Insulating oil plays a critical role in electrical systems, and its electrical performance must be regularly tested to ensure safe and reliable operation. The primary tests include dielectric strength testing, dielectric loss tangent (tan δ) measurement, and volume resistivity assessment. These tests evaluate the oil’s insulation capability, purity, and overall quality.
**First, Dielectric Strength Test**
The dielectric strength test, also known as the breakdown voltage test, measures the maximum voltage that insulating oil can withstand before it breaks down. This is crucial because if the oil loses its insulation properties, it may fail under normal operating voltages, leading to equipment damage or failure. The test setup typically involves an oil cup with electrodes and a high-voltage source. The voltage is gradually increased until breakdown occurs, and the value at which this happens is recorded.
For accurate results, the test equipment should have a rated output of 60kV or 80kV, be capable of smooth and continuous voltage adjustment, and maintain a sinusoidal waveform with a crest factor of 2 ± 5%. The device must also generate a minimum short-circuit current of 20 mA above 15 kV. The voltage increase rate is usually 2 kV/s for spherical electrodes and 3 kV/s for plate-shaped electrodes. The oil cup, made of glass, plastic, or ceramic, contains two electrodes—spherical, conical, or flat—each with specific dimensions and spacing (typically 2.5 mm). Proper cleaning, calibration, and handling are essential to avoid contamination and ensure accurate readings.
**Second, Dielectric Loss Tangent (tan δ) Measurement**
The tan δ test evaluates the dielectric loss of the oil, indicating the presence of impurities and moisture. A low tan δ value suggests good oil quality, while higher values may signal degradation. This test is commonly performed using an AC bridge, such as the QS3 type Xilin bridge, with a resolution of at least 10^-4. The oil cup used is typically a single-cylinder type made from stainless steel or brass, with an insulating material like epoxy or PTFE. The test procedure includes preheating the oil, measuring at room temperature, and repeating the process at 90°C to assess thermal stability. According to regulations, new oil should have a tan δ (90°C) of ≤1% for 330kV and below, and ≤0.7% for 500kV systems.
**Third, Volume Resistivity Measurement**
Volume resistivity measures how well the oil resists electric current, which is influenced by impurities and moisture. A higher resistivity indicates better insulation performance. The test involves applying a constant voltage to the oil sample and measuring the resulting current. The equipment used should cover a wide range, from 10^8 to 10^16 Ω·m, with an accuracy of no more than 10%. The electrode cup is cleaned, dried, and calibrated before use, and the sample is heated to 90°C for 30 minutes. Resistivity is measured at 20 seconds and 60 seconds after applying voltage. New oil should have a resistivity ≥6×10^10 Ω·m at 90°C, while operational oil must meet lower thresholds depending on the system voltage.
These three tests are essential for maintaining the integrity of insulating oil and ensuring the safe and efficient operation of electrical equipment. Regular monitoring helps detect early signs of deterioration, allowing for timely maintenance and replacement. Each test has specific procedures and standards, making it important to follow guidelines closely to achieve reliable results.
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