One of the most prevalent triggers for localized discharge lies in the presence of geometric irregularities on insulating components and metal structures within the transformer. Epoxy resin-encapsulated transformers rely on precise manufacturing to ensure smooth surfaces on conductors, metal housings, and insulating parts—but even minor deviations can have significant consequences.
When exposed to high electric field intensity (a common operating condition for dry-type transformers), charge particles naturally accumulate at areas with minimal curvature, such as sharp edges, protrusions, or residual burrs. This phenomenon, known as the “point effect” in electrical engineering, occurs because the electric field lines become densely packed around irregular surfaces, creating localized regions of extreme field strength. Over time, this concentrated charge exceeds the breakdown voltage of the surrounding medium (either epoxy resin or air gaps), initiating small-scale discharges that erode insulation materials at the point of concentration.
For example, during the fabrication of high-voltage or low-voltage leads, incomplete deburring or imprecise cutting can leave tiny metal protrusions. Even if these burrs measure just a few micrometers in height, they can amplify the electric field by a factor of 5–10 compared to smooth surfaces, making them hotspots for localized discharge.
The epoxy resin encapsulation process is designed to provide a seamless, insulating barrier—but inadequate process control can introduce microscopic or macroscopic voids (air bubbles) within the resin matrix, which act as potent sources of localized discharge.
Void formation typically stems from flaws in the vacuum casting process, a critical step in epoxy resin encapsulation. Common issues include insufficient degassing of the resin mixture prior to pouring, incomplete removal of air from the mold cavity, or uneven flow of resin during filling. Additionally, rapid curing of the epoxy (due to improper temperature control) can trap air pockets before they have a chance to rise to the surface and escape. Even small voids—less than 1mm in diameter—can pose risks, as their dielectric properties differ drastically from the surrounding resin.
A key technical principle behind void-induced discharge is the significant difference in dielectric permittivity between air (εᵣ ≈ 1.0) and epoxy resin (εᵣ ≈ 3.5–4.0). According to electrical field theory, when two materials with varying dielectric constants are exposed to the same electric field, the field intensity within the material with the lower permittivity (air, in this case) is proportionally higher. This means that the electric field inside a void can be 3–4 times stronger than in the adjacent epoxy resin. When this amplified field exceeds the breakdown strength of air (approximately 3 kV/mm at standard conditions), localized discharge occurs within the void, gradually damaging the surrounding resin and expanding the defect over time.
Electrical continuity between conductors, windings, and terminal connections is critical for stable transformer operation—any disruption or imperfection in these connections can lead to localized discharge, particularly through the generation of floating potentials.
Floating potentials arise when a metal component is not properly grounded or electrically connected to the transformer’s main circuit, leaving it at an undefined voltage relative to other parts. This often occurs due to poor contact between conductors, such as loose terminal connections, oxidized contact surfaces, or inadequate soldering/brazing. When current flows through these high-resistance connections, voltage drops occur, creating localized regions of unstable potential. These regions act as discharge sources, as the voltage difference between the floating component and adjacent parts can exceed the insulation’s withstand capacity.
Over time, repeated discharge at these connection points can cause further oxidation, corrosion, or melting of metal surfaces, worsening contact resistance and creating a vicious cycle of increasing discharge activity. This not only degrades the transformer’s electrical performance but also increases the risk of arcing and thermal damage.
Environmental conditions and handling practices play a significant role in triggering localized discharge, particularly through their impact on the transformer’s insulation system.
High ambient air humidity can penetrate the transformer’s encapsulation (even through minor seals or cracks) and be absorbed by insulating materials such as epoxy resin, paper, or pressboard. Moisture reduces the insulation’s breakdown voltage and increases its conductivity, making it more susceptible to localized discharge. For example, if the water content in epoxy resin exceeds 0.5%, the material’s dielectric strength can drop by 30–50%, creating conditions where even moderate electric fields can initiate discharge.
Physical damage to insulation during installation—such as scratches, impacts, or improper handling of windings—can create microcracks or exposed surfaces that act as discharge hotspots. Similarly, prolonged idleness (e.g., storage for more than 6 months without proper maintenance) can lead to moisture absorption, dust accumulation, or degradation of encapsulation materials. In such cases, the insulation’s performance degrades over time, and when the transformer is put into service, the weakened insulation cannot withstand normal operating voltages, resulting in localized discharge.
The foundational design of the transformer’s insulation system, the quality of raw materials, and the precision of manufacturing processes are all critical factors that influence the risk of localized discharge.
Insufficient attention to electric field distribution during the design phase can lead to inherent vulnerabilities. For instance, inadequate spacing between winding layers, improper arrangement of high- and low-voltage coils, or sharp bends in lead conductors can create regions of excessive electric field intensity. Even if the transformer is manufactured to specification, a poorly designed insulation structure will inevitably concentrate charges, leading to localized discharge during operation.
The performance of epoxy resin-encapsulated transformers is directly tied to the quality of insulating materials. Substandard epoxy resin—such as resin with impure fillers, inconsistent curing agents, or improper mixing ratios—may have uneven dielectric properties, creating internal interfaces that trap charges. Similarly, deficiencies in key manufacturing processes (e.g., inadequate drying of windings before encapsulation, imprecise temperature control during resin curing, or incomplete vacuum degassing) can introduce defects like burrs, voids, or uneven resin thickness.
The assembly process for high- and low-voltage leads is particularly critical, as any imperfection can create discharge hotspots. For example, imprecise cutting of leads, residual burrs on conductor ends, or incorrect spacing between leads and grounded components can all lead to electric field concentration. Additionally, poor alignment of windings during assembly can cause uneven pressure on the epoxy encapsulation, leading to microcracks or voids that become discharge sites.
Localized discharge in epoxy resin-encapsulated transformers is a multifaceted issue driven by geometric irregularities, manufacturing defects, environmental factors, design flaws, and poor maintenance practices. Addressing these factors requires a holistic approach—from optimizing insulation design and improving material quality to enhancing manufacturing process control and implementing rigorous maintenance protocols. By understanding the technical mechanisms behind each cause, manufacturers and operators can minimize the risk of localized discharge, extend transformer service life, and ensure reliable, safe operation in industrial, commercial, and utility applications.
