A93 A Review of IEC TR 60076-26:2020 - Functional Requirements of Insulating Liquids for use in Power Transformers

Introduction

According to IEC, as of the date this document IEC TR 60076-26:2020?was written, various suppliers are introducing many new insulating liquids for power transformers. End users of these liquids typically assume that each type has been thoroughly tested and proven to meet the necessary functional requirements as a coolant and electrical insulator, ensuring reliable performance throughout the intended lifespan of the device.

As of now, there is no IEC International Standard that serves as a comprehensive roadmap detailing the specific requirements for liquids used in power transformers, reactors, and other high-voltage equipment. Currently, these liquids are only required to meet general standard specifications, such as IEC 60296, IEC 62770, or IEC 61099.?

In this IEC document which is a Technical Report, the functional requirements deemed essential for the use of an insulating liquid in power transformer applications are outlined. IEC states that detailed technical information about these requirements and their validation is intentionally omitted. This approach is taken because the document is designed to serve as a reference for the transformer industry, including liquid suppliers and relevant scientific and technical bodies involved with insulating liquids.

Some functional requirements of an insulating liquid are interdependent with the design aspects and materials in contact with the liquid. For instance, tests for dielectric withstand capability and considerations of transformer systems aging often require that solid and liquid insulation materials be evaluated together as a cohesive system. Additionally, transformers equipped with tap-changers have specific requirements that are not yet included in current standards are applicable.

To address these complexities, a matrix of functional requirements are developed, which includes the current status of their validation through testing. This matrix is presented in this document. IEC recommends considering these requirements for any insulating liquid intended for use in power transformers, including reactors and HVDC transformers.

The individual requirements (parameters) in the matrix are given a relevance index as to their importance for design and service. Furthermore, for each parameter the status of the method is indicated as:

• Already existing as an IEC/ISO International Standard
• Existing as an IEC/ISO International Standard that requires updating / extension, or
• Does not exist as an IEC/ISO International Standard, thus would require future development

Some of the identified tests are only available as ASTM or IEEE standards. Such tests, as well as missing tests, are recommended as future work items for the relevant CIGRE and IEC bodies.

The IEC states its intention to regularly update this document based on the results produced by the previously mentioned bodies. This will ensure that the most current information on the functional requirements of insulating liquids used in the transformer industry is readily available.

Scope of Technical Report

In this Technical Report, the functional requirements deemed essential for the use of insulating liquids in power transformers, including reactors and HVDC transformers are outlined. Each requirement is accompanied by a relevance index indicating its importance for design and service, as well as the current status of its validation.

Categorized Functional Requirements of Insulating Liquids

The requirements are categorized into the following groups detailed from Table 1 to Table 6:

Table 1: General (physical and chemical properties)

Table 2: Dielectric and Insulation characteristics

Table 3: Thermal and Cooling capacities

Table 4: Ageing and Stability

Table 5: Liquid-solid system interactions

Table 6: Material compatibility

For example, in Table 4, the parameters for the functional category "Aging and Stability" are listed. For each parameter, the Relevance Index for Design and Service, IEC/ISO Test Method, and Comments are provided. The following parameters are included in this Table:

• ???????? Liquid aging marker characterization
• ???????? Oxidation stability
• ??????? UV stability
• ???????? Thermal and hydrolytic stability
• ??????? Stray gassing

Conclusion

IEC TR 60076-26 provides the functional requirements for insulating liquids necessary for use in power transformers, including reactors and HVDC transformers. These requirements are assembled and listed in the report. This Technical Report is designed to serve as a comprehensive reference for the transformer industry, including liquid suppliers and relevant scientific and technical bodies involved with insulating materials.

References

[1] IEC 60076-26:2020, Power transformers - Part 26: Functional requirements of insulating liquids for use in power transformers

[2] M. Eklund, Mineral insulating oils; functional requirements, specifications and production, Conference record of the 2006 IEEE Symposium on Electrical Insulation

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