Several typical structures and factors causing partial discharge in transformers

Several typical structures and factors causing partial discharge in transformers:
Lead-in:
In the insulation structure of a transformer, there are many lead arrangements. The electric field distribution between leads is extremely uneven. When two leads with the same radius are parallel or perpendicular to each other, the maximum electric field intensity appears at the surface of the two leads. Under the same conditions (ignoring the outer insulation layer), the maximum electric field intensity when the two leads are perpendicular to each other is about 10% higher than when they are parallel. The lead-out wire at the beginning of the high-voltage winding to the box wall and to the external voltage regulating winding is also a region where the electric field is concentrated and prone to partial discharge.

End insulation mechanism:
In the end insulation structure of ultra-high voltage power transformers, electrostatic rings are usually installed at the end of the windings. On the one hand, this improves the impulse voltage distribution of the windings; on the other hand, it serves as a shield to make the end electric field uniform. However, the wedge-shaped oil gap (also known as the oil wedge) formed between the electrostatic ring and the end ring is a region where the electric field is concentrated. The “oil wedge” is related to the larger electric field strength, the distance between the main insulation of the winding, the end insulation distance, the curvature radius of the electrostatic ring, and the insulation thickness.
The protruding metal electrode surfaces in the transformer, such as the weld seams on the inner wall of the oil tank and the weld slag attached to them, as well as the sharp corners and burrs left during the welding process, and the burrs formed at the edges of the core columns and core sheets during shearing, etc., all can cause electric field concentration, increasing the field strength several times (regardless of whether the electrode is charged or grounded). The sharp corners and burrs formed during the manufacturing process should be ground smooth. Impurities:
In the insulation structure of a transformer, the dielectric constant of oil is lower than that of the low-voltage board. In a composite insulation structure, the electric field that oil is subjected to is higher, and among the three insulating materials, the breakdown field strength of oil is the lowest. This determines that the thinnest part in the transformer insulation is the oil gap. Impurities in the oil, such as metal and non-metal particles, water content, and gas content, can cause distortion of the electric field in the oil.
Most partial discharges in transformers occur in high-voltage and high-electric-field areas. The nature of the discharge can be determined based on the discharge patterns observed during partial discharge, the starting and extinction voltages of the discharge, and the variation of the discharge quantity over time. The electrical location where the partial discharge occurs can be identified using electrical location methods.