Measures to Reduce Partial Discharge
1. Dust Control
One of the main causes of partial discharge is the presence of foreign particles and dust. Test results show that metal particles larger than 1.5μm can generate discharges exceeding 500pC under an electric field. Both metallic and non-metallic dust can create localized electric fields, which lower the initial discharge voltage and reduce the breakdown voltage of the insulation. Therefore, maintaining a clean environment during transformer manufacturing is crucial. Strict dust control measures must be implemented throughout the production process. A sealed, dust-free workshop should be established, especially when handling components such as flat wires, wire wraps, winding assemblies, core stacking, insulation manufacturing, body assembly, and finishing. Any residual foreign matter or dust entry must be strictly prohibited to ensure high-quality insulation performance.
2. Insulation Processing
Insulating parts are highly sensitive to metal dust, as it is extremely difficult to remove once it adheres. To prevent this, insulation processing should be conducted in a dedicated, isolated workshop away from dusty areas. This helps maintain the integrity and cleanliness of the insulating components, reducing the risk of contamination and subsequent discharge issues.
3. Control of Burrs on Silicon Steel Sheets
The silicon steel sheets used in the transformer core are formed through cutting and shearing, which often result in burrs. These burrs not only cause short circuits between the laminations but also increase no-load losses and reduce the number of laminations by increasing the core thickness. More importantly, if the burrs fall onto the body during operation or vibration, they may cause discharges. Even if they settle at the bottom of the tank, they might align under an electric field, leading to ground potential discharges. For 110kV transformers, the burr size should not exceed 0.03mm, while for 220kV transformers, it should not exceed 0.02mm.
4. Cold-Pressed Terminals
Using cold-pressed terminals is an effective way to minimize partial discharge. Unlike soldering, which produces spatter and slag, cold pressing reduces the risk of contamination within the transformer. Additionally, the welding area should be isolated with soaked asbestos rope to prevent moisture ingress. If moisture remains after insulation wrapping, it can significantly increase the likelihood of partial discharge.
5. Rounding of Component Edges
Rounding the edges of metal components improves the electric field distribution and increases the initial discharge voltage. It also prevents friction-induced iron filings. Components such as clamps, pressure plates, terminal brackets, and internal magnetic shields should all have rounded edges. Rounded lifting holes are also necessary to avoid contact with lanyards or hooks, reducing the risk of mechanical damage and particle generation.
6. Environmental Control and Body Finishing During Assembly
After vacuum drying, the transformer body should be finished in a controlled, dust-free environment before packaging. Larger transformers with complex structures require extended finishing times. Prolonged exposure to air can lead to moisture absorption and dust accumulation. Therefore, the body should be handled in a sealed area, and if exposed for more than 8 hours, it should be re-dried. After finishing, the tank is evacuated and filled with oil. Since insulation materials absorb moisture during the finishing stage, vacuum drying is essential to maintain insulation strength, particularly for high-voltage equipment.
7. Vacuum Oil Filling
Vacuum oil filling removes trapped air and ensures complete impregnation of the transformer body with oil. This process involves evacuating the transformer to eliminate dead spaces in the insulation structure, followed by oil injection under vacuum. After oiling, the transformer should be tested for at least 72 hours. The degree of oil penetration depends on factors like insulation thickness, oil temperature, and immersion time. Proper soaking reduces the risk of discharge, so sufficient rest time is critical.
8. Sealing of the Fuel Tank and Components
The quality of the sealing structure directly affects the risk of leakage. A leak allows moisture to enter the transformer, causing the oil and insulation materials to absorb moisture, which can trigger partial discharges. Ensuring a reliable and durable seal is therefore essential to maintain the long-term performance and safety of the transformer.
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