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

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.

Secondary Distribution System

Definition: The secondary distribution system is the part of the electric power distribution network that delivers electricity from local transformers to the end-users or consumers. It is the final stage of the electricity supply chain, ensuring that power reaches homes, businesses, and other establishments at the appropriate voltage level for consumption.

Working Principle: In a secondary distribution system, electricity is received from the primary distribution system through a step-down transformer, which reduces the voltage to a level suitable for end-use (typically 230V for single-phase or 400V for three-phase systems). This reduced voltage power is then distributed to consumers through a network of distribution lines, poles, and cables.

The system comprises distribution boards, feeders, service mains, and consumer connections. The feeders carry electricity from the transformer to distribution boards, from where it is further distributed to individual consumers via service mains. The key objective of the secondary distribution system is to ensure a reliable and efficient supply of electricity to consumers.

Advantages:

• Delivers electricity at a consumer-friendly voltage level, ensuring safety and compatibility with household and commercial appliances.
• Allows for efficient distribution of power to a large number of end-users within a localized area.
• Facilitates the use of modern metering and monitoring technologies to improve energy management and billing accuracy.

Disadvantages:

• Requires extensive infrastructure, including poles, cables, and transformers, which can lead to high installation and maintenance costs.
• Vulnerable to power losses due to resistance in the distribution lines and potential faults or failures in the network.

Applications: The secondary distribution system is used in urban, suburban, and rural areas to provide electricity to residential, commercial, and industrial consumers. It is a critical component of the power distribution network, ensuring that electricity reaches the end-user efficiently and reliably.

Correct Option Analysis:

The correct option is:

Option 4: Secondary distribution system

The secondary distribution system is specifically responsible for delivering electricity from local transformers to end-users. After the primary distribution system transmits power to local substations or transformers, the secondary distribution system takes over, stepping down the voltage and distributing it to individual consumers. This system directly connects with homes, offices, and other establishments, ensuring that they receive electricity at a usable voltage level.

Additional Information

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

Option 1: Generation system

The generation system is the initial stage of the electricity supply chain, where electrical energy is produced in power plants using various energy sources such as coal, natural gas, nuclear, hydro, wind, or solar energy. While the generation system is responsible for producing electricity, it is not involved in delivering power to end-users. Instead, it supplies power to the transmission system for further distribution.

Option 2: Primary distribution system

The primary distribution system is an intermediate stage between the transmission system and the secondary distribution system. It distributes electricity from substations to local transformers at medium voltage levels (usually between 11kV and 33kV). Although it plays a crucial role in the distribution network, it does not directly deliver electricity to end-users. Its primary function is to serve as a link between the transmission network and the secondary distribution system.

Option 3: Transmission system

The transmission system is responsible for transporting electricity over long distances from power plants to substations at high voltage levels (typically ranging from 110kV to 765kV). The high voltage minimizes power losses during transmission. However, the transmission system does not deliver electricity directly to end-users. Instead, it feeds electricity into the primary distribution system for further distribution.

Option 5: Not Applicable

This option is irrelevant to the question as the correct answer lies within the specified options (1 to 4). The secondary distribution system is the appropriate choice for delivering electricity from local transformers to end-users.

Conclusion:

The secondary distribution system plays a vital role in the electricity supply chain by ensuring that power reaches consumers at a usable voltage level. It is the final stage of the distribution network, connecting local transformers to individual homes, businesses, and other establishments. While other systems such as generation, transmission, and primary distribution are essential for producing and transporting electricity, they do not directly deliver power to end-users. Understanding the functions of each system helps in appreciating the complexity and efficiency of modern power distribution networks.

Radial distribution system

• A radial distribution system is a type of electrical power distribution system where each consumer is connected to a single power source or feeder.
• This configuration is simple and cost-effective, but it has a disadvantage in terms of reliability, as a fault in the feeder can result in loss of power to all connected consumers.

Radial layout with high voltage drop is unsuitable for industrial loads because:

• Radial layouts can experience significant voltage drops at points farther from the source, especially under high load conditions.
• Industrial loads (like motors, PLCs, automation systems) are sensitive to voltage variations—fluctuations can cause inefficiencies, overheating, or even equipment malfunction.
• Therefore, radial systems with high voltage drops are not preferred for such loads; more reliable layouts like ring or mesh systems are often used instead.

Three Wire DC Distribution Systems:

• It consists of two outer wires and a middle or neutral wire which is earthed at the substation.
• Availability of two voltages in a 3-wire system is preferred over the 2-wire system for d.c. distribution.
• The voltage between the outers is twice the voltage between the outer and neutral wire
• The principal advantage of this system is that it makes available two voltages at the consumer terminals.
• V volts between any outer and neutral and 2V volts between the outers.

This system leaves the following connection choices to a consumer:

• Between positive conductor and neutral (V volt)
• Between negative conductor and neutral  (V volt)
• Between the positive and negative conductor  (2V volt)

Indian electricity rules:

• All-electric supply lines and apparatus shall be sufficient in power and size of sufficient mechanical strength for the work they may be required to do, and, so far as is shall be constructed, installed, protected, worked, and maintained under the standards of the Indian Standards Institution to prevent danger.
• For carrying the overhead line, wooden poles, concrete poles, steel poles, and rail electric poles are used.
• Which poles are to be used, depends on the importance of load, location, and place, the cost-effectiveness of such construction, including maintenance cost, keeping its profit element in mind.

Wooden poles:

• In the earlier period, wooden poles are used for 400 volts and 230 volts L.T. line and II.K.V.H.T. line in a massive way.
• In some casess wooden poles are used for33 kV line.
• The cost-effectiveness of a wooden pole is much less in comparison to other electric poles and the expenditure incurred for its foundation is also comparatively very less.
• If proper maintenance and treatment are done on the wood, the wooden pole is lost for a long period.
• The recommended span in the case of wooden poles is 40 – 50 meters.
• The breakdown force is between 450 kg / cm² and / above 850 kg / cm².

Note:

Span: Span means the horizontal distance between two adjacent supporting points of an overhead conductor.

Normal span in meters \(= C\sqrt {\frac{{P - L}}{D}}\) 

Where,

P = height of conductor support for which the normal span is to be calculated (meter)

L = conductor clearance above level ground (meter)

C = ruling span (meter)

D = conductor sag for ruling span C (meter)

As per Indian Electricity rule in overhead systems, the recommended span of various support poles is given below

Single line diagram of power system

The primary distribution is carried out by a 3-phase 3-wire system.

Primary distribution handles large consumers such as factories and industries which need voltage in kV which is possible only 3-phase 3-wire system.

If it is carried out with 1ϕ, then there are chances of voltage drop.

Three-phase system has the following advantages as compared to the single-phase system:

• Low cost of Machinery: The power to weight ratio of a 3-ϕ alternator, three-phase induction motor and three-phase transformers is high as compared to the 1-ϕ alternator, single-phase induction motor and single-phase transformer respectively. That means for the generation of the same amount of electric power, the size of a 3-ϕ alternator is small as compared to a 1-ϕ alternator.
• Required less amount of conductor material: For electric power transmission and distribution of the same amount of power, the requirement of conductor material is less in the 3-ϕ system as compare to the 1-ϕ system.
• Vibration-free operation: The instantaneous power is almost constant over the cycle results in a smooth and vibration-free operation of the machine. Whereas in the 1-ϕ system the instantaneous power is pulsating hence change over the cycle, which leads to vibrations in machines.
• Better power factor and efficiency: Three-phase motor has better power factor and efficiency as compared to the 1-ϕ motor.
• Reliability: If a fault occurs in any winding of a 3-phase transformer, the rest of the two winding can be used in the open delta to serve the 3-phase load. The same is not possible in the 1-ϕ transformer. This ability of 3-phase transformer further increases the reliability of the 3-phase transformer.
• A 3-phase system can be used to feed a 1-ϕ load, whereas vice-versa is not possible.

The supply frequency of a single-phase AC system in India is 50 Hz.

Concept:

In a uniformly loaded distributor fed at one end, the maximum total voltage drop = IR/2

In a uniformly loaded distributor fed at both ends, the maximum total voltage drop = IR/8

The maximum voltage drop in the case of uniformly loaded distributor fed at both ends is one-fourth of the maximum voltage drop in the case of uniformly loaded distributor fed at one end.

Calculation:

Length of distributor = 200 m = 0.2 km

Current supplied by distributor = 2 amperes/meter

Total current supplied by distributor (I) = 200 × 2 = 400 A

The resistance of single wire = 0.3 Ω/km

Total resistance = 0.3 × 0.2 = 0.06 Ω

For Two wire, R = 0.06 × 2 Ω

Maximum voltage drop \( = \frac{{IR}}{2} = \frac{1}{2} × 400 × 0.06\times 2 = 24\;V\)

• Feeders are the conductors which have a large current carrying capacity
• The feeders connect the substation to the area where power is to be finally distributed to the consumers.
• It feeds to power end distributor
• No tapings are taken from the feeders
• The feeder current always remains constant
• The voltage drop along the feeder is compensated by compounding the generator

Primary transmission :

The electric supply (132 kV, 220 kV, 500 kV or greater) is transmitted to load center by three-phase three-wire (3 phase - 3 wires) overhead transmission system.

Secondary transmission :

At the receiving station, the level of voltage reduced by step-down transformers up to 132 kV, 66 or 33 kV and electric power is transmitted by three-phase three-wire (3 phase - 3 wires) overhead system to different substations.

Primary distribution :

At a substation, the level of secondary transmission voltage (132KV, 66 or 33KV) is reduced to 11 kV (in a three-phase three-wire overhead system) by step down transformers.

Secondary distribution :

• Electric power is given to (from primary distribution line (i.e.) 11 kV) distribution substation.
• This substation is located nearby consumers area where the level of voltage reduced by step down transformers is 415 V.
• In a 3 phase four wire system (3 phase - 4 wires), there are 415 volts (Three-phase supply system) between any two phases and 230 volts (single-phase supply) between neutral and any one of the phases (lives) wire.
• Residential load (i.e. Fans, light, and TV, etc) may be connected between any one phase and neutral wires, while three-phase loads may be connected directly to the three-phase lines.
• A three-phase 415V, supply is used for supplying small industrial and commercial loads such as garages, schools, and blocks of flats. A single-phase 230 V supply is usually provided for individual domestic consumers.

Concept of Uniformly Loaded Distributor Fed at One End:

Fig shows the single line diagram of a 2-wire DC distributor A B fed at one end A and loaded uniformly with i amperes per metre length.

• It means that at every 1 m length of the distributor, the load tapped is i amperes. Let l metres be the length of the distributor and r ohm be the resistance per metre run.
• Consider a point C on the distributor at a distance x metres from the feeding point A as shown in Fig.
• The current at point C is = (il − ix) A = i (l − x) A
   

Now, consider a small length dx near point C.

Its resistance is r dx and the voltage drop (dv) over length dx is given by,

dv = i (l − x) r dx = i r(l − x) dx

The total voltage drop (V) in distributor up to point C is given by,

V = \(\int_0^x ir(l-x)dx=ir(lx-\frac{x^2}{2})\)

Application:

We have,

Current loading (i) = 2 A/m

Resistance of distributor per metre run (r) = 2 × 0·3/1000 = 0·0006 Ω

Length of distributor (l) = 200 m

Now, Voltage drop upto a distance x metres from feeding point (V) is given by,

V = \(ir(lx-\frac{x^2}{2})\)

For, x = 150 m

Voltage Drop = \(2\times 0.0006(200\times 150 - \frac{150^2}{2})=22.5 \ V\)

Concept:

In a uniformly loaded distributor fed at one end, the maximum total voltage drop = IR/2

In a uniformly loaded distributor fed at both ends, the maximum total voltage drop = IR/8

The maximum voltage drop in case of uniformly loaded distributor fed at both ends is one fourth of the maximum voltage drop in case of uniformly loaded distributor fed at one end.

Calculation:

For a uniformly loaded dc distribution fed at one end, the maximum voltage drop = 10 V

If it is fed at both ends at the same voltage, the maximum voltage drop will be one fourth of 10 V.

= 0.25 × 10 = 2.5 V

Concept:

On the basis of how DC distributors are fed by the feeders, they are classified as:

→ Distributor fed at one end

→ Distributor fed at both ends

→ Distributor fed at the centre.

→ Ring distributor.

Now the type of distribution given in the question is of type “Distributor fed at one end”.

• In this type of feeding, the distributor is connected to the supply at one end and loads are taken at different points along the length of the distributor.
• In the above figure end P is also called singly fed distributor and loads I₁, I₂ and I₃ tapped off at points Q, R, S respectively.

Points to remember in this type of distribution:

1) The current in the various sections of the distributor away from the feeding point goes on decreasing. Thus the current in the section PQ is more than current in the section QR and the current in the section QR is more than current in section RS.

2) The voltage across the loads away from the feeding point goes on decreasing. Therefore minimum voltage occurs at point S.

3) In case a fault occurs at/on any section of the distributor, the whole distributor will have to be disconnected from the supply mains.

Calculations:

Given- conductor resistance per km = 0.02 Ω

But in 2 wire DC distributor system 2 conductors are present

∴ Resistance per km for 2 wire DC distributor = 0.02 × 2 = 0.04 Ω 

∴ Resistance of section AB = 0.04 × 2 = 0.08 Ω (R_AB)

∴ Resistance of section BC = 0.04 × 2 = 0.08 Ω (R_BC)

Also, I₂ = 200 A, I₁ = 100 A

∴ Current in section AB = I₁ + I₂ = 100 + 200 = 300 A

∴ current in section BC = I₂ = 200 A

i.e. I_AB = 300 A, I_BC = 200 A

Now, Voltage available at load point B

V_B = Voltage at A – Voltage drop in AB

V_B = 500 V – I_AB × R_AB

V_B = 500 V – (300 × 0.08) V

V_B = (500 - 24) V

V_B = 476 V

Now, voltage available at point C

V_C = voltage at B – voltage drop in BC

V_C = 476 V – I_BC × R_BC

V_C = 476 V – (200 × 0.08) V

V_C = 476 V – 16 V

V_C = 460 V

Therefore the voltage available at the farthest point (C) in the system is 460 V.

Note:

There are a few advantages of other types of the distribution system.

• In this type of distribution, if a fault occurs on any feeding point of the distributor or on any section of the distributor, the continuity of the supply is maintained from the other operating feeding point.
• Also the area of cross-section required for doubly-fed distributors is much less than that of a singly fed distributor.

Fig. Distributor fed at the center

• Distributor fed at the center is equivalent to two singly fed distributors, each distributor having a common feeding point and length equal to half of the total length.
• In-ring main distribution, the distributor is in the form of a closed ring. It is equivalent to a straight distributor fed at both ends with equal voltages, where the two ends being brought together to form a closed ring.
• The distributor ring may be fed at one or more than one point.