On Preventive Test of Transformer

In the diagnosis of transformers, it is often difficult to find some partial faults and heating defects due to the electrical test method alone, and the method of chemical detection through gas chromatographic analysis of transformer oils is used to detect certain latent faults inside the transformer and its The early diagnosis of the development level is very sensitive and effective, which has been proved by a large number of fault diagnosis practices. The principle of oil chromatography is based on the fact that the production rate of any specific hydrocarbon gas varies with temperature. At a certain temperature, the gas production rate of a certain gas will tend to be the maximum; as the temperature increases, the production The gas with the highest gas rate is CH4, C2H6, C2H4, and C2H2. This also proves that there is a corresponding relationship between fault temperature and dissolved gas content, and local overheating, corona and arcing are the main causes of faulty characteristic gas generation in oil-impregnated paper insulation. Under normal operating conditions, the transformer will gradually deteriorate due to oil and solid insulation, and degenerate into a very small amount of gas (mainly including hydrogen H2 methane CH4 ethylene C2H4 acetylene C2H2 carbon monoxide CO carbon dioxide CO2 and other gases). When the transformer internal overheating fault, discharge fault or internal insulation damp, the content of these gases will rapidly increase. Most of these gases are dissolved in the insulating oil, and a few rise to the surface of the insulating oil and enter the gas relay. Experience has shown that the content of various components of the gas in the oil is related to the nature and extent of the failure. The characteristics of the gas produced by different faults or different energy densities are different. Therefore, during the operation of the equipment, the measured dissolved in the oil is periodically measured. The gas composition and content have very important significance and practical results for early detection of latent faults existing in oil-filled electrical equipment. In the prophylactic test procedures for electrical equipment promulgated and implemented in 1997, gas chromatographic analysis of transformer oil has been performed. It has been put into the forefront position, and has achieved remarkable results through the widespread application and experience accumulation in recent years. The main internal faults of power transformers are overheating faults, discharge faults, and damp insulation. According to relevant data, statistics on faulty transformers show that overheating faults account for 63%; high-energy discharges account for 18. 1%; Overheating and high energy discharge fault accounts for 10%; Spark discharge fault accounts for 7%; Moisture or partial discharge fault accounts for 1. 9%. In the case of overheating faults, the tap-changer contact failure accounts for 50%; the multi-point grounding of the iron core and the local short-circuit or flux leakage circulation account for about 33%; the overheating of the wire and the defective joint or loosening of the fastener cause the overheating to account for about 14. 4%; the remaining 2. 1% for other faults, such as silicone oil into the body caused by local oil blockage, resulting in local heat dissipation caused by overheating failure. The arc discharge is dominated by winding turns and interlayer insulation breakdown, followed by faults such as wire breakage or flashover to the ground and flying fox tap switches. Spark discharge is usually caused by the discharge of the casing lead to the uninserted casing conductive tube, the equal pressure ring, etc.; discharge due to poor local contact of the lead wire or poor contact of the iron core grounding plate; potential levitation of the tap-changer switch fork or metal screw The resulting discharge and so on.

Accurate judgment of transformer fault location depends on a thorough understanding of its internal structure and operating conditions. It also compares the chromatographic data and other preventive tests (direct resistance, insulation, ratio, leakage, no-load, etc.) over the years. At the same time, we must also pay attention to the non-failure gas produced due to fault gas production and normal operation. It is technically inseparable. In some cases, some gases may not be caused by equipment failure. For example, if the oil contains water, it can react with iron to generate hydrogen gas. Interlayer oil film cracking can also generate hydrogen. New stainless steels may also adsorb hydrogen during processing or welding and release slowly after operation. In addition, some operations may also generate fault gases, such as switching in on-load tap-changer transformers. The switch oil leaks to the main tank of the transformer or selects the influence of the floating potential of the switch when the switch is operated at a certain position. The equipment tank is filled with oil to repair welding. The original injected oil contains certain gas components and the filter oil does not completely leave residual gas after overhaul.