Abstract: GB/T 1094.3-2017, as the third part of the GB/T 1094 series national standards in China, specifies the insulation levels, dielectric test requirements and minimum external clearances in air for power transformers. This standard, which was released on December 29, 2017 and implemented on July 1, 2018, is revised and adopted with modification based on IEC 60076-3:2013, incorporating technical adjustments adapted to China’s power system conditions. This article elaborates on the core content of GB/T 1094.3-2017, including its scope of application, key technical definitions, insulation test requirements, external insulation clearance specifications, differences from international standards, and its significance in ensuring the safe and reliable operation of power transformers.

1. Introduction

Power transformers are crucial equipment in power transmission and distribution systems, and their insulation performance directly determines the safety and stability of the entire power grid. GB/T 1094.3-2017 “Power Transformers—Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air” is an important normative document that regulates the insulation-related requirements of power transformers in China. It replaces the previous version GB/T 1094.3-2003, incorporating advanced international technical experience while adapting to the development needs of China’s high-voltage and ultra-high-voltage power grids. The formulation and implementation of this standard provide clear technical guidelines for the design, manufacturing, testing and operation of power transformers, and play a vital role in improving the overall reliability of power equipment and reducing insulation-related faults.

2. Scope of Application

GB/T 1094.3-2017 applies to power transformers specified in GB/T 1094.1 (the first part of the GB/T 1094 series: General Requirements). For certain types of power transformers and reactors that have their own independent standards, this standard is only applicable when explicitly referenced by those standards.

The standard specifies the relevant insulation tests and minimum insulation test levels for power transformers. When the user does not put forward specific requirements, it also recommends the minimum air insulation clearances between the external live parts of the transformer and between these parts and the ground. It should be noted that the insulation levels and insulation tests specified in this standard are only applicable to internal insulation. Although it is reasonable to take the rated withstand voltage value of the transformer’s internal insulation as the benchmark for external insulation, this approach may not be consistent with all actual working conditions. Therefore, users can appropriately increase the test voltage value of the transformer’s internal insulation according to safety needs.

3. Key Technical Definitions

GB/T 1094.3-2017 defines a series of core technical terms related to transformer insulation, which lays a foundation for the unified understanding and application of the standard. The main key definitions are as follows:

• Highest Voltage for Equipment (Um): The root-mean-square value of the highest phase-to-phase voltage in a three-phase system for which the insulation of the transformer winding is designed. This parameter is the core basis for determining the insulation level of the transformer.
• Rated Voltage of a Winding (Ur): The specified voltage applied between the terminals of a tapped winding at the main tap or between the terminals of a non-tapped winding, or the voltage induced under no-load conditions. For three-phase windings, it refers to the line-to-line voltage.
• Rated Insulation Level: A set of rated withstand voltages that characterize the dielectric strength of the insulation, which is an important indicator to measure the insulation performance of the transformer.
• Rated Withstand Voltage: The specified test voltage value used in standard insulation withstand tests to verify that the insulation meets the required performance requirements.
• Uniform Insulation of a Transformer Winding: The insulation structure where the insulation level of all parts of the winding is the same, which is usually used in transformers with lower voltage levels or specific application scenarios.
• Non-uniform Insulation of a Transformer Winding: The insulation structure where the insulation level of different parts of the winding is different. For example, the insulation level of the terminal part of the winding is higher than that of the middle part, which is often used in high-voltage and ultra-high-voltage transformers to optimize the insulation design.

4. Core Technical Requirements

4.1 Insulation Test Requirements

GB/T 1094.3-2017 adjusts the insulation test sequence and test items compared with the previous version, and clarifies the specific requirements of various tests. The main insulation tests include the following categories:

• Lightning Impulse Test: Including lightning full-wave impulse test (LI), lightning chopped-wave impulse test (LIC) and neutral terminal lightning full-wave impulse test (LIN). The standard classifies these tests into type tests and special tests, and adjusts the test voltage levels and application methods. For example, in 13.4.1, it is added that for fully insulated transformers with non-leaded neutrals, the lightning full-wave impulse test voltage value and application method are specified.
• Switching Impulse Test: For transformers with Um=126 kV, the standard requires switching impulse test, which is an important measure to verify the insulation performance of the transformer against overvoltage generated during system switching.
• AC Withstand Voltage Test: The standard separates the winding terminal AC withstand voltage test as an independent test item, clarifies its test voltage benchmark and test procedures, and effectively verifies the insulation performance of the transformer under AC working voltage.
• Induced Withstand Voltage Test: Adjusted to be based on the rated voltage (Ur), and the pre-voltage, measurement voltage, measurement time and partial discharge quantity limit for partial discharge measurement are adjusted, which helps to more accurately detect potential insulation defects of the transformer.

4.2 External Insulation Air Clearance Specifications

The standard specifies the minimum air insulation clearances between the external live parts of the transformer and between these parts and the ground. It adjusts the external insulation air clearances compared with the 2003 version, and supplements the insulation levels, neutral point insulation levels and external insulation air clearances for transformers with system nominal voltages of 750 kV and 1000 kV, to meet the technical needs of China’s ultra-high-voltage power grid development.

The determination of external insulation air clearance is based on factors such as the highest voltage for equipment (Um) and the actual operating environment. The standard provides corresponding minimum air clearance values in Appendix E (Informative Appendix), which can be directly referenced in engineering design and manufacturing.

5. Differences from IEC 60076-3:2013

GB/T 1094.3-2017 uses the method of revising and drafting to modify and adopt IEC 60076-3:2013. There are technical differences between the two standards, and these differences are marked with vertical single lines in the margin of the standard. Appendix A (Informative Appendix) provides a list of corresponding technical differences and their reasons. At the same time, the standard also makes a series of editorial modifications to adapt to China’s standardization habits and actual application needs. The main differences and modifications are as follows:

• Adjustment of Examples: The voltage values in the examples throughout the text are adjusted according to China’s actual power system conditions to ensure the applicability of the examples in China.
• Deletion of Notes: The content of Note 3 in 3.6 and the note on “lightning chopped-wave impulse test” in 7.1 of IEC 60076-3:2013 are deleted.
• Addition of Prompt Content: In 7.3.2.1, prompt content is added that the phase-to-phase induced voltage may exceed the specified insulation level during the test, to remind the operator to pay attention to safety during the test process.
• Adjustment of Test Classification: The “lightning chopped-wave impulse test (LIC)” and “neutral terminal lightning full-wave impulse test (LIN)” in 7.3.2.2 of IEC 60076-3:2013 are adjusted to type tests, and the other 3 test items in this section are continued as special tests; similar adjustments are also made to the test classification in 7.3.3.1 and 7.3.3.2.
• Adjustment of Appendix Content: The order of appendices is adjusted. The standard deletes the appendix “Test Voltage for ACSD Test” in the international standard, and adds two new appendices: “Information on Transformer Insulation Requirements and Insulation Tests to be Provided During Inquiry and Ordering” (Appendix B) and “Basic Principles for Determining Insulation Tests, Insulation Levels and Insulation Clearances” (Appendix D). At the same time, the content of some appendices in the international standard is adjusted to the corresponding appendices of this standard.

6. Adjustments Compared with GB/T 1094.3-2003

As a replacement for GB/T 1094.3-2003, GB/T 1094.3-2017 has made a series of important technical changes, mainly including:

• Adjustment of Normative Reference Documents: Updated and adjusted the normative reference documents to ensure the timeliness and applicability of the referenced standards.
• Addition of Definitions: The definition of the rated voltage of the winding is added to make the technical terms of the standard more complete.
• Adjustment of Insulation Level Marking: The marking method of insulation level is adjusted to make it more scientific and intuitive.
• Supplementation of Ultra-high Voltage Requirements: The insulation levels, neutral point insulation levels and external insulation air clearances for transformers with system nominal voltages of 750 kV and 1000 kV are supplemented, and the relevant test requirements are improved to meet the development needs of China’s ultra-high-voltage power grid.

7. Significance of Implementation

The implementation of GB/T 1094.3-2017 has important practical significance for China’s power industry. Firstly, it unifies the technical requirements for the insulation level, insulation test and external insulation air clearance of power transformers, which helps to standardize the design, manufacturing and testing processes of transformer manufacturers and improve the quality and consistency of transformer products. Secondly, by adopting advanced international standards and combining with China’s actual conditions, the standard promotes the technological progress of China’s transformer industry and enhances the international competitiveness of domestic transformer products. Thirdly, the clear insulation requirements and test methods specified in the standard help to effectively detect potential insulation defects of transformers, reduce the occurrence of insulation-related faults, and ensure the safe and stable operation of the power grid. Finally, the standard provides a reliable technical basis for the maintenance, operation and accident investigation of power transformers, which is conducive to improving the level of power grid operation and maintenance.

GB/T 1094.3-2017 is an important national standard regulating the insulation performance of power transformers in China. It integrates international advanced technology and China’s practical experience, and has the characteristics of scientificity, advanced nature and applicability. The standard clearly defines the scope of application, key technical terms, core technical requirements, and differences from international standards, providing comprehensive technical guidance for the entire life cycle of power transformers from design, manufacturing, testing to operation. The strict implementation of this standard is of great significance for improving the reliability of power transformers, ensuring the safe and stable operation of the power grid, and promoting the healthy development of China’s power industry. In the future, with the continuous development of power technology, the standard will also be continuously revised and improved to adapt to new technical challenges and application scenarios.

References

• GB/T 1094.3-2017, Power Transformers—Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air[S]. General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, Standardization Administration of the People’s Republic of China, 2017.
• IEC 60076-3:2013, Power Transformers—Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air[S]. International Electrotechnical Commission, 2013.
• GB/T 1094.3-2003, Power Transformers—Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air[S]. General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, 2003.