Fully-sealed oil-immersed transformers utilize transformer oil as both insulation and cooling medium, replacing traditional conservators with a permanent hermetic structure. The tank cover and flange are joined through full-welding or high-strength bolting, forming a rigid sealed assembly with elastic sealing materials. This design isolates oil from air contact, reducing oil aging rate to <1/3 of conventional units, achieving maintenance-free operation while eliminating oil leakage.
I. Core Structure & Technical Principles
- Sealing System
Employs multilayer metal-nonmetal composite sealing: Oil-resistant fluororubber gaskets embedded in tank flange, supplemented by stainless steel compensation diaphragms. Diaphragms absorb ±15kPa pressure fluctuations during thermal cycling through elastic deformation. Corrugated tank walls (15-30mm depth) accommodate 2%-5% volumetric expansion for dynamic sealing.
- Thermal Management
Exemplified by Fule Electric's solution: Radiator tubes feature gradient fin design (upper: 15 fins/cm; lower: 22 fins/cm), establishing bottom-to-top thermal gradient. Combined with nano-modified transformer oil (thermal conductivity ≥0.15 W/m·K), maintains oil temperature rise ≤55K at 40°C ambient. Eddy-current driven cooling module boosts oil flow velocity by 300% during overload.
- Insulation Enhancement
S11-type LV windings adopt helical layered composite structure, with copper conductors clad in 0.2mm polyimide-boron nitride insulation. Axial 3mm oil ducts between layers, combined with HV winding's radial segmented insulation, limit partial discharge to ≤5pC (GB: ≤10pC). Vacuum oil-filling ensures moisture content ≤8ppm and breakdown voltage ≥70kV/2.5mm.
II. Performance Metrics & Trends
- Efficiency Data
S11-type reduces no-load losses by 30% vs. S9 (1000kVA: 1.04kW→0.72kW). Step-lap cores (joint gap ≤0.05mm) decrease no-load current by 70%. EU Eco-design Directive (2025/1780) mandates ≥80% biodegradable oil, projecting >35% market share for ester fluids.
- Research Frontiers
- Magnetic thermal particles (Fe₃O₄@rGO): Form directional thermal channels under magnetic fields, +200% heat dissipation
- Self-healing insulation: Microencapsulated alkylbenzene (20μm) repairs insulation defects during local overheating
- Digital twins: Multi-physics lifetime prediction models achieve <3% error rate
III. Application-Specific Adaptation
| Scenario | Technical Solution | Key Performance Indicators |
|---|---|---|
| Urban Distribution | Corrugated tank + ONAN cooling | Noise ≤45dB, 25-year maintenance-free |
| Offshore Wind | Titanium alloy tank + OFAF cooling | Salt fog resistance, 1.8kVA/kg power density |
| High-altitude Substations | Negative-pressure compensation + BP enhancer | 5500m altitude operation, ΔT≤45K |
| Data Centers | Dual-split winding + forced oil | 8% short-circuit impedance, 99.999% redundancy |
Fully-sealed oil-immersed transformers advance toward maintenance-free operation, high reliability, and intelligence through structural innovation and material upgrades. Accelerating adoption in wind power and data centers will drive global market beyond $40 billion by 2025. Vegetable-oil insulated transformers will grow at 15.2% CAGR, while intelligent diagnostic systems exceed 60% penetration, underpinning security in modern power systems.