What are bundled conductors, and how do they reduce corona?

Short Answer:

Bundled conductors are a group of two or more conductors used per phase in a transmission line instead of a single large conductor. These conductors are placed close together and tied using spacers at fixed intervals. Bundled conductors are mostly used in extra high voltage (EHV) and ultra-high voltage (UHV) transmission systems.

They help reduce corona discharge by increasing the effective diameter of the conductor, which lowers the electric field intensity on the surface. This makes it harder for the air around the conductor to become ionized, thus minimizing corona formation, energy loss, and radio interference.

Detailed Explanation:

Bundled Conductors and Corona Reduction

In high-voltage power transmission systems, especially above 220 kV, using a single large conductor per phase creates very high electric field intensity on the surface. This strong field can cause corona discharge, which leads to power loss, noise, and electromagnetic interference. To overcome this, engineers use bundled conductors, which are multiple conductors per phase, arranged and held together using spacers.

These bundled conductors act as a single electrical unit while physically distributing the current through multiple paths. By spreading the current and enlarging the surface area, the electric field on any single conductor is reduced, which significantly lowers the chance of corona discharge.

Structure and Working of Bundled Conductors

  • A bundled conductor consists of two or more sub-conductors for each phase. Common arrangements include two, three, or four conductors per phase.
  • These sub-conductors are spaced a short distance apart (typically 30 to 60 cm) and are held using metallic spacers to maintain uniform distance.
  • All sub-conductors in the bundle carry equal portions of the total phase current.
  • Although the conductors are separated physically, electrically they behave like a larger single conductor, which helps reduce the overall electric field strength.

How Bundled Conductors Reduce Corona

  1. Increased Effective Diameter
    • The main reason bundled conductors reduce corona is because they increase the effective diameter of the conductor system.
    • A larger effective diameter results in a lower surface electric field, reducing the chances of air ionization and corona discharge.
  2. Even Electric Field Distribution
    • Bundled conductors distribute the electric field more evenly across a larger area.
    • This prevents the formation of localized strong electric fields that can trigger corona at sharp or dirty points on the conductor surface.
  3. Reduced Surface Gradient
    • Surface gradient is the electric field strength at the conductor surface.
    • Bundled conductors reduce this gradient, keeping it below the critical disruptive value, thereby avoiding corona.
  4. Lower Radio and Audible Noise
    • By minimizing corona discharge, bundled conductors also reduce the buzzing sound and radio interference typically caused by corona in high-voltage lines.
  5. Minimizing Corona Loss
    • Since corona causes energy to be lost in the form of light, heat, and chemical reactions, reducing corona means better efficiency and lower power losses.

Additional Advantages of Bundled Conductors

  • Reduced Line Reactance: Improves power transfer capacity and system stability.
  • Lower Voltage Gradient: Helps protect surrounding equipment and insulation.
  • Improved Current Distribution: Helps in evenly distributing current, avoiding hot spots.
  • Mechanical Flexibility: Reduces vibration and wind pressure on conductors.

Applications in Power Systems

  • Widely used in EHV (400 kV) and UHV (765 kV and above) transmission lines.
  • Found in long-distance transmission networks where corona loss would otherwise be a major concern.
  • Often combined with other anti-corona techniques like corona rings and proper conductor spacing.
Conclusion

Bundled conductors are multiple conductors used per phase to improve the performance of high-voltage transmission lines. They are designed to reduce corona discharge by lowering the electric field at the surface through increased effective diameter and better electric field distribution. This leads to reduced power loss, radio noise, and insulation stress. Bundled conductors play a vital role in making EHV and UHV transmission systems more efficient, safe, and reliable in modern power networks.