During transformer operation, the main factors affecting the insulation performance of transformers are temperature, humidity, oil protection methods and overvoltage effects. Therefore, controlling these factors within a reasonable range is a key element to ensure the safe use of transformers.
1, the influence of temperature
Power transformers are insulated with oil-paper, and the moisture in the oil-paper has different balance curves at different temperatures. Under normal circumstances, when the temperature rises, the moisture in the paper will precipitate into the pool; Otherwise, the paper absorbs moisture from the oil. Therefore, when the temperature is higher, the moisture content of the insulating oil in the transformer is larger; Conversely, the water content is small.
When the temperatures are different, the degree of dissolution and chain breakage of the cellulose produced by the accompanying gas varies. At a certain temperature, the yield of CO and CO2 is constant, that is, the gas content of CO and CO2 in oil is linearly related to time. As the temperature continues to rise, The rate of CO and CO generation tends to increase exponentially. Thus, the content co and CO in oil is directly related to the thermal aging of the insulation paper, and the content change can be used as one of the criteria for judging whether the paper layer in the sealing transformer is abnormal.
The life of the transformer depends on the degree of aging of the insulation, which in turn depends on the operating temperature. Under the rated load, the average temperature of the windings of the oil-immersed transformer rises to 65°C, and the hottest temperature rises to 78°C. If the average ambient temperature is 20°C, the hottest spot temperature is 98°C; At this temperature, the transformer can run for 20-30 years, if the transformer is overloaded, the temperature will rise, thereby shortening the life.
The International Electrotechnical Commission (IEC) believes that class A insulated transformers are in the temperature range of 80 to 140°C, and for every 6°C increase in temperature, the effective life of the transformer insulation will be doubled. This is the 6°C rule, which illustrates the limitations of heat. It is stricter than the 8°C rule accepted in the past.
2, the impact of humidity
The presence of moisture accelerates the degradation of paper cellulose. Therefore, the production of CO and CO2 is also related to the moisture content of cellulosic materials. When the humidity is constant, the higher the water content, the more carbon dioxide is decomposed. Conversely, the lower the water content, the more CO decomposes.
Trace moisture in insulating oil is one of the important factors affecting insulation performance. The presence of trace amounts of moisture in the insulating oil has great harm to the electrical and physical and chemical properties of the insulating medium. Moisture will reduce the spark discharge voltage of the insulating oil, increase the dielectric loss factor tg8, promote the aging of the insulating oil, and make the insulation performance worse. Equipment moisture will not only reduce the operational reliability and service life of power equipment, but also cause equipment damage and even endanger personal safety.
3. The influence of oil-proof mode
The action of oxygen in transformer oil accelerates the insulation decomposition reaction, and the oxygen content is related to the protection method of the oil.In addtion,different pools of water protect different ways in which CO and CO2 dissolve and diffuse in the oil. For example, the dissolved amount of CO is small, which makes it easy for open transformer CO to diffuse into the oil surface space. Therefore, the volume fraction of CO in open transformers generally does not exceed 300x10-6. For sealed transformers, because the oil surface is insulated with air, so that CO and CO2 are not easy to volatilize, so their content is relatively high.
4. The influence of overvoltage
(1) The influence of transient overvoltage
The relative ground voltage generated by the normal operation of the three-phase transformer is 58% of the phase voltage, but when a single-phase fault occurs, the main insulation voltage increases by 30% for the neutral point grounding system and 73% for the neutral point ungrounded system,which may damage the insulation.
(2) The influence of lightning overvoltage
Due to the steep lightning overvoltage wave, the voltage distribution on the longitudinal insulation (inter-turn, parallel, insulation) is very uneven, which may leave discharge marks on the insulation, thus destroying the solid insulation.
(3) The influence of the working overvoltage
Since the operating overvoltage head is relatively smooth, the voltage distribution is approximately linear. When the operating overvoltage wave is transferred from one winding to another, it is roughly proportional to the number of turns between the two windings, which is likely to lead to deterioration and damage to the main or interphase insulation.
5. The influence of short-circuit electromotive force
The electromotive force when the outlet is short-circuited may deform the transformer windings and displace the egress line, thereby changing the original insulation distance, making the insulation heat, accelerating aging or damage, causing discharge, arcing, and short-circuit failure.