I. Definition of Preventive Tests
Preventive tests are inspection, testing or monitoring activities carried out on operating equipment to identify potential hazards and prevent accidents or equipment damage. They form the basis for the safe operation and condition-based maintenance of power equipment.
II. Standards and Procedures for Test Execution
The execution of preventive tests for transformers is mainly based on national standards and power industry standards. Some power enterprises may also formulate stricter enterprise standards as supplementary execution bases.
Core reference standards include:
GB 50150-2016 “Code for Handover Test of Electrical Installation Projects – Electrical Equipment”
DL/T 596-2021 “Regulations for Preventive Tests of Electrical Equipment”
GB/T 1094.1-2013 “Power Transformers – Part 1: General” and other transformer series standards
III. Main Preventive Test Items and Objectives
Transformer preventive tests are divided into routine tests and diagnostic tests. Routine tests are regular conventional tests, while diagnostic tests are only conducted when problems are found in routine tests or when in-depth research on the equipment status is required. The following are common routine test items and their corresponding test purposes:
(1) Insulation Characteristic Tests
Insulation resistance and absorption ratio / polarization index: To check if the overall insulation of the transformer is damp and if there is dirt or local defects on the surface of components.
Dielectric loss factor (tanδ): Sensitive to overall insulation dampness, aging, and deterioration of the transformer.
(2) Winding DC Resistance Test
To check the welding quality of winding conductors, the contact condition of tap changers, the connection of leads, and whether there are short circuits between winding turns.
(3) Oil Analysis Tests
Conduct tests such as breakdown voltage, dielectric loss, water content, and chromatographic analysis of insulating oil. Among them, oil chromatographic analysis (DGA) is a core project that can sensitively diagnose latent faults such as overheating and discharge inside the transformer; the other items are mainly used to check the insulation strength of insulating oil.
(4) Electrical Strength Tests
Leakage current: To check the dampness and surface dirt of the transformer insulation. The test voltage is relatively high, and the detection effect is more effective than the insulation resistance test.
AC withstand voltage test: A key test to assess the main insulation strength of the transformer, to check the equipment’s ability to withstand overvoltage during operation and to detect concentrated insulation defects.
(5) Characteristic Parameter Tests
Ratio test: To check if the voltage ratios of each tap are correct, if the tap changer indication is normal, and if there are short circuits between winding turns.
No-load current and no-load loss: To check the insulation between silicon steel sheets of the core, the insulation of the through bolts, and if there are short circuits between winding turns.
Short-circuit impedance test: To check if the windings have deformed.
(6) Special Inspection Tests
Winding deformation test (frequency response method / FRA): To diagnose whether the windings have shifted, twisted, or bulged after being impacted by short-circuit currents.
Partial discharge test: To detect whether there is partial discharge inside the transformer, which is an important indicator for evaluating the insulation performance of the equipment.
IV. Test Cycles and Judgment Criteria
The test cycles and judgment criteria for transformer preventive tests are mainly based on DL/T 596-2021, with the latest released standards and regulations taking precedence.
(1) Test Cycles
Regular tests: Conventional test items such as insulation resistance, winding DC resistance, simplified insulating oil tests, tanδ, etc., are usually conducted every 1-3 years; oil chromatographic analysis is conducted at different frequencies based on the voltage level of the equipment, with 330kV and above transformers tested every 3 months, 110-220kV transformers every 6 months, and important 35kV and below transformers annually.
Newly put into operation: All handover tests must be completed before operation. After commissioning, the frequency of tests should be increased at specified time points (such as 72 hours, 1 month, and 3 months), with oil chromatography analysis being a key test item.
After major overhauls: Regardless of whether the original test cycle has expired, tests for relevant items must be conducted.
When necessary: If a transformer trips due to a nearby short circuit, the heavy gas protection operates, or it is suspected to have a fault, relevant tests and inspections should be carried out immediately.
(2) Partial test item determination standards (taking oil-immersed transformers of 35kV and above as an example)
Insulation resistance: The current measurement value should not show a significant decrease compared to the previous measurement, usually not less than 70% of the previous value; the absorption ratio (R60″/R15″) should not be less than 1.3 in an environment of 10℃ to 30℃; the polarization index (R600″/R60″) should not be less than 1.5.
Winding DC resistance: For transformers above 1.6MVA, the difference in resistance between each phase winding should not exceed 2% of the average value of the three phases (for those without a neutral point lead-out, it is the line-to-line difference); the resistance between taps of the same phase should not change by more than 2% compared to the previous measurement at the same location.
tanδ (20℃): For transformers of 330kV and above, it should not exceed 0.5%; for 35-220kV transformers, it should not exceed 0.8%; the current test value should not show a significant increase compared to the previous test value.
Insulating oil breakdown voltage: For equipment below 15kV, it should be ≥25 kV; for 15-35kV equipment, it should be ≥35 kV; for 66-220kV equipment, it should be ≥40 kV; for 330kV equipment, it should be ≥50 kV; for 500kV equipment, it should be ≥60 kV.
AC withstand voltage: The test voltage value should be determined according to the equipment’s voltage level and insulation level as per standard regulations. For example, the factory test voltage for the 35kV winding is 85kV, and the preventive test voltage is 72kV.
Post time: Jan-22-2026