Classification, Mechanism and Comparative Analysis of Power System Overvoltages

📋 Table of Contents

External Overvoltages: Lightning-Induced Surges

Internal Overvoltages: System-Generated Transients

• Temporary Overvoltages
• Switching Overvoltages
• Resonant Overvoltages

Summary Comparison Table

External Overvoltages: Lightning-Induced Surges

 

Also known as atmospheric overvoltages, these originate from lightning discharges and are classified into three subtypes:

• 1.1 Direct Lightning Strikes

When lightning directly strikes an overhead line conductor or exposed equipment, the resulting overvoltage can reach millions of volts in microseconds. This extreme stress can cause:

Flashover between phases or to ground

Permanent insulation damage

Conductor melting or equipment destruction

• 1.2 Backflashovers (Counterattacks)

When lightning strikes a transmission tower or grounding structure, the tower potential rises rapidly. If the grounding resistance is too high, the potential rise can exceed the insulation level and discharge back to the live conductors - causing an effect almost as severe as a direct strike.

• 1.3 Induced Lightning Overvoltages

Even without a direct hit, a nearby lightning strike can induce transient voltages on power and communication lines through electromagnetic field coupling. These induced surges are lower in magnitude but far more frequent, and can still damage sensitive electronic control systems.

 

⚡ Lightning Overvoltage Characteristics:

Attribute	Value
Duration	Tens of microseconds (impulse waveform)
Amplitude	Up to >1 million volts
Frequency	Dependent on local lightning activity (keraunic level)
Impact Range	Most significant for systems ≤220 kV

🛡️ Common Protection Measures:

Overhead shield wires (ground wires)

Low-impedance grounding systems

Surge arresters at line entrances and equipment terminals

Shielded cables for critical feeders

Internal Overvoltages: System-Generated Transients

 

Unlike lightning, internal overvoltages originate from changes in the power system's own operating state - such as switching operations, faults, or resonance conditions.

• 2.1 Temporary Overvoltages (Power-Frequency Overvoltages)

These are sustained rises in power-frequency voltage that last from several cycles up to several seconds. They typically occur during:

Ferranti Effect - On long, lightly loaded transmission lines, the line's distributed capacitance causes the receiving-end voltage to rise above the sending-end voltage. This is particularly relevant for long cable feeders in urban distribution networks.

Single-Phase-to-Ground Faults - In ungrounded or resonant-grounded systems, the voltage on healthy phases can rise to √3 times (≈1.73×) the normal phase voltage during a ground fault.

Load Rejection - When a large load is suddenly disconnected, generator automatic voltage regulators may overshoot before stabilizing, causing a brief voltage rise.

Characteristic: Moderate overvoltage multiples (typically <1.5× rated) but long duration - which can cause thermal stress on equipment.

• 2.2 Switching Overvoltages

These are fast-decaying transient overvoltages triggered by circuit breaker or switch operations. Common scenarios include:

Energization of no-load lines - When closing a breaker to energize a long transmission line or cable.

Reclosing operations - Automatic reclosing after a transient fault.

De-energization of transformers or reactors - Interrupting small inductive currents.

Arc-grounding overvoltages - Intermittent arcing faults in ungrounded systems (common in 6kV–35kV distribution networks).

Characteristic: Random occurrence, decaying within milliseconds, but can reach 2.5–3.5× rated voltage under worst-case conditions.

• 2.3 Resonant Overvoltages

Resonant overvoltages occur when the inductive and capacitive reactances in a network become balanced at or near power frequency (or a harmonic frequency). They are particularly dangerous because they can sustain high voltages for extended periods.

Subtype	Mechanism	Common Locations
Linear resonance	Constant L and C parameters	Long cables, capacitor banks
Ferroresonance	Saturable iron-core inductors (e.g., transformers)	Ungrounded transformer neutrals, VT circuits
Parametric resonance	Time-varying parameters	Rotating machines, special drives

⚠️ Why Ferroresonance Deserves Special Attention:

In medium-voltage distribution systems, ferroresonance can occur when a transformer is energized through a cable with high capacitance, or when single-phase switching leaves a transformer connected through only one or two phases. The result can be:

Sustained overvoltage up to 3× rated

Severe audible noise and vibration

Thermal damage to transformer cores and windings

Protection strategies include: neutral grounding resistors, damping circuits, and controlled switching devices.

Summary Comparison Table

 

Overvoltage Type	Primary Cause	Typical Duration	Amplitude Range	Dominant Risk For
⚡ Lightning	Direct/induced strikes	Microseconds (impulse)	Up to >1 MV	≤220 kV systems
⏳ Temporary	Ground faults, load rejection, Ferranti effect	Cycles to seconds	1.1–1.5× rated	Long cable feeders, EHV lines
🔘 Switching	Breaker operations, reclosing	Milliseconds	2.5–3.5× rated	≥300 kV systems (but also MV networks)
🌀 Resonant	LC resonance (linear/ferro/parametric)	Sustained (steady-state)	Up to 3× rated	Ungrounded MV systems, transformer neutrals