1. BASICS OF TRANSMISSION AND DISTRIBUTION

1.1 Single Line Diagrams with Components of the Electric Supply Transmission and Distribution Systems

A Single Line Diagram (SLD) is an essential schematic used to represent electrical power systems. It simplifies complex systems by showing only one conductor for each phase, even if there are multiple conductors in the system.

Key Components in the SLD:

Power Generation: This represents the power plants (thermal, hydro, nuclear) that produce electricity. The power generated is typically in the range of 11 kV to 33 kV.
Step-Up Transformer: After generation, electricity is stepped up to high voltages (110 kV, 220 kV, 400 kV) for efficient long-distance transmission. This reduces energy losses due to the resistance in the transmission lines.
Transmission Line: These are the high-voltage lines (110 kV, 220 kV, 400 kV) that carry power across long distances. Overhead transmission lines are most common.
Step-Down Transformer: Once the electricity reaches local substations, the voltage is reduced to safer levels (33 kV, 11 kV) for distribution.
Distribution Lines: These are lower-voltage lines that deliver electricity to residential, commercial, and industrial users.
Consumer Load: The final endpoint where power is consumed. This includes households, factories, and businesses that use electrical energy.

In an SLD, all these components are shown with connecting lines representing the flow of power.

Example Diagram of a Transmission and Distribution System:

Power Plant --> Step-Up Transformer --> Transmission Line (110 kV/220 kV) --> Step-Down Transformer --> Distribution Line (33 kV/11 kV) --> Consumer Load (Homes/Industries)

1.2 Classification of Transmission Lines

Transmission lines are classified based on several factors, which help in selecting the correct type of line for the specific requirements of distance, voltage, and type of current.

Classification Based on Voltage Level:

Low Voltage (LV): Typically under 1 kV, used in local distribution.
Medium Voltage (MV): From 1 kV to 33 kV, used for sub-regional transmission.
High Voltage (HV): From 33 kV to 220 kV, for inter-region transmission.
Extra High Voltage (EHV): Above 220 kV, used for long-distance transmission to minimize losses.

Classification Based on Length:

Short Transmission Line: Up to 50 km, where the line resistance and reactance are small, and losses are not significant.
Medium Transmission Line: Between 50 km to 200 km, where the line inductance and capacitance need to be considered.
Long Transmission Line: Above 200 km, where the voltage drop and line impedance need to be closely monitored.

Classification Based on Type of Current:

AC Transmission Lines: Most commonly used for electricity transmission, especially for long distances. AC lines are classified further based on the number of phases.
DC Transmission Lines: Used for very long-distance, high-capacity transmission (HVDC). It is more efficient over distances exceeding 500 km.

1.3 Primary and Secondary Transmission

Primary Transmission: Refers to the high-voltage transmission of electricity from the generating station to the substations. Voltage levels in this stage can go as high as 400 kV, depending on the distance and capacity requirements.
- Example: A typical 400 kV line that transmits power from a thermal power plant to a city.
Secondary Transmission: This involves reducing the voltage to a more manageable level for distribution, typically between 11 kV to 33 kV. The power is transmitted to local areas for final distribution.
- Example: A 33 kV line that carries power from a substation to a local area.

Key Point: Primary transmission lines deal with long-distance power transport, while secondary transmission lines are used to deliver electricity to local areas or regions.

1.4 Standard Voltage Levels Used in India

India follows specific voltage standards that help in maintaining uniformity across the country:

Generation Voltage: Power plants typically generate electricity at 11 kV to 33 kV.
Transmission Voltage: The electricity is stepped up to higher voltages to reduce losses during long-distance transmission:
- 110 kV, 220 kV, 400 kV
Distribution Voltage: Power is then stepped down for use by residential and commercial consumers:
- 33 kV (for larger areas)
- 11 kV (for local areas)
- 400 V (for three-phase systems)
- 220 V (for single-phase systems)

Voltage Levels in India ensure efficient transmission and distribution by using higher voltages in transmission and lower voltages in the final distribution.

1.5 Characteristics of High Voltage for Power Transmission

High-voltage transmission is essential for efficient power delivery. Here’s why:

Lower Current: According to Ohm’s Law ( $V = IR$ ), for a given power transmission, increasing the voltage reduces the current. This results in lower losses because the power loss in transmission lines is proportional to the square of the current ( $P_{\text{loss}} = I^2R$ ).
Reduced Energy Losses: By transmitting at high voltage, the line losses (both resistive and reactive) are minimized. This improves the efficiency of the transmission network.
Economic Benefits: While building high-voltage infrastructure is costly, the reduced losses make it more economical in the long run.

Example: In long-distance transmission (e.g., 400 kV lines), high voltage helps reduce the power losses over distances exceeding hundreds of kilometers.

1.6 Method of Construction of Electric Supply Transmission System

Steps to construct high-voltage transmission systems (110 kV, 220 kV, 400 kV):

Survey and Planning: Selecting the route for the transmission line, considering geography, environmental conditions, and safety regulations.
Foundation Work: Constructing foundations for towers that will support the transmission lines.
Erection of Transmission Towers: These are tall steel structures that hold the conductors. They are designed to handle the mechanical stresses imposed by wind, ice, and electrical load.
Stringing of Conductors: High-voltage conductors, typically made of aluminum or aluminum steel, are strung between towers.
Substation Construction: Substations are located at strategic points to step up or step down voltage levels.
Testing and Commissioning: After construction, the system undergoes thorough testing before it is energized.

Example: The construction of a 400 kV transmission line involves careful selection of the route, installation of steel towers, and high-quality insulation to avoid electrical faults.

1.7 Method of Construction of Electric Supply Distribution Systems

For 220V, 400V, 11 kV, and 33 kV distribution systems:

Selection of Distribution Network: Identifying the geographical areas to be served by the distribution network.
Erection of Poles and Towers: Wooden or concrete poles are used for lower voltage (e.g., 400 V, 220 V), while steel towers are used for higher voltage levels (e.g., 11 kV, 33 kV).
Underground Cables: In densely populated urban areas or regions with harsh weather conditions, underground cables are used for both safety and aesthetic reasons.
Step-Down Transformers: These are used to reduce voltage levels from high to low. For example, a 33 kV line is stepped down to 11 kV or 400 V for local distribution.
Connection to Consumers: Finally, the power is distributed through a network of local lines to residential, commercial, and industrial users.

Example: A typical home is powered by a 220 V or 400 V supply, depending on whether it’s a single-phase or three-phase connection.

Summary of Key Points:

Transmission Lines carry high-voltage electricity over long distances, while Distribution Lines deliver electricity at lower voltages to end-users.
High Voltage is used for long-distance transmission because it reduces losses and improves efficiency.
Single Line Diagrams (SLDs) provide a simplified, easy-to-read representation of the power system.
India's Standard Voltage Levels ensure optimal transmission and distribution of power across vast distances and varied regions.

Practice Questions:

Explain why high voltage is preferred for long-distance power transmission.
What are the major components of a single-line diagram in the power transmission system?
Differentiate between primary and secondary transmission.
List and explain the steps involved in the construction of a 220 kV transmission system.
What is the significance of step-up and step-down transformers in the transmission and distribution system?

1. BASICS OF TRANSMISSION AND DISTRIBUTION