Transmission and Distribution MCQ Quiz - Objective Question with Answer for Transmission and Distribution - Download Free PDF

Explanation:

Underground vs. Overhead Power Systems

Definition: Power distribution systems can be broadly classified into overhead and underground systems. An overhead system involves power lines that are installed above ground, typically using poles or towers. An underground system, on the other hand, involves cables that are laid below the surface of the ground. Each system has its own advantages and disadvantages, and the choice between them depends on factors such as cost, safety, reliability, and aesthetics.

Correct Option Analysis:

The correct option for the given question is:

Option 4: Tapping for loads and service mains is easier.

This option is NOT an advantage of the underground system over the overhead system. In fact, this is one of the main disadvantages of underground systems. The process of tapping into underground cables for additional loads or service mains is significantly more challenging compared to overhead systems. Here's why:

• Complex Installation: Underground cables are buried beneath the surface, often encased in protective conduits or ducts. To tap into these cables, excavation work is required, which can be time-consuming, labor-intensive, and costly.
• Specialized Equipment: Unlike overhead lines, where tapping can be done relatively easily using basic tools, underground systems require specialized equipment and skilled personnel to access and modify the cables.
• Disruption: Excavation work for tapping into underground cables can cause significant disruption to traffic, pedestrians, and the surrounding environment. It may also require obtaining permits from local authorities.
• Risk of Damage: During the process of tapping, there is a risk of damaging the existing underground cables, which can lead to power outages and expensive repairs.

In contrast, tapping for loads and service mains is much easier in overhead systems. The lines are easily accessible, and modifications can be made without the need for excavation or specialized equipment. This accessibility is one of the key advantages of overhead systems over underground systems.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Reduced voltage drops.

This is an advantage of underground systems over overhead systems. Underground cables are typically shorter and have better insulation, which reduces resistance and minimizes voltage drops. Additionally, underground systems are less affected by environmental factors such as temperature and weather conditions, which can impact the resistance and efficiency of overhead lines.

Option 2: Better aesthetics and safety.

Underground systems offer significant advantages in terms of aesthetics and safety. Since the cables are buried, they do not obstruct the visual landscape, making them ideal for urban areas, residential neighborhoods, and locations with strict aesthetic requirements. From a safety perspective, underground systems eliminate the risk of accidental contact with live wires and are less prone to damage from storms, falling trees, or other external factors.

Option 3: Lower chances of power failures.

Underground systems are generally more reliable than overhead systems. They are less susceptible to power failures caused by environmental factors such as high winds, lightning, and snowstorms. Additionally, underground cables are protected from physical damage caused by animals or human activities, further reducing the likelihood of power outages.

Option 4: Tapping for loads and service mains is easier.

As previously explained, this is NOT an advantage of underground systems. Tapping into underground cables is more complex, costly, and disruptive compared to overhead systems.

Conclusion:

While underground power systems offer numerous advantages, such as reduced voltage drops, better aesthetics, enhanced safety, and lower chances of power failures, they are not without their drawbacks. One of the significant disadvantages is the difficulty in tapping for loads and service mains, which is why Option 4 is the correct answer to the question. Understanding the trade-offs between underground and overhead systems is crucial for making informed decisions in power distribution planning and infrastructure development.

Explanation:

Voltage Drop in Residential Areas

Definition: Voltage drop refers to the reduction in voltage levels across electrical systems due to the resistance of conductors and other components. It occurs when electrical current flows through a circuit, causing energy loss as heat. In residential areas, voltage drop can be caused by various factors, such as high power consumption, long wiring distances, or inadequate electrical infrastructure.

Understanding Statement (I): High voltage drop in residential areas can cause lights to dim or flicker.

This statement is accurate and true. When there is a significant voltage drop in the electrical system of a residential area, the available voltage for appliances decreases. Devices such as lights, which rely on stable voltage, can exhibit behavior such as dimming or flickering due to insufficient power supply. This is a common symptom of voltage drop, especially during peak power consumption periods or in circuits with long wiring runs.

Explanation:

• Lights are designed to operate within a specific voltage range. When the voltage drops below this range, the brightness of the lights diminishes, leading to dimming.
• Flickering occurs when the voltage fluctuates due to intermittent or unstable power supply, often caused by sudden changes in electrical load or faulty connections.

Statement (I) is therefore correct as it accurately describes the effects of voltage drop on lighting systems in residential areas.

Understanding Statement (II): Voltage drop occurs due to excessive power consumption in a household.

This statement is incorrect. While excessive power consumption can strain the electrical system and lead to other issues, it is not the direct cause of voltage drop. Voltage drop primarily occurs due to the resistance of electrical conductors and components in the circuit. Factors contributing to voltage drop include:

• Long distances between the power source and the load, which increase resistance.
• Undersized wiring or inadequate conductor gauge that cannot efficiently carry the required current.
• Poor connections or aging infrastructure that add resistance to the circuit.

Excessive power consumption may exacerbate voltage drop but is not the root cause. Therefore, Statement (II) is incorrect.

Correct Option Analysis:

The correct option is:

Option 3: Only Statement (I) is true.

This option correctly identifies that high voltage drop in residential areas can cause lights to dim or flicker, as described in Statement (I). Statement (II), however, is not true because voltage drop results from resistance in electrical systems rather than excessive household power consumption.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Only Statement (II) is true.

This option is incorrect because Statement (II) is not true. Voltage drop is caused by resistance in the electrical system, not excessive power consumption. While high power usage may amplify the effects of voltage drop, it is not the primary cause.

Option 2: Neither Statement (I) nor Statement (II) is true.

This option is incorrect because Statement (I) is true. High voltage drop in residential areas does cause lights to dim or flicker. However, Statement (II) is false, as explained earlier.

Option 4: Both Statement (I) and Statement (II) are true.

This option is incorrect because while Statement (I) is true, Statement (II) is false. Voltage drop is not directly caused by excessive power consumption; it is primarily a result of resistance in the electrical system.

Conclusion:

Voltage drop is a common phenomenon in electrical systems and can lead to noticeable effects in residential areas, such as dimming or flickering lights. While excessive power consumption may highlight the presence of voltage drop, it is not the direct cause. Understanding the distinction between causes and symptoms is essential for diagnosing and addressing electrical issues effectively. Statement (I) correctly identifies the effects of voltage drop, making Option 3 the correct choice.

Ring Main System

Definition: A ring main system is an electrical distribution system in which the distribution network forms a closed loop or ring. This system enables power to be supplied to consumers from two directions, which enhances the reliability and flexibility of the power supply. Ring main systems are commonly used in urban and industrial areas where power supply reliability is critical.

Working Principle: In a ring main system, power is distributed through a closed loop of cables or conductors. The system is designed so that each consumer or load point can receive power from two directions. If there is a fault or maintenance requirement in one section of the ring, power can still be supplied from the other direction, minimizing interruptions in the power supply.

Advantages:

• Improved Reliability: Power supply is maintained even if a fault occurs in one section of the network, as the load can be fed from the opposite direction.
• Flexibility: The ring structure allows for easy addition of new load points or modifications to the system without significant disruption.
• Reduced Voltage Drop: Power being supplied from two directions reduces the overall voltage drop, ensuring better quality of supply.
• Efficient Load Distribution: The ring main system helps in balancing the load across the network.

Applications: Ring main systems are commonly used in urban distribution networks, industrial plants, and commercial complexes where reliability and flexibility are essential.

Correct Option Analysis:

The correct option is:

Option 2: It allows power to be supplied from two directions.

This option correctly explains the primary advantage of a ring main system. By forming a closed loop, the system ensures that power can be fed to any load point from two directions. This feature significantly enhances the reliability of the power supply because, in the event of a fault or maintenance activity in one part of the network, the other part can continue to supply power. This dual feeding arrangement is the hallmark of the ring main system and is particularly beneficial in critical applications where uninterrupted power is a priority.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: It reduces the number of distributors.

This option is incorrect. While a ring main system may reduce the complexity of the network to some extent, its primary advantage does not lie in reducing the number of distributors. Instead, the focus is on enhancing reliability and flexibility through the dual feeding arrangement.

Option 3: It reduces voltage drop in the system.

This statement is partially true but not the primary reason for improved reliability. The ring main system can reduce voltage drop because power is supplied from two directions, thereby balancing the load. However, this is a secondary benefit and not the main feature that enhances system reliability.

Option 4: It eliminates the need for a transformer.

This option is incorrect. The ring main system does not eliminate the need for transformers. Transformers are still required to step down or step up voltage levels as needed within the distribution network. The ring main system is concerned with the configuration of the distribution network, not the elimination of transformers.

Conclusion:

The ring main system is a robust and reliable electrical distribution configuration that ensures uninterrupted power supply to consumers. Its ability to supply power from two directions is the key feature that improves reliability, making it suitable for critical applications. While the system offers additional benefits such as reduced voltage drop and flexibility, these are secondary to its primary function of enhancing supply reliability. Understanding the advantages and limitations of the ring main system is essential for designing efficient and dependable electrical distribution networks.

Explanation:

Key Advantage of Voltage Drop in Radial Power Distribution System:

Definition: Voltage drop in a radial power distribution system refers to the reduction in voltage as electrical energy flows through the network, due to the resistance and reactance of the conductors. While excessive voltage drop can be detrimental, controlled voltage drop has certain advantages in power distribution systems.

Correct Option: Option 4: It limits excessive current flow, aiding in overcurrent protection.

Detailed Explanation:

Voltage drop plays a critical role in radial power distribution systems by influencing the current flow within the network. When electrical energy is transmitted over long distances, the resistance and impedance of the conductors cause a gradual reduction in voltage. This phenomenon has several implications, one of which is aiding in overcurrent protection.

Radial power distribution systems are designed to supply electricity from a single source to multiple endpoints. These systems often experience varying loads at different points along the distribution line. The inherent resistance and reactance of the conductors create a natural limitation on the amount of current that can flow through the system. This limitation becomes an advantage in the following ways:

• Prevention of Excessive Current Flow: Voltage drop reduces the potential difference available for current flow. As per Ohm's Law (I = V/R), a lower voltage results in a lower current for a given resistance. This natural limitation helps prevent excessive current flow, which can otherwise lead to overheating, damage to equipment, or even fire hazards.
• Aid in Overcurrent Protection: Overcurrent protection devices like circuit breakers and fuses are designed to trip or blow when current exceeds a safe limit. Controlled voltage drop ensures that the current remains within manageable levels, reducing the likelihood of overloading these protective devices. This contributes to the safety and reliability of the power distribution system.
• Enhanced System Stability: By limiting excessive current flow, voltage drop helps maintain the stability of the electrical network. Uncontrolled high currents can lead to voltage sags, equipment malfunctions, and instability in the power system.

While voltage drop is often viewed as a challenge to be minimized, its role in limiting excessive current flow and aiding in overcurrent protection highlights its importance in ensuring the safety and efficiency of radial power distribution systems. Engineers carefully design distribution networks to balance the need for minimizing voltage drop with its benefits in current limitation.

Important Information:

To further understand the analysis, let’s evaluate the other options:

Option 1: It increases power dissipation in the form of heat.

This option is partially correct but does not highlight the key advantage of voltage drop. While voltage drop does lead to power dissipation in the form of heat due to the resistance of the conductors, this is generally considered a disadvantage rather than an advantage. Excessive heat generation can lead to energy losses, reduced efficiency, and potential damage to the distribution infrastructure. Thus, this option does not align with the benefits of voltage drop.

Option 2: It ensures voltage remains constant throughout the network.

This statement is incorrect. Voltage drop inherently causes a reduction in voltage as electrical energy flows through the network. Ensuring constant voltage throughout the system requires additional equipment such as voltage regulators or compensators. Voltage drop does not contribute to maintaining constant voltage; instead, it represents a deviation from the ideal scenario where voltage remains uniform across the network.

Option 3: It eliminates the need for voltage regulators.

This option is incorrect. Voltage regulators are specifically used to counteract the effects of voltage drop by maintaining a stable voltage at different points in the network. Voltage drop does not eliminate the need for voltage regulators; instead, it necessitates their use to ensure consistent voltage levels for end-users. This option misrepresents the role of voltage drop in power distribution systems.

Option 5: (Option missing)

The fifth option is not provided in the question, so it cannot be analyzed. However, the correct answer remains option 4, as explained above.

Additional Information:

Conclusion:

Voltage drop in a radial power distribution system has a key advantage in limiting excessive current flow, which aids in overcurrent protection. This phenomenon contributes to the safety and reliability of the electrical network by preventing overheating, equipment damage, and fire hazards. While voltage drop has other implications, such as energy losses and the need for voltage regulation, its role in current limitation underscores its importance in power system design and operation. Engineers carefully manage voltage drop to balance its advantages and disadvantages, ensuring optimal performance and safety in radial power distribution systems.

Explanation:

Transmission Line Sag Calculation:

Definition: Sag in a transmission line refers to the vertical distance between the highest point of the conductor (at the support) and the lowest point of the conductor (mid-span). Proper sag calculation is essential to ensure the mechanical stability and safe operation of the transmission line.

Given Data:

• Span length (L) = 300 metres
• Conductor weight (w) = 1.2 N/m
• Tension in the conductor (T) = 6000 N

Formula for Sag Calculation:

The sag for a transmission line conductor can be calculated using the following formula:

Sag (S) = (w × L²) / (8 × T)

Where:

• w = weight of the conductor per unit length
• L = span length of the conductor
• T = tension in the conductor
• S = sag

Step-by-Step Solution:

Substitute the given values into the formula:

S = (1.2 × 300²) / (8 × 6000)

First, calculate the numerator:

Numerator = 1.2 × 300² = 1.2 × 90000 = 108000

Next, calculate the denominator:

Denominator = 8 × 6000 = 48000

Now, divide the numerator by the denominator:

Sag (S) = 108000 / 48000 = 2.25 metres

Final Answer:

The sag of the transmission line conductor is 2.25 metres.

Correct Option Analysis:

The correct answer is Option 2, which states that the sag is 2.25 metres. This calculation is accurate based on the formula and the given values.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: 6 metres

This option is incorrect. If we calculate the sag using the formula provided, the result is 2.25 metres, not 6 metres. The value of 6 metres might be applicable for a different set of parameters, but it does not correspond to the given data.

Option 3: 5 metres

This option is also incorrect. Using the formula and the given data, the sag is calculated as 2.25 metres. The value of 5 metres is significantly higher and does not match the calculation.

Option 4: 1 metre

This option is incorrect. The sag of 1 metre is too low compared to the calculated value of 2.25 metres. This discrepancy indicates that the tension in the conductor and the weight of the conductor have not been correctly accounted for if this value were to be chosen.

Conclusion:

Understanding the sag calculation is crucial for designing and maintaining transmission lines. The correct sag ensures that the conductor remains mechanically stable and operates safely under various conditions. The calculated sag value of 2.25 metres is accurate and matches the given data, making Option 2 the correct choice. Proper sag calculation also helps prevent excessive stress on the conductor and minimizes the risk of contact with ground or nearby objects due to sagging.

The minimum clearance distance that equipment should be kept away from power lines of different voltage levels is shown in below table.

Transmission lines are classified based on three criteria.

a) Length of transmission line

b) Operating voltage

c) Effect of capacitance

The table below summarizes the classification of transmission lines.

The following types of cables are generally used for 3-phase service:

1. Belted cables - up to 11 kV

2. Screened cables - from 22 kV to 66 kV

3. Pressure cables - beyond 66 kV

Belted cables:

• These cables are used for voltages up to 11 kV but in extraordinary cases, their use may be extended up to 22 kV
• The belted type construction is suitable only for low and medium voltages as the electrostatic stresses developed in the cables for these voltages are more or less radial i.e., across the insulation
• For high voltages (beyond 22 kV), the tangential stresses also become important
• These stresses act along the layers of paper insulation
• As the insulation resistance of paper is quite small along the layers, therefore, tangential stresses set up leakage current along the layers of paper insulation
• The leakage current causes local heating, resulting in the risk of breakdown of insulation at any moment

Dielectric Strength:

It reflects the electric strength of insulating materials at various power frequencies.

It is the voltage per unit thickness at which a material will conduct electricity.

Concept:

Real power for single-phase P = VI cosθ

Reactive power for single-phase Q = VI sinθ

For purely capacitive circuit Q = v²ω c

Note: To suppress the inductive effect of load, we add purely capacitive load across load.

Calculation:

Given V = 250 volt

I = 50 amp

θ = 30°

ω = 2 × π × 50 = 314

Reactive power = 250 × 50 × sin30° = 6250 VAr

For unity power factor, we add capacitance across load

V²ωC = 6250 VAr

⇒ 250² × 314 × C = 6250 VAr

Concept:

• Pin insulators are used for holding the line conductors on the straight running of poles. These are commonly used in power networks up to 33 kV system.
• Suspension insulators consist of a number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross- arm of the tower. For high voltage (>33KV), it is a usual practice to use suspension type insulators.
• When there is a dead-end of the line or there is a corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used.
• For low voltage lines (<11 kV) shackle insulators are used as strain insulators.
• Stay insulators are also known as strain insulators and are generally used up to 33 kV line. These insulators should not be fixed below three meters from the ground level. These insulators are also used where the lines are strained.

• The phenomenon arising due to unequal distribution of current over the entire cross-section of the conductor is referred to as the skin effect.
• Such a phenomenon does not have much role to play in case of a very short transmission line, but with an increase in the effective length of the conductors, skin effect increases considerably.
• The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
• But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current.

	The cross-sectional area of a round conductor available for conducting DC current. (DC resistance)
	The cross-sectional area of the same conductor available for conducting low-frequency AC (AC resistance)
	The cross-sectional area of the same conductor available for conducting high-frequency AC (AC resistance)

Factors affecting skin effect in transmission lines are:

• Frequency – The skin effect increases with the increase in frequency.
• Diameter – It increases with the increase in the diameter of the conductor.
• The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
• Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).

Important Points:

• The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than the 1cm.
• In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
• The concentration of current near the surface enabled the use of an ACSR conductor.

Classification of underground cables on the basis of voltage level is given below

• During the no-load condition, the current flowing is only charging current due to line capacitance. It increases the capacitive var in the system.
• Since the line is under no load the line inductance will be less. Therefore, the capacitive var becomes greater than inductive var during no load or light load condition.
• Due to this phenomenon, the receiving end voltage becomes greater than the sending end voltage. This effect is also called the Ferranti effect.