Measures to Reduce Partial Discharge:
1. Dust Control
One of the major causes of partial discharge is the presence of foreign particles and dust. Studies have shown that metal particles larger than 1.5μm can generate discharges exceeding 500pC under an electric field. Whether metallic or non-metallic, these particles create concentrated electric fields, which lower the initial discharge voltage and reduce the breakdown voltage of the insulation system. Therefore, maintaining a clean environment during transformer manufacturing is essential. Strict dust control measures must be implemented throughout the production process, including the use of sealed, dust-free facilities. For example, during processes such as flat wire handling, winding wrapping, core stacking, insulation manufacturing, and final assembly, any residual foreign matter or dust entry must be strictly prohibited. This helps ensure the integrity of the insulation and minimizes the risk of internal discharges.
2. Insulation Processing
Insulating components are highly sensitive to metal dust, as it is extremely difficult to completely remove once adhered. To prevent contamination, insulation parts should be processed in dedicated workshops with controlled environments. These areas should be physically separated from other zones where dust may be generated, ensuring a clean and safe working condition for critical components.
3. Controlling Burrs on Silicon Steel Sheets
During the cutting and shearing of silicon steel sheets, burrs are commonly formed. These burrs not only cause short circuits between laminations but also create internal currents, increasing no-load losses and reducing the effective number of laminations. More importantly, when the core is inserted into the yoke or vibrated during operation, burrs may fall onto the body and cause discharges. Even if they settle at the bottom of the tank, they can align under electric fields and lead to ground potential discharges. Therefore, burr size must be minimized. For 110kV transformers, burrs should not exceed 0.03mm, while for 220kV transformers, they should be limited to 0.02mm.
4. Cold-Pressed Terminals
Using cold-pressed terminals is an effective way to reduce partial discharges. Unlike welding, which produces spatter and slag, cold pressing avoids introducing contaminants into the insulation system. Additionally, sealing the welding area with soaked asbestos rope prevents moisture ingress. If moisture remains after insulation wrapping, it can significantly increase the likelihood of partial discharge.
5. Rounding Component Edges
Rounding sharp edges improves electric field distribution and increases the initial discharge voltage. Components such as clips, pressure plates, bushings, and magnetic shields should all have rounded edges to prevent localized field concentration. Rounded edges also help avoid friction-induced iron filings, especially around lifting holes where contact with ropes or hooks may occur.
6. Environmental Control and Body Finishing
After vacuum drying, the transformer body should be finished in a dust-free area before packaging. Larger units with complex structures require extended finishing times, during which exposure to air can lead to moisture absorption and dust accumulation. It’s important to limit the time the body is exposed and, if necessary, re-dry it. After finishing, the tank is evacuated and filled with oil. Since insulation materials absorb moisture during this phase, dehumidification through vacuuming is crucial to maintain high-voltage insulation performance.
7. Vacuum Oil Filling
Vacuum oil filling ensures that all trapped air is removed from the transformer. By creating a vacuum, the insulating structure is fully saturated with oil, eliminating dead spaces. This process enhances the dielectric strength of the insulation. The oil immersion time is determined based on factors like material thickness, oil temperature, and soaking duration. A minimum of 72 hours of testing after oiling is recommended to ensure optimal saturation and minimize the risk of partial discharges.
8. Sealing of the Fuel Tank and Components
The quality of the sealing structure directly affects the transformer's resistance to moisture ingress. Any leakage allows water to enter, leading to oil degradation and insulation failure—both of which can trigger partial discharges. Ensuring a reliable and well-maintained seal is therefore critical to the long-term performance and reliability of the transformer.
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