Power Transformer Buying Guide | Rating, Losses and Type Selection
1. Fix the electrical rating before comparing prices
Start with the load study and the single-line diagram, then confirm:
- Rated capacity in kVA, with continuous and peak load
- Primary and secondary voltage, including the tap range needed
- Rated frequency: 50Hz or 60Hz
- Vector group and neutral arrangement
- Impedance percentage required by the fault-level study
- Indoor or outdoor installation and ambient temperature range
Two quotations for the same kVA can hide very different core grades, conductor sections and loss figures. Fix the rating first, then compare the data sheet line by line.
2. Choose oil-immersed or dry-type construction
| Type | Typical use | Key considerations |
|---|---|---|
| Sealed oil-immersed (S11-M, S13) | Outdoor distribution, pad-mounted and pole-mounted projects | Corrugated tank cooling, no conservator, low loss, lowest cost per kVA |
| Cast-resin dry-type (SCB) | Indoor rooms, basements, hospitals, data centres, fire-sensitive sites | No insulating oil, low partial discharge, IP20 to IP54 enclosures, higher cost |
| Pad-mounted renewable units | Solar and wind step-up stations | 6kV to 38.5kV HV windings, 0.27kV to 0.4kV LV, outdoor enclosure |
Choose dry-type when oil leakage, fire load or indoor ventilation rules make oil unacceptable. Choose sealed oil-immersed units when efficiency and first cost dominate and the unit can be installed outdoors or in a ventilated chamber.
3. Compare losses over the life of the unit
No-load loss runs 24 hours a day; load loss follows the load curve. Ask for guaranteed no-load loss, load loss and impedance at rated capacity, and calculate the capitalized cost using your energy price and expected loading. A unit with a lower purchase price but higher losses can cost more within a few years. Confirm the reference temperature and the measurement method used for the loss figures.
4. Specify taps, impedance and vector group
State the off-circuit or on-load tap range, usually plus or minus 5% or two taps of 2.5%. Confirm the impedance that the protection study assumes, because a different value changes fault levels and breaker settings. Give the vector group, for example Dyn11, and confirm how the neutral is earthed. For parallel operation, taps and impedance must match the existing units.
5. Confirm tests, standards and documentation
Our transformers are designed and tested to IEC 60076 and GB 1094. Routine tests normally cover winding resistance, ratio and polarity, impedance and load loss, no-load loss and current, and applied and induced voltage withstand. Ask whether lightning impulse, temperature rise and sound-level tests are required as type or special tests, and whether a third-party witness is needed. Documentation should include the data sheet, outline drawing, nameplate details and routine test report.
6. Plan transport, foundation and commissioning
Check total mass, dimensions, lifting points and oil volume for transport and crane planning. For oil-immersed units, confirm whether the transformer ships oil-filled or with oil separate. Allow for a level foundation, cable or busbar entry, earthing terminals, fire separation and ventilation for dry-type units. On arrival, check impact recorder data, insulation resistance, ratio and tap position before energising.
7. RFQ checklist
- Rated capacity, primary and secondary voltage, frequency
- Vector group, impedance and tap range
- Oil-immersed or dry-type, indoor or outdoor
- Enclosure or tank finish, IP rating and ambient conditions
- Required losses, efficiency class and energy cost data
- Standards, type tests and inspection scope
- Terminal arrangement, cable or busbar entry
- Quantity, destination port and required delivery date
A complete RFQ lets the factory confirm the design, loss guarantees and delivery schedule without repeated drawing revisions.
