#highvoltageinsulation

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The hidden weak link that can take your transformer offline

Stem: In high-voltage transformer design, the interface between pressboard and oil is often the weakest link in the insulation chain; understanding creep breakdown along this boundary is critical for engineers who need to prevent surface discharges that slowly develop into permanent conduction paths.

Key Takeaway: Localised electrical stress causes surface discharge on solid insulation material at normal operating voltages, slowly developing into a permanent conduction path leading to dielectric failure; different ratios of liquid/solid insulation permittivities cause shifts in electric field distribution.

Supporting Material: Research shows that "the different ratios of liquid and solid insulation materials permittivities at interfaces cause a shift in electric field distribution within the insulation system of a transformer."

The Silent Threat Inside Every Large Transformer

You have likely heard of transformer insulation failures. But have you heard about creep breakdown? Unlike a sudden flashover that announces itself loudly, creep breakdown is a slow, insidious process that occurs at the interface between pressboard and oil. It is one of the most dangerous fault conditions in large power transformers—because it can develop quietly, under normal operating conditions, for months or even years before culminating in catastrophic failure.

Surface discharges at the oil-pressboard interface are sustained for long periods without immediate flashover or breakdown, making them notoriously difficult to detect until significant damage has already occurred. Once these discharges create permanent carbon tracking on the pressboard surface, the insulation system is compromised permanently. Replacement becomes the only option.

So what causes this phenomenon? And more importantly, how can you prevent it?

The Root Cause: A Fundamental Mismatch

The oil-pressboard system is a composite insulation where two very different materials share electrical stress. The relative permittivity (dielectric constant) of pressboard is approximately 4.4, while transformer oil sits around 2.2. This ratio of roughly 2:1 is the starting point of the problem.

Under AC voltage, the electric field distributes itself inversely proportionally to each material's permittivity. Because pressboard has the higher permittivity, the oil bears the greater stress. The electric field strength in the oil becomes roughly twice that in the pressboard.

But here is the critical catch: The breakdown field strength of liquid dielectric (oil) is always lower than that of the solid dielectric (pressboard). So the material that receives the higher stress—the oil—is simultaneously the weaker material. This fundamental mismatch creates the perfect conditions for surface discharge initiation.

Dielectric constants: Oil ≈ 2.2, pressboard ≈ 4.4. Ratio ≈ 2:1 Stress distribution (AC): Oil bears ≈ 2× the electric stress of pressboard Consequence: Highest stress on the weakest material → Surface discharge initiation

One patent notes this very issue: "Poor matching between pressboard and oil with respect to dielectric constants results in that the oil is highly stressed under capacitive voltage". The industry has long recognized that decreasing the density of pressboard (typically 1.0–1.3 g/cm³) can reduce the permittivity ratio and improve the situation, but practical manufacturing constraints have limited how far this can be pushed.

From Discharge to Catastrophe: The Creep Breakdown Process

Once the electric stress exceeds the oil's withstand capability, partial discharges occur in the oil gap adjacent to the pressboard surface. The Sokolov creeping discharge model breaks down the failure progression into three connected steps:

Step 1: Partial Breakdown of the Oil Gap

Small discharges appear in the oil near the pressboard surface. At this stage, the insulation still functions normally.

Step 2: Surface Discharges on the Pressboard

The discharges begin to creep along the pressboard surface. The oil gap permits electrical current to flow, but rather than propagate purely in the oil, the discharges more readily creep along the pressboard surface.

Step 3: Carbon Tracking and Permanent Damage

Over time, the surface discharges leave behind carbon tracks on the pressboard. These tracks are conductive paths that progressively lengthen with each discharge event. Pitting evolves into the starting points of electrical trees, gradually leading to final breakdown.

At this point, the insulation is permanently compromised. The carbonized pathways cannot be reversed, and the transformer faces either immediate failure or a significantly reduced remaining life.

Research confirms that creeping discharge at the oil-pressboard interface "is a serious fault condition because it can lead to catastrophic failure under normal operating conditions of large transformers".

Critical Factors That Accelerate Creep Breakdown

1. Temperature

Heat is a double-edged sword. Temperature has little influence on the relative permittivity of oil and pressboard, but great influence on their resistivity. As temperatures rise, resistivity falls—but at different rates for oil and pressboard. This shifting ratio under thermal cycling can significantly alter the field distribution, particularly in DC applications where the voltage division is proportional to volume resistivities. The ratio of PB to oil volume resistivity can reach "several tens or over one hundred", creating extreme stress concentration at the interface.

2. Moisture

Moisture is a known catalyst for surface discharge activity. Even small amounts of water ingress dramatically reduce the surface breakdown voltage of the oil-pressboard interface. Drying the insulation system during manufacturing and maintaining it throughout service is not just good practice—it is essential for preventing creep breakdown.

3. Ageing of Pressboard

Interestingly, research shows that pressboard ageing "affected the discharge inception but had no influence on the interface breakdown strength". In other words, older pressboard may begin discharging at lower voltages, but once the process starts, the progression to breakdown is similar regardless of age. This finding underscores that prevention is far more effective than monitoring when it comes to creep breakdown.

4. Hydraulic Pressure

Recent studies indicate that surface partial discharge intensity decreases with increasing hydraulic pressure. Higher pressure suppresses discharge activity, which explains why well-sealed, pressurized transformer designs tend to exhibit better resistance to creep breakdown.

Design Strategies to Prevent Creep Breakdown

Strategy 1: Optimize Creepage Distance

The shortest distance along an insulating surface between conductive parts is called the creepage distance. Optimizing creepage distance through proper design and material selection can prevent up to 90% of carbonization failures while extending transformer life by eight years or more. Industry standards like IEC 61558 define clear creepage requirements based on voltage class and pollution degree.

Strategy 2: Pressboard Barrier Design

Concentric pressboard barriers placed in the main duct between HV and LV windings are a proven solution for managing creep stress. These barriers subdivide the oil gap, preventing the development of long creepage paths while also providing mechanical support. However, the barriers themselves must be carefully designed to avoid creating new stress concentration points where they contact the duct pieces.

Strategy 3: Lower Permittivity Materials

Utilizing pressboard barriers with lower relative permittivity—closer to that of the liquid—lowers the dielectric stress in the liquid, allowing for smaller oil gaps. This is an active area of material development, with manufacturers exploring lower-density pressboard formulations and alternative fibers to achieve a more favorable permittivity match with the insulating liquid.

Strategy 4: Alternative Dielectric Liquids

Natural ester fluids offer an intriguing advantage. Research shows that "the lower relative permittivity ratio of natural ester impregnated paper to natural ester is beneficial to its dielectric strength". Transformers using natural ester (vegetable oil) as the dielectric fluid exhibit improved resistance to thermal decomposition and potentially reduced creep breakdown risk due to the more favorable permittivity matching with the solid insulation.

Strategy 5: Aramid Composite Barriers

For high-stress applications, aramid composites are increasingly used as barrier sheets that extend the creepage path and shield sharp edges, reducing the likelihood of partial discharge. These materials offer high thermal stability and improved dielectric performance compared to standard pressboard.

The Takeaway for Engineers

Creep breakdown at the oil-pressboard interface is not a theoretical concern—it is a leading cause of field failures in high-voltage transformers. Surface discharges over pressboard surfaces are "one of the prominent causes of a number of failures in transformers". Understanding why it happens—and how to design against it—separates engineers who merely specify insulation from those who truly understand it.

This knowledge is especially critical for professionals on both sides of the industry. If you are an equipment engineer, knowing how to specify creepage distances and barrier configurations directly impacts the reliability of the assets you manage. If you work on the vendor or supplier side, this understanding allows you to communicate technical solutions to customers more effectively, positioning yourself as a true subject-matter expert rather than just another sales contact.

🎓 Ready to Deepen Your Transformer Knowledge?

Creep breakdown is just one of the many hidden risks that can compromise transformer reliability.

The Electrical Transformers Fundamentals course, taught by Mike, is designed specifically for professionals who want to master the practical, real-world knowledge that university courses often miss. With years of hands-on experience in the power utility industry, Mike translates complex engineering concepts into actionable insights—whether you are preparing to specify equipment, troubleshoot field issues, or communicate more effectively within your organization.

The course is built for equipment engineers, utility professionals, and supplier-side technical staff who need more than textbook theory. It covers not just the physics of insulation, but the practical knowledge of how transformers are designed, procured, installed, and maintained in today's demanding operating environment.

Knowledge is power. Make it yours today.