When selecting a power transformer, insulation is often discussed as if it were a single parameter. In reality, transformer insulation design involves two fundamentally different aspects: thermal insulation performance and dielectric insulation strength.

The thermal side determines how the insulation system withstands operating temperature, temperature rise, hot-spot temperature and thermal ageing. The dielectric side determines whether the transformer can withstand power-frequency voltage, lightning impulse and other transient overvoltages.

Therefore, thermal class and dielectric insulation level must be selected separately. Confusing these two concepts can lead to incorrect transformer specifications, unnecessary costs or inadequate insulation coordination.

1. What Is Transformer Insulation Class?

The terms Class A, E, B, F and H are commonly used to describe the thermal classification of electrical insulation systems or insulating materials.

IEC 60085 establishes criteria for evaluating the thermal endurance of electrical insulating materials and electrical insulation systems and assigning thermal classes. Importantly, the standard distinguishes between the thermal classification of insulation materials and that of complete insulation systems. ([IEC Webstore][1])

Typical reference thermal classes include:

These temperatures should not be interpreted as universal transformer winding hot-spot limits.

A transformer manufacturer must evaluate the complete insulation system, temperature-rise design, cooling arrangement, loading conditions and applicable transformer standard before determining the allowable operating temperature.

Thermal Class Does Not Equal Temperature Rise

This distinction is essential.

For example:

Class F ≠ 100 K temperature rise

and

Class H ≠ 125 K temperature rise

Thermal class describes the thermal endurance of the insulation system or material. Transformer temperature-rise limits are specified separately according to the transformer design, applicable standard and operating conditions.

For liquid-immersed transformers, IEC 60076-2 specifically addresses temperature-rise requirements and temperature-rise tests. ([IEC Webstore][2])

2. Thermal Class vs Dielectric Insulation Level

This is the most important concept in transformer insulation selection.

Therefore:

Thermal class determines the thermal endurance of the insulation system, while dielectric insulation level determines the ability of the transformer insulation to withstand electrical stresses.

Increasing the transformer voltage rating does not automatically mean that the thermal class must be increased.

For example, a high-voltage oil-immersed power transformer may use a conventional cellulose-based insulation system while having a much higher dielectric insulation level than a low-voltage dry-type transformer.

These are two independent design dimensions.

3. How to Select the Transformer Thermal Class

The correct thermal-class selection should begin with the actual operating conditions rather than simply choosing a higher class.

A practical selection process can be divided into five steps.

Step 1: Identify the Transformer Type and Insulation System

First determine whether the transformer is:

• Mineral-oil-immersed

• Ester-filled

• Cast-resin dry-type

• Air-insulated dry-type

• Another specialized design

The insulation system should then be evaluated as a complete system rather than by the name of one insulating material.

Oil-immersed transformers

Conventional mineral-oil-immersed transformers commonly use cellulose-based insulation systems. Thermally upgraded paper and other high-temperature insulation technologies can also be used when the design requires improved thermal performance.

For transformers using high-temperature insulation materials, IEC 60076-14 specifically addresses design, testing and loading considerations. ([IEC Webstore][2])

Therefore, it is better to specify the complete insulation system and its thermal performance rather than simply saying that an oil transformer is “Class A.”

Dry-type transformers

Class F and Class H insulation systems are widely used in modern dry-type transformers.

The appropriate choice depends on:

• Ambient temperature

• Load profile

• Cooling conditions

• Required temperature rise

• Overload requirements

• Insulation system construction

• Installation environment

IEEE standards also distinguish dry-type transformer requirements and testing from liquid-immersed transformer requirements. ([IEEE Xplore][3])

4. Consider Ambient Temperature and Load Profile

Ambient temperature is one of the most important factors in thermal transformer selection.

A transformer operating continuously at 45°C ambient temperature has considerably less thermal margin than one operating at 25°C, even if both have the same rated power.

The following conditions should be identified during specification:

Harmonic loads deserve particular attention. Rectifiers, variable-frequency drives, photovoltaic inverters and other power-electronic loads can produce nonsinusoidal currents and additional transformer losses.

IEEE C57.110 provides methods for evaluating transformer capability when supplying nonsinusoidal load currents and applies to both liquid-immersed and dry-type transformers within its scope. ([IEEE Xplore][4])

5. How to Evaluate Winding Hot-Spot Temperature

The winding hot-spot temperature is more important for thermal ageing than the average winding temperature.

A simplified engineering concept is:

Hot-spot temperature = Ambient temperature + relevant temperature-rise components + hot-spot allowance

However, this should not be treated as a universal calculation using one fixed hot-spot gradient.

The actual hot-spot temperature depends on:

• Transformer construction

• Winding geometry

• Loss distribution

• Cooling method

• Load level

• Oil circulation or air circulation

• Thermal characteristics of the insulation system

IEEE has dedicated guidance for determining hottest-spot temperature in both liquid-filled and dry-type transformer applications. For example, IEEE C57.134 addresses methodologies for determining steady-state winding hottest-spot temperature in dry-type transformers. ([IEEE Xplore][5])

For liquid-immersed transformers, the temperature-rise design should be evaluated according to the applicable requirements and test methods of IEC 60076-2 or the corresponding IEEE requirements.

A Better Engineering Approach

Instead of saying:

45°C + 115 K + 8 K = 168°C → therefore Class H is required

the correct engineering approach is:

Maximum ambient temperature → load profile → transformer thermal model → winding temperature rise → hot-spot temperature → insulation-system thermal capability → required service life

If the calculated or tested hot-spot temperature is excessive, possible solutions include:

• Increasing transformer capacity

• Reducing continuous loading

• Improving cooling

• Reducing losses

• Optimizing winding design

• Selecting a suitable insulation system

• Using high-temperature insulation technology where justified

Therefore, upgrading from Class F to Class H is only one possible solution, not an automatic requirement.

6. Consider Thermal Ageing and Service Life

Transformer insulation gradually deteriorates under thermal, electrical and environmental stresses.

For many conventional insulation systems, a 10°C increase in insulation temperature is often used as a rule of thumb to illustrate accelerated thermal ageing. However, actual insulation ageing depends on the complete insulation system and factors such as:

• Temperature

• Moisture

• Oxygen

• Insulation material

• Oil condition

• Load profile

• Operating duration

Therefore, the commonly quoted “10°C rule” should be treated as an engineering approximation rather than a universal lifetime equation.

For applications requiring long service life, the transformer specification should define the expected load profile and operating conditions rather than simply requiring a higher thermal class.

7. When Should You Consider a Higher Thermal Class?

A higher thermal class may be appropriate when the transformer is exposed to demanding thermal conditions.

Typical factors include:

High ambient temperature

Examples include:

• Desert installations

• Tropical environments

• Poorly ventilated electrical rooms

• Enclosed substations

• Containerized substations

Frequent overload

If the transformer is expected to operate above rated load for significant periods, the thermal design should be evaluated according to the actual loading profile.

Harmonic loads

Industrial drives, rectifiers, data centers, renewable-energy converters and other nonlinear loads may increase transformer losses and heating.

Limited cooling

For dry-type transformers, inadequate room ventilation can significantly affect the actual operating temperature.

Special operating requirements

Mining, metallurgy, rail transit, renewable energy and other applications may require customized thermal and insulation designs.

However, these conditions do not automatically mean that Class H must be selected. The manufacturer should evaluate the complete thermal design first.

8. Environmental Conditions Should Not Be Solved by Thermal Class Alone

Another common mistake is assuming:

Harsh environment → Class H insulation

This is not necessarily correct.

Different environmental problems require different engineering measures.

For example, high humidity may require appropriate moisture protection and insulation-system selection, but simply changing from Class F to Class H does not automatically solve the problem.

Similarly, high altitude affects both thermal and dielectric design, so it should be considered separately rather than treated as a thermal-class issue alone.

9. Select the Dielectric Insulation Level Separately

After the thermal design has been established, the transformer dielectric insulation level should be selected separately.

The basic process is:

System voltage → Highest voltage for equipment (Um) → Insulation coordination → AC withstand → Lightning impulse → Switching impulse where applicable

For medium- and high-voltage transformers, important dielectric parameters may include:

• Power-frequency withstand voltage

• Lightning impulse withstand voltage (LI or LIWV)

• Switching impulse withstand voltage (SI or SIWV), where applicable

• Internal insulation coordination

• External clearances

• Creepage distances

IEC 60076-3 specifically covers transformer insulation levels, dielectric tests and related external clearances. Therefore, it is a much more precise reference for this part of transformer specification than simply citing the general IEC 60076 series.

Typical Application Considerations

Overhead-line connection

Lightning impulse insulation coordination is particularly important because of the exposure to atmospheric overvoltages.

Cable-connected transformer

The overvoltage environment is different from an overhead-line connection and should be assessed based on the actual network configuration.

GIS-connected transformer

Fast transient phenomena and the actual GIS-transformer interface may require additional consideration.

The important point is:

The transformer thermal class should not be used as a substitute for dielectric insulation coordination.

10. Thermal Design and Dielectric Design Must Be Verified Separately

A well-designed transformer specification should therefore contain two independent sections.

This separation makes the technical specification much clearer and reduces the risk of selecting an insulation system based solely on voltage or temperature class.

11. Transformer Insulation Tests and Verification

The test program should also be divided into thermal and dielectric verification.

Thermal verification

Depending on transformer type and applicable standard, this may include:

• Temperature-rise test

• Winding temperature-rise verification

• Hot-spot temperature evaluation

• Loading capability assessment

For liquid-immersed transformers, IEC 60076-2 provides the relevant temperature-rise framework and test methods. ([IEC Webstore][2])

Dielectric verification

Depending on transformer design and specification, tests may include:

• Applied voltage test

• Induced voltage test

• Lightning impulse test

• Partial discharge measurement where applicable

• Insulation resistance measurements as specified

These tests verify dielectric performance and should not be confused with thermal-class verification.

For dry-type transformers, IEEE C57.12.91 provides test methods for tests specified by the applicable dry-type transformer standards and user specifications. ([IEEE Xplore][6])

12. Practical Transformer Insulation Selection Examples

Instead of using a simple rule such as “high temperature = H class,” consider the complete application.

Example 1: Conventional oil-immersed distribution transformer

Conditions:

• 10/0.4 kV

• Ambient temperature within the specified standard conditions

• Stable load

• Low harmonic content

• Conventional distribution application

A conventional cellulose-based insulation system may be appropriate. The actual temperature-rise and loading limits should be established according to the applicable transformer standard and manufacturer design.

Example 2: Indoor cast-resin transformer

Conditions:

• 10/0.4 kV

• Commercial building

• Normal ambient temperature

• Moderate load variation

A Class F insulation system may be suitable if the complete thermal design meets the required temperature-rise and service-life requirements.

Example 3: Dry-type transformer in a high-temperature electrical room

Conditions:

• High ambient temperature

• Restricted ventilation

• Significant load variation

• Harmonic-producing equipment

The manufacturer should first calculate the actual thermal performance. Depending on the resulting hot-spot temperature and required service life, the solution may involve higher capacity, improved cooling, reduced temperature rise, or a higher-temperature insulation system.

Example 4: High-voltage power transformer

A high-voltage transformer does not automatically require a higher thermal class simply because its voltage is higher.

Its dielectric insulation level must be selected according to the system voltage and insulation coordination, while its thermal design must be determined independently.

This is one of the most important principles in transformer insulation selection.

13. Common Transformer Insulation Selection Mistakes

Mistake 1: Higher voltage means higher thermal class

Incorrect.

Voltage primarily affects dielectric insulation coordination, while thermal class relates to thermal endurance.

Mistake 2: Class H is always better than Class F

Not necessarily.

A higher thermal class may be unnecessary if the transformer already meets the required temperature-rise, hot-spot and service-life requirements.

The correct approach is to select the insulation system according to actual operating conditions.

Mistake 3: Thermal class directly determines temperature rise

Incorrect.

Thermal class and transformer temperature-rise limits are related but are not interchangeable.

Mistake 4: Average winding temperature is the same as hot-spot temperature

Incorrect.

The winding hot spot can be significantly higher than the average winding temperature, and hot-spot temperature is important for evaluating thermal ageing.

Mistake 5: A higher thermal class solves every harsh environmental condition

Incorrect.

Humidity, dust, salt fog, chemical contamination and altitude require their own design considerations.

Mistake 6: Insulation thermal class determines BIL

Incorrect.

Lightning impulse insulation level is part of the dielectric insulation design and should be selected according to the applicable insulation coordination and transformer standard.

14. Transformer Insulation Selection Checklist

Before finalizing a transformer specification, verify the following:

15. Which Standards Should Be Considered?

The exact standards depend on the transformer type and target market, but the following are important references when developing an international transformer specification:

IEC 60085 — Thermal evaluation and designation of electrical insulation. It establishes criteria for evaluating the thermal endurance of electrical insulating materials and insulation systems. ([IEC Webstore][1])

IEC 60076-2 — Power transformers, Part 2: Temperature rise for liquid-immersed transformers. ([IEC Webstore][2])

IEC 60076-3 — Power transformers, Part 3: Insulation levels, dielectric tests and external clearances.

IEC 60076-14 — Liquid-immersed power transformers using high-temperature insulation materials. ([IEC Webstore][2])

For North American projects, applicable IEEE/ANSI standards should be selected according to transformer type and project requirements. For example, IEEE C57.12.01 covers general requirements for dry-type distribution and power transformers, while IEEE C57.110 addresses transformer capability with nonsinusoidal load currents. ([IEEE Xplore][3])

Conclusion

Accurate transformer insulation selection should never be reduced to simply choosing Class A, B, F or H.

A reliable selection process should separate two independent design questions:

Thermal design

Ambient temperature → Load → Cooling → Temperature rise → Hot-spot temperature → Thermal ageing → Service life

and

Dielectric design

System voltage → Um → Insulation coordination → AC withstand → LI/BIL → SI where applicable

The thermal class describes the thermal endurance of the insulation system, while the dielectric insulation level determines its ability to withstand electrical stress. IEC 60085, IEC 60076-2, IEC 60076-3 and IEC 60076-14 address different parts of this design process rather than providing one single “transformer insulation grade” for every application. ([IEC Webstore][1])

For transformer buyers and engineers, the best approach is therefore not to choose the highest insulation class, but to specify the insulation system, thermal performance and dielectric insulation level according to the actual voltage, load, environment and service conditions.

Youth Transformer Co.,Ltd.