Transformer oil in the state of water, the transformer in the transportation, storage, use process may be entered by the outside or oil itself oxidation of water, the water generated will exist in the following states: one is free water. The second is that extremely fine particles dissolve in water. The insulating fiber molecule is glucose (C6H12O6) molecule. When water enters the fiber molecule, it reduces its attraction, promotes its hydrolysis into low-molecular substances, and reduces the mechanical strength and degree of polymerization of the fiber.
At present, power transformer is not only one of the most important and expensive equipment in power system, but also one of the equipment that causes the most accidents in power system. Before the sudden failure of the transformer, the deterioration of insulation and the latent fault will produce a series of effects and information such as light, electricity, sound, heat and chemical changes under the action of the operating voltage. Therefore, not only preventive maintenance based on preventive tests should be carried out regularly at home and abroad, but also predictive maintenance strategies based on online monitoring are being studied in order to monitor and diagnose potential faults or defects online in real time or on a regular basis. The moisture content in transformer insulation oil is also a parameter to determine the insulation quality of transformer. Transformer online intelligent diagnosis equipment can automatically collect, analyze the content of water in the oil and find out the cause of failure, to provide solutions, users timely solve the hidden dangers existing in the transformer, to prevent accidents.
1. The state and harm of moisture in transformer oil
Transformer in the process of transportation, storage and use may be entered by the outside world or the oxidation of oil itself to produce water, generated water will exist in the following states:
One is free water. For the external invasion of water, such as not easy to stir with water. Does not affect the breakdown voltage of the oil, but also does not allow, indicating that there may be dissolved water in the oil, immediate treatment.
The second is that extremely fine particles dissolve in water. Usually from the air into the oil, sharply reducing the breakdown voltage of the oil. Medium loss increase, vacuum filter oil.
Third, emulsified water. Oil refining is poor, or long-term operation causes oil aging, or oil is contaminated by emulsification, will reduce the interface tension between oil and water, such as oil and water mixed together, will form the emulsified state. Add demulsifier.
Its harm: one is to reduce the breakdown voltage of oil products. 100 ~ 200mg/kg breakdown voltage dropped to 1.0kV, the fiber impurities in the oil easily absorb water, under the action of the electric field, the formation of conductive “bridge” between the electrodes, so easy to break down.
The second is to increase the loss factor of the medium. Suspension emulsified water has the greatest effect and is not uniform. The insulating fiber molecule is glucose (C6H12O6) molecule. When water enters the fiber molecule, it reduces its attraction, promotes its hydrolysis into low-molecular substances, and reduces the mechanical strength and degree of polymerization of the fiber. Experiments show that 120℃, the insulation fiber in the moisture increases by 1 times, the mechanical strength of the fiber decreases by 1/2, when the temperature rises, the water in the oil increases, the water in the fiber decreases, the temperature decreases, the opposite. Therefore, the moisture content in the oil should be monitored, and thus the aging of the insulating fiber should be monitored. The fourth is that water promotes the corrosion ability of organic acids and accelerates the corrosion of metal parts. To sum up, the more water content in oil, the faster the aging of oil itself, the aging of equipment insulation and the corrosion of metal parts, so it is necessary to monitor the water content in oil, especially the content of dissolved water.
2. Test method for moisture in transformer insulation oil
The evaluation of humidity in insulation material is an important factor to ensure the reliability and service life of transformer. Humidity in insulating oil is constantly changing, which may adversely affect quality. In addition, most of the humidity is distributed in the insulating paper. Humidity affects the dielectric breakdown strength of solid and liquid insulating materials, and affects the aging rate of cellulose insulating materials and the tendency of bubble formation during overload.
Ambient temperature, load, aging, leakage, and other factors can cause constant changes in humidity. Therefore, continuous monitoring and diagnosis are needed with the cyclic change of transformer temperature. This is especially necessary for overloaded or peaking load transformers. The total humidity in the transformer insulation system is determined by the moisture content of the cellulose and liquid. The humidity relationship between insulating paper and insulating oil depends only on temperature. As the temperature increases, the solubility of water in the insulating oil (the water-holding capacity of the solution) increases, and water will be transferred from the insulating paper to the insulating oil.
When the temperature drops, the process goes in reverse, but the flow of water from the liquid medium to the solid insulating material is quite slow. Therefore, the water content of insulating oil during cooling is higher than that during heating. Therefore, in order to accurately grasp the humidity distribution in the transformer, it is necessary to know the position of the equipment in its thermal cycle. To understand the true humidity of insulating paper by monitoring the moisture content in the liquid medium, the transformer must be in a relatively stable temperature state.
The relative saturation of water in the insulating oil needs to be standardized because the humidity in the insulating oil is closely related to temperature changes and there is a certain temperature gradient in the transformer main box (usually the top of the box is warmer than the bottom). To complete standardization, the expert system analysis deduce the percentage of relative saturation at the bottom. This can be obtained by using the temperature reported by the sensor and its sampling location.
The analysis assumes that the top temperature of the transformer is 10℃ higher than the bottom temperature. If a location other than the top or bottom is used as a sampling point, the temperature deviation must be specified. Through the relative saturation of humidity and specific measured temperature, the expert system will be a certain percentage of relative saturation for insulation paper humidity calculation results. It is important to note that this calculation is based on information from a single measurement and may not reflect the true humidity concentration of the insulating paper, especially after the transformer has undergone a dramatic temperature change.
If the expert system determines that the transformer is in equilibrium (the insulating paper neither releases nor absorbs humidity), a second percentage of relative saturation is calculated, which is the average of the last 30 measurements of relative saturation percentage when the transformer is in equilibrium. The temperature and variation recorded above determine the criteria for determining the presence or absence of equilibrium.
article source: https://www.transformer-home.com/news/how-to-detect-the-state-of-trace-moisture-in-transformer-oil/
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