Guideline for Full‑process Quality Control of Oil Dielectric Loss and Resistivity Tester

Guideline for Full‑process Quality Control of Oil Dielectric Loss and Resistivity Tester

I. Why is the credibility of insulation oil dielectric loss detection data so important?

1.1 Million-dollar loss caused by a set of distorted data

During an annual oil quality testing campaign at a 220 kV substation, the dielectric loss factor measured for the insulating oil of one main transformer was 0.008 – significantly below the warning threshold of 0.02 – leading the testing personnel to conclude that the oil quality was satisfactory. However, three months later, this main transformer experienced an inter-turn short-circuit fault. Upon opening the transformer cover for inspection, it was discovered that the insulating oil had suffered severe degradation, and the actual dielectric loss factor exceeded 0.05. Post-incident analysis revealed that the oil cup used for this test had not been thoroughly cleaned in accordance with the standard procedure; the residual oil from a previous test with high loss factor had led to an abnormally low test result, thereby directly masking the underlying aging issue in the equipment.

This is not an isolated case. Statistical data from the power industry indicate that more than 60% of abnormal results obtained from dielectric loss measurement tests on insulating oil do not stem from issues with the oil itself, but rather from deficiencies in quality control during the testing process—factors such as inadequate cleaning of the oil cup, deviations in temperature control, improper selection of test voltage, or sample contamination can all cause the test data to deviate from the true values. In mild cases, this leads to redundant testing and wasted manpower; in severe cases, it can result in misjudgment of the equipment's condition, thereby creating significant safety hazards.

1.2 Dielectric Loss and Resistivity: Dual-Core Indicators for Assessing the Health Status of Insulating Oil

The dielectric loss factor (tanδ) reflects the energy loss in insulating oil under an alternating current electric field and is one of the most sensitive indicators for assessing the degree of moisture ingress, aging, or contamination in oil products; the DC resistivity, on the other hand, reflects the electrical conductivity of the oil under a direct current electric field and is directly related to the concentration of ionized impurities in the oil. The combined use of these two parameters enables a comprehensive evaluation of the electrical performance status of insulating oil and makes them mandatory test items for preventive testing of oil-filled power equipment such as transformers, current transformers, and oil-immersed reactors.

Since the test results of these two indicators directly influence the assessment of equipment condition and the formulation of maintenance decisions, the credibility and traceability of the data itself are more critical than mere testing efficiency. The Guodian Zhongxing Oil Dielectric Loss and Resistivity Tester is designed in accordance with the GB/T 5654-2007 standard. Leveraging core technologies—including a three-electrode oil cup configuration, precise mid-frequency induction temperature control, and SF6 standard capacitors—along with comprehensive empty-cup calibration and data management functions, this device provides a reliable hardware foundation for laboratories to establish a full-process quality control system covering the entire workflow from sample collection to report generation.

II. Six Core Factors Affecting Test Data Quality

2.1 Temperature factor: The sensitivity of dielectric loss value to temperature is far greater than one might expect.

The dielectric loss factor of insulating oil exhibits strong temperature dependence – for every 10°C increase in temperature, the tanδ value can increase by 1.5–2 times. Dielectric loss values measured at different temperatures are not comparable; this is the fundamental reason why standards require testing to be conducted at a specified temperature (typically 90°C).

Quality control point: The temperature deviation must be kept within ±0.5°C; testing should only commence after the oil temperature has fully stabilized to avoid measurement errors caused by the temperature difference between the interior and exterior of the oil cup. This instrument utilizes medium-frequency induction heating combined with a PID temperature control algorithm, offering excellent non-contact heating uniformity and a temperature measurement accuracy of ±0.5°C, thereby ensuring the accuracy of the temperature conditions at the hardware level.

2.2 Oil sump cleanliness: The most overlooked source of measurement error

Even the presence of trace amounts of oil contamination or impurities on the electrode surfaces of a three-electrode oil cup can significantly alter the dielectric loss value of an empty cup, thereby affecting the test results for the sample. In particular, when the testing sequence switches from a high-loss oil sample to a low-loss oil sample, if the oil cup has not been thoroughly cleaned, the residual oil film can cause the test results to be systematically higher or lower; moreover, this type of deviation is often concealed and difficult to detect during a single test.

Quality Control Point: Before each formal test, a blank cup calibration verification must be performed. The dielectric loss value for the blank cup shall be less than 5×10⁻⁵, and the capacitance shall fall within the range of 60±5 pF. Only after confirming that the oil cup's cleanliness and assembly status meet the required specifications may sample testing proceed. This instrument features a built-in blank electrode cup calibration function that automatically saves calibration data, enabling precise calculation of relative dielectric constant and DC resistivity; this ensures the reliability of the blank baseline throughout the testing process.

2.3 Test Voltage: Normative requirements for electric field strength selection

GB/T 5654-2007 specifies that the electric field strength for the dielectric loss measurement of liquid insulating materials should range from 0.1 kV/mm to 1 kV/mm. For an oil cup with an electrode spacing of 2 mm, the corresponding AC test voltage range is 200 V to 2000 V. If the voltage is too low, the signal is weak and the signal-to-noise ratio is poor; if the voltage is too high, it may induce partial discharge in the oil, thereby altering the properties of the oil. Both scenarios can affect the accuracy of the measured data.

Quality Control Point: Select an appropriate test voltage based on the type of oil being tested and the purpose of the test; lower voltages may be used for new oil, while standard electric field intensity testing with recorded voltage values is recommended for aged operating oil. This instrument features a continuously adjustable AC test voltage range of 200–2200 V and a continuously adjustable DC test voltage range of 200–500 V, fully covering the test ranges specified by relevant standards and meeting the testing requirements for various types of oil and different standards.

2.4 Sample preparation and injection: The key to preventing secondary contamination

Moisture or impurity contamination can occur during the collection, transportation, or storage of oil samples. Improper handling during the filling of the oil cup, which may generate bubbles, or the use of contaminated sampling instruments can directly affect the accuracy of the test results. In particular, trace amounts of moisture have a highly significant impact on both dielectric loss and resistivity.

Quality control point: Oil samples shall be stored in a sealed, light-proof container and allowed to return to room temperature before testing; the sampling utensil must be clean and dry; when filling the oil cup, pour the oil slowly along the inner wall of the cup to avoid bubble formation; after filling, let the sample stand for a short period until the bubbles have risen and dispersed before proceeding with the test.

2.5 Standard Capacitor Stability: Benchmark for Instrument Accuracy

The principle of using the AC bridge method to measure dielectric loss involves comparing the capacitance of the test sample with that of a standard capacitor. If the dielectric loss and capacitance of the standard capacitor itself vary with environmental conditions, the reference point of the entire measurement system will shift, introducing systematic error into all test results.

Quality control point: Standard capacitors should be of a type that is minimally affected by temperature and humidity, and should be regularly submitted for metrological calibration. This instrument incorporates an SF6-filled three-terminal standard capacitor; its dielectric loss and capacitance are not influenced by factors such as ambient temperature or humidity, ensuring that the instrument's accuracy remains stable and reliable even after prolonged use.

2.6 Environmental Conditions: The Impact of Laboratory Temperature and Humidity

When the ambient humidity is too high, the insulation resistance between the instrument surface and the external electrode of the oil cup decreases, which can introduce leakage current interference; when the ambient temperature is too low, the performance parameters of the instrument's internal electronic components may also drift, thereby affecting measurement accuracy.

Quality control points: The laboratory ambient temperature should be maintained between 0°C and 40°C, with the relative humidity kept below 75%; prior to testing, the instrument should be allowed to remain in the laboratory for an adequate period to achieve thermal equilibrium. This instrument is designed for use under ambient temperature conditions ranging from 0°C to 40°C and relative humidity below 75%; optimal measurement accuracy can be achieved under standard laboratory conditions.

III. Core Technical Specifications and Testing Capability Matrix

IV. Standardized Seven-Step Testing SOP (including Quality Control Points)

Step 1: Laboratory Environment and Instrument Preparation

Environmental confirmation: Verify that the laboratory temperature is between 0–40°C and the relative humidity is below 75%; ensure the instrument is placed on a stable, vibration-free test bench, away from sources of strong electric or magnetic field interference. Confirm that the power supply voltage is stable and that the instrument's housing is reliably grounded – in addition to grounding the power cord, the instrument's chassis grounding terminal must also be grounded; this dual grounding ensures both safety and measurement stability.

Power-on preheating: Turn on the power switch; once the instrument enters the main menu, allow it to preheat for at least 15 minutes to ensure that the internal electronic components reach thermal equilibrium, thereby minimizing the impact of temperature drift on low-loss measurements. During the preheating period, oil cup cleaning and sample preparation can be performed simultaneously.

Step 2: Oil Cup Cleanliness Verification

This is the most critical step in ensuring data credibility—and it must never be skipped:

1. Place the cleaned and dried empty oil cup into the oil cup slot, connect the test cable, and close the box lid.
2. Enter the parameter settings, select "Empty Cup" as the test type, set the temperature to 50°C, and choose the dielectric loss test.
3. Click "Start Test"; once the temperature reaches the set value, the system will automatically perform the measurement of empty-cup capacitance and dielectric loss.
4. Verification criteria: The empty-capacitance value shall be within the range of 60 ± 5 pF; the empty-dielectric-loss value shall be less than 5 × 10⁻⁵.
5. If the results fall outside the specified range, it indicates that the oil cup was not cleaned thoroughly or was improperly assembled; the oil cup must be re-cleaned and re-assembled, followed by another verification, until the data for an empty cup meets the acceptance criteria.
6. Valid empty-bottle calibration data is automatically saved; the instrument uses this data as a reference to calculate the relative permittivity and resistivity.

Step 3: Sample preparation and injection

Oil sample pretreatment: Extract the oil sample from its storage container in advance and place it at room temperature in a laboratory environment to prevent condensation on the inner wall of the oil container due to temperature variations. Inspect the oil sample for any visible impurities or water sedimentation.

Oil filling procedure: Open the box cover and remove the oil cup. Using a clean, dry syringe, slowly inject the oil sample along the inner wall of the oil cup until the electrode is fully submerged (approximately 40 mL); be careful not to introduce any air bubbles. After filling, allow the mixture to stand for 2–3 minutes until all air bubbles have risen and dissipated.

Installation and Positioning: Carefully place the oil cup containing the oil sample into the oil cup slot; connect the test cable and the temperature sensor; verify that all connections are secure; then close the box lid. The box lid features a closing protection function: when the lid is open, the voltage-rise procedure will be automatically terminated to ensure operational safety.

Step 4: Test Parameter Configuration

Enter the "Start Test" parameter settings interface and configure each test condition individually:

1. AC test voltage: Set according to standard requirements or operating instructions; for conventional transformer oil tests at 90°C, 1000 V is commonly used (corresponding to an electric field strength of 0.5 kV/mm); the range is adjustable from 200 V to 2200 V.
2. DC test voltage: Resistivity measurement voltage; default setting: 500 V; adjustable range: 200–500 V
3. Test temperature: Set according to standard; the dielectric loss of transformer oil is typically at 90°C, and its resistivity is typically at 90°C or 80°C; the temperature range is adjustable from 40°C to 120°C.
4. Test Type: Select "Sample"
5. Test Item: Select the item to be tested – dielectric loss factor and/or DC resistivity; light green indicates selection.

Quality Requirements: All test parameters must be recorded in the original documentation to ensure that test conditions are traceable and reproducible.

Step 5: Start Automatic Testing

After confirming that all parameter settings are correct, press the "Start Test" button; the instrument will then enter the fully automated testing procedure:

1. Heating phase: Medium-frequency induction heating is activated; the PID algorithm provides precise temperature control, with the current temperature displayed in real time on the screen. The heating rate is uniform, preventing localized overheating that could lead to localized degradation of the oil.
2. Constant-temperature stabilization: Once the temperature reaches the set value, the system enters the constant-temperature phase to ensure that the overall temperature of the oil sample remains uniform and stable; the temperature deviation is controlled within ±0.5°C.
3. Dielectric loss test: An AC test voltage is automatically applied, and the capacitance and dielectric loss factor of the oil sample are measured using the AC bridge method; the entire testing process is fully automated and requires no manual intervention.
4. Resistivity measurement: If the resistivity option is selected, upon completion of the dielectric loss measurement, the system will automatically switch to DC high voltage to perform a DC resistivity measurement.
5. Result Output: Upon completion of all test items, the screen displays the final test results, including all data such as capacitance, relative permittivity, dielectric loss factor, and DC resistivity.

Safety Note: During testing, the instrument contains high voltage and high temperature inside; do not open the enclosure cover, and do not touch the oil cup or cable connectors.

Step 6: Data Recording and Printing

After completing the test, perform data retention:

1. Automatic Storage: Test results are automatically saved to the instrument's internal memory, complete with the test date and time stamp, facilitating subsequent traceability and review. Past records can be viewed using the "Historical Data" function.
2. Printing Output: The device can be configured to operate in automatic printing mode; upon completion of the test, the built-in thermal printer will automatically print a test receipt, which should be pasted into the test record book as the original documentation. The printed output includes the date, time, temperature, voltage, and the measured values of all parameters – ensuring comprehensive information.
3. Manual verification: The tester verifies that the printed data matches the data displayed on the screen; upon confirming that the data is normal, the tester signs off to confirm it. If the data is found to deviate significantly from the normal range, it is marked as suspicious data, and a re-test is arranged.

Step 7: Final cleaning and instrument placement

Upon completion of all tests:

1. Wait until the instrument temperature has dropped to a safe level (recommended: below 50°C) before removing the oil cup to prevent burns.
2. Transfer the oil sample; clean the oil cup according to the standard procedure; dry the cup and store it properly.
3. Clean any oil residue that may have splashed into the oil reservoir to keep the instrument interior clean.
4. Turn off the power supply; tidy up the test cables and accessories; cover the instrument with a dust cover.
5. Organize all original test records and printed receipts for archiving and safekeeping.

V. Oil Cup Cleaning Procedure – The First Line of Defense for Data Integrity

5.1 Why cleaning the oil cup is so critical

The electrode plates of the three-electrode oil cup feature an exceptionally high surface finish; even trace amounts of oil contamination, moisture, or dust can alter the surface condition of the plates, leading to an increase in the dielectric loss of an empty cup or a deviation in capacitance – effects that are subsequently superimposed on the sample test results. In particular, when a low-loss oil sample is tested after a high-loss oil sample has already been tested, residual contamination can cause significant cross-contamination errors; however, such errors are often overlooked and treated as inherent variations in the oil quality itself.

5.2 Identification Test – Standard Seven-Step Cleaning Method

For applications where extremely high data accuracy is required—such as arbitration tests or new oil acceptance inspections—a complete data cleansing process must be implemented:

1. Step 1: Solvent Pre-washing – Fully disassemble the oil container and sequentially clean all components using chemically pure petroleum ether (boiling range: 60–90°C) and benzene to remove most of the residual oil contamination.
2. Step 2: Acetone rinsing – Rinse all components with acetone to remove any residual solvent or polar contaminants from the previous step.
3. Step 3: Neutral detergent cleaning – Carefully clean all components using a neutral detergent to remove organic dirt and ionized impurities.
4. Step 4: Boiling with Trisodium Phosphate – Place all components into a 5% trisodium phosphate distilled water solution and boil for 5 minutes to thoroughly remove polar contaminants and grease from the electrode surface.
5. Step 5: Rinse with distilled water – Rinse repeatedly with distilled water to thoroughly remove any residual sodium tripolyphosphate or detergent.
6. Step 6: Boil distilled water – Place all components into distilled water and boil for at least 1 hour to perform the final purification and cleaning step.
7. Step 7: Drying and Assembly – Place all components into an oven set at 40–45°C and dry them for at least 1 hour; once cooled to a temperature that is no longer hot to the touch, assemble the oil cup according to the standard spacing.

5.3 Simplified Cleaning Method for Routine Testing

For routine testing scenarios such as daily preventive tests, a simplified procedure may be adopted: skip the two steps of boiling with trisodium phosphate and boiling with distilled water; instead, follow these steps: solvent rinsing → acetone rinsing → multiple rinses with distilled water → direct drying. This simplified procedure reduces the overall processing time; however, it must be accompanied by an empty-bottle calibration validation to confirm that the cleaning efficacy meets the required standards before use.

5.4 Rapid Transition Method for Continuous Testing within the Same Batch

When testing a batch of oil samples with similar loss values consecutively, if the dielectric loss of the previous sample falls within the acceptable range, it may not be necessary to thoroughly clean the oil cup before testing the next sample; however, the oil cup must be repeatedly rinsed with the next sample at least three times to remove any residual oil from the cup walls, thereby minimizing cross-contamination as much as possible.

VI. Eight Common Data Anomalies and Troubleshooting Procedures

1. Abnormally high dielectric loss value with poor repeatability

Possible causes: ① Incomplete cleaning of the oil reservoir, leaving residual contaminants on the electrode surfaces; ② The oil sample is damp or contains impurities; ③ The test temperature is too high, exceeding the set value; ④ Excessive ambient humidity increases the surface leakage current.

Troubleshooting procedure: First, perform a blank-test to determine whether the issue stems from the oil cup; if the blank test is successful, proceed to inspect the oil sample and temperature; if the blank test fails, re-clean the oil cup; during the rainy season when humidity levels are high, activate the laboratory dehumidification equipment.

2. The resistivity data are relatively low, showing a significant deviation from historical values.

Possible causes: ① Increased ionic impurities in the oil sample indicate oil aging or degradation; ② Inadequate cleaning of the oil cup has left behind conductive ions; ③ The DC test voltage setting is too low; ④ The test temperature exceeds the standard temperature.

Troubleshooting plan: Verify whether the test temperature and voltage settings match the historical records; confirm that the oil cup cleaning procedure is complete; perform a comparative test using a known compliant standard oil sample to determine whether the issue stems from the oil product or the instrument.

3. The capacitance of the empty capacitor deviates from the standard value of 60 ± 5 pF.

Possible causes: ① Improper oil cup assembly, resulting in altered electrode plate spacing; ② Loose RF head on the protective electrode cover; ③ Poor contact at the test cable connector.

Troubleshooting procedure: Reassemble the oil cup according to the standard clearance; check whether the RF head on the signal cable's protective electrode cover is securely tightened; if necessary, reconnect it; inspect the cable terminals for any signs of oxidation or loosening.

4. No temperature signal detected during the heating process

Possible causes: ① The temperature signal cable is not connected or has poor contact; ② The temperature sensor is faulty; ③ The cable plug is inserted in the wrong position.

Troubleshooting procedure: After powering off the system, verify that the connections at both ends of the temperature signal cable are correct and secure; re-engage the cable to confirm; if the issue recurs multiple times, contact the manufacturer to replace the temperature sensor.

5. Displays "Electrode Cup Short Circuit" during voltage increase

Possible causes: ① Improper oil cup assembly resulting in a short circuit between the high-and low-voltage electrodes; ② The presence of metal particles or impurities in the oil sample causing an inter-electrode short circuit; ③ Oil contamination or foreign objects in the oil cup groove causing a creepage fault.

Troubleshooting Procedure: After powering off the device, remove the oil cup and check whether the electrode plates are properly aligned and whether there is any contact; inspect the oil sample for any visible impurities; clean the interior of the oil cup cavity to remove any oil contamination or dust.

6. The power indicator light does not illuminate; the screen shows no display.

Possible causes: ① The power socket is not powered; ② Poor contact in the power cable; ③ The power board fuse has blown.

Troubleshooting procedure: Verify that the power supply is functioning normally; test the device by replacing the power cord; inspect the fuse at the instrument's power input port – if it has blown, replace it with a fuse of the same specification. Frequent fuse blowouts indicate an internal fault; refer the device for repair and further diagnosis.

7. Significant fluctuations in test results obtained from multiple analyses of the same oil sample

Possible causes: ① Testing commenced before the temperature had fully stabilized; ② Air bubbles in the oil sample have not yet dissipated; ③ Poor oil sample homogeneity, with possible stratification or suspended solids.

Inspection procedure: Extend the constant-temperature holding time to ensure uniform oil temperature distribution; allow the oil to stand for a sufficient period after filling to purge any air bubbles; gently shake the oil sample before testing (to avoid introducing new bubbles) to ensure uniformity of the sample.

8. Blurry printout or no paper ejection

Possible causes: ① Thermal printer paper is exhausted; ② The printer paper is inserted incorrectly, with the thermal coating facing away from the print head; ③ The print head is contaminated with dust.

Troubleshooting procedure: Open the paper feed compartment to inspect the paper roll; replace the paper roll if any is missing; ensure that the thermal coating side is facing the print head; for printers that will not be used for an extended period, gently wipe the print head with alcohol-wetened cotton to remove dust.

VII. Test Data Interpretation and Insulating Oil Condition Classification

7.1 Transformer Oil Dielectric Loss Reference Standard

According to DL/T 596 "Code for Preventive Testing of Electrical Equipment," the reference values for the dielectric loss factor (at 90°C) of transformer oil under operating conditions are as follows:

7.2 DC Resistivity Reference Standard

The DC resistivity (at 90°C) of operating transformer oil shall generally not be lower than:

• New oil / Recycled oil: ≥ 6 × 10¹⁰ Ω·cm (i.e., 600 GΩ·cm)
• Operating oil resistance value: ≥ 1 × 10¹⁰ Ω·cm (i.e., 100 GΩ·cm)

A decrease in resistivity is typically associated with an increase in polar impurities or organic acid content in the oil, and often occurs alongside the deterioration of other parameters such as acid value and dielectric loss.

7.3 Level-Based Status Classification and Management Recommendations

VIII. Laboratory Data Management and Traceability System Implementation

8.1 Three Key Elements for Data Traceability

The test data for qualified insulating oil must be able to answer three questions: under what conditions the measurement was performed, which instrument was used for the measurement, and by whom the measurement was conducted. These constitute the three essential elements of data traceability:

Traceable test conditions: All method parameters—such as test temperature, test voltage, and test item selection—must be fully recorded. All test parameters of this instrument are stored and printed together with the test results to ensure that no parameter settings are lost.

Instrument status is traceable: the instrument calibration certificate remains valid, the empty-bottle calibration verification has been successfully completed, and the instrument is operating normally. It is recommended to perform an empty-bottle verification upon each power-on; the verification data should be archived together with the corresponding batch testing records.

Personnel operation traceability: Test personnel must sign off on the procedures, and all operational processes must comply with SOP specifications. The laboratory shall establish an authorized personnel system; only personnel who have completed training and assessment may perform testing operations.

8.2 Historical Data Trend Analysis

The historical trend curves of dielectric loss and resistivity for individual equipment units hold greater diagnostic value than the individual measurement values. The instrument features a built-in historical data storage function, enabling the preservation of multiple sets of test records; users can browse these historical data records and print them out. It is recommended that laboratories maintain an oil quality record for each oil-filled equipment unit, compile the historical testing data into trend curves, analyze the rate of change, and proactively identify potential degradation inflection points.

8.3 Periodic Verification and Instrument Calibration

The instrument shall be submitted annually to a metrology service provider for comprehensive calibration and shall obtain a calibration certificate. Between two calibrations, periodic interim verification shall be performed using standard oil samples with known characteristics or empty-bottle parameters to verify whether the instrument's performance is normal, ensuring that the instrument remains under controlled conditions between these two calibrations.

IX. Five Typical Application Scenarios

Scenario 1: Electric Power Testing Research Institute – Oil and Chemicals Laboratory

Challenge: The large volume and wide variety of oil samples submitted for testing impose stringent requirements on both testing efficiency and data quality; these requirements must be met in compliance with the quality control requirements recognized by CNAS.

Solution: An integrated, fully automated design enables automatic completion of temperature-rise testing and printing once the parameters are set; a single operator can monitor multiple devices simultaneously, significantly increasing the testing throughput. The system features a three-electrode standard oil cup with empty-cup calibration functionality and comprehensive data recording – meeting the stringent laboratory accreditation requirements regarding the traceability of testing methods and the credibility of data – making it easy to prepare for audits and blind sample assessments.

Scenario 2: Oil and Chemicals Team, Substation Maintenance Section, Power Supply Company

Challenge: The jurisdiction encompasses a large number of main transformers and instrument transformers, with a concentrated annual pre-inspection workload; therefore, rapid result delivery is required to support maintenance decision-making.

Solution: The mid-frequency induction heating system offers rapid heating, precise PID temperature control, and a short single-sample testing cycle, making it ideal for batch sample analysis. Test results are automatically stored and printed, enabling direct entry into the equipment status record. The desktop design ensures stability and reliability, making it suitable for long-term use in field laboratories; its weight of 25 kg also facilitates easy relocation between different work areas.

Scenario 3: Transformer manufacturing plant's outgoing inspection

Key requirement: Before leaving the factory, each product must undergo testing of its insulating oil performance; the testing conditions must be standardized and the data must demonstrate good consistency, serving as the basis for the factory compliance certificate.

Solution: Unified test parameter settings + precise temperature control + a stable standard capacitor reference ensure excellent consistency and comparability of measurement data across different batches and operators. The built-in printer generates test slips directly, which can serve as original documentation for factory inspection and be archived alongside the products.

Scenario 4: Third-party power inspection agency

Challenge: Handling oil sample testing services for a diverse range of clients requires professionally reliable data and standardized reports, while also accommodating the varying testing standard requirements of different clients.

Solution: Fully adjustable parameters (AC voltage: 200–2200 V; DC voltage: 200–500 V; temperature: 40–120°C) enable compatibility with various testing requirements across different standards and oil types. The test data includes timestamps for traceability; when integrated with standardized original record management systems, this solution meets the requirements of third-party testing agencies regarding data fairness and traceability.

Scenario 5: Self-owned power plants of large industrial and mining enterprises

Challenge: Corporate-owned substations and plant transformers require regular oil analysis, yet there is no dedicated oil analysis laboratory available; therefore, there is a need for equipment that is easy to operate and yields intuitive results, enabling even non-specialists to use it effortlessly.

Solution: 5.7-inch color touchscreen with a fully Chinese-language menu – the operation process is simple and intuitive; users can start operating the system immediately after receiving basic training. The fully automated testing functionality minimizes human-operated errors; test results are displayed and printed directly, eliminating the need for complex calculations or interpretation, thereby meeting the daily oil quality monitoring requirements of industrial and mining enterprises.

X. Daily Instrument Maintenance and Quality Assurance

10.1 Daily Operational Maintenance Key Points

Cleaning and Maintenance: Wipe the instrument's exterior with a clean, soft cloth; avoid using any hard objects to scratch the touchscreen. After each use, clean the oil reservoir to prevent oil buildup. When not in use for an extended period, cover the device with a dust cover and store it in a dry, well-ventilated location.

Oil cup storage: After cleaning and drying, the oil cups should be stored in a clean, dry, dust-proof container to prevent dust accumulation or moisture exposure. As oil cups are precision components, handle them with care when removing or placing them; avoid any impact or deformation that could affect the inter-electrode spacing.

Cable protection: Test cables – particularly high-voltage and RF connectors – should be properly protected to avoid bending or impact. Regularly inspect the connector areas for any loosening or oxidation to ensure good electrical contact.

10.2 Regular Performance Verification

It is recommended to perform a "empty cup" test once per month, recording the empty cup capacitance and dielectric loss value, and comparing these measurements with historical data. If there are significant changes in the empty cup parameters, this indicates a change in the oil cup's condition, necessitating thorough cleaning or inspection of the assembly. This is also the simplest and most effective method for periodic verification; it allows you to determine whether the instrument's basic operating condition is normal without requiring any additional reference materials.

10.3 Safety Usage Guidelines

• The instrument must be reliably grounded; dual grounding (power supply cable + grounding terminal) is required to ensure safety.
• During testing, it is strictly prohibited to open the box cover or touch the oil cup or the high-voltage connector.
• The medium-frequency heating furnace is equipped with a 120°C temperature limit protection feature that automatically cuts off power in case of overheating, preventing dry burning.
• Equipped with overvoltage, overcurrent, and high-voltage short-circuit protection; in the event of an abnormal condition, the high-voltage Shall rapidly cut off and an alarm is triggered.
• When replacing a fuse, use a fuse of the same specification and model; using a fuse with a larger specification is prohibited.