Ferranti Effect vs. Corona Effect vs. Reactive Power Effects

A complete technical comparison for long AC transmission lines

This is a new, standalone authority article, explaining how these three major AC phenomena differ, how they interact, and how engineers analyze them. Highly optimized for search engines and excellent as an anchor post.

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⚡ 1. Introduction

Long AC transmission lines exhibit several electrical phenomena that affect voltage, power flow, stability, and insulation requirements. Three of the most important are:

• Ferranti Effect — voltage rises at the receiving end

• Corona Effect — ionization of air around conductors

• Reactive Power Effects — charging currents, VAR flow, power factor challenges

These phenomena are often confused, but they arise from different mechanisms, have different impacts, and require different mitigation strategies.

This article gives a deep technical comparison.

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⚡ 2. Ferranti Effect

Voltage rise due to distributed capacitance

The Ferranti Effect occurs on long, lightly loaded AC transmission lines, causing the receiving-end voltage to exceed the sending-end voltage due to line capacitance and inductance interactions.

Root Mechanism

• Line capacitance generates leading reactive power

• This current leads voltage by 90°

• Line inductance causes additional voltage rise

• The rise compounds along the line (distributed effect)

$$V_R = V_S \cosh(\gamma l)$$

Where $\gamma = \sqrt{LC}$.

When It Occurs

• Line length > 200–250 km (AC)

• Voltage level ≥ 220 kV

• Light or zero load

Key Symptoms

• Overvoltage at receiving end

• VAR generation from line

• Stress on insulators, arresters, transformers

Mitigation

• Shunt reactors

• Series compensation

• Controlled switching

• FACTS devices (STATCOM/SVC)

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⚡ 3. Corona Effect

Ionization of air surrounding conductors under high electric stress

The Corona Effect is a localized surface phenomenon where air molecules near the conductor are ionized, causing:

• power loss

• audible noise

• radio interference

• UV light and ozone generation

Root Mechanism

Occurs when the electric field gradient exceeds the critical disruptive voltage:

$$E_c = \delta \, g_0 \, m_0$$

Where:

• $\delta$ = air density factor

• $g_0$ = 30 kV/cm (breakdown electric field)

• $m_0$ = surface irregularity factor

Key Symptoms

• Purple glow around conductors

• Audible humming

• Radio interference (RIV)

• Power loss due to ionization

Conditions That Increase Corona

• High voltage

• Bad weather (rain, fog, humidity)

• Rough or dirty conductors

• Small-diameter conductors

Mitigation

• Bundled conductors (increase effective diameter)

• Smooth, polished, or coated conductors

• Corona rings at line terminations

• Maintaining proper conductor spacing

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⚡ 4. Reactive Power Effects

Behavior of inductive and capacitive VARs in long AC systems

Reactive power affects voltage control, stability, and line loading.

Root Mechanism

In AC systems:

• Inductive components absorb VARs

• Capacitive components generate VARs

Transmission lines generate capacitive VARs due to distributed capacitance:

$$I_c = \omega C V$$

Transformers and motors absorb inductive VARs.

Key Symptoms

• Voltage instability (over/under-voltage)

• Increased current flow

• Reduced real power transfer capability

• Oscillation risk

Causes in Transmission Lines

• Light loading → capacitive VAR dominance

• Heavy loading → inductive VAR dominance

• Long 400–765 kV lines → high charging currents

Mitigation

• SVC, STATCOM

• Shunt reactors / capacitors

• Series compensation

• On-load tap-changing transformers (OLTC)