I. Overview of Transformer Usage Technology
In the power system, transformers play a core role in converting current and voltage, and their operational performance directly determines the accuracy of electric energy measurement, as well as the stability and reliability of the relay protection system. Regarding the quality control and status operation and maintenance of transformers, there are two parallel and complementary standardized technical paths in the industry, which are respectively adapted to different working scenarios and technical goals.
The first path is the legal metrological verification path, with the core objective of ensuring the compliance of electric energy measurement, strictly following the national metrological verification regulations to conduct periodic verification of the error characteristics of transformers, ensuring fair and just trade settlement measurement. The second path is the equipment operation and maintenance path, with the core objective of ensuring the safe and stable operation of power grid equipment, relying on the power preventive test regulations to conduct regular status detection and fault排查 of transformers’ insulation performance， electrical characteristics， etc.
Transformer calibration instruments and transformer testing instruments are the core specialized equipment for the above two technical paths. However， in actual on-site operations， the functional positioning and application scenarios of these two types of equipment are often confused， leading to improper equipment selection and deviations in the execution of operation standards. This article， in combination with the current power and metering standards system， systematically sorts out the technical principles， standard basis， core functions and applicable scenarios of these two types of instruments， clarifies their technical boundaries， and provides professional references for power operation and maintenance， metering detection technicians.
II. Technical Standard System： Dual Path Framework
2.1 Standard Basis of the Measurement Verification Path
Transformers are the core components of electric energy metering devices， and their accuracy grades directly determine the fairness and accuracy of power trade settlement. According to DL/T 448 “Technical Management Regulations for Electric Energy Metering Devices”， the regulations clearly define the accuracy configuration requirements for current and voltage transformers of different accuracy grades for electric energy metering devices. The current and voltage transformers for Class I metering devices must have an accuracy grade of 0.2 or higher.
To strictly ensure the accuracy of transformer measurement， the country has established a complete metrological verification regulation system， with core applicable regulations including two. The first is JJG 313 “Verification Regulations for Measuring Current Transformers”， applicable to current transformers with a rated frequency of 50Hz or 60Hz and accuracy grades of 0.001 to 0.5 for the first verification， subsequent verification， and in-use inspection. The second is JJG 314 “Verification Regulations for Measuring Voltage Transformers”， with the same coverage as the former， covering the full cycle metrological verification of voltage transformers of the same grade.
Both of these regulations are currently in an effective implementation state. According to the standard update regulations， both regulations will be abolished on October 2， 2026， and will be replaced by new revised regulations， which are the core technical basis for transformer metrological verification work at present.
2.2 Standard Basis of the Characteristic Testing Path
The operation， maintenance and fault diagnosis of transformers， status detection and fault diagnosis， follow an independent power equipment test standard system， which is completely different from the technical logic and assessment indicators of the metrological verification system. DL/T 596 “Power Equipment Preventive Test Regulations” is the core regulation in this field， and its dedicated chapter clearly stipulates the preventive test technical requirements for various transformers.
This regulation， from the perspective of equipment safe operation， focuses on core indicators such as insulation performance， electrical parameters， and withstand voltage strength of transformers， and defines regular test items， mainly including insulation resistance testing， dielectric loss factor testing， ratio and polarity verification， excitation characteristic testing， and power frequency withstand test， comprehensively covering the technical requirements for evaluating the operating status of transformers and identifying defects.
At the product technical specification level， GB 1208 “Current Transformers” and GB 1207 “Voltage Transformers” are industry-wide general basic standards. Among them, GB 1208 is applicable within the frequency range of 15Hz – 100Hz, for the newly manufactured current transformers used in power measurement and protection devices; GB 1207 corresponds to the technical parameters and test methods of voltage transformers. These two national standards provide unified technical guidelines for the factory tests, on-site commissioning tests, and periodic preventive tests of transformers, and are also the core functional design basis of transformer testers.
III. Transformer Calibration Instrument: Special Equipment for Transformer Error Verification
3.1 Working Principle
The core function of the transformer calibration instrument is to accurately detect the ratio error (ratio difference) and phase error (phase difference) of the transformer. The core working principle is the difference comparison method. During operation, the standard transformer and the tested transformer are placed in the same primary electrical condition and operated in parallel. The secondary output signals of the two devices are connected to the difference loop of the calibration instrument, and the instrument accurately collects and decomposes the tiny difference signals. Through a dedicated algorithm, it accurately calculates the measurement error of the tested transformer relative to the standard reference equipment.
Due to the limitations of the technical principle, the transformer calibration instrument cannot independently complete the entire verification operation. A compliant transformer measurement verification system requires a set of standard transformers, current/voltage load boxes, current/voltage boost power supplies, and the calibration instrument itself. Among them, the core benchmark standard transformer must have an accuracy level two grades or more higher than the tested transformer to ensure minimal benchmark error and stable performance, providing a reliable reference standard for error calculation.
3.2 Core Function and Application Scenarios
The core output indicators of the transformer calibration instrument are ratio difference and phase difference. The detection and judgment basis is the error limit standard corresponding to the accuracy level of the transformer. Taking the dedicated 0.2-class current transformer as an example, under the rated working condition, its ratio difference allowable deviation does not exceed ±0.2%, and the phase difference allowable deviation does not exceed ±10′. For protection transformers, the equipment can specifically detect special protection characteristic parameters such as composite error and instrument security coefficient, meeting the verification requirements of protection-type transformers.
The core application scenarios of this equipment are laboratory verification of transformer measurement in the power metering field and periodic on-site inspection of the power grid. According to the requirements of DL/T 448 regulations, transformer measurement verification must strictly follow the national measurement verification system table and corresponding verification regulations. All verification original records must be retained for at least three verification cycles to ensure that the measurement data is traceable and verifiable.
In terms of verification cycle norms, the rotation cycle of high-voltage transformers and on-site inspection cycles are generally 10 years, and the inspection cycle of low-voltage current transformers is 20 years. As a legal measurement verification equipment, the test data issued by the transformer calibration instrument can be used to compile formal verification certificates and is the core basis for determining whether the transformer’s measurement performance is compliant and meets the trade settlement requirements.
IV. Transformer Tester: Comprehensive Characteristic Test Platform
4.1 Working Principle
The technical design logic of the transformer tester is fundamentally different from that of the calibration instrument. It can operate independently without relying on external standard transformers. The equipment is equipped with a variable frequency power supply and a power amplifier unit, which can independently output standard voltage and current excitation signals to the specified winding of the transformer, and simultaneously collect the real-time response data of the transformer. Through the built-in algorithm, it accurately calculates various characteristic parameters such as insulation, electrical, excitation, and transformation ratio, achieving single-machine integrated testing without the need for external reference equipment and auxiliary power supply, and adapting to the convenient operation requirements of the field.
4.2 Core Test Items and Standard Basis
According to DL/T 596 “Power Equipment Preventive Test Regulations” and GB 1208, GB 1207 national standards, the transformer tester can comprehensively cover the core detection items of the transformer operation and maintenance stage, as follows:
Firstly, insulation resistance test. Use a 2500V megohmmeter to measure the insulation resistance between the windings of the transformer and between the windings and the ground. Taking a 10kV current transformer as an example, the standard value of its insulation resistance between the primary and secondary windings, as well as to the ground, should be no less than 1000MΩ, and the absorption ratio should be no less than 1.3. If the insulation resistance value drops abnormally, it can directly reflect the defects such as moisture and aging of the equipment insulation.
Secondly, the dielectric loss factor test. This project is applicable to oil-paper insulated and adhesive-paper capacitive transformers. It is the core indicator for the refined assessment of the insulation state. The specification requires that the dielectric loss factor of oil-paper capacitive transformers should not exceed 0.8%, and that of oil-filled transformers should not exceed 3%. The deviation of the actual measured capacitance of the equipment from the factory-rated value needs to be controlled within ±5%, which can accurately identify the hidden moisture and insulation deterioration problems of the equipment.
Thirdly, the excitation characteristic test. By applying a gradient voltage to the secondary winding of the transformer and simultaneously measuring the excitation current and drawing the volt-ampere characteristic curve, it can accurately evaluate the saturation performance of the iron core of the equipment, and determine the signal transmission capability of the protection-level transformer under the fault large current condition. It is a key test item to verify the compliance of the 10% error curve of the protection-use transformer.
Fourthly, the ratio and polarity check. The equipment automatically outputs an excitation signal, detecting the deviation of the actual ratio of the transformer from the rated ratio, and simultaneously verifying the polarity of the equipment. Industry standards require that the transformer should meet the negative polarity standard. For 0.5-class measuring current transformers, the ratio error should not exceed ±0.5%, and abnormal polarity will directly cause the relay protection device to malfunction or fail to operate, affecting the safe operation of the power grid.
Fifthly, the power frequency withstand voltage test. It is used to test the insulation withstand strength between the windings of the transformer and between the windings and the ground. The test voltage is strictly set according to national standards and the higher operating voltage of the equipment. The standard test duration is 1 minute, which can effectively detect major safety hazards such as insulation breakdown and inter-turn short circuits.
4.3 Core Function Boundary with the Calibration Instrument
In on-site operations, the concepts of the ratio-related parameters for the two types of equipment are completely different and cannot be confused or substituted. The ratio error detected by the transformer tester reflects the numerical deviation between the actual ratio of the equipment and the rated design ratio, and is mainly used to verify whether the winding turns of the transformer are accurate and whether there are inter-turn short circuits and other manufacturing or equipment defects.
While the ratio difference detected by the transformer calibration instrument is a comprehensive measurement error indicator after superimposing the excitation current compensation effect under standard load and standard power factor conditions. It is mainly used to determine whether the transformer meets the corresponding measurement accuracy grade requirements, and is directly related to the accuracy of electric energy trade settlement. The technical definitions, assessment standards, and application purposes of the two are completely different and cannot be substituted.
V. Equipment Selection and On-site Application Decision Framework
5.1 Selecting Based on the Nature of the Work Task
If the work task is legal metrological verification, for the transformers used for power trade settlement, when it is necessary to issue a legally valid verification report and certificate, the transformer calibration instrument must be selected. At the same time, the metrological verification operation must meet the environmental requirements of the specification, and the verification area of voltage and current transformers should be set up in zones, with sufficient safe working distance from high voltage, and the corresponding safety protection facilities should be provided to ensure the accuracy, compliance, and effectiveness of the verification data.
If the work task is power grid operation maintenance, equipment condition assessment, and fault investigation, for the health status detection of operating transformers in substations, and the verification of abnormal protection circuits, the transformer tester should be selected first. The single-machine integration, simple operation, and portable usability of the equipment are highly compatible with the rapid on-site operation requirements in outdoor and substation environments.
5.2 Selecting Based on the Accuracy Requirements of the Detection
The accuracy requirements of the transformer calibration instrument are strict. The measurement uncertainty of the equipment itself needs to be precisely matched with the accuracy grade of the inspected transformer. The industry standard is that the measurement error of the calibration instrument itself should not exceed one-third to one-fifth of the error limit of the inspected transformer to ensure the authority and accuracy of the metrological verification data.
The accuracy requirements of the transformer tester are set differently for each item, and the overall level is lower than the metrological verification standard. Among them, insulation resistance testing and power frequency withstand voltage test are mainly qualitative judgments, mainly to determine whether the equipment is qualified and whether there are defects, and the accuracy requirements are relatively low; ratio testing, DC resistance testing, etc. require basic quantitative accuracy, but do not need to reach the high-precision traceability standard of legal metrology.
5.3 Industry Application Development Trend
At present, there are some integrated comprehensive detection equipment in the industry, which also have both calibration and testing functions. However, the core design of the equipment has a clear emphasis, and it is impossible to achieve the standardized full coverage of both functions. From the perspectives of technical specifications, accuracy traceability, and operational compliance, the technical systems and accuracy standards of metrological verification and preventive tests are greatly different. A single device is difficult to simultaneously meet the traceability requirements of legal metrology and the diagnostic needs of operation and maintenance tests.
In actual engineering applications, to ensure the legal validity of verification data and the diagnostic accuracy of operation and maintenance tests, the principle of using dedicated equipment for specific purposes should be adhered to. Select equipment based on the core operation goals, avoiding the compromise of multi-functional equipment performance, which may lead to invalidation of metrological data and inaccurate defect detection, and ensuring the compliance of power transformer metrology and operational safety.
Post time: Sep-23-2026