Dust Control
Foreign particles and dust are major contributors to partial discharge. Tests show that metal particles with a diameter of ф1.5μm can trigger discharges exceeding 500pC under electric fields.
Both metallic and non-metallic dust can cause electric field concentration, reducing the initial and breakdown voltage of insulating materials.
Therefore, strict cleanliness control must be maintained throughout the manufacturing process, especially during conductor leveling, winding, coil assembly, core stacking, insulation preparation, body assembly, and finishing. These operations should be carried out in sealed, dust-free workshops to prevent foreign matter and dust ingress.
Centralized Processing of Insulating Components
Once metallic dust adheres to insulating parts, it is extremely difficult to remove. Since metallic dust is a potential source of partial discharge, all insulating components must be processed in a dedicated insulation workshop, completely isolated from areas where dust might be generated.
Control of Silicon Steel Burrs
Transformer core sheets are formed through longitudinal or transverse shearing processes, which often produce burrs.
These burrs can cause interlaminar short circuits, form internal circulating currents, increase no-load losses, raise the effective core thickness, and reduce the number of stacked sheets.
More critically, burrs may dislodge during core yoke assembly or due to operational vibrations, fall into the transformer body, and create electric field concentrations that trigger discharges.
Even burrs lying at the bottom of the tank can align under an electric field and induce ground potential discharge.
Therefore, burr height must be controlled within 0.03mm for 110kV class products.
Use of Cold-Pressed Terminals for Leads
The welding process using phosphor bronze generates splattered weld slag that may adhere to insulation surfaces.
Asbestos ropes soaked in water are often used for thermal insulation during welding, which may introduce moisture into the insulation layer.
If this moisture is not completely removed after wrapping, it significantly increases the risk of partial discharge.
Cold-pressed terminal connections can effectively reduce the likelihood of such issues.
Rounding of Component Edges
There are two main purposes for rounding component edges:
To improve electric field distribution, raise the partial discharge inception voltage, and reduce discharge probability;
To prevent iron filings caused by friction during assembly or operation.
Therefore, the following metal structural parts should be rounded: core clamps, tie plates, bracket edges, press plate edges, lead edges, bushing pipes, and internal shielding boxes.
Contact areas such as clamp hanging holes should also be rounded to prevent sharp corners from causing discharges.
Environmental Control and Body Cleaning During Final Assembly
After vacuum drying and before oil filling, the larger and more complex the transformer, the longer the exposure during the finishing process.
During this stage, the body may absorb moisture and dust due to compression, fastening, and environmental exposure.
If exposure exceeds 8 hours, re-drying is required.
All finishing work must be done in dust-controlled areas to maintain insulation integrity.
Vacuum Oil Filling Process
The purpose of vacuum oil filling is to:
Completely remove air from the insulation structure by vacuuming;
Inject transformer oil under vacuum so the entire body is thoroughly impregnated and free of bubbles;
After filling, the transformer should stand for at least 72 hours to allow full oil penetration into insulation materials.
The effectiveness of the oiling process depends on material thickness, oil temperature, and soaking time, all of which must be properly ensured.
Sealing Control of Oil Tank and Components
The quality of sealing directly impacts transformer insulation performance.
If seals are inadequate, moisture can easily enter the equipment.
Once transformer oil and other insulating components become damp, the risk of partial discharge increases significantly.
Therefore, all sealing areas must be strictly controlled to ensure no leakage, no moisture ingress, and high sealing reliability.
In summary, the above multi-pronged process control measures are essential to improving insulation performance, effectively suppressing partial discharge, and extending the operational life of transformers.
