Power System Overvoltage Types: Causes, Effects, and Protection Strategies for Medium-Voltage Equipment

Power System Overvoltage Types: Causes, Effects, and Protection Strategies for Medium-Voltage Equipment

 

Understanding overvoltage is essential for protecting transformers, switchgear, and cable systems - especially in distribution networks where equipment selection directly impacts reliability and lifespan.

📋 Table of Contents

What Is Power System Overvoltage?

Why Overvoltage Matters for Equipment Selection

Overvoltage Protection Strategies for Distribution Systems

Frequently Asked Questions

What Is Power System Overvoltage?

 

Power system overvoltage refers to any voltage rise that exceeds the normal operating voltage of an electrical network by more than 10% of its rated value. These abnormal voltage events can last from microseconds to seconds - and in extreme cases, may reach millions of volts.

While most electrical equipment is designed to handle brief voltage fluctuations, sustained or high-magnitude overvoltages can:

Accelerate insulation aging and degradation

Cause premature failure of transformers and switchgear

Trigger nuisance tripping of protective relays

Lead to costly unplanned outages and equipment replacements

For engineers and procurement specialists working with medium-voltage distribution systems (6kV–35kV), understanding overvoltage is not just an academic exercise - it directly influences equipment specification, insulation coordination, and long-term maintenance planning.

Why Overvoltage Matters for Equipment Selection

 

When selecting transformers, surge arresters, or switchgear for a project, one of the first questions should be: What overvoltage conditions will this equipment face?

Equipment Type	Overvoltage Risk	Consequence of Inadequate Protection
Dry-type transformers	Lightning strikes, switching transients	Winding insulation puncture, short-circuit failure
MV switchgear	Internal arc faults, resonance	Contact erosion, catastrophic arc flash
Cable systems	Temporary overvoltages	Partial discharge, dielectric breakdown
Surge arresters	All types	Improper sizing leads to premature failure or inadequate protection

Key takeaway: Proper insulation coordination - matching equipment insulation levels to the expected overvoltage environment - is the most cost-effective way to ensure long-term system reliability.

Overvoltage Protection Strategies for Distribution Systems

 

For medium-voltage equipment buyers and system designers, the following protection methods should be considered based on the dominant overvoltage type at the installation site:

Overvoltage Type	Recommended Protection	Application Notes
Lightning surges	Surge arresters (MOV type), shield wires	Select based on system voltage and energy rating
Temporary overvoltages	Proper insulation coordination, neutral grounding	TOV capability of arresters must be verified
Switching transients	Pre-insertion resistors, RC snubbers	Controlled switching devices for critical breakers
Ferroresonance	Neutral grounding resistors, damping transformers	Avoid single-phase switching of ungrounded transformers

For dry-type transformer users specifically:

Modern cast resin transformers (such as SCB series) are designed with:

Class F or H insulation - providing higher thermal margin during temporary overvoltages

Epoxy encapsulation - which offers excellent partial discharge resistance under impulse conditions

Embedded temperature monitoring - to detect abnormal heating caused by resonance events

These features make epoxy resin dry-type transformers inherently more resilient to many overvoltage scenarios compared to traditional oil-immersed designs - particularly in indoor substations, high-rise buildings, and offshore platforms where lightning protection and fire safety are critical.

Frequently Asked Questions

 

Q1: What is the most common cause of overvoltage in medium-voltage distribution networks?

For 6kV–35kV systems, lightning-induced surges and single-phase ground faults are the most frequent causes. In urban cable networks, switching transients from breaker operations are also very common.

Q2: How do I know if my dry-type transformer needs surge protection?

If your transformer is connected to overhead lines (even for a short distance) or located in an area with moderate to high lightning activity (keraunic level >30 days/year), surge arresters are strongly recommended at the transformer terminals.

Q3: Can resonant overvoltages damage epoxy cast resin transformers?

Yes - sustained resonance can cause thermal buildup and insulation stress. However, epoxy resin transformers have better thermal conductivity and higher partial discharge inception voltage than oil-filled types, offering an extra margin of safety.

Q4: What is the difference between a surge arrester and a lightning rod?

A lightning rod intercepts direct strikes to protect structures. A surge arrester limits transient voltage on electrical circuits by diverting surge current to ground - the two devices serve complementary but different functions.