Corona Loss in Transmission Lines; Hissing sound in transmission lines

⚡ Corona Effect in Transmission Lines

🔹 Introduction

In high-voltage transmission lines, the surrounding air acts as a dielectric medium. When the electric field intensity around conductors exceeds a certain critical value, it ionizes the surrounding air molecules. This ionization leads to partial discharge of electricity, accompanied by bluish glow, hissing noise, and ozone production.

This phenomenon is known as the Corona Effect.

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🔹 Mechanism of Corona Formation

1. Free electrons are always present in air (from cosmic rays, UV radiation, radioactivity).

2. As voltage increases → electric field gradient at conductor surface increases.

3. Electrons accelerate → collide with neutral air molecules → release more electrons.

4. This leads to an electron avalanche → ionization of surrounding air.

5. Discharge appears as faint luminous glow + hissing sound.

👉 If conductor spacing is too small (spacing-to-radius ratio < 15), flashover occurs before corona starts.

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🔹 Visual & Audible Symptoms

• Visual glow: Bluish, violet, or faint luminous glow around conductors at night.

• Audible noise: Crackling or hissing, louder in rainy/stormy weather.

• Ozone smell: Due to O₂ dissociation forming O₃.

• Energy loss: Appears as Corona Loss.

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🔹 Corona Loss (Power Dissipation)

The corona loss (empirical Peek’s formula) is:

$$P_c = 241 \times 10^{-5} (f+25)\sqrt{\frac{r}{d}} \cdot \frac{(V_p - V_0)^2}{d} \; \text{(kW/km/phase)}$$

Where:

• $f$ = frequency (Hz)

• $r$ = conductor radius (cm)

• $d$ = spacing between conductors (cm)

• $V_p$ = phase-to-neutral RMS voltage (kV)

• $V_0$ = critical disruptive voltage (kV)

• $d$ = air density correction factor

$$V_0 = g \, r \, d \, \ln\left(\frac{D}{r}\right)$$

with $g ≈ 30 \, kV/cm$ (at NTP).

👉 Key takeaways:

• Corona loss ∝ $(V_p - V_0)^2$ → increases sharply above disruptive voltage.

• Larger conductor diameter reduces corona.

• Higher spacing also reduces corona effect (up to a limit).

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🔹 Factors Affecting Corona

1. Conductor Parameters

• Diameter: Larger → lower electric field → reduced corona.

• Surface condition: Polished/smooth → higher disruptive voltage; rough/dirty → more corona.

• Bundled conductors: Used in EHV lines (400 kV, 765 kV) to reduce corona.

2. Line Voltage

• Below 30 kV → negligible corona.

• 110 kV – 220 kV → corona starts appearing.

• 400 kV → severe corona → special design needed.

3. Spacing Between Conductors

• Small spacing → flashover risk.

• Very large spacing → weak field, corona less likely.

• Optimized spacing balances mechanical & electrical design.

4. Atmospheric Conditions

• Humidity & Rain: Increases corona (more free ions).

• Pressure: At higher altitudes (lower air density), corona appears at lower voltages.

• Temperature: Higher temperature reduces air density → easier ionization.

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🔹 Effects of Corona on Transmission Lines

✅ Advantages

• Acts as a safety valve: reduces steepness of lightning surges.

• Provides some protective cushioning for insulation.

❌ Disadvantages

• Power Losses: Continuous energy dissipation as corona loss.

• Noise Pollution: Hissing/crackling in EHV lines.

• Interference: Causes radio/television disturbances.

• Material Degradation: Ozone produced corrodes nearby insulators & conductors.

• Voltage Regulation Issues: Loss increases with voltage, affecting efficiency.

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🔹 Practical Engineering Solutions to Reduce Corona

1. Increase conductor diameter

   • Use hollow conductors or ACSR (Aluminium Conductor Steel Reinforced) for strength & larger radius.

2. Bundled Conductors

   • Common in 400 kV, 765 kV lines: splitting one phase into 2–4 sub-conductors → reduces field intensity.

3. Smooth conductor surface

   • Prevents dirt, dust, and corrosion which increase corona.

4. Optimized spacing

   • Proper phase-to-phase spacing minimizes losses without causing flashover.

5. Weather considerations

   • Lines in humid/polluted regions designed with extra margins for corona inception voltage.

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🔹 Real-World Example

• 400 kV Single Circuit Line (India):

• Uses quad-bundled conductors to minimize corona.

• If single conductor were used, corona loss could reach 50–100 kW/km/phase in wet conditions.

• With bundled conductors, loss reduces to <10 kW/km/phase.

Key Insights into Corona Phenomenon

1. Why the Violet Glow?

• The glow arises from ionized nitrogen molecules emitting photons as they return to a lower energy state—primarily in the blue-violet and ultraviolet spectra WikipediaEEP - Electrical Engineering Portal.

• It often becomes visible at higher voltages, especially under nighttime or low-light conditions wazipoint.com.

2. What Causes Corona?

• When the field gradient at the conductor surface exceeds ~30 kV/cm (at sea level), air ionization begins, leading to corona discharge Wikipedia+1.

• Sharp or irregular conductor surfaces intensify field stress, making corona more likely. This is why smooth, polished conductors or corona rings are often used to mitigate the effect Wikipedia.

3. Associated Effects

• Audible noise: The ionization process generates small pressure waves, heard as hissing or crackling, especially in wet weather EEP - Electrical Engineering Portal.

• Ozone and NOₓ formation: Ionization leads to dissociation of oxygen and nitrogen molecules, producing ozone and nitrogen oxides, which can be corrosive EEP - Electrical Engineering Portal.

• Electromagnetic interference (EMI): The corona-generated current pulses inject noise into nearby communication systems EEP - Electrical Engineering Portal.

4. Engineering Countermeasures

• Increase conductor diameter: Reduces electric field intensity.

• Use bundled conductors: Spread the field over a larger area.

• Apply corona rings or grading rings: These rings help smoothen the electric field gradient, lowering the chance of ionization Wikipedia.

• Maintain conductor surface: Clean and smooth surfaces resist corona initiation.

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🔹 Conclusion

The corona effect is an inevitable phenomenon in EHV (Extra High Voltage) transmission lines, but its impact on efficiency, noise, and interference must be carefully minimized through design. Engineering solutions such as increasing conductor size, using bundled conductors, maintaining proper spacing, and ensuring smooth surfaces are essential for reliable and economic power transmission.

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