Corona Discharge on HV Lines — Causes, Effects and Mitigation

Corona discharge is a localized electrical discharge phenomenon that occurs around high-voltage conductors when the electric field intensity exceeds the ionization threshold of air (approximately 30 kV/cm under standard conditions).

What Is Corona Discharge?

The phenomenon is named after its visual appearance — a faint bluish-violet glow surrounding the conductor, resembling a halo or crown. Physically, it is a partial ionization of the air caused by an excessively localized electric field — but not strong enough to cause complete breakdown.

Conditions for Corona to Occur

Corona appears when the operating voltage exceeds the critical corona voltage. This threshold depends on:

• Conductor radius — smaller conductors have higher surface field intensity, so corona appears at lower voltages
• Phase-to-phase and phase-to-ground spacing
• Altitude above sea level — thinner air at higher elevations reduces the ionization threshold, making corona more likely
• Weather — rain, fog, and snow significantly increase corona activity

Critical Corona Voltage Formula (Peek’s Equation)

The critical corona voltage is calculated using Peek’s formula:

Parameter	Symbol	Unit
Critical corona voltage (peak)	V₀	kV
Air density factor	δ	dimensionless
Conductor radius	r	cm
Phase-to-phase spacing	d	cm

V₀ = 21.1 × δ × r × ln(d/r) (kV, peak value)

Effects of Corona Discharge

1. Power Loss (Corona Loss)

Electrical energy is dissipated as heat and sound. On 220 kV and 500 kV lines in Vietnam, corona losses can account for 1–3% of total transmitted power under heavy rain conditions.

2. Radio Interference (RI)

Corona generates electromagnetic waves in the 0.15–30 MHz frequency range, causing interference to AM radio and television broadcasts in areas near the line.

3. Audible Noise (AN)

Particularly in wet conditions, corona produces a low-frequency hum and frying sound that can be heard up to 50–100 m from the line.

Mitigation Methods

1. Bundled conductors — 2, 3, or 4 sub-conductors per phase instead of a single conductor. This is the most common method on 220 kV and 500 kV lines
2. Increase conductor diameter — reduces surface electric field intensity
3. Corona rings — installed at the ends of insulator strings to redistribute the electric field
4. Surface maintenance — remove small protrusions, protect against corrosion

Corona on Vietnam’s Transmission Lines

Vietnam’s North–South 500 kV transmission system uses quad bundle conductors (4 sub-conductors per phase) to control corona. Each phase consists of 4 × ACSR-400 conductors spaced 45 cm apart in a square configuration — effectively increasing the equivalent radius by a factor of 4–5 compared to a single conductor.

Corona mitigation design is a mandatory requirement under Vietnamese standard TCVN 5715 and IEC 60826 for high-voltage transmission lines.

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