Transformer Selection Guide for Industrial Projects
A practical guide for selecting transformers in industrial projects. Covers rating calculation, type selection, cooling method, and installation requirements.
Transformer Types
Oil-immersed transformer: Lower cost, needs maintenance, suitable for outdoor installation.
Dry-type transformer: Safer, low maintenance, suitable for indoor installation (hospitals, commercial buildings).
Cast resin transformer: Better moisture resistance, suitable for tropical climate.
Rating Calculation
1. Calculate total load (kVA) from connected equipment
2. Apply demand factor (typically 0.7-0.8 for industrial plants)
3. Add 20-30% spare capacity for future expansion
4. Consider harmonic distortion (may need to oversize)
Voltage Regulation
On-load tap changer: Adjusts voltage under load (±10% in 5 steps)
Off-circuit tap changer: Adjusts voltage when de-energized (±5% in 2 steps)
For areas with unstable grid voltage, on-load tap changer is recommended.
Cooling Method
ONAN (Oil Natural Air Natural): Up to 2500kVA
ONAF (Oil Natural Air Forced): 2500kVA and above
For hot climates, consider ONAF or even OFAF (Oil Forced Air Forced).
Installation Requirements
Minimum clearance: 1.5m from walls, 3m from other equipment
Ventilation: 10-15 air changes per hour for indoor installation
Foundation: Reinforced concrete, able to support transformer weight (including oil)
Drainage: Oil containment pit to prevent fire spread
Common Specification Mistakes
1. Undersizing (no spare capacity for expansion)
2. Wrong impedance (causes voltage drop or fault current issues)
3. Ignoring altitude correction (insulation fails at high altitude)
4. No harmonic specification (transformer overheats with non-linear loads)
5. Wrong tap changer type (on-load vs off-circuit)