There are many factors that affect the electrical performance of oil immersed transformer, among which the main factors that affect the compressive strength of transformer are water content, air content, impurities, temperature, flow, etc. By analyzing the influence of these factors on the voltage intensity of transformer oil, it is found that the higher the voltage level of transformer is, the more strict the control requirements for water content, gas content and impurities in oil are. The transformer oil is dehydrated and degassed by advanced technology, or the impurities in the oil are removed by coarse and fine equipment.
Various factors affecting the electrical performance of the transformer are analyzed as follows:
1. oil content
Water exists in transformer oil in three forms: deposition, dissolution and combination. The lower the water content in the oil, the higher the power frequency breakdown voltage. When the water content is more than 200 × 10-6, the breakdown voltage remains unchanged, because the excess water sinks to the bottom of the oil, which will not affect the breakdown voltage during the oil test.
当油中含水量为(300 ~ 400)×10-6时，水含量超过饱和溶解度，水沉淀到井底，油压承载值与饱和溶解度承载值相同。随着含水量的增加，浸油纸的击穿电压呈曲线快速下降。500kv变压器绝缘纸含水率控制在0.5%以下。
When the water content of the oil is (300 ~ 400) × 10-6, the water content exceeds the saturated solubility, and the water precipitates to the bottom of the well. The bearing value of the oil pressure is the same as that of the saturated solubility. With the increase of water content, the breakdown voltage of the oiled paper decreased rapidly in a curve. The moisture content of 500 kV transformer insulating paper is controlled below 0.5%.
降低油水含量对提高变压器运行安全性、延缓油品老化起着重要作用。为了降低油品的含水量，可以采用真空加热的方法对油品进行处理，将油品温度加热至60 ~ 70℃，抽吸高真空，降低油品中的含水量。
Reducing the content of oil and water plays an important role in improving the safety of transformer operation and delaying oil aging. In order to reduce the water content of oil products, the vacuum heating method can be used to treat oil products. The temperature of oil products can be heated to 60 ~ 70 ℃, and high vacuum can be drawn to reduce the water content of oil products.
2. Impurities in oil
The breakdown field of pure oil is very strong. When the oil contains impurities and water, the breakdown voltage of the oil decreases obviously. The transformer has many insulating materials, and the oil contains fiber impurities, among which the water fiber is more conductive.
Under the action of power frequency voltage, the influence of suspended particles in oil on the insulation strength is related to the number, size and property of particles.
目前，油过滤器被用来处理油中的杂质。对于500kv的变压器，应使用粗滤器和精滤器两种过滤器清除油中的杂质，以保证油压电阻等级达到60kv / 2.5mm。
At present, oil filter is used to deal with impurities in oil. For 500 kV transformer, two kinds of filters, coarse filter and fine filter, should be used to remove impurities in the oil, so as to ensure that the oil pressure resistance level reaches 60 kV / 2.5 mm.
3. Air content of oil
Air content is one of the main control indexes of transformer oil. During operation, the gas content of transformer oil shall not exceed 4%, and the gas content of 500 kV transformer oil shall be controlled within 0.5%. For transformers above 60 kV, electrostatic discharge treatment is needed before vacuum oil injection and finished product test. Its purpose is to eliminate the bubbles in the transformer body and oil and prevent the bubbles from discharging during the product test.
4. Uniformity of electric field
The breakdown voltage of high quality transformer oil can be improved by improving the uniformity of electric field. For the oil containing impurities under impulse voltage, impurities do not have enough time to form a "small bridge", so improving the uniformity of electric field can improve the voltage resistance of oil.
5. oil flow rate
When the transformer oil flows, static electricity is generated by friction with insulating materials. The higher the flow rate, the higher the voltage. The phenomenon of electrified oil flow in transformer is called electrified oil flow. The electric field of transformer will be distorted by live oil flow. When the applied electric field strength exceeds the partial discharge strength or breakdown strength of the insulating material, the superposition of the live oil voltage and the test voltage will endanger the safe operation of the transformer. Therefore, the transformer must control the oil flow speed, increase the cross-section of the oil flow channel, reduce the live voltage of the oil flow, and prevent the phenomenon of partial discharge or insulation breakdown caused by the live oil flow.