1.Structural Components

The core assembly of a three-phase oil-immersed transformer comprises a sealed magnetic circuit system consisting of laminated silicon steel cores and windings. The complete structure includes the following key components:

(1) Iron Core Assembly
As the magnetic flux pathway, the iron core minimizes energy losses through the use of 0.35-0.5mm-thick silicon steel laminations coated with insulating oxide films. The vertical section (core column) supports both high and low voltage windings, while the horizontal yoke forms a closed magnetic loop. This configuration optimizes magnetic flux distribution and reduces core losses.

(2) Transformer Winding 
The electrical circuit consists of primary and secondary windings:

• Primary windings (connected to power source) and secondary windings (connected to load) are concentrically arranged on the core column
• Low-voltage windings (fewer turns) are positioned inside for optimal insulation, separated from the core by insulating sleeves
• High-voltage windings (more turns) are externally wound with reinforced insulation barriers between layers
• Intervening oil channels facilitate heat dissipation through natural convection

(3) Tank Assembly
The welded steel tank serves multiple functions:

• Structural containment for core/winding assembly
• Dielectric insulation medium (transformer oil)
• Heat transfer system (natural convection)
• Capacity expansion options: Radiators/heat exchangers for large-capacity units

Transformer oil specifications:

• Mineral oil with dielectric strength ≥30 kV
• Three grades: No.10 (-10°C), No.25 (-25°C), No.45 (-45°C) based on environmental requirements

(4) Conservator System
The expandable oil reservoir (10% tank volume) features:

• Capsule-type air separation design
• Automatic volume compensation (±8% capacity variation)
• Integrated oil level indicator with temperature-compensated scales:
  • +40°C (maximum full load level)
  • +20°C (normal operating reference)
  • -30°C (minimum no-load level)
• Breathing apparatus with silica gel moisture absorption

(5) Bushing System
Insulating assemblies connect internal windings to external circuits:

• High-voltage bushings (≥110kV class)
• Low-voltage bushings (6-35kV class)
• Creepage distance ≥25mm/kV

(6) Explosion-proof Device
Safety valve system:

• Rupture disc design (set pressure 50kPa±10%)
• Automatic pressure relief during internal faults
• Integrated fault signaling mechanism

(7) Gas Relay Protection
Differential protection system:

• Dual-stage protection (signal + trip)
• Gas analysis capability (air vs. combustible gases)
• Oil flow monitoring with directional sensing

(8) Temperature Monitoring
Critical parameters:

• Top oil temperature limit: 95°C (continuous), 85°C (recommended)
• Winding temperature limit: 105°C (emergency)
• Alarm activation at 80°C (oil)/100°C (winding)

(9) Neutral Point Grounding
System protection requirements:

• Compulsory grounding for 110kV graded insulation systems
• Preventive measure against ferroresonance overvoltages
• Maintenance grounding protocol during shutdowns

(10) On-load Tap Changer
Voltage regulation system:

• Separate conservator with non-capsule design
• Operation sequence: Stepwise adjustment with 1-minute intervals
• Emergency stop procedure for multiple position shifts
• Maintenance cycle: Every 5000 operations or 3 years

2.Operational Practices

(1) Normal Operating Modes

A. Rated Conditions

• Ambient temperature: -25°C to +40°C
• Oil temperature rise: ≤55K (top oil)
• Load current: 100% rated value

B. Permissible Overloads

• Continuous: 1.1 times rated current (≤8h)
• Short-term: 1.3 times rated current (≤2h)
• Seasonal: 1.2 times rated current (ambient ≤+15°C)

(2) Abnormal Conditions & Responses

A. Immediate Shutdown Criteria

1. Internal explosions or violent discharges
2. Temperature runaway (>10°C above normal)
3. Oil ejection from explosion-proof device
4. Oil level below minimum mark
5. Carbonaceous deposits in oil
6. Severe bushing flashover

B. Overload Management Protocol

1. Verify load vs. cooling capacity correlation
2. Check cooling system integrity
3. Monitor winding temperatures (thermocouples)
4. Initiate forced cooling if ΔT >10°C

C. Gas Relay Activation Procedure

1. Perform dissolved gas analysis (DGA)
2. Check oil sampling for combustibility
3. Inspect core and winding insulation
4. Implement oil processing if necessary

(3) Maintenance Requirements
A. Routine Inspection Items

1. Acoustic monitoring for partial discharges
2. Oil level verification (±5% tolerance)
3. Thermographic analysis of connections
4. Breather system functionality check
5. Cooling fan vibration analysis

B. Periodic Maintenance Schedule

• Quarterly: Bushing leakage test, oil dielectric strength test
• Annually: Core and clamp insulation resistance, winding deformation test
• Biennially: Furan content analysis, oil chromatography

C. Specialized Testing

1. Impulse withstand test (1.5/5μs waveform)
2. Partial discharge mapping (>100pC threshold)
3. Short-circuit withstand verification

This technical specification provides comprehensive guidelines for the operation, maintenance, and troubleshooting of three-phase oil-immersed transformers, emphasizing safety compliance and equipment longevity through systematic condition monitoring.