What is the critical disruptive voltage in corona discharge?

Short Answer:

The critical disruptive voltage in corona discharge is the minimum voltage at which the surrounding air around a transmission line conductor becomes ionized and starts to conduct electricity. When the voltage reaches this level, the electric field strength at the surface of the conductor becomes strong enough to break down the air insulation, resulting in corona formation.

Below this voltage, the air acts as a perfect insulator, and no corona discharge occurs. But once the voltage crosses this threshold, corona begins, leading to energy loss, noise, and interference. The critical disruptive voltage depends on factors like conductor size, surface condition, spacing, and air conditions.

Detailed Explanation:

Critical Disruptive Voltage in Corona Discharge

Corona discharge occurs when the electric field intensity around a conductor becomes high enough to ionize the surrounding air. The point at which this ionization starts is known as the critical disruptive voltage. It marks the exact threshold where the dielectric strength of air is no longer enough to prevent ionization, and a faint bluish glow, hissing sound, and power loss begin to appear due to the discharge.

Understanding the critical disruptive voltage is important in designing power transmission lines, especially in extra high voltage (EHV) systems, because engineers must either avoid this voltage level (to prevent corona) or manage it with protective equipment and design techniques.

Factors Determining Critical Disruptive Voltage

The critical disruptive voltage is not a fixed value. It depends on several physical and atmospheric factors:

  1. Conductor Radius (r)
    • Larger radius conductors have a lower surface electric field for a given voltage.
    • Hence, they can handle higher voltages before reaching critical conditions.
  2. Spacing Between Conductors (d)
    • Larger spacing reduces the electric field interaction between conductors, increasing the voltage required to initiate corona.
  3. Surface Condition of the Conductor
    • Smooth and clean surfaces delay corona formation, raising the critical disruptive voltage.
    • Rough or dirty surfaces concentrate electric fields and lower the threshold.
  4. Air Density Factor (δ)
    • Air density changes with altitude, temperature, and pressure.
    • Higher altitudes or warmer air reduce air density, lowering the dielectric strength and thus reducing the critical voltage.
  5. Weather Conditions
    • Humid, rainy, or dusty weather reduces the air’s insulating capability.
    • Corona may start at lower voltages in such conditions.

Formula for Critical Disruptive Voltage

The theoretical formula for calculating the critical disruptive voltage (Vc) for a single-phase line is:

Vc = m₀ × δ × r × g × ln(d/r)

Where:

  • Vc = Critical disruptive voltage (in kV)
  • m₀ = Surface irregularity factor (usually less than 1)
  • δ = Air density factor (depends on altitude and temperature)
  • r = Radius of the conductor (in cm)
  • d = Spacing between conductors (in cm)
  • g = Breakdown strength of air (approx. 21.1 kV/cm under standard conditions)
  • ln = Natural logarithm

This formula shows that increasing conductor radius, spacing, and air density increases the critical disruptive voltage, making corona less likely to occur.

Importance in Power System Design

  1. Prevention of Energy Loss
    • By designing the line to operate below the critical voltage, corona loss can be avoided or reduced.
  2. Improved Efficiency
    • Preventing corona helps in reducing electromagnetic interference, audible noise, and chemical emissions.
  3. Better Equipment Life
    • Preventing the onset of corona protects insulation and reduces corrosion of conductors.
  4. Design of EHV and UHV Lines
    • At very high voltages, corona is difficult to avoid, so engineers aim to control it using bundled conductors, smooth surfaces, and corona rings.

Managing the Critical Disruptive Voltage

  • Bundled Conductors: Help reduce surface field intensity, allowing higher voltages without reaching critical field strength.
  • Corona Rings: Distribute the electric field at terminals and insulators to avoid local breakdown.
  • Smooth Conductor Design: Using polished or coated conductors increases the critical voltage.
  • Adequate Spacing: Properly spaced conductors reduce the mutual field and raise the critical voltage.
Conclusion

The critical disruptive voltage is the minimum voltage at which corona discharge begins in a transmission line. It represents the limit beyond which air surrounding a conductor gets ionized and starts leaking energy. This value depends on conductor size, spacing, surface condition, air density, and weather. To avoid the negative effects of corona, power engineers focus on maintaining system voltages below this level or use techniques like bundling and spacing to raise the disruptive voltage threshold. Understanding and managing this voltage is essential for safe and efficient power system operation.