Why Furan Testing Is Important for Evaluating Transformer Insulation Health

Understanding the Benefits of Furan Analysis in Transformer Dielectric Fluids

When evaluating the condition of an oil-filled transformer, routine dielectric fluid testing provides valuable information about the quality and condition of the insulating liquid. Tests such as dissolved gas analysis (DGA), moisture content, dielectric breakdown voltage, acidity, and interfacial tension help identify potential problems with the transformer and its insulating fluid.

However, there is another important diagnostic test that can provide insight into a different aspect of transformer health: furan analysis.

Furan testing is used to evaluate the condition and deterioration of the cellulose paper insulation inside a transformer. Because the paper insulation is one of the most critical components of a transformer's internal insulation system, understanding its condition can help utilities, industrial facilities, and transformer owners make better decisions about maintenance, repairs, and equipment replacement.

For transformers that have been in service for many years, furan testing can provide particularly valuable information about the aging process and the remaining mechanical strength of the paper insulation.

What Is Furan Testing?

Furan testing is a laboratory analysis performed on a sample of transformer dielectric fluid to identify and measure chemical compounds known as furanic compounds.

These compounds are produced as cellulose-based materials deteriorate. Inside a transformer, cellulose paper and pressboard are commonly used to insulate windings, conductors, and other internal components. Over time, exposure to heat, moisture, oxygen, and electrical stresses causes the cellulose insulation to age and break down.

As this deterioration occurs, chemical compounds are released from the paper insulation. Some of these compounds migrate into the surrounding dielectric fluid, where they can be detected through laboratory analysis.

The most commonly evaluated furanic compound is 2-furfuraldehyde (2-FAL), also referred to as furfural.

Although several furanic compounds can be identified, 2-FAL is generally considered the most useful indicator for evaluating the aging of cellulose insulation in mineral-oil-filled transformers.

Furan analysis is commonly performed using High-Performance Liquid Chromatography (HPLC), following ASTM D5837, Standard Test Method for Furanic Compounds in Electrical Insulating Liquids.

Why Is the Condition of Transformer Paper Insulation Important?

A transformer relies on two primary insulation systems:

• Liquid insulation: The dielectric fluid provides electrical insulation, cooling, and heat transfer.
• Solid insulation: Cellulose paper and pressboard provide electrical separation between windings, conductors, and grounded components.

While dielectric fluid can often be processed, filtered, or replaced to restore certain properties, cellulose paper insulation is much more difficult to repair or replace without extensive transformer disassembly.

As paper insulation ages, the cellulose molecules begin to break down into shorter molecular chains. This process is known as depolymerization.

The deterioration of these molecular chains reduces the mechanical strength of the paper. Eventually, the paper can become brittle and lose its ability to withstand the mechanical forces associated with transformer operation.

This is particularly concerning during electrical faults, short circuits, and other events that place significant mechanical stress on transformer windings.

A transformer may continue operating even when its paper insulation has experienced substantial deterioration. However, weakened insulation increases the risk of damage during abnormal operating conditions.

Furan testing provides an opportunity to identify this deterioration before it becomes an obvious operational problem.

Understanding the Relationship Between Furan Levels and Degree of Polymerization

One of the most important benefits of furan testing is its relationship with the Degree of Polymerization (DP) of cellulose paper insulation.

Degree of Polymerization is a measurement of the average length of the cellulose molecular chains that make up the paper insulation. It is commonly used as an indicator of the mechanical condition of the paper.

New cellulose insulation typically has a DP value in the range of approximately 1,000 to 1,200, although actual values vary depending on the material.

As the paper ages, its DP value decreases.

The following ranges provide a general illustration of how DP relates to paper insulation condition:

• DP 1,000–1,200: Relatively new cellulose insulation with good mechanical strength.
• DP 600–800: Some aging has occurred, but the paper generally retains substantial mechanical strength.
• DP 400–500: Advanced aging with a noticeable reduction in mechanical strength.
• DP 200–250: Severely deteriorated insulation with significantly reduced mechanical strength.

A DP value approaching 200 is generally associated with the loss of most of the original mechanical strength of the cellulose insulation.

Because furanic compounds are produced as cellulose deteriorates, the concentration of 2-FAL in the dielectric fluid can be used to estimate the aging condition of the paper insulation.

It is important to understand that this is an indirect relationship. Furan concentration does not provide an actual measurement of DP, and estimating DP from furan results requires consideration of transformer design, insulation materials, operating history, and other conditions.

Nevertheless, furan analysis is a valuable diagnostic tool for evaluating the condition of insulation that would otherwise be difficult to access without opening the transformer.

Five Important Benefits of Performing Furan Testing

1. Evaluating the Age and Condition of Paper Insulation

A transformer can have dielectric fluid that meets acceptable testing requirements while the internal paper insulation is significantly deteriorated.

This is one of the most important reasons to include furan analysis in a transformer testing program.

Traditional dielectric fluid testing primarily evaluates the condition of the liquid insulation. DGA helps identify gases associated with thermal and electrical faults. Furan testing provides additional information about the deterioration of the solid cellulose insulation.

By incorporating furan analysis into a routine testing program, transformer owners can develop a more complete understanding of the condition of their equipment.

2. Identifying Potential End-of-Life Conditions

The condition of the paper insulation is one of the primary factors influencing the useful service life of a transformer.

Unlike dielectric fluid, which can be processed or replaced, deteriorated paper insulation cannot easily be restored.

As the paper loses mechanical strength, the transformer becomes increasingly vulnerable to damage during short circuits and other abnormal operating conditions.

Elevated furan concentrations may indicate advanced cellulose deterioration and can help identify transformers that require additional investigation.

This information can be particularly valuable when determining whether an older transformer should continue operating, undergo additional evaluation, or be considered for replacement.

Furan testing does not establish an exact remaining service life. However, it can provide important information when evaluating the overall condition and future reliability of a transformer.

3. Supporting Preventative Maintenance Programs

Preventative maintenance is intended to identify potential problems before they result in equipment failures.

Furan analysis adds another dimension to transformer condition monitoring by helping identify deterioration that may not be apparent through conventional dielectric fluid testing.

When performed periodically, furan testing allows transformer owners to monitor changes in the concentration of furanic compounds over time.

An increasing 2-FAL concentration may indicate continuing deterioration of the cellulose insulation.

Tracking these changes can help maintenance personnel identify transformers that warrant closer monitoring, additional diagnostic testing, or more frequent inspections.

Rather than relying exclusively on transformer age or operating hours, owners can use actual testing data to make more informed maintenance decisions.

4. Improving Transformer Purchasing and Selling Decisions

Furan testing can be particularly beneficial when purchasing, selling, or evaluating used transformers.

A transformer may have an acceptable appearance, operate normally, and pass routine dielectric fluid testing. However, these conditions do not necessarily indicate that the internal paper insulation is in good condition.

For companies purchasing used transformers, furan testing provides additional information that can help evaluate the equipment before completing a transaction.

For sellers, having documented furan results can help demonstrate the condition of the transformer and provide prospective buyers with additional confidence in their purchase.

This is especially important for older transformers where the remaining condition of the paper insulation may significantly influence equipment value.

When evaluating used transformers, furan results should be considered alongside DGA, dielectric fluid quality testing, maintenance history, loading history, and other available inspection information.

5. Providing Information That Complements Dissolved Gas Analysis

Dissolved Gas Analysis is one of the most widely used diagnostic tools for identifying potential transformer problems.

DGA evaluates gases generated by electrical and thermal stresses within a transformer. These gases can help identify conditions such as overheating, arcing, partial discharge, and other developing faults.

However, DGA and furan analysis provide different types of information.

DGA primarily evaluates gases associated with electrical and thermal activity, while furan analysis provides insight into the deterioration of cellulose-based insulation.

Using both tests together provides a more comprehensive understanding of transformer condition.

For example, a transformer may not exhibit significant abnormal gas generation but may still have experienced years of gradual paper insulation deterioration.

Furan testing can help identify this type of aging that may not be fully characterized by DGA alone.

What Do Elevated Furan Results Mean?

Furan results are generally reported in micrograms per liter (µg/L), which is equivalent to parts per billion (ppb).

Higher concentrations of 2-FAL generally indicate greater cellulose degradation. However, there is no single concentration that universally determines whether a transformer is approaching the end of its useful life.

The interpretation of results should consider several factors, including:

• Transformer age and operating history.
• Type and condition of the cellulose insulation.
• Operating temperatures and loading conditions.
• Moisture content within the insulation system.
• Type of dielectric fluid being used.
• Previous oil processing, oil replacement, or fluid change-outs.
• Previous furan test results and the rate of change over time.

A single furan result provides useful information, but comparing multiple results over time can provide a much better indication of the insulation's aging progression.

It is also important to recognize that furan concentrations can be affected by the distribution of compounds between the paper and the dielectric fluid, operating temperature, and other factors.

An important consideration is transformer oil replacement. Replacing the dielectric fluid can remove a significant portion of the furanic compounds that have accumulated in the oil. Consequently, a transformer that has undergone an oil change may produce artificially lower furan results, even though the condition of the paper insulation has not improved.

For this reason, previous oil changes and processing history should always be considered when interpreting furan results.

How Often Should Furan Testing Be Performed?

The appropriate frequency of furan testing depends on the age, importance, operating conditions, and overall condition of the transformer.

For newer transformers, routine furan testing may not be necessary as frequently as it is for older units.

For transformers that have been in service for several decades, periodic furan analysis can be a valuable part of a comprehensive maintenance program.

Testing frequency should be established based on the equipment's condition and the recommendations of qualified transformer engineers and maintenance professionals.

Furan testing should also be considered when:

• Purchasing or selling used transformers.
• Evaluating transformers with extensive operating histories.
• Investigating unexplained changes in transformer performance.
• Assessing transformers that have experienced prolonged high-temperature operation.
• Developing long-term transformer replacement plans.
• Establishing baseline condition data for critical electrical assets.

The Importance of Comprehensive Transformer Fluid Testing

No single laboratory test can provide a complete assessment of transformer condition.

Furan analysis is most valuable when used in conjunction with other diagnostic tests that evaluate both the dielectric fluid and the internal insulation system.

A comprehensive transformer testing program may include:

• Dissolved Gas Analysis (DGA).
• Furan analysis.
• Moisture content.
• Dielectric breakdown voltage.
• Acidity (neutralization number).
• Interfacial tension.
• Color and visual examination.
• Power factor or dissipation factor.

Together, these tests provide valuable information about the condition of the dielectric fluid, potential developing faults, and the aging of the transformer's internal insulation.

For transformer owners, this information supports better maintenance planning, more informed asset management decisions, and improved long-term reliability.

Protect Your Transformer Investment with Professional Dielectric Fluid Testing

Understanding the condition of a transformer's internal paper insulation is just as important as understanding the condition of its dielectric fluid.

Furan testing provides valuable insight into cellulose insulation deterioration and can help transformer owners identify aging equipment, evaluate potential end-of-life conditions, and make more informed decisions about maintenance and replacement.

Power Substation Services provides comprehensive dielectric fluid testing services to help utilities, industrial facilities, and transformer owners evaluate the condition of their equipment.

Our onsite dielectric fluid testing laboratory offers a wide range of analytical services, including furan analysis, dissolved gas analysis, and PCB testing.

With our focus on accurate testing and fast turnaround times, we help customers obtain the information they need to make informed decisions about their transformer assets.

Contact Power Substation Services to learn more about our dielectric fluid testing capabilities and how furan analysis can help you better understand the condition of your transformers.

Visit Power Substation Services to learn more about our services.