A three phase transformer is the workhorse of nearly every distribution network, industrial plant and commercial building. It steps voltage down from the 10 kV or 35 kV feeder to the 400 V that equipment actually uses, and it usually stays in service for 20 to 30 years. That long life is why the buying decision matters far more than the invoice: the specification you sign off today determines your energy losses, your fault level, your maintenance burden and your replacement date. This overview covers the basics a procurement engineer, EPC contractor or plant manager needs before requesting quotations.

What a three phase transformer does, in plain terms

Three phase power arrives on three conductors with a 120-degree phase displacement. A three phase transformer transfers that power between voltage levels through a shared magnetic circuit, with no moving parts and very little loss. The main components are the same across almost every design:

• Magnetic core — laminated silicon steel or, in amorphous alloy units, a thin amorphous metal ribbon; it carries the flux and drives no-load loss.
• Windings — high-voltage and low-voltage coils in copper or aluminium, wound around the core limbs.
• Cooling medium — insulating oil in a sealed tank, or air in a cast-resin dry-type design.
• Tap changer — off-circuit links for occasional voltage adjustment, or an on-load tap changer (OLTC) where the supply fluctuates and must be regulated live.
• Protection and accessories — bushings, temperature sensors, pressure relief, Buchholz relay on oil units, fans on forced-cooled units.

Two core constructions dominate the market: the conventional three-limb stacked core and the wound core with a three-dimensional magnetic path. The wound core design is common in distribution ratings because its shorter flux path reduces no-load loss.

Oil-immersed or dry-type: match the unit to the site

The first real decision is the cooling and insulation system, because it determines where the transformer can be installed and what fire protection the site needs.

• Oil-immersed transformers (S11, S13, S20, S22, and amorphous alloy SH15) offer the lowest cost per kVA and the widest capacity range, from 30 kVA up to 6300 kVA and beyond. They are the default for outdoor pads, pole-mounted installations, industrial yards and renewable energy projects. The trade-off is oil containment, bunding and fire separation distances.
• Dry-type transformers (SCB10, SCB13, SCB14, SCB18) use cast-resin insulation and air cooling. No oil means no leakage risk and no bunding, which is why they dominate indoor substations, hospitals, metro systems, data centres and high-rise buildings. They normally come with an IP20 to IP23 enclosure and need good ventilation.

A rough rule of thumb: outdoor, high capacity and cost-sensitive points to oil-immersed; indoor, fire-sensitive and public-occupancy points to dry-type. Non-standard duties — mining, rectifier loads, lightning arrester testing, capacity regulation — are served by purpose-built special transformers rather than by derating a standard catalogue unit.

Reading the nameplate: the parameters that set the price

Quotations for the same apparent capacity can differ by 30 percent or more. The gap is explained by these six items:

• Rated capacity (kVA): 30–6300 kVA is the usual distribution range. Capacity should follow a calculated load, not a guess with a large safety margin.
• Voltage ratio and tapping range: for example 10/0.4 kV with ±5% off-circuit taps, or a wider OLTC range where the supply is unstable.
• Vector group: Dyn11 is the standard distribution connection; Yyn0, YNd11 and others appear in specific duties. The vector group must match upstream protection settings and any parallel operation.
• Impedance: typically 4–6% for distribution ratings. It governs downstream fault current and voltage regulation, and it is not a free parameter to change.
• Loss class: the guaranteed no-load and load loss figures, which are usually the largest single driver of both price and operating cost.
• Cooling class and temperature rise: ONAN, ONAF or AN/AF, plus the specified winding and oil temperature rises.

Losses are the real price tag

No-load loss is consumed around the clock as long as the transformer is energised, whether the plant is running or not. Load loss rises with the square of the load and peaks during your busiest shifts. A higher-efficiency grade — moving from S11 to S13, S20 or S22, or choosing an SH15 amorphous alloy unit — costs more upfront but pays back through a smaller electricity bill, and the payback is fastest on units that stay energised under light load, such as commercial buildings, water utilities and solar or wind collector substations.

A simple evaluation method works well: price the transformer plus the capitalised value of its guaranteed losses over your expected service life, at your own electricity tariff. Compare that total, not the quotation.

Five specification mistakes that quietly cost money

• Oversizing “just in case”. A unit running at 30 percent load wastes its no-load loss and pays interest on unused copper.
• Ignoring site conditions. High altitude, high ambient temperature and restricted ventilation all require derating or a modified cooling design.
• Treating harmonic-rich loads as normal. Rectifiers, VFDs, induction furnaces and inverter outputs raise eddy-current and stray losses; they need K-factor derating or a dedicated rectifier transformer.
• Forgetting the installation details. Enclosure IP rating, cable entry direction, clearance for door opening, anchor dimensions and weight all cause site delays when missed.
• Buying on price alone. A unit without verifiable test reports is an unknown asset for the next two decades.

How to vet a three phase transformer manufacturer

Every unit should be routine-tested before shipment: winding resistance, voltage ratio, impedance and load loss, no-load loss, applied voltage and induced voltage withstand, insulation resistance, and oil dielectric strength for oil-immersed designs. Type tests — temperature rise, lightning impulse and short-circuit withstand — should be available as reports, ideally from an independent laboratory. Ask for the quality management certificate, the winding and core photographs, and a clear statement of what documents ship with the unit.

Suppliers such as Runsheng Transformer Co., Ltd. cover this ground with S11, S13, S20 and S22 oil-immersed transformers, SCB10, SCB13, SCB14 and SCB18 dry-type transformers, SH15 and SCBH15 amorphous alloy units, 35 kV preassembled and compact substations, and high and low voltage switchgear. Each unit is delivered with a manual, inspection report and test certificate, backed by lifelong quality tracking — the documentation package that matters most when an inspector or insurer asks questions later.

A short procurement checklist

• Confirm capacity from a load calculation, with the future expansion clearly stated.
• Fix voltage ratio, tapping range, vector group and impedance with your electrical designer.
• Specify the loss class and ask for guaranteed values in writing.
• State ambient temperature, altitude, installation location and enclosure requirement.
• Require routine test reports for the actual serial number, plus the manual and inspection certificate.
• Agree on delivery schedule, packing, and who handles commissioning and oil filling on site.

A three phase transformer is a long-term asset bought once and paid for continuously through its losses. Getting the type, the nameplate parameters and the test documentation right at the quotation stage is the cheapest decision you will make in the whole project.

For specification questions or a factory-direct quotation, contact Runsheng Transformer Co., Ltd.: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.