In the maintenance and installation of electrical equipment, it is essential to collect and analyze insulating oil from transformers and reactors. After passing the tests, the equipment is ready for operation. However, improper sampling methods, storage, transportation, or prolonged exposure can lead to inaccurate test results, requiring repeated sampling and increasing operational costs. To address this issue, a specialized negative pressure oil extraction device has been developed for transformers, along with an improved sampling method. This system ensures that both the inside of the oil pipe and the sampling bottle remain in a closed environment, preventing exposure to atmospheric contaminants. This reduces the risk of secondary pollution, improves the accuracy of measurements, and makes the entire process safer, more reliable, and easier to perform. Currently, China's GB/T 7595-2008 standard includes particle size testing for 500kV-grade transformer oil. According to DL/T 1096-2008, the maximum number of particles larger than 5μm in 100ml of oil should not exceed 2000 before operation (after hot oil circulation) and 3000 during operation. For DC commutation at 500kV and above, the limit is 1000 particles per 100ml before hot oil circulation. The PLD0201 oil particle size analyzer from Proudit is commonly used for such measurements. Next, we will discuss Proud’s approach to measuring and controlling transformer oil quality. **Particle Size Test Sampling Method and Device** **Introduction:** **A. Oil Sampling Requirements** 1. **Sampling Container:** The container must be cleaned thoroughly using distilled or deionized water, followed by rinsing with isopropanol or high-purity ethanol. It should be dried before use. If the container is rinsed with the sample itself, it should be used immediately. Alternatively, a 250ml NAS16380 cleaning bottle can be purchased for better control. 2. **Sampling Tube:** The tube must be made of high-quality material that resists corrosion from insulating oil and does not react chemically with it. Before use, the tube should be cleaned with distilled or deionized water, then rinsed with isopropanol or high-purity ethanol and dried. 3. **New Oil Sampling:** When sampling new oil, take it from the middle to the bottom of the drum. If free water is present, sample from the bottom. The sampling tool should be cleaned with isopropanol or high-purity ethanol and dried before use. Stir the oil thoroughly before sampling to avoid sedimentation. 4. **Running Oil Sampling:** Clean the sampling valve with isopropanol or high-purity ethanol before sampling. At least 1–2 liters of liquid should be flushed out before taking the sample, and the valve should not be adjusted during flushing or sampling. **B. Sampling Device** The oil sampling kit includes a puller, a negative pressure oil sampler, a NAS16380 grade hose, and a 250ml NAS16380 cleaning bottle. These components are pre-purified to meet NAS16380 standards. During sampling, the oil is drawn under negative pressure, minimizing exposure to the atmosphere and reducing contamination from airborne dust. This helps ensure more accurate particle size measurements. **C. Important Notes** When sampling, always take the sample from the lower valve of the transformer, including the sealed sampling valve. Clean the valve with clean grade A cotton or gauze before sampling. Unscrew the nut, connect the oil-resistant tube, flush the pipeline, and collect the flushed oil—do not discharge it directly on-site. After rinsing, take the sample and seal the bottle properly. It is important to replace the sampling tube after each use to prevent cross-contamination between samples. Avoid sampling when the oil temperature is below ambient temperature, or when relative humidity exceeds 75%. Also, avoid sampling during windy or rainy weather unless special precautions are taken. Before measurement, inspect the oil for visible impurities, bubbles, or water droplets. If there are large particles (>500μm), they may block the sensor, so the instrument should not be used. High moisture content can significantly affect the results, leading to false readings. In such cases, dehydration or oil replacement may be necessary. Additionally, consider the viscosity of the oil. For normal sampling and measurement, the viscosity should not exceed 100 mm²/s. For high-pressure online measurements, the limit is generally 500 mm²/s. If the oil is too viscous, it should be diluted before measurement to ensure smooth flow into the system.

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