The energy loss caused by transformer oil under the action of electric field is called the dielectric loss of oil. Usually, the loss of transformer oil is measured under the specified conditions and expressed by the tangent of dielectric loss angle TG δ. The measurement of dielectric loss tangent of insulating oil can sensitively reflect the aging degree of insulating oil under the action of electric field, oxidation, sunlight, high temperature and other factors. It can also sensitively find that when the insulating oil contains water or other impurities, the polar impurities and charged colloidal substances gradually increase. Therefore, TG δ test of insulating oil is an important electrical characteristic test. The dielectric loss of transformer oil can be expressed as follows:

　　由上式可知，油的介质损耗因数正比于电导系数γ，因此分析油介损超标或有大的增长趋势的原因，也应主要从分析绝缘油的电导系数γ变化情况入手。

　　It can be seen from the above formula that the dielectric loss factor of the oil is directly proportional to the conductivity coefficient γ. Therefore, the reason why the dielectric loss of the oil exceeds the standard or has a large growth trend should be analyzed mainly from the change of the conductivity coefficient γ of the insulating oil.

　　1.1油中浸入溶胶杂质

　　1.1 sol impurities immersed in oil

　　变压器在出厂前残油或固体绝缘材料中存在着溶胶杂质，在安装过程中可能再一次浸入溶胶杂质（如采用了黑色橡胶管等），在运行中还可能产生溶胶杂质。根据调查，原变压器油运行一段时间以后出现油介损增大的原因，主要是由于变压器原油生产厂家对油品的管理混乱，变压器残油回收利用不当，致使含有溶胶杂质的变压器残油混入变压器原油中。油中存在溶胶后，引起电导系数可能超过介质正常电导的几倍或几十倍，从而导致油tgδ值增大。

　　There are sol impurities in the residual oil or solid insulating materials of the transformer before leaving the factory. During the installation process, sol impurities may be immersed again (such as the use of black rubber tube, etc.), and sol impurities may also be generated during the operation. According to the investigation, the reason for the increase of oil dielectric loss after the operation of the original transformer oil for a period of time is mainly due to the confusion of the management of oil products by the crude oil manufacturer of the transformer, the improper recovery and utilization of the residual oil of the transformer, resulting in the mixing of the residual oil of the transformer containing sol impurities into the crude oil of the transformer. When there is sol in the oil, the conductivity may be several times or dozens of times higher than the normal conductivity of the medium, resulting in the increase of TG δ value of the oil.

　　1.2油质老化程度较深

　　1.2 oil aging degree is deep

　　油质老化将引起油中酸值的增大、油的粘度减小、界面张力的减小等。但目前油介损偏大的变压器，绝大多数是运行时间不长的变压器，由老化引起油介损升高比较少见。

　　The aging of oil will cause the increase of acid value, the decrease of viscosity and the decrease of interfacial tension. But at present, most of the transformers with large oil dielectric loss are those with short operation time, and the increase of oil dielectric loss caused by aging is rare.