Corona in Power System – Definition, Causes, Effects, and Questions

🔍 What is Corona in Power System?

Corona is an electrical phenomenon that occurs when the voltage gradient around a conductor is high enough to ionize the surrounding air, producing a faint bluish glow, hissing noise, ozone gas, and energy loss. It typically occurs in overhead high voltage transmission lines.

⚡ Where and Why Does Corona Occur?

Corona appears when the electric field intensity exceeds the breakdown strength of air (~30 kV/cm). It's commonly seen in:

High voltage lines above 33 kV
Lines with small conductor spacing
Rough or non-uniform conductor surfaces
Moist or dusty atmosphere

👇 Conditions Favoring Corona:

High system voltage
Low air pressure or bad weather
Smaller diameter of conductor
Non-uniform conductor surface

🔑 Critical and Disruptive Voltages

Disruptive Critical Voltage: Minimum voltage at which ionization starts around the conductor.
Visual Critical Voltage: Voltage at which corona becomes visible.

Peek’s formula helps in calculating disruptive voltage:

$V d$ : Disruptive voltage
$m 0$ : Surface factor (1 for smooth, <1 for rough)
$δ$ : Air density factor
$r$ : Radius of conductor
$D$ : Distance between conductors

🛠️ How to Minimize Corona Effect?

Increase conductor diameter – Reduces electric field intensity.
Use bundled conductors – Common in EHV (400 kV+) lines.
Increase conductor spacing – Lowers potential gradient.
Use smooth-surfaced conductors – Reduces chances of ionization.
Corona rings – Used in substations to uniformly distribute electric field.

✅ Advantages of Corona

Lowers voltage gradient at conductor surface.
Helps in uniform voltage distribution across line insulators.
Useful in high voltage testing.

❌ Disadvantages of Corona

Power loss (corona loss)
Audible noise (hissing sound)
Ozone generation that corrodes insulation
Interference with communication signals

📘 Previous Year Questions on Corona – SSC JE, PGCIL DT, UPPCL JE/AE

Q1. Corona loss in transmission lines is: (SSC JE 2018)
a) More in fair weather
b) Less in fair weather
c) Independent of weather
d) Same in all weather
✅ Ans: b) Less in fair weather

Q2. What is the effect of increasing conductor size on corona loss? (SSC JE 2017)
a) Increases
b) Decreases
c) No effect
d) Depends on voltage
✅ Ans: b) Decreases

Q3. Corona discharge generally occurs at:
a) Low voltage lines
b) Medium voltage lines
c) High voltage lines
d) All of the above
✅ Ans: c) High voltage lines

Q4. Which of the following is used to reduce corona effect? (PGCIL DT 2021)
a) Increasing conductor surface roughness
b) Reducing conductor size
c) Corona rings
d) Decreasing conductor spacing
✅ Ans: c) Corona rings

Q5. Which factor is not responsible for corona? (UPPCL JE 2019)
a) Humidity
b) Temperature
c) Frequency
d) Magnetic field
✅ Ans: d) Magnetic field

Q6. Which one of the following helps in reducing corona loss in EHV transmission? (UPPCL AE 2020)
a) Use of single conductor
b) Reducing conductor spacing
c) Using bundled conductors
d) Increasing transmission frequency
✅ Ans: c) Using bundled conductors