Transformer oil withstand voltage tester

Transformer oil dielectric strength tester technology analysis

• core functionality: Accurate measurement of the dielectric strength, i.e. Breakdown Voltage (BDV), of transformer insulating oils or other liquid insulating media.

• Test Principle: Under standardized test conditions, a continuously rising industrial frequency alternating current (AC) voltage is applied to a pair of electrodes immersed in an oil sample until the oil sample arcs through at the electrode gap, at which time the voltage value is recorded as the breakdown voltage.

• primary purpose: Evaluates the electrical insulating properties of insulating oils to determine whether they have deteriorated due to moisture, particulate impurities or gases.

• technology standard: The testing process strictly follows international and national standards, mainly IEC 60156 and ASTM D1816 / ASTM D877, to ensure comparable and valid results.

• applied value: As a key part of transformer condition monitoring and preventive maintenance, regular insulating oil voltage testing is the basis for safe and reliable transformer operation.

I. The basic role of insulating oil and the need for voltage withstand test

Insulating oils (usually mineral or synthetic) in power transformers have a double critical mission:isolaterespond in singingradiator. As an insulating medium, it fills the space between the windings, the core and the case to prevent electrical discharges from occurring inside. As a cooling medium, it transfers the heat generated by the windings and core to the heat sink through convection circulation.

The dielectric strength of insulating oils is highly susceptible to trace contaminants:

• Moisture: Moisture is the number one enemy of insulating oil performance. Dissolved or suspended water drastically reduces the breakdown voltage of the oil.

• Particulate Matter:: Metal or fiber particles from aging equipment, external intrusion, and the formation of "conductive bridges" in the presence of an electric field can significantly reduce the strength of the insulation.

• Dissolved Gases: Gases generated or air mixed in during operation may free and initiate partial discharges under an electric field, eventually leading to an overall breakdown.

Therefore, regular use of insulating oil withstand voltage tester to detect its breakdown voltage, is a direct, rapid determination of the insulating oil is contaminated, whether the performance of the most effective means of deterioration.

II. Principle of operation and standardized test procedure

1. Core operating principles.
The instrument generates an AC voltage that is smoothly increased from zero at a preset rate (e.g. 2 kV/s) by means of a built-in step-up transformer. This voltage is applied to a pair of electrodes in a standard oil cup. As the voltage increases, the electric field strength at the electrode gap increases. When the electric field strength reaches the limit of the oil sample can withstand, the oil molecules ionization, the formation of conductive channels, instantaneous arc discharge, that is, "breakdown". The high-speed overcurrent detection circuit inside the instrument will immediately detect the breakdown current and instantly cut off the high-voltage output, and at the same time, lock and record the peak value of the voltage before the breakdown occurs, which is the breakdown voltage value of the measurement.

2. Standardized testing process.
In order to ensure accurate and reproducible test results, modern voltage testers are usually equipped with a fully automated test process:

1. Sampling and preparation: Obtain a representative sample of oil from the transformer and prepare a clean, dry special test oil cup.

2. Oil Filling and Standing: Inject the oil sample slowly into the oil cup along the cup wall to prevent air bubbles and ensure that the oil sample completely submerges the electrode. Afterwards, the oil sample is allowed to stand for 5-10 minutes according to the standard requirements (e.g. IEC 60156 requires 5-10 minutes of standing time) in order to eliminate small air bubbles generated during the oil filling process.

3. parameterization: Select the appropriate test standard (e.g., IEC 60156) on the instrument, and the instrument will automatically load the preset parameters for that standard, including boosting rate, stirring time, resting time, and number of tests.

4. automatic testingAfter starting the test, the instrument will automatically complete the cycle of "Stirring - Standing - Pressurizing - Breakdown - Recording - Pressure Relief - Stirring...". After starting the test, the instrument will automatically complete the cycle of "Stirring-Stationary-Raising-Breakdown-Recording-Relieving-Stirring...". Typically, 6 consecutive breakdown tests are performed.

5. Results processing: After 6 tests are completed, the instrument automatically calculates the average and standard deviation of the breakdown voltage of the 6 tests and judges whether the set of data is valid or not according to the selected criteria. Finally, the test results are displayed on the screen and can be printed out through the built-in printer.

III. Core components and key international standards

1. Core components of the instrument.

• High Pressure Generation and Regulation Systems: It consists of a high-precision, distortion-free step-up transformer and voltage regulator, which ensures a standard output voltage waveform and constant step-up rate.

• Standard test oil cup with electrodes: Oil cups are usually made of highly translucent glass or Plexiglas for easy viewing. The shape and size of the electrodes follow strict standards, commonly spherical electrodes (IEC 60156) and mushroom head shaped electrodes (ASTM D1816). The precision of the electrode gap (usually 2.5mm) is critical to the accuracy of the test.

• Microprocessor control and measurement systemsAs the brain of the instrument, it is responsible for the automated control of the entire test process, high-precision voltage measurement, fast breakdown detection, as well as the calculation, storage and printing of data.

2. Comparison of key international standards.

Frequently Asked Questions (FAQ)

1. What is the breakdown voltage of transformer oil?
The qualification criteria depend on the voltage class of the transformer and the relevant operation and maintenance procedures. Usually, there are clear qualification criteria for new oil or oil in operation. For example, according to Chinese standards, for transformers of 220 kV and above, the breakdown voltage of oil in operation should not be less than 50 kV. the latest national or industry standards should be referred to.

2. Why is it necessary to conduct multiple (e.g., six) tests and take an average?
The breakdown process of the insulating medium has a certain degree of randomness and dispersion. The results of a single test may be subject to chance and cannot accurately reflect the overall insulation level of the oil. By taking multiple measurements and averaging them, random errors can be eliminated and a more reliable and statistically significant result can be obtained to accurately assess the dielectric strength of the oil.

3. Can the pressure tester directly measure the water content (ppm) in the oil?
Cannot. The withstand voltage tester measures the breakdown voltage (in kV), which reflects the effect of all contaminants such as moisture, particles, etc. on the insulating properties of theComprehensive impact. It can qualitatively determine whether the oil is damp or not, but cannot quantitatively measure the specific content of moisture. To accurately measure the content of trace moisture in oil (in ppm), it is necessary to use specializedKarl Fischer microhydrometerThe

4. What is indicated by the high mean but high dispersion (high standard deviation) of the test results?
This usually indicates the presence of unevenly distributed contaminants in the oil, such as individually large suspended fibers or particulate impurities. Although the insulating properties of the oil matrix are fair (high average values), the presence of these impurities constitutes a weak point in the insulation and poses a significant safety hazard. This situation also requires treatment of the insulating oil.

5. What is the difference between portable and laboratory voltage testers?
The main differences are precision, functional complexity and environmental adaptability. Laboratory instruments are usually more accurate and versatile, but they are larger. Portable instruments are compact, rugged, have built-in batteries, are optimized for field use, and, while they may be slightly less accurate than laboratory models in terms of ultimate accuracy, they are fully capable of meeting the requirements of preventive maintenance testing in the field.