Understanding Partial Discharge: Causes, Effects, and Solutions

Causes of Partial Discharge

Partial discharge (PD) occurs in insulating materials when localized electrical stress exceeds the dielectric strength of the material in specific areas, typically within air gaps, oil films, or surfaces. Key causes include:

1. Localized Electric Field Concentration:

   • PD is triggered when the local field strength in a gas, liquid, or weak dielectric region exceeds the breakdown field strength. This often occurs in insulation structures with sharp edges or imperfections.
   • Example: The concentrated electric field at the edge of electrodes or slots of high-voltage motor coils.

2. Manufacturing Imperfections:

   • During manufacturing, insulating materials may develop air bubbles, oil films, or voids that lower the breakdown strength compared to solid insulation.
   • Example: In cast transformers, plastic cables, and oil-immersed insulation systems, such defects are common.

3. Electrical Stress:

   • AC Voltage: PD occurs most commonly under sinusoidal AC voltage due to the continuous stress imposed by alternating cycles.
   • DC Voltage: Although less frequent, PD under DC voltage can still occur but at much lower repetition rates than AC.

4. Corona Discharge:

   • At exposed electrode edges, corona discharge in air generates reactive byproducts like ozone, which can chemically degrade surrounding materials.

Impacts of Partial Discharge

Partial discharge significantly affects the lifespan and reliability of electrical insulation systems. Key impacts include:

1. Insulation Degradation:

   • Physical Effects: Each discharge produces high-energy electrons, causing micro-cracking, weakening the insulation structure.
   • Chemical Reactions: Charged particles and reactive gases generated by PD (e.g., ozone) degrade the molecular structure of insulation.

2. Internal and Surface Discharge:

   • Internal Discharge: Happens within voids, bubbles, or oil films, leading to localized insulation breakdown.
   • Surface Discharge: Occurs along the interface of insulation and air or oil, especially in areas with uneven field distribution.

3. Critical Component Damage:

   • Common in high-voltage equipment such as transformers, cables, capacitors, and motor windings.
   • Over time, repeated PD activity accelerates insulation aging, leading to equipment failure.

4. System Reliability:

   • PD-related insulation failures are a leading cause of faults in high-voltage power systems, especially in ultra-high voltage transformers.

Solutions to Partial Discharge

Addressing partial discharge requires proactive design, monitoring, and testing strategies. Below are effective approaches:

1. Design Improvements

• Minimize field concentration by using rounded electrode edges and optimized insulation structures.
• Eliminate voids and air gaps during manufacturing by applying vacuum impregnation techniques in oil-paper insulation.

2. Testing and Detection

• Factory Testing:
  • During manufacturing, conduct PD testing on each unit to ensure insulation integrity.
  • Ensure working field strengths are below the initial discharge field strength of the insulation material.
• On-Site Testing:
  • Perform partial discharge tests during transformer installation and handover to identify latent defects.
  • Monitor gas content in transformer oil; high gas concentrations indicate insulation breakdown.

3. Online Monitoring

• Real-time monitoring of insulation systems provides early warning of PD activity under operational conditions.
• Modern online monitoring systems track PD signals and gas formation trends, helping prevent unexpected failures.

4. Maintenance Practices

• Use partial discharge testers for periodic assessments, especially for transformers rated above 110 kV.
• Conduct repairs or retrofitting as needed to mitigate weak points identified during testing.

5. Standards and Regular Maintenance

• Follow international standards (e.g., IEC 60270) for PD testing and monitoring.
• Implement regular maintenance cycles to inspect and clean insulation surfaces, ensuring optimal performance.

Conclusion

Partial discharge is a critical factor in the degradation of high-voltage electrical equipment, directly impacting reliability and safety. Understanding its causes, impacts, and solutions is essential for effective insulation management. Through rigorous testing, advanced monitoring, and proper maintenance, engineers can mitigate the risks posed by PD, ensuring long-term performance and reliability of power systems.