Factors affecting transformer insulation failure

The main factors affecting the insulation performance of electrical transformers are temperature, humidity, oil protection mode and the influence of over-voltage.

The main factors affecting the insulation performance of electrical transformers are temperature, humidity, oil protection mode and the influence of over-voltage.

1. Effect of temperature

Power transformers are insulated by oil and paper, and the water content in oil and paper has different equilibrium curves at different temperatures. Under normal circumstances, the temperature rises, the moisture in the paper will precipitate in the mooring; On the contrary, the paper will absorb the water in the oil. Therefore, when the temperature is high, the moisture content of the insulation oil in the transformer is larger; On the contrary, the moisture content is small.

At different temperatures, the degree of cellulosic annulolysis, chain breakage and gas generation are different. Under a certain temperature, the production speed of CO and CO₂ is constant, that is, the content of CO and CO₂ gas in oil shows a linear relationship over time. When the temperature is rising, the production rate of CO and CO₂ tends to increase exponentially. Therefore, CO and CO₂ content in oil and insulating paper thermal aging has a direct relationship, and the change of content can be one of the criteria of abnormal paper layer in sealed transformer.

The life of the transformer depends on the degree of insulation aging, which in turn depends on the operating temperature. If the oil immersed power transformer is under rated load, the average temperature rise of the winding is 65℃, and the hottest temperature rise is 78℃. If the average ambient temperature is 20℃, the hottest temperature is 98℃. At this temperature, the oil filled transformer can run for 20-30 years. If the oil filled transformer is overloaded, the temperature will rise, and the life will be shortened.

The International Electrotechnical Commission (1EC) believes that the A-level insulation transformer is in the temperature range of 80 ~ 140C. Every 6℃ increase in temperature, the rate at which the effective insulation life of a transformer decreases will be doubled, this is the 6℃ law, indicating that the heat limit has been more strict than the 8℃ law approved in the past.

2 . Influence of Humidity

The presence of water will accelerate the degradation of paper cellulose. Therefore, the production of CO and CO₂ and cellulose material water content is also related. When the humidity is certain, the higher the water content, the more the decomposition of the CO₂. Conversely, the lower the water content, the more CO will be decomposed.

Trace moisture in insulating oil is one of the important factors affecting insulating properties. The existence of trace moisture in the insulating oil has great harm to the electrical and physical and chemical properties of the insulating medium. The moisture can lead to the decrease of the spark discharge voltage of the insulating oil and the increase of the dielectric loss factor TG8, which promotes the aging of the insulating oil and the deterioration of the insulating performance. Dampness of equipment not only reduces the reliability and life of power equipment, but also damages the equipment and even endangers personal safety.

3. The influence of oil protection mode

The effect of oxygen in transformer oil will accelerate the insulation decomposition reaction, and the oxygen content is related to the way of oil protection. In addition, ways of protection is different, so that the CO and CO₂ in oil solution and diffusion situation is different. If the dissolution of CO is small, the open transformer CO is easy to diffuse to the oil surface space. Therefore, the volume fraction of CO in the open transformer in general is not greater than 300x10-6. Sealed oil type transformer, the oil surface is insulated from the air, so that CO and CO₂ is not easy to volatile, so its content is higher.

4. The effect of overvoltage

① The influence of transient overvoltage

The voltage between phase and ground generated by the normal operation of three-phase transformer is 58% of the voltage between phases. However, when the single-phase fault occurs, the voltage of the main insulation will increase by 30% for the neutral point grounded system and 73% for the neutral point ungrounded system, which may damage the insulation.

② The influence of lightning overvoltage

Due to steep wave head, lightning overvoltage will cause that the longitudinal insulation (turn, interconnection, insulation) voltage distribution is very uneven, it may leave discharge marks on the insulation, so that the solid insulation is damaged.

③ The influence of operating overvoltage

As the wave head of the operating overvoltage is rather flat, the voltage distribution is approximately linear. When the operating overvoltage wave is transferred from one winding to another, it is approximately proportional to the number of turns between the two windings, which is easy to cause deterioration and damage of the main insulation or interphase insulation.

5. Short-circuit electrodynamic effect

The electric power of the outlet short circuit may make the transformer winding deformation, lead shift, so as to change the original insulation distance, so that the insulation will emit heat, the aging will be accelerated, and it will cause discharge, arc pull and short circuit fault.

To sum up, mastering the insulation performance and reasonable operation and maintenance of power transformers directly affects the safe operation, service life and power supply reliability of the transformer like oil filled transformer. Power transformer is an important and key main equipment in the power system. As the transformer operation and maintenance personnel and managers, you must understand and master the power transformer insulation structure, material performance, process quality, maintenance methods and scientific diagnosis technology, and carry out optimized and reasonable operation management, in order to ensure the use efficiency, life and power supply reliability of oil immersed power transformer.