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 modifies and adopts IEC 60076-3:2013, plays a crucial role in ensuring the safe and reliable operation of power transformers in power transmission and distribution systems. This article elaborates on the core content of GB/T 1094.3-2017, including its scope of application, key technical requirements, main technical changes compared with the previous version, and practical application significance, providing a comprehensive technical reference for professionals in the power industry.

1. Introduction

Power transformers are core equipment in power systems, responsible for voltage conversion, power transmission and distribution. The insulation performance of transformers directly determines their operational reliability and service life, and is also related to the stability of the entire power grid. To standardize the insulation design, testing and installation of power transformers, the General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China and the Standardization Administration of China jointly issued GB/T 1094.3-2017 “Power Transformers—Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air” on December 29, 2017, which was officially implemented on July 1, 2018 . This standard replaces the previous GB/T 1094.3-2003 and has been revised and improved in combination with the latest international standards and domestic technical development needs, making it more in line with the actual requirements of the current power industry.

2. Scope of Application

GB/T 1094.3-2017 specifies the relevant insulation tests and minimum insulation test levels for power transformers. When not specified by the user, this standard recommends the minimum air insulation clearances between external live parts of the transformer and between them and the ground . This standard applies to power transformers specified in GB/T 1094.1. For certain types of power transformers and reactors with their own standards, this standard shall only apply when explicitly referenced by these standards . It should be emphasized that the scope of application of this standard covers not only oil-immersed transformers, but also clearly defines the applicability to gas-insulated transformers and other types of transformers, adapting to the development of power technology and the application needs of new transformers .

3. Key Technical Requirements

3.1 Equipment Maximum Voltage and Rated Insulation Level

Each winding’s line terminal and neutral point terminal shall be marked with its equipment maximum voltage Um (see GB/T 1094.1) . The rules for transformer insulation tests vary according to the value of Um. When there is a contradiction between the test rules of different windings in a transformer, the entire transformer shall adopt the test rules suitable for the winding with the highest Um value . The test rules for windings with different Um are specified in Table 1 of the standard, and the standard values of Um are specified in Table 2 . Unless otherwise specified, the Um value used for transformer windings may be equal to or slightly higher than the winding rated voltage value .

The rated insulation level of a transformer is determined by the equipment maximum voltage and a set of specified rated withstand voltage values, which are verified by a series of insulation tests . The rated insulation level of each winding of the transformer shall be proposed as part of the inquiry and contract signing . If a winding has graded insulation, the equipment maximum voltage value and its rated insulation level at the neutral point terminal shall be in accordance with the provisions of Table 3 of the standard . The marking method of the rated insulation level is: Um/SI/LI/LIC/AC together with the corresponding values of each winding terminal . If the winding does not specify the SI or LIC withstand voltage level, SI or LIC shall be omitted from the marking . Among them, SI refers to the rated switching impulse withstand voltage at the line terminal of the winding with the highest Um value; LI refers to the rated lightning impulse withstand voltage of each independent winding terminal; LIC refers to the rated lightning impulse withstand voltage of each independent winding line terminal when performing the chopped wave lightning impulse test; AC refers to the highest rated AC withstand voltage of each winding terminal to ground .

3.2 Insulation Test Requirements

The insulation performance of transformers is verified by insulation tests. GB/T 1094.3-2017 specifies a series of insulation test items, operation procedures and criteria, including lightning impulse test, switching impulse test, applied voltage test, induced voltage test, line terminal AC withstand voltage test, etc. .

The lightning impulse test (LI, LIC, LIN, LIMT) is used to verify the ability of the equipment to withstand transient fast-rising typical lightning impulse voltages during operation . Among them, the line terminal full-wave lightning impulse test (LI) verifies the lightning impulse withstand strength of the tested transformer by applying impulse waves to the line terminals; the line terminal chopped wave lightning impulse test (LIC) includes full-wave impulse and wave tail chopped impulse that produces a sharp voltage change, and its voltage peak and frequency are higher than those of the full-wave impulse test; the neutral point terminal lightning impulse test (LIN) verifies the lightning impulse withstand strength of the neutral point terminal and its connected windings to ground, to other windings and the longitudinal insulation of the tested windings; the lightning impulse test (LIMT) performed on two or more terminals simultaneously verifies the internal voltage of the transformer when two or more terminals are subjected to lightning impulses at the same time .

The switching impulse test (SI) is used to verify the ability of the equipment to withstand typical slow-rising transient voltages related to switching operations during operation, and to verify the switching impulse withstand strength of the line terminals and their connected windings to ground, to other windings, as well as the phase-to-phase and longitudinal insulation of the tested windings . This test is a single-phase test, and the induced voltage is distributed on all windings of the transformer .

The applied voltage test (AV) verifies the AC voltage withstand strength of the line terminals, neutral point terminals and their connected windings to ground and to other windings. The test voltage is applied to all terminals of the winding, including the neutral point terminal, so there is no inter-turn voltage . The line terminal AC withstand voltage test (LTAC) verifies the AC voltage withstand strength of each line terminal to ground, and the voltage is applied to one or more winding line terminals, allowing the line terminals of graded insulation transformers to apply voltages suitable for the line terminals .

The induced voltage test (IVW) verifies the AC withstand strength of the line terminals and their connected windings to ground, to other windings, as well as the phase-to-phase and longitudinal insulation of the tested windings. The test wiring is carried out in accordance with the transformer’s operating conditions, and symmetric voltages appear at the line terminals and between turns during the test, with no voltage at the neutral point . The induced voltage test with partial discharge measurement (IVPD) is used to verify that the transformer will not have harmful partial discharges under normal operating conditions, and the test voltage is applied to the transformer in the same way as in operation . In addition, the standard also specifies the insulation test requirements for auxiliary wiring (AuxW) that is not connected to the transformer windings .

3.3 External Insulation Air Clearance Requirements

GB/T 1094.3-2017 clarifies the design principles of external insulation air clearances, covering multiple influencing factors such as altitude, environmental pollution, mechanical strength and operating temperature, and specifies the specific requirements for creepage distance and electrical clearance . The standard provides the minimum air clearances for winding line terminals and the minimum air clearances for live parts of neutral point bushings to ground . These requirements are formulated to ensure that the transformer can maintain good insulation performance under different environmental conditions and avoid insulation breakdown caused by insufficient air clearances.

4. Main Technical Changes Compared with GB/T 1094.3-2003

Compared with the previous version GB/T 1094.3-2003, GB/T 1094.3-2017 has undergone a series of important technical adjustments and updates, mainly including the following aspects :

• Adjustments to normative reference documents: The normative reference documents have been updated and adjusted to ensure the advanced nature and consistency of the standard content.
• Addition of definitions: The definition of the rated voltage of the winding has been added, enhancing the clarity and operability of the standard.
• Adjustments to insulation level marking: The marking method of insulation level has been adjusted to make it more standardized and intuitive.
• Adjustments to insulation test sequence and items: The sequence and items of insulation tests have been optimized, and the winding line terminal AC withstand voltage test has been set as a separate test item.
• New requirements for switching impulse tests: For transformers with Um=126kV, switching impulse tests are required.
• Adjustments to induced voltage tests: The induced withstand voltage test is adjusted to be based on Ur, and the pre-applied voltage, measured voltage, measurement time and partial discharge quantity limit for partial discharge measurement are adjusted.
• Expansion of voltage level coverage: The winding insulation levels, neutral point insulation levels and external insulation air clearances of transformers with system nominal voltages of 750kV and 1000kV are supplemented, and the relevant test requirements are added.
• Adjustments to external insulation air clearances: The requirements for external insulation air clearances are adjusted to adapt to the development of high-voltage and ultra-high-voltage power transmission and distribution technologies.
• Adjustments to appendices: The appendix “Test Voltage for ACSD Test” is deleted, and two new appendices, “Information to be Provided When Inquiring and Ordering Regarding Transformer Insulation Requirements and Insulation Tests” and “Basic Principles for Determining Insulation Tests, Insulation Levels and Insulation Clearances”, are added.

5. Practical Application Significance

GB/T 1094.3-2017 has important practical application value in the power industry. For transformer manufacturers, this standard provides clear technical guidelines for insulation design, production and testing, helping to improve product quality and consistency. For operation and maintenance units, the standard specifies the insulation test methods and criteria, which is conducive to accurately evaluating the insulation status of transformers, formulating scientific maintenance strategies, and reducing the risk of equipment failure. For power engineering design personnel, the standard’s provisions on external insulation air clearances provide an important basis for the rational layout and installation of transformers, ensuring the safety and reliability of the power grid .

In addition, as this standard modifies and adopts the international standard IEC 60076-3:2013, it helps to promote the internationalization of China’s power transformer products, facilitate technical exchanges and cooperation with foreign countries, and enhance the competitiveness of domestic transformer manufacturers in the international market . At the same time, the standard incorporates more considerations on environmental protection and sustainability, puts forward environmental protection requirements for the insulation materials used in transformers, and encourages the adoption of materials and technologies with low environmental impact, which is in line with the current social demand for green energy development .

GB/T 1094.3-2017 is an important national standard regulating the insulation performance of power transformers. It integrates the latest international and domestic technical achievements, and has the characteristics of comprehensiveness, advancement and operability. By standardizing insulation levels, dielectric test requirements and external insulation air clearances, this standard effectively ensures the safe and reliable operation of power transformers, promotes the technological progress and industrial upgrading of the power transformer industry, and provides strong technical support for the stable development of China’s power system. In the future, with the continuous development of power technology, it is necessary to pay attention to the revision and update of relevant standards to adapt to the new application scenarios and technical requirements.

References

[1] General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, Standardization Administration of China. GB/T 1094.3-2017 Power Transformers—Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air[S]. Beijing: China Standards Press, 2017.

[2] IEC 60076-3:2013 Power Transformers—Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air[S].