I. Introduction
As a crucial core component in the power system, the transformer is often referred to as the “heart” of the power grid. Its performance and insulation condition directly affect the safe and stable operation of the power grid. In case of a transformer failure, it may cause local power outages at best, or even trigger chain reactions and result in huge economic losses and social impacts. Therefore, in order to verify the design, manufacturing quality, and operational reliability of the transformer, a series of strict tests must be carried out in accordance with relevant standards.
Transformer tests can be classified into routine tests, type tests, special tests, as well as on-site commissioning tests and preventive tests; according to the type of transformer, different structures such as oil-immersed and dry-type need to be distinguished. Each type of test has clear standards, collectively forming a complete test standard system. This article will systematically review the main test items and their corresponding standards, with the aim of providing reference for transformer test work.
II. Main Test Items and Corresponding Standards
(1) Winding Resistance Measurement
Main corresponding standard: GB/T 1094.1, JB/T 501
Winding resistance measurement is one of the most fundamental and important items in transformer tests. This test measures the DC resistance of each winding to check the welding quality of the winding, inter-winding short circuits, and the contact condition of the tap changer. The imbalance rate of DC resistance is usually required to be no more than 4% between phases and no more than 2% between lines. If the measurement results exceed the limit, it often indicates defects such as poor welding, inter-winding short circuits, or poor contact of the tap changer, and further investigation is required.
(2) Voltage Ratio Measurement and Connection Group Marking Verification
Main corresponding standard: GB/T 1094.1, JB/T 501
This test aims to verify whether the transformer ratio meets the design requirements and whether the connection group is correct. The voltage ratio deviation is generally required to be no more than ±0.5%. The voltage ratio and connection group marking directly affect whether the transformer can operate in parallel correctly and the coordination of protection device settings. Therefore, this test must be strictly carried out after commissioning and maintenance.
(3) Insulation Resistance Measurement
Main corresponding standard: GB/T 1094.1, DL/T 596
Insulation resistance measurement is an important means to determine the overall insulation condition of the transformer, which can effectively detect moisture, dirt, or penetrating defects. The resistance values are measured at 15s, 60s, and 600s, and the absorption ratio or polarization index is calculated. The absorption ratio (R60s/R15s) is generally required to be no less than 1.3, and the polarization index (R600s/R60s) is generally required to be no less than 1.5. This test is simple to operate and sensitive to insulation defects, and is one of the most commonly used insulation diagnostic methods in the field.
(4) Dielectric Loss Factor (tanδ) Measurement
Main corresponding standard: GB/T 1094.1, JB/T 501
Dielectric loss factor measurement is used to reflect the deterioration and moisture content of insulation. The tanδ of the winding at 20℃ is usually required to be no more than 0.5%. The trend of the tanδ value often has more diagnostic value than a single measurement value, and through longitudinal comparison of historical data, the aging state of the insulation system can be effectively determined.
(5) No-load Loss and No-load Current Measurement
Main corresponding standard: GB/T 1094.1
The no-load test is used to check the manufacturing and assembly quality of the iron core, including insulation of silicon steel sheets, iron core clamping, and magnetic circuit integrity. The no-load loss is not allowed to have a positive deviation, that is, the measured value must not exceed the design value. The size of the no-load current reflects the magnetization performance of the iron core, and an abnormal increase may indicate local short circuits or magnetic circuit defects.
(6) Short-circuit Impedance and Load Loss Measurement
Main corresponding standard: GB/T 1094.1
The short-circuit test is used to check the design and manufacturing quality of the winding, including the geometric size, wire specification, and interposition method of the winding. The load loss is also not allowed to have a positive deviation. The deviation of the short-circuit impedance will affect the parallel operation of transformers and the calculation of system short-circuit current. Therefore, the results of this test have significant reference value for system operation.
(7) External Applied Voltage Test (AV)
Main corresponding standard: GB/T 1094.3
The external applied voltage test is used to evaluate the insulation strength of the main insulation, that is, the insulation performance between windings and between windings and ground. For example, at 35kV, a voltage of 35kV is applied to the high-voltage side for 1 minute, and no breakdown is considered qualified. This test is a key item for testing whether the main insulation of the transformer meets the operation requirements and belongs to the routine test category.
(8) Induction Voltage Test (IVW)
Main corresponding standard: GB/T 1094.3
The induction voltage test is used to evaluate the insulation strength of longitudinal insulation, that is, insulation performance between turns, layers, and sections. When the test voltage frequency is higher than the rated frequency, the test time is shortened but not less than 15 seconds. This test can effectively detect insulation defects between turns and is an important supplement to the external applied voltage test.
(9) Insulation Oil Test
Main corresponding standard: GB/T 7252, JB/T 501
The insulation oil test is for oil-immersed transformers and is an important means to evaluate the condition of the oil-paper insulation system. Key indicators include breakdown voltage (such as ≥ 40kV) and moisture content (such as ≤ 20mg/L). In addition, oil dissolved gas analysis (DGA) can effectively detect latent faults such as overheating and discharge in the transformer, and is a core item in preventive tests.
(10) On-load Tap Changer Test
Main corresponding standard: GB/T 1094.1, JB/T 501
The on-load tap changer test is used to verify the switching reliability of the tap changer. For example, 8 operation cycles are completed when the transformer is not excited; 1 cycle is completed at 85% of the rated voltage. The tap changer is a key moving part of the on-load adjustable transformer, and its switching characteristics directly affect the voltage regulation performance and operational safety of the transformer.
(11) Temperature Rise Test
Main corresponding standard: GB/T 1094.2
The temperature rise test is used to verify the heat generation of the transformer under rated load. The upper layer oil temperature rise limit is usually 60K, and the average winding temperature rise limit is 65K. Temperature rise directly determines the insulation life and load capacity of the transformer and is an important item in the type test.
(12) Sound Level Measurement
Main corresponding standard: GB/T 1094.10
Sound level measurement is used to measure the noise level of the transformer during operation. With increasingly strict environmental protection requirements, noise control of transformers has become one of the important indicators for equipment selection and acceptance.
(13) Lightning Impulse Test
Main corresponding standard: GB/T 1094.3
The lightning impulse test is used to evaluate the ability of the transformer to withstand lightning overvoltage and belongs to the type test or special test. This test simulates the insulation response of the transformer when lightning waves invade, and is an important means to test the insulation coordination level of the transformer.
(14) Pressure Sealing Test
Main corresponding standard: GB/T 6451, GB/T 1094.1
The pressure sealing test is used to test the strength of the oil tank and sealing structure. For example, there should be no leakage after applying the specified pressure for 12 hours. This test is of great significance for ensuring that oil-immersed transformers do not leak oil during operation.
III. Standard System Explanation
The standard system for transformer tests is well-structured and interconnected, mainly including the following categories:
GB/T 1094 series: Core basic standards for power transformers. Among them, GB/T 1094.1 (General Rules) stipulates most of the routine test items; GB/T 1094.3 (Insulation Level, Insulation Test) specifically stipulates various types of voltage and impulse tests; GB/T 1094.2 stipulates the temperature rise test; GB/T 1094.10 specifies the measurement of sound levels. This series of standards is basically in line with the IEC 60076 series of standards of the International Electrotechnical Commission and enjoys wide international recognition.
DL/T 596: Power Equipment Preventive Testing Procedures, mainly targets operating transformers, stipulating the items, cycles, and criteria for regular tests, which differ from the focus of factory and commissioning tests. This procedure pays more attention to the trend analysis of insulation deterioration under operating conditions.
GB 50150: Electrical Installation Engineering Electrical Equipment Commissioning Test Standards, specifically stipulates the on-site commissioning tests that new installed transformers should undergo before being put into operation. This standard is the key quality control basis for transformers during the process from installation to commissioning.
JB/T 501: Test Guidelines for Power Transformers, provides more detailed operational guidance for specific test methods and is an important reference for test personnel during on-site operations.
These standards are mutually coordinated, forming a full life cycle standard coverage system from factory tests, on-site commissioning tests to preventive tests during operation.
The determination of transformer test items requires comprehensive consideration of the type of transformer (oil-immersed or dry-type), voltage level, and test purpose (factory, commissioning, or preventive). The test priorities for different stages and different equipment vary: factory tests focus on verifying manufacturing quality, commissioning tests focus on checking whether damage has occurred during transportation and installation, and preventive tests focus on monitoring the trend of insulation deterioration during operation.
Regardless of which stage, the tests should be based on the corresponding standards to ensure the scientificity and authority of the test results. At the same time, attention should be paid to the longitudinal comparison and trend analysis of test data, combining single test results with historical data, to accurately determine the actual state of the transformer and provide reliable guarantees for the safe operation of the transformer.
Post time: Oct-09-2026