Partial discharge in transformers typically occurs within the internal insulation of the assetnd other high-voltage equipment when exposed to electric stress. Unlike a complete flashover and breakdown that spreads across the entire insulation, partial discharge remains confined to a localized position – hence it is called “partial”. The intensity of each discharge is minimal and cannot be perceived by human senses of vision or hearing, but over time, it gradually weakens and damages the insulation.

Even though partial discharge is often invisible, its impact on transformer reliability can be severe. In this blog, we will examine the key aspects of partial discharge in transformers and the associated risks.

What Causes Partial Discharge in Transformers?

During transformer operation, the insulation is exposed to continuous electrical stress from the applied voltage. As voltage levels rise, the electric field across the insulation increases, making defects more susceptible to partial discharge activity. The chance of partial discharge in transformers increases in non-uniform insulation systems, where differences in dielectric properties disturb the electric field and cause high-stress regions. It usually initiates at the weak points, especially where the dielectric constant changes abruptly, since such variations amplify the local electric field.

Some of the factors influencing partial discharge in transformers:

1. Material and design-related causes
   Partial discharge frequently arises from imperfections within the insulation system. Some common examples include voids, microcracks, or gas-filled cavities in solid insulation, as well as bubbles, suspended particles, or dissolved gases present in transformer oil. Severe electric field distortion at the boundary between two different dielectric materials is another cause.
2. Human and operational factors
   Manufacturing and maintenance practices also play a major role. Inadequate impregnation of solid insulation, contamination introduced during assembly or installation, or poor-quality repairs can all create weak points that act as partial discharge initiation sites.
3. Environmental and aging effects
   Insulation performance deteriorates with time due to external conditions and operational stresses. Moisture ingress due to faulty seals or ineffective oil preservation is the most common issue. Similarly, fluctuating ambient conditions, such as high humidity and repeated temperature cycling, accelerate degradation. Long-term exposure to thermal, electrical, and mechanical stresses further reduces dielectric strength, increasing the probability of partial discharge activity.

In practice, partial discharge in transformers tend to appear in specific components and under certain operating conditions. The most common sources include:

• Core and winding assembly – where concentrated stresses occur due to high electric fields
• Bushings – susceptible to voids, surge voltages, and moisture ingress
• Load Tap Changer (LTC) – prone to arcing, contamination, and aging stresses

What Are the Types of Partial Discharge in Transformers?

There are three primary types of Partial discharge:

1. Internal Partial discharge

Internal partial discharge occurs in defects within the insulation system, including voids, air pockets, or contaminations. These discharges are initiated when the electric field exceeds the breakdown strength of the defect region. Repeated partial discharges in transformers over a long period can cause electrical treeing, which gradually degrades the insulation.

2. Surface Partial Discharge

Surface partial discharge in transformers occurs on the surface of insulating materials, especially where high electric fields are present, such as near sharp points, contamination, or moisture. These discharges can cause surface tracking, which slowly deteriorates the insulation and increases the risk of failure.

3. Corona Partial Discharge

Corona partial discharge in transformers is initiated at sharp edges, protrusions, or high-voltage connections, where the electric field converges. It usually occurs in the air and is not dangerous in limited quantities, but can result in electromagnetic interference, ozone generation, or surface erosion over time if it persists.

The Effect of Partial Discharge in Transformers

Although each partial discharge activity lasts for a very short time and releases minimal energy, its cumulative effect is extremely destructive. Partial discharge in transformers damages insulation in the following ways:

1. Physical bombardment: Partial discharge directly erodes the material, leaving behind small spots or dendritic burns in solid insulation.
2. Chemical action: Byproducts such as heat, ozone, and nitrogen oxides accelerate insulation corrosion, increase conductivity, and trigger thermal aging.
3. Liquid dielectrics: In transformer oil, partial discharge breaks down hydrocarbons with the formation of gases such as hydrogen, methane, and acetylene, and micro-bubbles that further concentrate field stresses.
4. Localized overheating: High energy discharges can create hot spots that further stress surrounding insulation.

Aside from these physical and chemical effects, partial discharge has a direct impact on transformer reliability, operational safety, and maintenance costs, highlighting its significance for asset managers.

Impact of Partial Discharge in Transformers

Partial discharge not only deteriorates insulation but also poses operational and safety hazards that may impact the transformer and the grid:

• Unplanned outages – Partial discharge can initiate faults suddenly, leading to unexpected failures.
• Safety issues – In certain instances, partial discharge can result in fires, explosions, or oil leakage.
• Maintenance and replacement cost – Fixing damaged insulation or replacing components can be costly and time-consuming.
• Reduced reliability – Power availability and quality may suffer if partial discharge repeatedly occurs.
• Operational downtime – Utilities and industries can experience disruptions when transformers are offline for maintenance or repairs.

The consequences of partial discharge are clear: early aging, degraded reliability, higher maintenance costs, and a shortened transformer service life.

Understanding the partial discharge mechanism, its common sources, and the areas where it usually occurs enables asset managers to identify potential failures beforehand. Identifying these risks is the first step in safeguarding key assets and maintaining a continuous power supply.

In the second part of this series, we will explore practical solutions for mitigating partial discharge in transformers, highlighting monitoring technologies and maintenance strategies that can extend asset life and improve overall system reliability.