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

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.

Concept

The voltage drop per phase (in V/phase) is given by:

V = I × Z_T

where, V = Voltage drop

I = Current

Z_T = Total Impedance

Calculation

Given, cosϕ = 0.8 lag → ϕ = 36.86° 

I = 50∠-36.86° 

Length = 400 m = 0.4 km

Z_T = (0.15 + j0.2) × 0.4

ZT = (0.06 + j0.08) Ω/km

V = (50∠-36.86°) × (0.06 + j0.08)

V = 50(0.8 - j0.6) × (0.06 + j0.08)

V = (40 - j30) × (0.06 + j0.08)

V = 4.8 + j1.4

Concept:

Bulk power transmission requires voltage levels high enough to reduce current and therefore minimize I²R losses and improve efficiency over long conductors.

The classification of voltage levels is:

• Low Voltage (LV): Up to 1 kV
• Medium Voltage (MV): 1 kV to 33 kV – used in distribution networks
• High Voltage (HV): 33 kV to 220 kV – commonly used for bulk power transmission over medium distances
• Ultra-High Voltage (UHV): Above 765 kV – used for long-distance, high-capacity transmission

Final Answer:  High Voltage (HV)

Explanation

• Voltage drop analysis helps ensure the end-user receives the correct voltage needed for safe and reliable operation of electrical devices.
• Regulatory standards typically specify that the voltage at a user’s point of connection should not deviate more than ±5% for most systems (e.g., from 230V nominal, the range would be 218.5V to 241.5V).
• Excessive voltage drop can cause equipment malfunction or failure. Therefore, analyzing and minimizing voltage drop helps maintain power quality and protect end-user equipment.
• Also, engineers can size conductors correctly, reduce losses, and maintain voltage within this acceptable range.

Methods of laying underground cables

Direct Laying:

• This method requires digging a 1.5m deep and 0.45m wide trench, which is then covered with sand.
• The cables are laid in the trench and covered with a 10 cm-thick layer of sand. To protect against mechanical injury, the trench is then covered with bricks and other materials.
• If more than one cable is required to be laid in a trench, then a horizontal or vertical inter-axial spacing of 30 cm is provided to prevent mutual heating.
• Direct Laying (Direct Buried Method) requires re-excavation when load expansion or modification is needed, making it costly and labor-intensive.

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Draw in the system:

• Ducts or conduits of cast iron or concrete or glazed stone with manholes are placed at suitable locations along the cable route. The manholes are used for pulling the cable in position.

Troughing System:

• A troughing system, also known as a cable troughing system or cable raceway, is a specialized system designed to protect, organize, and route electrical cables within a defined pathway.
• Cables are laid in pre-cast concrete or fiberglass troughs which are then covered.

Solid system:

• Underground cables are laid in open pipes or troughs along the cable route. The troughs are usually made of asphalt, stoneware or cast iron.
• Asphaltic compound is used for filling the troughs once the cable is laid in position.

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:

• 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.

• 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.

• 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.

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