To buy a transformer from China, write a clear technical specification first (rating, voltages, vector group, losses, cooling, tap changer, tests and standard), then send that same specification to several verified manufacturers and compare their guaranteed figures line by line. Most problems with imported transformers come from vague enquiries, not from the factory. This guide covers what to specify, what to test and what to check before you pay a deposit.
Choose the transformer type first
Your first decision is the insulation and cooling system, because it drives price, footprint and where the unit can be installed.
- Oil-immersed transformers use mineral oil (or an ester fluid) for insulation and cooling. They suit outdoor substations, utility networks and higher ratings.
- Dry-type transformers, usually cast resin, have no liquid. They suit indoor plant rooms, hospitals, high-rise buildings, tunnels and other places where fire risk and oil containment matter.
If you are unsure, compare fire risk, installation location, maintenance and cost before deciding. For units that arrive pre-installed in an enclosure with switchgear, see prefabricated substations.
The core ratings to specify
A complete enquiry states each of these values. Leave one out and suppliers will fill the gap with their cheapest option.
- Rated power (kVA or MVA) at the stated cooling mode.
- Primary and secondary voltages, for example 11 kV / 0.415 kV or 33 kV / 11 kV, plus the highest voltage for equipment (Um).
- Frequency: 50 Hz or 60 Hz. Do not assume; Chinese factories build for 50 Hz by default.
- Number of phases and vector group, for example Dyn11 for most distribution transformers feeding a four-wire LV network.
- Short-circuit impedance (%), which must match any transformer you plan to run in parallel.
- Cooling class: ONAN or ONAF for oil-immersed; AN or AF for dry-type.
- Insulation levels: lightning impulse and power-frequency withstand voltages for each winding.
- Site conditions: altitude, maximum and minimum ambient temperature, humidity and pollution level. Ratings above 1,000 m altitude need derating or extra insulation.
Losses and efficiency
Transformer losses are split into no-load loss (core loss, present whenever the unit is energised) and load loss (copper loss, rising with the square of the load current). Both are measured in watts and should be guaranteed in the offer.
Many markets set minimum efficiency levels. The European Union applies its Ecodesign rules for transformers, and China has its own national energy-efficiency grades under GB standards. Chinese model codes such as S13 for oil-immersed or SCB13 for cast resin indicate a loss series, but the code alone is not a guarantee. Ask for the actual no-load and load loss values, the tolerance that applies, and the penalty or rejection rule if test results exceed them.
If you evaluate offers on total cost of ownership, give suppliers your capitalisation values for no-load and load losses. A cheaper unit with higher losses can cost more over its life.
Vector group and tap changer
The vector group describes how the windings are connected and the phase shift between primary and secondary. Dyn11 (delta primary, star secondary with neutral, 30-degree shift) is the usual choice for distribution. Generator step-up units and some utility networks use other groups. Match the group to your existing network, especially for parallel operation.
The tap changer adjusts the turns ratio to hold the secondary voltage steady:
- Off-circuit tap changer (OCTC): changed only when the transformer is de-energised. Standard on distribution transformers, often with a range of ±2 × 2.5 %.
- On-load tap changer (OLTC): changes taps while energised, controlled by an automatic voltage regulator. Used on larger power transformers and where supply voltage varies a lot.
State the tap range, step size and which winding carries the taps.
Tests: routine, type and special
The main standard is the IEC 60076 series (IEC 60076-11 for dry-type). Ask which edition the factory tests to, and require the test plan in the offer.
| Test category | What it covers | Who gets it |
|---|---|---|
| Routine tests | Winding resistance, voltage ratio and phase displacement, short-circuit impedance and load loss, no-load loss and current, applied and induced voltage tests; partial discharge for dry-type; leak test for oil tanks | Every unit shipped |
| Type tests | Temperature-rise test, lightning impulse test (a type test for most distribution ratings) | One unit of a design, or a valid report for the same design |
| Special tests | Short-circuit withstand, sound level, and other tests by agreement | Only if specified in the contract |
Type-test reports should come from an independent, accredited laboratory and match your design: same rating, voltage class and construction. A report for a different kVA or voltage does not prove your unit. Short-circuit withstand tests are costly, so if your project requires one, agree it before the price is fixed.
Plan a factory acceptance test (FAT). Attend in person or appoint a third-party inspector to witness routine tests on your actual units.
Specification checklist
Copy this list into your enquiry so every supplier quotes against the same basis.
- Standard and edition (for example IEC 60076), plus any national standard your utility requires.
- Type, rated power, voltages, Um, frequency, phases, vector group.
- Impedance, insulation levels, cooling class, temperature-rise limits.
- Guaranteed no-load and load losses, with tolerances and penalties.
- Tap changer type, range and steps.
- Winding material (copper or aluminium) and core steel grade.
- Accessories: oil level gauge, pressure relief device, Buchholz relay or hermetic sealing, winding and oil temperature indicators, bushings and cable boxes.
- For dry-type: insulation thermal class, enclosure IP rating, environmental, climatic and fire classes.
- Tests, FAT arrangements and documents (drawings, test reports, manuals).
- Packing, shipping (oil-filled or nitrogen-filled), Incoterms and warranty.
For metering and protection CTs and VTs ordered with the job, specify them separately and check the ranked instrument transformer manufacturers.
What to check with a Chinese supplier
Before shortlisting, confirm that the company is real and that it builds what it sells.
- Unified Social Credit Code: every registered Chinese company has an 18-character code. Check that the name and scope on the business licence match the quote.
- Factory or trader: ask for the factory address, a video call on the winding and test floor, and the test-bay equipment list. Traders are not always a problem, but you should know who winds the coils.
- Certificates: ISO 9001, plus product certificates relevant to your market. CCC applies to certain products sold in China; for export, IEC compliance and independent type-test reports matter more.
- Type-test reports: check laboratory name, report number, date and the exact model tested.
- References: ask for export projects of similar rating in a similar climate.
Our rankings score companies on data completeness, not product quality. Read the methodology to see what the score means, then see the ranked list of transformer manufacturers and autotransformer manufacturers to build a shortlist.
Frequently asked questions
What information do Chinese transformer factories need for a quotation?
At minimum: rated power, voltages, frequency, vector group, impedance, cooling, tap changer, losses, standard and site conditions. A full specification sheet produces comparable quotes and fewer surprises.
Are routine test reports enough, or do I need type tests?
Routine tests prove each unit was built correctly; type tests prove the design meets temperature rise and impulse requirements. Ask for both, and check the type-test report matches your design.
Should I choose copper or aluminium windings?
Both can meet IEC 60076 when correctly designed. Copper gives a smaller unit and is preferred by many utilities; aluminium is lighter and lower in material cost. State your preference in the specification so all offers are comparable.
Can a 50 Hz transformer run on a 60 Hz network?
Not without checking the design with the manufacturer, because frequency affects core flux, losses and impedance. Order the transformer for your actual network frequency.