Transformer Testing: Guide to Transformer Dielectric Integrity Testing (Part-4)

Dielectric tests are designed to assess the integrity of transformers under voltage stresses encountered during both normal and abnormal operations.

Normal operation is defined as long time exposure to voltages close to rated voltage at the transformer terminals, together with possible transient over-voltages.?

Typically, over-voltages are categorized into three groups:?

• Over-voltages in the power frequency range with a duration in the order of seconds.
• Switching over-voltages with a duration in the order of a fraction of a second.
• Lightning over-voltages with a duration in the order of microseconds.

These different groups of over-voltages are considered in the test code, which may specify one or several tests to be conducted, either individually or in combination. The specific test code for a transformer primarily depends on its size, rated voltages, and the standards specified for the transformer.

By rigorously testing each transformer before it leaves the factory, manufacturers can identify and address any defects that might lead to failures, ensuring that only high-quality components are installed in the field. This proactive approach to quality assurance plays a critical role in protecting the electrical grid from disruptions and accidents, thereby enhancing the overall stability and efficiency of power distribution networks.

References / Standards?

Factory testing on distribution and power transformers are performed according to IEC and IEEE standards.

Standards for Liquid-Immersed Transformers?

• IEC 60076-3 (2013): Power transformers – Part 3: “Insulation levels, dielectric tests and external clearances in air”.

IEC 60076-3:2013 specifies the insulation requirements and the corresponding insulation tests with reference to specific windings and their terminals. It also recommends external clearances in air. It gives details of the applicable dielectric tests and minimum dielectric test levels. Requirements and tests for three categories of transformers are identified and summarized in Table 1, along with the relevant test requirements for each category.?

• IEEE C57.12.90-2021: IEEE Standard Test Code for Liquid-Immersed Distribution, Power and Regulating Transformers, clause 10: “Dielectric tests”.
• IEEE C57.12.00-2015: IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.

The IEEE C57.12.90-2021 standard outlines procedures for performing tests on liquid-immersed distribution, power, and regulating transformers, as specified in IEEE C57.12.00-2021 and other relevant standards. This standard covers a wide range of tests, including resistance measurements; polarity and phase-relation tests; ratio tests; no-load loss and excitation current measurements; impedance and load loss measurements; dielectric tests; temperature tests; short-circuit tests; audible sound level measurements; and the calculation of related data.

The IEEE C57.12.00-2015 standard specifies the electrical and mechanical requirements for liquid-immersed distribution, power, and auto-transformers, as well as regulating transformers. It covers both single-phase and polyphase transformers with voltages of 601 V or higher in the highest voltage winding. This standard serves as a foundation for defining performance criteria and establishing limited electrical and mechanical interchangeability requirements for such equipment. Additionally, it provides guidance for the proper selection of these transformers. The requirements set forth in this standard are applicable to all liquid-immersed distribution, power, and regulating transformers.?

Standards for Dry-Type Transformers?

• IEC C57.12.91-2020: IEEE Standard Test Code for Dry-Type Distribution and Power Transformers
• IEEE C57.12.01-2020: IEEE Standard for General Requirements for Dry-Type Distribution and Power Transformers

The IEEE C57.12.91-2020 standard outlines methods for conducting tests on dry-type distribution and power transformers, as specified in IEEE Std C57.12.01 and other related standards. These methods apply to transformers with a voltage of 601 V or higher in the highest voltage winding. Designed to serve as a foundational guide for performance evaluation and proper testing, this standard is applicable to all dry-type transformers, including those with solid cast and resin-encapsulated windings. It provides detailed procedures for testing these types of transformers.?

The IEEE C57.12.01-2020 standard specifies the electrical and mechanical requirements for single and polyphase dry-type distribution and power transformers, including ventilated, non-ventilated, and sealed models, or autotransformers, with a voltage of 601V or higher in the highest voltage winding. It applies to all dry-type transformers, encompassing those with solid-cast and resin-encapsulated windings.

REFERENCES

[1] IEC 60076-3 (2013): Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in air.

[2] IEEE C57.12.90-2021: IEEE Standard Test Code for Liquid-Immersed Distribution, Power and Regulating Transformers.

[3] IEEE C57.12.00-2015: IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.

[4] IEC C57.12.91-2020: IEEE Standard Test Code for Dry-Type Distribution and Power Transformers

[5] IEEE C57.12.01-2020: IEEE Standard for General Requirements for Dry-Type Distribution and Power Transformers