The electrical performance of insulating oil is typically evaluated through three key tests: dielectric strength, dielectric loss tangent (tan δ), and volume resistivity. These tests are crucial for ensuring the reliability and safety of electrical equipment. The dielectric strength test measures the breakdown voltage of the oil, which indicates its ability to withstand high voltages without failing. When insulation properties degrade, the oil may break down even under normal operating conditions, potentially causing damage to equipment. The tan δ value reflects the quality of the oil by indicating the level of impurities or moisture present. Volume resistivity, on the other hand, shows how much contamination exists in the oil, as even small amounts of impurities can significantly reduce its resistance.
### First, Dielectric Strength Test
Also known as the breakdown voltage test, this procedure measures the maximum voltage that the oil can withstand before it breaks down. The setup is similar to an AC withstand voltage test, as shown in Figure 2-22. The test equipment must have a rated output voltage of 60kV or 80kV, with a continuously adjustable voltage supply that produces a sinusoidal waveform with a crest factor of 2 ± 5%. The device should also generate a minimum short-circuit current of 20 mA above 15 kV and measure the breakdown voltage with an accuracy of at least 3%. The voltage increase rate should be 2kV/s for spherical electrodes and 3kV/s for plate-shaped ones. Proper grounding and protective resistors are essential.
The test oil cup consists of two parts: the cup body and the electrode. Electrodes come in different shapes—spherical, hemispherical, and flat—and each type yields different results. Spherical electrodes provide the highest breakdown voltage, followed by hemispherical, and then flat. The oil cup is made of transparent glass, plastic, or ceramic and has a volume between 200mL and 500mL. The electrodes, usually made of polished copper or stainless steel, should be clean and free of pits. They must be placed 2.5 mm apart and immersed to a depth of about 40mm.
The testing procedure involves sampling the oil using a clean, dry container. The oil cup is cleaned with the sample before being filled, and care is taken to avoid air bubbles. After covering the cup and waiting for 10 minutes, the voltage is gradually increased until breakdown occurs. This process is repeated six times, with the electrodes cleaned after each test. The results are compared against standard values to determine if the oil meets specifications.
### Second, Measurement of Tan δ
Measuring the dielectric loss tangent (tan δ) is essential for both new and operational insulating oils. It helps assess the condition of the oil and detect early signs of degradation. For routine testing, an AC bridge with a resolution of 10â»â´ (such as the QS3 Xilin bridge) is commonly used. For samples with capacitance below 100pF, a higher-resolution bridge is recommended.
The oil cup used for tan δ testing comes in various types, such as single-cylinder, double-cylinder, or three-electrode configurations. The most common is the single-cylinder design, made from materials like stainless steel or brass. The insulating material is often epoxy glass fiber or Teflon. Additional equipment includes ovens, thermometers, hygrometers, and chemical reagents for cleaning.
The test procedure begins with cleaning the oil cup with solvents like carbon tetrachloride or petroleum ether, followed by drying in an oven. The cup is then rinsed with the sample oil and allowed to stand for 10 minutes to release any bubbles. The tan δ is measured at room temperature and again at 90°C. The results are compared against the Pre-Regulation standards, which specify limits for new and operating oils based on voltage levels.
### Third, Measurement of Volume Resistivity
Volume resistivity is a critical parameter that reflects the purity of the oil. Impurities, especially ionized or colloidal particles, and water content can drastically reduce the oil’s resistivity. A volume resistivity tester with a range of 10⸠to 10¹ⶠΩ·m is used, with a measurement error not exceeding 10%.
Other equipment includes a thermostat for controlling the temperature between 50–100°C, a Y-type electrode cup made of stainless steel, a capacitance meter, and a desiccator. The electrode cup is cleaned, dried, and checked for cleanliness and insulation resistance before use.
The testing procedure involves filling the electrode cup with the sample, allowing it to reach a constant temperature, and measuring the resistivity at 20 seconds and 60 seconds. The results are compared against the Pre-Regulation standards, which specify minimum resistivity values depending on the voltage level of the equipment.
By conducting these three tests, engineers can accurately assess the condition of insulating oil and ensure the safe and efficient operation of electrical systems.
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