Insulating oil serves as the core dielectric medium in oil-filled equipment such as transformers, reactors, and instrument transformers, fulfilling dual roles of insulation and heat dissipation. Once the oil deteriorates, both the electrical strength and cooling efficiency of the equipment decline simultaneously, potentially leading to insulation breakdown and internal overheating under severe conditions. Among various oil quality indicators, the dielectric loss factor (tanδ) is highly sensitive to polar impurities, aging byproducts, and moisture content in the oil, making it a key parameter for early detection of oil degradation.
This article systematically reviews the measurement principles, standards, and selection criteria for dielectric loss factor testers for insulating oil, based on current testing standards and procedures, providing professional reference for power operation and maintenance, equipment inspection, and research institutions.
Measurement Principles and Technological Evolution
The dielectric loss factor (tanδ) is defined as the ratio of active current to reactive current in an insulating medium under alternating voltage. For ideally pure insulating oil, tanδ should be close to zero; however, in actual operation, the presence of polar substances such as moisture, acidic compounds, metallic soaps, and fiber contaminants causes tanδ values to rise.
Traditional measurements are based on the classical Schering bridge principle, which offers intuitive and high-accuracy results but involves cumbersome manual operations requiring skilled personnel to adjust balance and perform calculations. Modern testers generally employ fully automatic digital measurement technology, integrating within the instrument a highly stable sine wave signal source, a high-precision vector current-voltage measurement unit, and a high-speed analog-to-digital converter. These components enable synchronized high-speed sampling of the standard voltage applied to the oil sample and the current flowing through it. Using algorithms such as digital filtering and Fourier transformation, the system directly separates and calculates the active and reactive components of the current vector, enabling real-time computation of tanδ and relative permittivity εr.
High-precision tanδ measurement relies on carefully designed hardware systems. Precision measurement electrode cells typically use a three-electrode configuration (high-voltage electrode, measuring electrode, and guard electrode) to eliminate edge effects and surface leakage currents. The electrode surfaces must be highly polished, with precisely controlled spacing to ensure stability and repeatability of capacitance values.
Standard and Regulatory Framework
National Standards
GB/T 5654-2007 “Liquid Insulating Materials—Determination of Relative Permittivity, Dielectric Loss Factor and DC Resistivity” is the core method standard. This standard is identical to IEC 60247:2004 and specifies methods for measuring the dielectric loss factor, relative permittivity, and DC resistivity of liquid insulating materials at test temperature. It emphasizes that strict control of test conditions—such as temperature (90°C ± 0.5°C) and frequency (50 Hz)—is critical to measurement accuracy; even a 1°C temperature fluctuation can cause significant deviations in dielectric loss values.
Industry Standards
DL/T 846.14-2023 “General Technical Requirements for High Voltage Testing Equipment—Part 14: Testers for Dielectric Loss Factor and Volume Resistivity of Insulating Oil,” implemented on November 26, 2023, fully replaces the previous standard. This standard specifies technical requirements, test methods, inspection rules, marking and documentation, packaging, transportation, and storage for testers, applicable to their production and verification.
In-service Oil Quality Standards
GB/T 7595-2017 “Quality of In-service Transformer Oil” defines the permissible limits for dielectric loss in operating transformer oil. Depending on voltage level, the dielectric loss limit is no more than 0.020 for 500 kV to 1000 kV systems and no more than 0.040 for 330 kV and below. The volume resistivity requirement at 90°C shall not be less than 1×10¹⁰ Ω·cm.
Key Selection Criteria
Compliance with Standards
The primary criterion in equipment selection is strict compliance with current standards. The equipment must clearly declare compliance with the GB/T 5654-2007/IEC 60247 standard and the DL/T 846.14-2023 industry standard, which is a prerequisite for ensuring the validity of test data. Equipment that fails to meet these standards cannot have its test results used as evidence of compliance and may also pose safety risks.
Measurement Accuracy
DL/T 846.14-2023 explicitly defines the measurement accuracy requirements for testing instruments. When selecting equipment, key considerations include: the dielectric loss factor measurement range should cover 0.00001–1, and the resistivity measurement range should reach 2.5 MΩ·m to 20 TΩ·m. Typical technical specifications include: dielectric loss accuracy of ±(reading × 0.5% + 0.0001), and resistivity accuracy of ±(reading × 10%).
Temperature Control Performance
Temperature is a significant factor affecting tanδ in insulating oil. The standard requires measurements to be conducted at a specified constant temperature (e.g., 90°C). The instrument must integrate a high-precision constant-temperature heating system to ensure uniform and stable oil sample temperature at the set value, typically with a fluctuation range not exceeding ±0.5°C. Modern instruments commonly use medium-frequency induction heating combined with PID temperature control algorithms, enabling non-contact heating between the oil cup and heating element, offering even heating and fast response.
Oil Cup Design
The oil cup should adopt a standardized three-electrode structure made of high-quality materials (such as quartz glass or stainless steel) for easy disassembly and cleaning. The three-electrode system effectively shields electromagnetic interference and separates surface leakage current, ensuring measurement accuracy.
Automation and Intelligence Level
Modern instruments should support fully automatic testing. Users only need to load prepared oil samples, set the test temperature and voltage, and the instrument will automatically complete the entire process—including heating, temperature stabilization, measurement, calculation, and data storage. Priority should be given to models equipped with touch-screen operation, automatic empty-cup calibration, and data management functions. Data storage and upload capabilities facilitate the creation of an oil “health record,” meeting traceability requirements under the standard.
Safety Protection
The equipment must be equipped with overvoltage, overcurrent, overheating protection, and high-voltage cutoff upon opening the cover, complying with electrical safety standards to comprehensively safeguard both operators and the device.
Post time: Sep-15-2026