Dielectric loss factor (tanδ) is one of the most sensitive electrical indicators for evaluating the deterioration state of insulating oil. This article starts from the measurement principle and systematically reviews the core selection indicators, typical application scenarios, and current test standard system of the transformer oil dielectric loss tester. It focuses on analyzing the technical requirements of GB/T 5654-2007 and DL/T 846.14-2023, and combines the requirements of the full life cycle management of power equipment to provide reference for equipment selection and testing practice.
I. Overview
Insulating oil in oil-immersed transformers performs both insulation and heat dissipation functions simultaneously. The deterioration of its performance will directly threaten the operational reliability of the equipment. Among the numerous indicators for evaluating oil quality, the dielectric loss factor (tanδ) can sensitively reflect the changes in the content of polar impurities, moisture, acidic products, and colloidal substances in the oil, and is a key means for screening the early deterioration of insulating oil.
The dielectric loss factor is defined as the ratio of active current to reactive current in an insulating medium under AC voltage. Ideally, pure insulating oil has a tanδ close to zero; when water, fibers, metal soap substances, or oxidation products are mixed into the oil, the polar substances increase, causing the tanδ value to rise. Therefore, the precise measurement of tanδ essentially quantitatively assesses the purity and aging degree of the oil.
This article discusses the transformer oil dielectric loss tester from three dimensions: selection indicators, application scenarios, and standard system, aiming to provide systematic technical references for power operation, oil supervision, and equipment procurement personnel.
II. Core Selection Indicators
The selection of a dielectric loss tester requires comprehensive consideration from five aspects: standard compliance, measurement accuracy, temperature control capability, anti-interference performance, and oil cup structure.
2.1 Standard Compliance: The prerequisite for valid data
The first principle of selection is to strictly comply with current standards. The equipment must clearly state compliance with GB/T 5654-2007 (equivalent to IEC 60247:2004) and DL/T 846.14-2023 industry standards. DL/T 846.14-2023 has been officially implemented in November 2023 and has completely replaced the original standard, making systematic regulations on the technical requirements, test methods, and inspection rules of the tester. Equipment that does not comply with the standard cannot be used as a basis for compliance and may pose risks to equipment safety assessment.
2.2 Measurement Accuracy: The ability to detect early deterioration
The dielectric loss value of insulating oil is usually in a low range (new oil is required ≤ 0.005) when the oil is in good condition. Therefore, the resolution and stability of the instrument in the low dielectric loss range are crucial. When selecting, attention should be paid to: the tanδ measurement range usually covers 0.0001 to 1, with an accuracy of ±(0.5% reading + 0.0002) or better; the capacitance measurement range should cover 1 pF to 200 pF; the volume resistivity measurement range is generally from 2.5 MΩ·m to 20 TΩ·m.
2.3 Temperature Control Capability: The key variable for data accuracy
The dielectric loss factor is extremely sensitive to temperature. A change of 1℃ in temperature may cause a significant deviation in the dielectric loss value. GB/T 5654-2007 stipulates that the standard test temperature for transformer oil is 90℃ ± 0.5℃. Modern instruments mostly adopt medium-frequency induction heating to achieve non-contact uniform heating of the oil cup and heating body, combined with PID control algorithm, to stabilize the temperature within the error range of the set value. Some models also have a cooling function to meet the special requirements of resistance rate measurement for anti-oil at 20℃.
2.4 Anti-interference Performance: The guarantee of reliability for on-site testing
In the strong electromagnetic environment of substations, power frequency interference and harmonics can seriously affect the measurement accuracy. The mainstream anti-interference technologies include the off-frequency measurement method (using 45Hz/55Hz or 40Hz/60Hz combinations to avoid the interference frequency band) and adaptive filtering technology. For on-site testing of 110kV and above substations, the off-frequency anti-interference equipment is the most mature solution at present, with the measurement error of dielectric loss controlled within ±0.5%.
2.5 Oil cup structure: The hardware foundation of measurement accuracy
The oil cup is the core sensing component of dielectric loss testing. According to GB/T 5654-2007, the oil cup must adopt a three-electrode structure (high-voltage electrode, measurement electrode, and protection electrode), with the inter-electrode spacing fixed at 2mm to eliminate the influence of stray capacitance and surface leakage current. The electrode material should be selected from inert materials such as stainless steel, and the surface should be highly polished. The dielectric loss value of an empty cup should be extremely small and stable, which is an important basis for judging the cleanliness and assembly status of the oil cup.
III. Application scenarios
The application of the transformer oil dielectric loss tester runs through the full life cycle management of power equipment.
New oil acceptance is the first checkpoint for oil product entry into the network. According to DL/T 596-2021, new oil before commissioning should have tanδ no more than 0.005 at 90℃, and the test results are the basis for determining the qualification of the oil product.
Regular inspections during operation are the core content of preventive tests. DL/T 596-2021 stipulates the testing cycles and determination limits for transformers of different voltage levels: for transformers of 330kV and above, tanδ of the operating oil should be ≤ 0.02, for those of 220kV and below, ≤ 0.04 (at 90℃). By regularly monitoring the trend of tanδ changes, it is possible to effectively track the aging rate of the oil.
After major overhauls and after oil filtration, the evaluation also relies on dielectric loss testing. After major overhauls or vacuum oil filtration treatment of transformers, the change in dielectric loss value is used to determine whether the processing method has effectively reduced the content of polar impurities in the oil, providing a basis for the re-activation of the equipment.
In terms of fault diagnosis assistance, when abnormal gas chromatography analysis detects abnormal gas production, dielectric loss testing can help determine whether the insulation anomaly is related to oil quality deterioration, and form a complement with dissolved gas analysis.
IV. Test standard system
4.1 National standard: GB/T 5654-2007
GB/T 5654-2007 “Measurement of Relative Permittivity, Dielectric Loss Factor and DC Resistance Rate of Liquid Insulating Materials” is the core method standard for dielectric loss testing of liquid insulating materials in China, equivalent to IEC 60247:2004. This standard is applicable to reference tests of unused liquid insulating materials and also to liquids in operating transformers, cables, etc.
The core technical conditions stipulated in the standard include: test temperature 90℃ ± 0.5℃ (for transformer oil); test frequency 50Hz; electric field strength 0.5 ～ 1 kV/mm, and the three-electrode oil cup usually applies a 2kV test voltage. The standard specifies in detail the cleaning, drying process and oil sample preparation steps of the electrode cup, emphasizing the decisive influence of temperature control on the accuracy of the results.
4.2 Power industry standards
DL/T 846.14-2023 “General Technical Conditions for High Voltage Test Equipment – Part 14: Dielectric Loss Factor and Volume Resistance Rate Tester for Insulating Oil” was implemented in November 2023, stipulating the technical requirements, test methods, inspection rules, marking, packaging, transportation and storage requirements of the tester, and applicable for the production and manufacturing and procurement acceptance of the tester.
DL/T 596-2021 “Power Equipment Preventive Test Procedures” stipulates the implementation timing and result determination standards for dielectric loss testing. The dielectric loss limit of the transformer oil in operation is classified according to the voltage level, and this regulation also specifies the test cycle requirements.
4.3 Quality Standards for Operating Oil
GB/T 7595-2017 “Quality of Transformer Oil in Operation” stipulates the dielectric loss limit for operating transformer oil: for 500kV to 1000kV, the tanδ of operating oil should not exceed 0.020 at 90℃, and for 330kV and below, it should not exceed 0.040; the volume resistivity (90℃) should be no less than 1×10¹⁰ Ω・cm.
4.4 Standard Test Procedure
The standard test procedure ensures the reliability of the data. The sampling环节 should strictly follow the provisions of GB/T 7597. Use clean and dry dedicated sampling bottles to avoid contamination and moisture. Before testing， complete the calibration of the empty cup， measure the capacitance and dielectric loss value of the empty electrode cup as the background， and confirm that the cleanliness and assembly status of the oil cup meet the requirements. When injecting the oil sample， do it slowly to avoid generating bubbles. After the test is completed， immediately empty the oil sample and thoroughly clean the oil cup to prevent residual contamination from affecting the next measurement.
Post time: Sep-23-2026