Power transformers, as the core hubs for power transmission and distribution in the power system, directly determine the safety and stability of the power grid and the quality of electrical energy through their operational reliability. During the factory acceptance, on-site installation handover, and preventive tests during operation, five conventional tests – winding DC resistance measurement, ratio and connection group test, insulation resistance and absorption ratio (polarization index) test, dielectric loss factor (tanδ) test, and AC withstand voltage test – are key means to comprehensively evaluate the electrical performance, insulation condition, and potential defects of the transformer. This article, in accordance with core standards such as GB 50150-2016 “Code for Handover Test of Electrical Installation Projects” and DL/T 596-2021 “Regulations for Preventive Tests of Electrical Equipment”, systematically analyzes the technical principles, test equipment, operation points, and qualification criteria of these five conventional tests, providing professional references for on-site test operations, data interpretation, and fault detection in engineering.
I. Winding DC Resistance Measurement: Core Detection of Winding Conductivity and Connection Status
Winding DC resistance measurement is a fundamental test for transformers, with the core purpose of detecting the welding quality of winding conductors, the contact reliability of tap changers, inter-turn short circuits in windings, and the integrity of lead connections. The measurement results directly reflect the conductivity of the winding circuit and are a key means to identify early latent faults in windings.
From the perspective of the test principle, this test is based on Ohm’s Law, where a constant DC voltage is applied to the winding, and the DC current in the winding circuit is measured. Through calculation, the DC resistance value of the winding is obtained. Since the inductance of the transformer winding has no hindrance effect on DC signals, it can effectively eliminate inductance interference and accurately obtain the resistance parameters of the winding conductors and connection parts, avoiding the influence of inductance and capacitance on the results in AC measurement.
The test equipment should use a dedicated DC resistance tester, with core technical indicators meeting the following requirements: measurement range covering 0.01mΩ to 10kΩ, measurement accuracy not less than ±0.2%, equipped with automatic discharge and temperature compensation functions, and adaptable to the winding test requirements of transformers of different capacities and voltage levels. For large-capacity transformers, a tester with an auxiliary magnetic function can be selected to shorten the test time and reduce the impact of winding heating on the measurement results.
During the practical operation, the following norms must be strictly followed: First, the transformer windings should be fully discharged before measurement, and all external connection lines should be removed to ensure the independence of the test circuit; second, the ambient temperature and winding temperature should be recorded during measurement for subsequent temperature conversion; third, for transformers with tap changers, the DC resistance at each tap position should be measured sequentially to ensure the flexibility and good contact of the tap changer; fourth, after the measurement is completed, the windings should be discharged again to avoid safety hazards caused by residual charges.
The qualification criteria strictly follow the current standard requirements: for transformers of 1600kVA and above, the inter-phase difference of the winding DC resistance should not exceed 2% of the average value of the three phases; for transformers below 1600kVA, the inter-phase difference should not exceed 4%, and the line-to-line difference should not exceed 2%. At the same time, the measured value should not show significant changes compared to the factory value and historical test values (after temperature conversion), with a difference not exceeding 2%. Otherwise, defects such as inter-turn short circuits and poor contact of tap changers should be investigated.
II. Ratio and Connection Group Test: Verification of Voltage Transformation Accuracy and Correct Wiring
The core purpose of the ratio and connection group test is to verify whether the voltage ratio between the windings of the transformer meets the design requirements, whether the actual position of the tap changer is consistent with the marking, and whether the connection group (or the polarity of the lead-out line of a single-phase transformer) is consistent with the nameplate marking. This test helps to identify defects such as incorrect winding turns and inter-turn short circuits, ensuring the voltage transformation accuracy of the transformer and avoiding abnormal operation of the power grid due to incorrect wiring. The test principle is based on the electromagnetic induction law of transformers, which states that the ratio of the number of turns in the primary and secondary windings is directly proportional to the voltage ratio (U₁/U₂ = N₁/N₂). By applying an AC voltage of a certain amplitude to the primary winding through a dedicated test instrument, the actual voltages of the primary and secondary windings are measured, and the actual transformation ratio is calculated and compared with the rated transformation ratio marked on the nameplate. At the same time, the wiring group or polarity is determined through phase detection.
The test equipment should be a dedicated transformation ratio and group test instrument, which should have the following functions: a transformation ratio measurement range of 1:1 to 1000:1, a measurement accuracy of no less than ±0.1%, the ability to automatically detect the wiring group (Y/Y, Y/Δ, Δ/Y, etc.) and polarity, data storage and printing functions, and be adaptable to different wiring methods of transformers.
Key points of practical operation include: first, before testing, ensure that the transformer tap changer is in the rated tap position, and the wiring is firm and free from looseness; second, strictly follow the instrument operation specifications to connect the test leads, distinguish the primary and secondary windings, and avoid reversing the connections to prevent measurement errors; third, for multi-winding transformers, the transformation ratio and wiring group of each winding pair should be measured sequentially; fourth, during the measurement process, the test environment should be quiet to avoid electromagnetic interference affecting the measurement accuracy.
The criteria for qualification are clear: at the rated tap position, the transformation ratio error of the transformer should not exceed ±0.5%; for transformers with a voltage level of 35kV or below and a voltage ratio less than 3, the allowable deviation of the transformation ratio can be relaxed to ±1%; for other tap positions, the deviation of the transformation ratio should be within 1/10 of the transformer’s impedance voltage value (%) and not exceed ±1%; the wiring group and the polarity of the lead-out lines must be completely consistent with the nameplate marking and the symbols on the casing; otherwise, the winding connections or the tap changer position should be rechecked.
III. Measurement of Insulation Resistance, Absorption Ratio, and Polarization Index: Preliminary Assessment of Insulation Condition
The measurement of insulation resistance, absorption ratio (R60/R15), and polarization index (R10min/R1min) is a simple and efficient method for evaluating the overall insulation condition of a transformer. The core purpose is to detect moisture, dirt, aging, and through faults in the insulation material, providing a basis for subsequent insulation tests. Among them, insulation resistance reflects the overall insulation level of the insulation, while the absorption ratio and polarization index are used to determine the degree of insulation moisture. They are particularly suitable for assessing the insulation condition of high-voltage and large-capacity transformers.
The test principle is based on the DC conduction characteristics of insulation materials: when a DC high voltage (output by a megohmmeter) is applied to the transformer windings, leakage current and polarization current will occur in the insulation material. As the application time increases, the polarization current gradually decays, and the leakage current tends to stabilize. The resistance value measured at this time is the insulation resistance. The absorption ratio is the ratio of the insulation resistance at 60 seconds to that at 15 seconds, and the polarization index is the ratio of the insulation resistance at 10 minutes to that at 1 minute. Both can effectively reflect the moisture condition of the insulation – the polarization process of moist insulation accelerates, and the absorption ratio and polarization index will significantly decrease.
The test equipment should be an insulation resistance tester (megohmmeter), and the voltage level should be reasonably selected based on the rated voltage of the transformer: for transformers with a rated voltage of 10kV and below, a 2500V megohmmeter should be used; for transformers with a rated voltage of 35kV to 220kV, a 2500V/5000V megohmmeter should be used; for transformers with a rated voltage of 220kV or above or a capacity of 120MVA or above, a 5000V megohmmeter should be used to measure the polarization index. The measurement range of the instrument should cover 1MΩ to 100000MΩ, with a measurement accuracy of no less than ±5%, and it should have timing and data locking functions.
During the practical operation, the following points should be noted: first, before measurement, the transformer windings, core, and clamps should be fully discharged, external connection lines should be removed, and the surface dirt and dust of the windings should be cleaned; second, during measurement, the ambient temperature and humidity should be recorded, and the ambient humidity should not exceed 80% to avoid its influence on the measurement results. Third, for oil-immersed transformers, the measurement should be conducted after the insulating oil has been left to stand and qualified. Fourth, during the pressurization process, the instrument must be kept stable to avoid voltage fluctuations caused by shaking. The pressurization time must strictly follow the specified requirements (15 seconds, 60 seconds, 10 minutes).
The criteria for qualification are as follows: the insulation resistance value should not be lower than the values stipulated in the regulations and the factory values (after temperature conversion); for 35kV transformers, the absorption ratio should be ≥ 1.2, and for 110kV and above transformers, the absorption ratio should be ≥ 1.3; for high-voltage and large-capacity transformers, the polarization index should be ≥ 1.5 to 2.0. If the insulation resistance is low, or the absorption ratio or polarization index does not meet the requirements, the results of dielectric loss tests and insulating oil tests should be combined to comprehensively determine the degree of insulation moisture or aging.
Four. Measurement of Dielectric Loss Factor (tanδ) and Capacitance: Sensitive Detection of Insulation Defects
The measurement of dielectric loss factor (tanδ) and capacitance is a more sensitive method for detecting the insulation state than insulation resistance testing. The core purpose is to detect the energy loss of the transformer’s insulating medium, and to identify issues such as insulation moisture, aging, oil deterioration, bushing defects, and internal partial air gaps. It is particularly suitable for discovering latent defects within the insulation and is an important indicator for evaluating insulation reliability.
Test Principle: Under the action of alternating voltage, the insulating medium of the transformer will produce polarization loss and conductance loss. The ratio of the total loss to the capacitive current of the insulating medium is the dielectric loss factor (tanδ). The larger the tanδ value, the greater the energy loss of the insulating medium and the poorer the insulation performance; capacitance reflects the capacitive characteristics of the insulating medium, and its changes can indirectly reflect the integrity of the insulation structure, such as damage to the winding insulation layer or aging of the bushing, which can lead to abnormal changes in capacitance.
Test equipment should use a high-voltage dielectric loss tester, which must have anti-electromagnetic interference capabilities to effectively eliminate on-site power frequency interference. The measurement accuracy should not be lower than ±0.001, the tanδ measurement range should be 0 to 10%, and the capacitance measurement range should be 10pF to 100,000pF. The instrument should have automatic temperature compensation and data storage functions, and be capable of simultaneously measuring tanδ and capacitance. For oil-immersed transformers, a dedicated dielectric loss tester can also be used to simultaneously detect the dielectric loss of the insulating oil.
Practical operation norms include: First, the test equipment should be calibrated before measurement to ensure instrument accuracy. Second, when connecting the test leads, shielded cables should be used to avoid external interference. Third, the ambient temperature should be recorded during measurement to facilitate temperature conversion of the tanδ value (usually converted to 20°C). Fourth, when measuring the windings along with the bushings, the non-measured windings should be short-circuited and grounded to ensure the accuracy of the measurement circuit. Fifth, during the measurement process, the changes in the tanδ value and capacitance displayed by the instrument should be observed. If there is a sudden change, the test wiring or equipment defects should be checked.
The qualification criteria strictly follow the regulations: at 20°C, for transformers of 110kV and below, tanδ should be ≤ 0.8%, and for 220kV and above, tanδ should be ≤ 0.6%. The measured tanδ value compared to the factory value should not exceed 130% of the factory value; if it does, other test results should be combined for comprehensive judgment. The capacitance compared to the factory value and historical test values should not deviate by more than ±3% (for oil-immersed transformers) or ±5% (for dry-type transformers). If the deviation is too large, insulation structure defects or bushing faults should be investigated.
Five. AC Withstand Voltage Test: Assessment of Main Insulation Strength
The AC withstand voltage test is a highly destructive test among the five conventional tests for transformers. The core purpose is to assess the insulation strength of the main insulation (between windings, between windings and the core, and between windings and the casing), to identify concentrated insulation defects (such as cracks, bubbles, severe moisture, local damage, etc.), and to verify whether the insulation can withstand the rated working voltage and overvoltage. It is the last line of defense to ensure the safe operation of the transformer. Test Principle: By applying a specified amplitude of power frequency alternating voltage (or equivalent sinusoidal alternating voltage) to the transformer windings through test equipment and maintaining it for a specified period of time, observe whether the winding insulation experiences breakdown, flashover or overheating. If the insulation can withstand the specified voltage without any abnormality, it indicates that the main insulation strength is qualified; if breakdown or flashover occurs, it indicates that there are serious defects in the insulation, and maintenance is required.
Test equipment should be reasonably selected based on the transformer capacity and voltage level: for small-capacity, low-voltage transformers, a power frequency test transformer + voltage regulator + protective resistor combination is used; for large-capacity, high-voltage transformers, due to the large required test capacity, a variable-frequency series resonant test device is usually selected. This device can generate high voltage and large current with a small capacity power source through series resonance, meeting the test requirements while avoiding the cost waste caused by overly large test equipment capacity. The output voltage accuracy of the test equipment should not be less than ±3%, the voltage waveform should be close to a sine wave, and it should have overcurrent and overvoltage protection functions.
The practical operation process must strictly follow safety regulations and operation procedures: first, the first four routine tests must be completed and all results must be qualified before conducting the AC withstand voltage test; second, before the test, the transformer must be thoroughly dried and degassed (for oil-immersed transformers), all external connection lines must be removed, and non-tested windings must be short-circuited and grounded; third, during the test, the voltage must be increased slowly, with a rate controlled at 1-2 kV/s. After reaching the specified test voltage, maintain it for 1 minute (conventional requirement) and observe the equipment status; fourth, after the test, the voltage must be slowly reduced to zero, then the power supply must be cut off, and the windings must be fully discharged to avoid residual voltage causing safety accidents; fifth, a safety warning area must be set up at the test site, and insulated protective equipment must be provided. Unrelated personnel are strictly prohibited from entering.
The criteria for qualification are clear: during the test, there should be no breakdown, no flashover, and no obvious heating in the transformer windings. After the test, measure the insulation resistance, and if there is no significant decrease compared to before the test, it is qualified. The test voltage standards for transformers of different voltage levels are different. For example, the handover test voltage for a 10kV transformer is 35kV, and the preventive test voltage is 28kV; the handover test voltage for a 35kV transformer is 72kV, and the preventive test voltage is 57.6kV. Specific standards must strictly follow the provisions in Appendix of GB 50150-2016.
Post time: Apr-16-2026