| Transformer oil is widely used in a variety of power equipment because of its good insulation performance.However,in the long-term operation process,water will inevitably be mixed in the transformer oil.At the same time,organic acids and other aging products produced in the aging process of oil paper insulation will also be dissolved in the oil,which will degrade the insulation performance of transformer oil.In order to find out the mechanism of the influence of moisture and aging on the conductivity and breakdown characteristics,this paper carried out experimental and simulation research,in order to enrich the theory of transformer oil conductivity and breakdown,and provide theoretical guidance for the design,manufacture and operation of power equipment.In this paper,the liquid dielectric conductivity measurement system and the breakdown test platform are built,and the transformer oil samples with different moisture content and different aging time are prepared.Taking temperature,moisture content and aging time as variables,the change rule of transformer oil conductivity is measured,and the breakdown characteristics of transformer oil under different temperature and moisture content are obtained.It is found that the conductivity of transformer oil increases exponentially with the increase of temperature and water content.After aging,the acid value of transformer oil increases,and the conductivity increases greatly.From room temperature to-10℃,the breakdown voltage decreases with the decrease of temperature.In order to find out the influence mechanism of moisture and aging acid on the conductivity of transformer oil,a model of transformer oil was established and molecular simulation was carried out.The results show that the local state of water is introduced at the bottom of conduction band of transformer oil,which narrows the band gap and increases the conductivity.With the increase of temperature,the diffusion ability of water molecules increases,the potential barrier of electron transition is lower when water molecules collide with oil molecules,and the probability of water molecules capturing electrons increases.Therefore,the higher the temperature and moisture content of transformer oil,the more water ions produced in the oil,resulting in the increase of oil conductivity.After aging,there are water molecules and aging acid molecules in transformer oil at the same time,which will introduce continuously distributed local states at the bottom of conduction band,and the trap energy level is lower,which is conducive to the occupation of electrons and makes the band gap narrow obviously.Therefore,the electrons in the valence band of oil molecules are prone to trap transition,resulting in the increase of carrier content in the oil,which explains the reason why the conductivity of transformer oil increases significantly with the increase of aging time.After water molecules capture electrons and convert into water ions,they will accelerate the migration under the action of electric field and collide with oil molecules or acid molecules,resulting in unsaturated carbon fragments that degrade the dielectric properties of transformer oil in local areas.This is another important factor to increase the conductivity of aging transformer oil.In order to study the breakdown process of transformer oil caused by suspended water in oil,the numerical simulation method is used in this paper.The results show that the water droplets suspended in the oil will deform along the direction of the electric field and form a tip,which distorts the electric field distribution and leads to the ionization of the oil molecules,and then the rapid migration of electrons makes a large number of positively charged space charges accumulate,resulting in the enhancement of the electric field at the streamer head and the rapid development of the streamer.The results of electric field distribution show that the streamer discharge process can be divided into three stages:initiation,accumulation and development,and the streamer development speed is gradually accelerated.When the average applied electric field reaches 12.5k V/mm,the discharge will occur,which is close to the breakdown test results.At this time,the maximum electric field strength at the lower end of the droplet can reach 2.5×10~8V/m.When the electric field at the lower end of the droplet is relatively low,only partial discharge occurs,while when the electric field is relatively high,the high field area at the head of the streamer expands,resulting in a larger range of oil molecules ionizing.The higher the applied voltage,the larger the head radius of streamer discharge and the higher the average development speed.The simulation results well simulate the discharge process. |
