| 1 | Match the Transformer Type to the Site | Oil-immersed: commonly used outdoors and for medium- to high-capacity substations; efficient heat dissipation but requires oil containment and fire-risk controls.
Dry-type: suitable for indoor installations and locations where fluid management is undesirable; generally requires more ventilation space.
Cast-resin: a dry-type design with improved moisture resistance and reduced maintenance needs, often selected for demanding indoor environments. | Use oil-immersed equipment when high capacity, outdoor installation, or lower total cost is important. Consider dry-type or cast-resin equipment for buildings, tunnels, hospitals, and environmentally sensitive areas. |
| 2 | Calculate the Required Capacity | Transformer capacity is normally specified in kVA or MVA. A basic estimate is:
Required kVA = Maximum Demand kW ÷ Power Factor × (1 + Design Margin)
A design margin of approximately 10%–25% is commonly evaluated, depending on load growth, duty cycle, and project policy. | Avoid selecting a transformer solely from connected load. Use measured or calculated maximum demand, motor-starting requirements, harmonics, ambient temperature, and future expansion plans. |
| 3 | Verify Voltage and Connection Requirements | Common system combinations include medium-voltage primary systems such as 6.6 kV, 11 kV, 13.8 kV, 22 kV, or 33 kV and low-voltage secondary systems such as 400 V or 415 V at 50 Hz, or 480 V at 60 Hz.
Important specifications include vector group, phase arrangement, neutral availability, tap range, impedance, and short-circuit withstand capability. | Confirm the utility interface, operating frequency, grounding method, allowable voltage variation, and downstream protection requirements before finalizing the transformer ratio. |
| 4 | Compare Efficiency at the Real Load Profile | Distribution transformers commonly achieve approximately 98%–99.7% efficiency, depending on rating, design, and operating load. No-load losses remain present whenever the transformer is energized, while load losses increase approximately with the square of current.
Maximum efficiency often occurs near the load at which variable load loss is approximately equal to fixed no-load loss. | Compare no-load loss, load loss, and total ownership cost—not only the nameplate efficiency. A slightly higher purchase price may be justified where the transformer operates continuously or at a high average load. |
| 5 | Check Thermal Performance and Cooling | Cooling classifications may include ONAN for oil-immersed natural cooling and AN for dry-type natural-air cooling. Larger units may use assisted cooling such as forced air or forced oil systems.
Temperature rise, ambient temperature, altitude, enclosure ventilation, and harmonic current all affect usable capacity. | Apply the manufacturer’s derating data for high-altitude or high-ambient sites. Provide adequate ventilation and avoid relying on short-term overload capability as a substitute for correct continuous sizing. |
| 6 | Evaluate Safety, Protection, and Environmental Conditions | Key considerations include fire separation, oil containment, pressure relief, surge arresters, temperature monitoring, overcurrent protection, differential protection for larger units, and suitable enclosure ratings.
Dry-type units eliminate liquid leakage concerns, while oil-immersed units require appropriate spill-control and fire-protection measures. | Select equipment and protection in accordance with applicable local electrical, fire, environmental, and utility requirements. Consider dust, humidity, salt exposure, vibration, and corrosive atmospheres. |
| 7 | Confirm Standards, Testing, and Lifecycle Cost | Common reference standards include IEC 60076 for power transformers, IEC 60076-11 for dry-type transformers, and applicable IEEE C57 standards. Routine tests generally include winding resistance, ratio verification, insulation tests, and no-load and load-loss measurements. | Request certified test results, guaranteed losses, impedance data, sound levels, maintenance requirements, spare-parts information, expected service life, and warranty conditions. Compare purchase price, energy losses, maintenance, downtime, and disposal costs. |
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