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The trend of power transformers to high voltage and large capacity has made the phenomenon of transformer oil flow charging that is not a problem in the past obvious. 2ml crimp gas chromatography vials After requiring large-capacity miniaturization to improve cooling efficiency and accelerating oil circulation, the problem of transformer oil flow charging is more prominent and becomes a new topic of transformer oil insulation.
Mechanism and characteristics of transformer oil flow charging and analysis method of gas chromatograph in oil
1. Mechanism of transformer oil flow charging
High-voltage large-scale power transformers, whether or not they are connected to alternating current, have a charge in their oil flow. In the following, the oil flow is described before the AC power is turned on and after the power is turned on.
The winding insulation of the transformer and the insulating laminate pad for fixing are all paper insulation or wood insulation. The chemical composition of these solid insulations is cellulose plus lignin, which carries hydroxyl (-OH), aldehyde (-CHO) and carboxyl (-COOH). Under the high-speed flow of transformer oil, the oil and solid insulation friction, The cellulose and lignin molecules are covered by the positive charge of -H8+ produced by the electronic shift of the above genes. The positively charged -H8+ forms an electrical layer with the corresponding negative ion at the oil-paper interface. Due to the high-speed flow of the transformer oil, the charge of the electrical layer is separated, the negative charge still adheres to the surface of the insulating paperboard, and the positive charge flows with the oil, forming a positive charge of the oil flow. As the transformer oil circulates, the difference in concentration of the positive charge taken in the oil is revealed.
The above is a relatively common view. They have different views on the generation and aggregation of ionization, but they are consistent with the idea that the current is positively charged and the insulating surface is negatively charged. So no matter which statement is contradictory to the discussion in this article.
The transformer in operation, after the addition of an alternating electric field, exacerbates the electrification of the above static charge. At present, there is no unified statement on how the high-voltage AC electric field plays a role in accelerating the electrification of static electricity. It is more acceptable to say that in the low field strength region (field strength <0.5kV/mm), the positive ion group that has diffused into the oil is affected by the alternating electric field, making its apparent charge distribution wider; In the high field strength region (field strength > 1kV/mm), the alternating electric field accelerates the paper insulation - the formation of positive and negative charges at the oil interface, resulting in an increase in the positive charge density in the oil. For example, the 511MVA step-up transformer insulation breakdown accident of the US nuclear power plant occurred when the change was still at a light load, indicating the aggravation effect of the high-strength field AC power on the oil flow electrification.
2. Basic characteristics of oil flow charging
Test methods for measuring oil flow charging are: partial discharge measurement method, gas analysis method in oil and measurement of neutral point leakage current method. The gas chromatograph analysis method in oil is the recommended method in this paper.
The determination of the dissolved gas content in the insulating oil by gas chromatography is an effective means for the power supply enterprise to judge whether there is a latent overheating or discharge failure in the oil-filled power equipment in operation to ensure safe and effective operation of the power grid. It is also a necessary means for the oil-filled electrical equipment manufacturers to carry out factory inspection of their equipment.
The special oil chromatograph for transformer oil is a special gas chromatograph specially developed for the determination of dissolved gas components in insulating oil for electric power. The instrument adopts advanced three detector flow with TCD detector and two. The FID detector can complete the full analysis of the contents of the seven gas components dissolved in the insulating oil in one injection in one injection. Among them, H2 is detected by TCD; hydrocarbon gases (CH4, C2H4, C2H6, C2H2) are detected by FID1, CO and CO2 are detected by FID2, which overcomes the influence of a large amount of CO and CO2 on hydrocarbon gas, especially the impact on C2H2. The detection time also greatly improves the detection sensitivity.
3. Factors affecting the degree of oil flow
The above oil temperature and oil speed are the main factors affecting the degree of oil flow, in addition to: the type and composition of the transformer oil, the type of solid insulation and its surface morphology, 2ml crimp gas chromatography vials the structural characteristics of the oil passage, the degree of drying of the insulation and the operation of the transformer Voltage.
3.1 The type of solid insulation and the influence of its surface morphology
Several solid insulators commonly used in power transformers, the potentials measured at a certain oil flow rate test indicate that the amount of their charge is arranged in the order of cotton tape > crepe paper > laminate > kraft paper. This refers to unprocessed new materials, which may change when their surface roughness is different. The same material, due to scratches or large burrs on the surface, will increase its oil flow.
3.2 Influence of oil channel structure
If the oil passage is evenly symmetrical, such as oil flow disorder or high oil flow rate, it will have a greater impact on the oil flow. Therefore, in the design and process, the shape of the solid insulation should avoid corners, burrs and scars, and more round or curved, the oil flow of the same section can be even, and the adjacent oil flow section should avoid sharp changes.
3.3 Influence of insulation dryness
The dryness of solid insulation has an effect on both the oil flow charge and the neutral point leakage current. High dryness will result in high insulation resistance. The degree of electrification of the oil stream also increases as the water in the transformer oil decreases. When the water content in the oil is lower than 15×10-4%, it has a higher charging tendency. Therefore, when the AC voltage and other conditions are the same, the amount of charge in the oil flow increases as the dryness increases, and the insulation resistance also increases. At this point, the level of partial discharge will increase. When the water content of the qualified high-voltage transformer oil is less than 10×10-4%, the oil level of the oil flow in operation is more prominent, which is a problem worthy of reminding.
4. Electrostatic discharge
After the oil flow rate exceeds 1 pu, an electrostatic discharge will occur in the oil flow. The aforementioned domestic 500kV-class power transformer hears intermittent discharge sounds in the transformer when the oil-cooled oil pump cycle is not energized, that is, the performance of electrostatic discharge. 2ml crimp gas chromatography vials The occurrence of electrostatic discharge is caused by a negative charge on a solid insulator; some is caused by a positive charge in the oil flow; or it is caused by an excessive concentration of locally accumulated charge.
A test has been carried out in which a crepe paper is placed in a high-speed oil flow having an average oil flow rate of 2.5 m/s, and photographs taken by electrostatic discharge in the oil are dendritic. This indicates that positive and negative charges are in the boundary layer and continue to generate new charges after discharge, so that a longer discharge can be formed. This is similar to the partial discharge of the transformer oil flow, which produces a gas containing H2 and C2H2.
5. Oil flow electrification is closely related to electrical conductivity and dielectric loss
The test results of 10 kinds of transformer oils on the market were tested with new oil and degraded oil after artificial degradation (150 °C1h), indicating that different types of new oils have different oil flow degrees; the same variety in the process of oil degradation The flow electrification is increasing continuously; when the same degree of artificial deterioration, the oil flow electrification varies from species to species. 2ml crimp gas chromatography vials The test shows that if measures are taken to suppress the deterioration rate of the transformer oil during operation, the increase rate of the oil flow electrification can be suppressed, thereby reducing the discharge amount; reducing the discharge amount can slow down the deterioration rate of the oil between them. A virtuous cycle.
The addition of a thermally stable alkylbenzene hydrocarbon compound to the transformer oil or the addition of benzotriazole (BTA) which is inert to the copper surface and inhibits oil degradation can reduce the chargeability in the oil stream. When the concentration of BTA exceeds 5 ppm, the degree of electrification decreases, but if the concentration of BTA is too large, reverse polarity charging occurs, so the concentration of the additive should be moderate. When the concentration of BTA is ≤5×10-6, the degree of electrification of the oil flow does not change. This is because the main role of BTA at this concentration is to moderate the oxidation of the copper surface, and copper oxide has a tendency to increase the degree of charge.2ml crimp gas chromatography vials
This is the end of the introduction of Mechanism and characteristics of transformer oil flow charging and gas chromatograph analysis in oil. I hope it can help you.
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