Extra High Voltage Transmission Notes for Engineering Students

Rajasthan Polytechnic (BTER) March 08, 2025

 

Extra High Voltage Transmission Notes for Engineering Students

This section will cover Extra High Voltage (EHV) transmission, its necessity, components, advantages, limitations, and applications, along with the specifics of High Voltage DC (HVDC) transmission and its characteristics.

3.1 Extra High Voltage AC (EHVAC) Transmission Line: Necessity

Definition:

  • EHVAC transmission refers to the transmission of electrical power at very high voltages, typically greater than 220 kV (kilovolts) and up to 800 kV.

Necessity of EHVAC:

  • Long Distance Transmission: To transmit large amounts of power over long distances with minimal loss.
  • Economic Efficiency: Reduces the cost of power transmission by minimizing losses and reducing the size of conductors needed.
  • Reduced Line Losses: Power losses (I²R losses) are directly proportional to the current. By increasing the voltage, the current is reduced, which reduces transmission losses.
  • Improved Stability: Helps maintain a stable grid with better voltage regulation.

Example:

  • Transmission lines used for power supply between regions or across countries, like the Eastern grid in India.

3.2 High Voltage Substation Components

Components:

  • Transformers: Step-up or step-down voltage levels between transmission lines and distribution systems.
  • Circuit Breakers: Protect the system by disconnecting the flow of electricity during faults.
  • Isolators: Disconnect parts of the system for maintenance.
  • Busbars: Conductors used to connect different circuits in a substation.
  • Switchgear: Equipment used for switching, controlling, and protecting electrical circuits.
  • Surge Arresters: Protect the system from voltage spikes or lightning strikes.

3.3 Advantages, Limitations, and Applications of EHVAC

Advantages:

  1. Reduced Losses: Higher voltages result in lower currents and hence lower transmission losses.
  2. Cost-Efficient: Decreases the number of conductors and overall transmission costs.
  3. Improved System Stability: Higher voltage transmission helps to stabilize the system and ensure efficient distribution of power.
  4. Long Distance Transmission: Ideal for transmitting power across long distances with minimal power loss.

Limitations:

  1. High Initial Cost: High voltage equipment (transformers, circuit breakers, etc.) is expensive.
  2. Voltage Instability: Fluctuations can cause voltage instability and require careful monitoring.
  3. Corona Losses: At higher voltages, corona discharge can occur, resulting in energy loss and signal interference.
  4. Physical Size: EHV lines require larger physical infrastructure, including towers and insulators.

Applications:

  • Interstate Power Transmission: Connecting different states or regions.
  • Long-Distance Power Transmission: Used in applications where electricity needs to be sent over vast distances (e.g., power grids).

3.4 EHVAC Lines in India

  • India has implemented several EHVAC lines for transmitting power across vast distances.
  • Example: The Power Grid Corporation of India operates transmission lines with voltage ratings of 400 kV and 765 kV, and these lines are key to India's national grid.
  • Impact: EHVAC lines help in efficiently transmitting power from power plants (located far from urban areas) to demand centers.

3.5 Corona Effect

What is Corona Effect?

  • Corona discharge occurs when the voltage exceeds a certain threshold, causing ionization of the air around conductors, leading to energy loss and noise.
  • Consequences: Power loss, radio interference, and sometimes damage to the conductors.

Diagram:

  • Corona Discharge: Visualize corona as a violet glow around a conductor at high voltages.

3.6 High Voltage DC (HVDC) Transmission Line: Necessity, Components, Advantages, Limitations, and Applications

Necessity of HVDC:

  • Long-Distance Transmission: HVDC is more efficient for transmitting power over very long distances (typically greater than 600 km).
  • Interconnecting Grids: Helps connect asynchronous power grids, making it ideal for international power exchange.
  • Reduced Losses: HVDC systems reduce losses compared to AC for long-distance transmission.

Components of HVDC Systems:

  1. Rectifiers: Convert AC to DC.
  2. Inverters: Convert DC back to AC at the receiving end.
  3. Converter Stations: Control the flow of power between AC and DC systems.
  4. Electrodes: Used in the case of underground HVDC transmission.

Advantages of HVDC:

  1. Less Line Loss: Compared to AC, HVDC lines have less loss over long distances.
  2. Improved Stability: Easier control of power flow, especially in long-distance and high-power applications.
  3. Cost-Effective Over Long Distances: More efficient over distances greater than 600 km, as AC systems incur more losses.

Limitations of HVDC:

  1. High Initial Cost: The cost of converters and transformers is high.
  2. Complex Technology: Requires advanced control systems for maintaining stability.
  3. Lack of Flexibility: Once set up, the power flow is difficult to reverse, unlike AC systems.

Applications of HVDC:

  • Underwater Cables: Used to connect different countries across seas, like the Norway to Netherlands HVDC link.
  • Long-Distance Power Transmission: Power transmitted from remote power plants (e.g., large hydroelectric plants) to cities.

3.7 Layout of Mono-polar, Bi-polar, Homo-polar Transmission Lines

Mono-polar HVDC Line:

  • Description: A single conductor is used, with the return path being through the ground or sea.
  • Application: Shorter distances or lower power needs.
  • Example: Used in some older systems.

Bi-polar HVDC Line:

  • Description: Two conductors are used with opposite polarities, and the return current flows between the two.
  • Application: More common for long-distance high-power transmission.
  • Example: The Delhi-Mumbai HVDC line in India.

Homo-polar HVDC Line:

  • Description: Similar to bi-polar, but both conductors have the same polarity.
  • Use: Limited application, typically in special configurations.

3.8 HVDC Lines in India

  • India has several HVDC transmission projects, including:
    • Mundra (Gujarat) to Ajmer (Rajasthan): A major HVDC link.
    • Chamundeshwari Power Corporation’s HVDC line.
    • HVDC lines in India help stabilize the grid and connect different parts of the country’s power network.

3.9 Features of EHVAC and HVDC Transmission Lines

FeatureEHVACHVDC
Voltage LevelHigher (220 kV to 800 kV)Very High (100 kV to 800 kV)
LossesHigher losses over long distancesLower losses over long distances
ComplexityEasier to operateMore complex, requires advanced equipment
CostLower initial costHigher initial cost, but cheaper for long distances
Grid StabilityLess stable, voltage fluctuationsMore stable, controlled power flow

Key Questions for Exam Preparation

  1. What is the need for EHVAC transmission lines?

    • They are necessary for efficient, long-distance transmission of electrical power with reduced losses.

  2. Explain the components of an EHV substation.

    • Include transformers, circuit breakers, isolators, switchgear, and busbars.

  3. Discuss the advantages and limitations of HVDC transmission lines.

    • Advantages: Lower transmission losses, better for long-distance, more stable.
    • Limitations: High initial cost, complex technology, limited flexibility.

  4. What is the corona effect, and how does it impact transmission lines?

    • It results in energy loss and interference due to ionization of the air around the conductor.
Rajasthan Polytechnic (BTER)

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Hi! I’m Garima Kanwar, founder of Rajasthan Polytechnic (BTER). This platform provides notes, PDFs, and video lectures for all Polytechnic branches (ME, CS, EE) and semesters to make learning easy .I ensure exam-focused, updated, and simple-to-understand content, along with academic updates and career guidance.

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