt reactors for reactive power control, and voltage transformers for measurement and protection purposes. About exciters: Exciters are devices used to supply DC field current to the rotor winding of synchronous machines such as generators or motors. They are installed in power plants or generator systems, not in substations. Reason: Since substations do not contain generating units, there is no need for exciters, making them absent from substation installations. Conclusion: Among the given options, exciters are not installed in substations.

Tap changing transformers are used to regulate the output voltage by changing the turn ratio. Tappings are typically provided on the high voltage winding rather than the low voltage side. This is because the current in the high voltage winding is lower , making it easier and safer to implement tap changes without handling large currents. Changing taps on the high voltage side also reduces the size and cost of the tap-changing equipment. Option A and Option B are incorrect because although technically possible, it is not common practice to place tappings on the primary or secondary specifically unless they are also the high voltage side. Option D is incorrect because while taps can theoretically be placed on any winding, they are practically and most commonly placed on the high voltage winding . Therefore, Option C: High voltage winding is the correct answer.

The angular displacement between two interconnected stations refers to the phase angle difference between their terminal voltages. While armature reactance and synchronous reactance of alternators do contribute to this displacement , their effects are relatively small compared to the impact of the interconnecting transmission line. The primary cause is the reactance of the interconnector . This reactance causes a voltage drop proportional to the power transmitted, which directly changes the phase angle between the two stations. The higher the interconnector's reactance , the greater the angular displacement for the same amount of transmitted power. Therefore, the main reason is the reactance of the interconnector , making Option B correct.

Option A: 1-phase 3-wire This represents a single-phase electrical system with three wires. It is commonly used for residential and small commercial applications where a split-phase system is required, providing two live conductors and one neutral conductor. Option B: 1-phase 4-wire This is another single-phase system, but it includes an additional ground wire (4 wires in total). It is commonly used for applications where a ground connection is necessary for safety, such as in residential distribution panels. Option C: 3-phase 3-wire This represents a three-phase electrical system with three wires and no neutral conductor. It is used in some industrial and commercial applications where a neutral conductor is not required, and balanced loads are expected across the three phases. Option D: 3-phase 4-wire This is a standard three-phase electrical system with four wires, including three live conductors and one neutral conductor. It is widely used for industrial and commercial applications, providing a neutral for unbalanced loads and additional safety. The option that is not generally used in standard electrical distribution systems is Option C: 3-phase 3-wire . While this configuration may be used in some specific industrial applications, it is less common in comparison to the other options provided. Most three-phase systems include a neutral conductor (Option D) for flexibility and safety. Therefore, Option C: 3-phase 3-wire is the system that is not generally used in common electrical distribution.

Corona losses are minimum : This is not the correct answer. While minimizing corona losses is important for efficient power transmission, it doesn't directly determine the maximum power transmission. Maximum power transmission is influenced by factors such as sending and receiving end voltages. Reactance is high : This is not the correct answer. High reactance can lead to increased voltage drop and reduced power transfer efficiency. Sending end voltage is more : This is the correct answer. The power transmitted in a transmission line is given by the formula \( P = \frac{V_s^2}{Z} \), where \( P \) is power, \( V_s \) is sending end voltage, and \( Z \) is the total impedance of the line. Therefore, increasing the sending end voltage increases the power transmitted. Receiving end voltage is more : This is not the correct answer. While receiving end voltage is important, the question specifically asks about maximizing power transmission, which is influenced by the sending end voltage. Therefore, in the context of the question, the power transmitted will be maximum when the Sending end voltage is more (Option C).

Booster transformer is the correct answer to “Which of the following equipment, for regulating the voltage in distribution feeder, will be most economical”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Adjusting the governor is the correct answer to “The frequency of voltage generated, in case of generators, can be increased by”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Increase the tensile strength is the correct answer to “In aluminium conductors, steel core is provided to”. This question is standard in competitive examination syllabi for Transmission And Distribution.

400 kV is the correct answer to “Overhead system can be designed for operation up to”. This question is standard in competitive examination syllabi for Transmission And Distribution.

33 kV is the correct answer to “Pin type insulators are generally not used for voltages beyond”. This question is standard in competitive examination syllabi for Transmission And Distribution.

8.8 kV is the correct answer to “Which of the following is usually not the generating voltage”. This question is standard in competitive examination syllabi for Transmission And Distribution.

The stations will fall out of step because of large angular displacement between the stations is the correct answer to “When the interconnector between two stations has large reactance”. This question is standard in competitive examination syllabi for Transmission And Distribution.

R.C.C. poles is the correct answer to “Distribution lines in India generally use”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Secondary distribution is the correct answer to “A 3-phase 4-wire system is commonly used on”. This question is standard in competitive examination syllabi for Transmission And Distribution.

A.C.S.R. conductors is the correct answer to “Overhead lines generally use”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Both (A) and (B) is the correct answer to “By which of the following systems electric power may be transmitted”. This question is standard in competitive examination syllabi for Transmission And Distribution.

1.1 kV is the correct answer to “Low voltage cables are meant for use up to”. This question is standard in competitive examination syllabi for Transmission And Distribution.

2.7 is the correct answer to “Aluminium has a specific gravity of”. This question is standard in competitive examination syllabi for Transmission And Distribution.

No insulation is required is the correct answer to “In AC.S.R. Conductors, the insulation between aluminium and steel conductors is”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Non-sinusoidal is the correct answer to “The current drawn by the line due to corona losses is”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Service mains is the correct answer to “are the conductors, which connect the consumer's terminals to the distribution”. This question is standard in competitive examination syllabi for Transmission And Distribution.

All of the above is the correct answer to “Which of the following are the constants of the transmission lines”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Any of the above is the correct answer to “Which of the following materials is used for overhead transmission lines”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Mho's relay is the correct answer to “Which of the following relays is used on long transmission lines”. This question is standard in competitive examination syllabi for Transmission And Distribution.

All of the above is the correct answer to “Which of the following methods is used for laying of underground cables”. This question is standard in competitive examination syllabi for Transmission And Distribution.

7.0 m is the correct answer to “The minimum clearance between the ground and a 220 kV line is about”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Series wound generator is the correct answer to “A booster is a”. This question is standard in competitive examination syllabi for Transmission And Distribution.

All of the above is the correct answer to “The effect of corona can be detected by”. This question is standard in competitive examination syllabi for Transmission And Distribution.

balancer connected to lightly- loaded side runs as a motor is the correct answer to “In a D.C. 3-wire distributor using balancers and having unequal loads on the two sides”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Suspension insulators is the correct answer to “High voltage transmission lines use”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Increase of short-circuit current of the system is the correct answer to “Which of the following is the demerit of a 'constant voltage transmission system'”. This question is standard in competitive examination syllabi for Transmission And Distribution.

All of the above is the correct answer to “Due to which of the following reasons the cables should not be operated too hot”. This question is standard in competitive examination syllabi for Transmission And Distribution.

Remain unchanged is the correct answer to “When an alternator connected to the bus-bar is shut down the bus-bar voltage will”. This question is standard in competitive examination syllabi for Transmission And Distribution.

All of the above is the correct answer to “The corona is considerably affected by which of the following”. This question is standard in competitive examination syllabi for Transmission And Distribution.