What Are the Causes and Hazards of Partial Discharge
Learn the causes, types, and hazards of partial discharge in high-voltage electrical equipment. Discover how air gaps, moisture, insulation defects, and corona discharge damage insulation systems and reduce equipment reliability.
Partial discharge (PD) is a common electrical insulation problem found in transformers, cables, switchgear, motors, generators, and other high-voltage equipment. Although partial discharge may begin as a small local defect, continuous discharge can seriously damage insulation systems and eventually lead to equipment failure.
Understanding the causes, types, and dangers of partial discharge is very important for electrical maintenance, insulation testing, and power system reliability.
What Is Partial Discharge?
Partial discharge refers to a localized electrical discharge that only affects part of the insulation system.
Unlike complete insulation breakdown, partial discharge does not fully bridge the insulation between conductors.
In simple words:
- Only a small area of insulation breaks down
- The entire insulation system does not fail immediately
- The discharge happens repeatedly under electric stress
Over time, these repeated discharges gradually damage the insulation.
Why Does Partial Discharge Occur?
The main reason for partial discharge is uneven electric field distribution inside the insulation system.
When insulation materials are not completely uniform:
- Some areas experience stronger electric fields
- Other areas experience weaker electric fields
If the electric field in one small area becomes higher than the breakdown strength of the material, local discharge occurs.
How Do Air Gaps Cause Partial Discharge?
Air gaps are one of the most common causes of partial discharge.
Electrical insulation systems may contain:
- Tiny air pockets
- Gas bubbles
- Voids
- Cracks
Because air has a lower dielectric constant than solid insulation materials, the electric field inside the air gap becomes much stronger.
When the field strength reaches a critical level, partial discharge begins.
Can Moisture Cause Partial Discharge?
Yes.
Moisture is another major cause of insulation problems.
When insulation absorbs water:
- The insulation properties weaken
- Water may decompose under electric stress
- Gas bubbles may form inside the insulation
These bubbles can create local electric field concentration and trigger partial discharge.
How Do Impurities and Defects Cause Partial Discharge?
Insulation systems may contain:
- Dust
- Metal particles
- Manufacturing defects
- Poor electrical connections
These defects can concentrate the electric field in one location, increasing the risk of discharge.
What Are the Main Types of Partial Discharge?
Partial discharge is generally divided into three main types:
- Internal discharge
- Surface discharge
- Corona discharge
What Is Internal Partial Discharge?
Internal discharge occurs inside the insulation material.
It is commonly caused by:
- Air gaps in solid insulation
- Gas bubbles in liquid insulation
When voltage stress increases, discharge occurs inside these weak areas.
Internal discharge is one of the most dangerous types because it slowly damages insulation from inside.
What Is Surface Partial Discharge?
Surface discharge happens along the surface of insulation materials.
It usually occurs when:
- Electric field concentration is high
- Surface contamination exists
- Moisture is present
- Creepage distance is insufficient
Surface discharge often appears at high-voltage terminals and insulation surfaces.
What Is Corona Discharge?
Corona discharge occurs in gas around high-voltage conductors.
It usually appears near:
- Sharp points
- Needle electrodes
- High-voltage transmission lines
The electric field becomes strongest at sharp edges, ionizing the surrounding air.
Corona discharge is common in high-voltage systems and overhead lines.
Why Is Partial Discharge Dangerous?
Although partial discharge may appear small at first, it continuously damages insulation over time.
If not detected early, it can eventually cause complete insulation failure.
How Does Partial Discharge Damage Insulation?
Partial discharge damages insulation in several ways.
Electrical Damage
Charged particles such as electrons and ions strike the insulation surface and damage its molecular structure.
This weakens the insulation material gradually.
Thermal Damage
Partial discharge generates heat inside the insulation.
Local overheating may cause:
- Insulation aging
- Carbonization
- Melting
- Thermal breakdown
Chemical Damage
Partial discharge produces harmful gases such as:
- Ozone (O₃)
- Nitrogen oxides (NO, NO₂)
When moisture is present, these gases may form nitric acid, which further corrodes insulation materials.
Oil Decomposition
In oil-insulated equipment, partial discharge may decompose insulating oil.
This can create:
- Sludge
- Gas
- Carbon deposits
These contaminants reduce insulation performance and heat dissipation capability.
How Does Partial Discharge Affect Equipment Life?
Continuous partial discharge gradually worsens insulation deterioration.
As insulation weakens:
- Equipment reliability decreases
- Insulation strength drops
- Failure risk increases
- Service life becomes shorter
Eventually, complete insulation breakdown may occur.
Which Electrical Equipment Is Most Affected by Partial Discharge?
Partial discharge commonly occurs in:
- Power transformers
- High-voltage cables
- Switchgear
- GIS systems
- Motors
- Generators
- Bushings
- Capacitors
How Is Partial Discharge Detected?
Modern partial discharge testing equipment can identify insulation defects before serious failures occur.
Common detection methods include:
- Ultrasonic testing
- UHF detection
- TEV testing
- Acoustic emission testing
- Online PD monitoring
Professional testing instruments from companies like Wrindu help engineers accurately detect and analyze partial discharge in high-voltage equipment.
How Can Partial Discharge Be Prevented?
Several methods can reduce partial discharge risk.
Improve Insulation Quality
High-quality insulation materials reduce internal defects.
Eliminate Air Voids
Proper manufacturing processes help avoid air gaps and bubbles.
Control Moisture
Keeping insulation dry improves insulation strength.
Avoid Sharp Electric Field Concentration
Proper electrode design reduces local electric field stress.
Perform Regular Testing
Routine partial discharge monitoring helps detect early insulation problems.
FAQs About Partial Discharge
Q1: What is partial discharge?
Partial discharge is a localized electrical discharge that only affects part of the insulation system without causing complete breakdown.
Q2: What causes partial discharge?
Common causes include:
- Air gaps
- Moisture
- Insulation defects
- Contamination
- Electric field concentration
Q3: What are the three main types of partial discharge?
The three main types are:
- Internal discharge
- Surface discharge
- Corona discharge
Q4: Why are air gaps dangerous in insulation systems?
Air gaps create high local electric field strength, which can trigger partial discharge.
Q5: How does partial discharge damage insulation?
It causes:
- Electrical erosion
- Heat damage
- Chemical corrosion
- Insulation aging
Q6: Can partial discharge lead to complete equipment failure?
Yes. Continuous partial discharge may eventually cause full insulation breakdown and equipment failure.
Q7: What gases are produced during partial discharge?
Common gases include:
- Ozone (O₃)
- Nitrogen oxides (NO, NO₂)
Q8: Which equipment commonly experiences partial discharge?
Transformers, cables, switchgear, generators, and other high-voltage equipment are commonly affected.
Q9: How do engineers detect partial discharge?
They use specialized PD testing instruments such as ultrasonic detectors, UHF analyzers, and online monitoring systems.
Conclusion
Partial discharge is a serious insulation problem in high-voltage electrical equipment. It is mainly caused by uneven electric fields, air gaps, moisture, contamination, and insulation defects.
Although partial discharge may begin as a small local phenomenon, continuous discharge can gradually destroy insulation, shorten equipment life, and eventually cause catastrophic electrical failures. Regular partial discharge testing and proper insulation maintenance are essential for improving power system safety and reliability.
