A three phase power transformer sits at the heart of almost every industrial, commercial and utility installation. It takes the medium-voltage supply delivered by the grid, usually 10 kV or 35 kV, and steps it down to the 400/230 V that motors, drives, lighting and building systems actually run on. Three-phase transmission and distribution carry a given load with far less conductor material than an equivalent single-phase arrangement, which is why the three phase power transformer remains the most economical and most widely deployed transformation solution worldwide. This overview explains what these units do, how the main types differ, which parameters decide quality and price, and how to avoid the most common purchasing mistakes.

How a three phase power transformer works

Every transformer relies on electromagnetic induction. Alternating current in the primary winding creates an alternating magnetic flux in a laminated steel core, and that flux induces a voltage in the secondary winding. Because the voltage ratio follows the turns ratio, a transformer can step voltage up or down with only small losses. In a three phase power transformer, three winding sets share a common magnetic circuit, normally built as a three-limb core. The high-voltage and low-voltage windings are wound concentrically around each limb, insulated from each other and from the core, then clamped and braced to withstand the mechanical forces produced by short-circuit currents.

Two parameters describe the losses. No-load loss occurs whenever the unit is energised, even with no load connected, and depends mainly on core material and flux density. Load loss varies with the square of the current and depends mainly on winding conductor cross-section and design current density. Together they determine the efficiency class of the transformer and, over a 20 to 30 year service life, usually outweigh the purchase price difference between models.

Oil-immersed or dry-type: the first decision

• Oil-immersed transformer. The core and windings are submerged in mineral insulating oil inside a sealed steel tank. The oil both insulates and carries heat to the radiators. These units offer high overload capability, excellent heat dissipation and the lowest cost per kVA, which is why they dominate outdoor distribution networks, substations and industrial plants. Typical designs include the hermetically sealed tank and the conservator type with breather and silica gel.
• Dry-type transformer. Windings are cast in epoxy resin or impregnated with Class F or Class H insulation, with cooling by natural air circulation or forced fans. There is no oil to leak, catch fire or require periodic testing, so dry-type units are preferred indoors: high-rise buildings, hospitals, metro stations, data centres, shopping malls and clean production areas where fire load must be minimised.

As a rule of thumb, choose an oil-immersed transformer wherever the unit can be installed outdoors or in a dedicated oil containment room, and choose dry-type where the installation is inside an occupied building.

Reading the model codes: S11, S13, S20, S22, SCB13, SCB14, SCB18, SH15

The letters and numbers in a Chinese model designation are not arbitrary. In oil-immersed types, S means three-phase and the number indicates the loss level of the design, so S11, S13, S20 and S22 represent progressively lower no-load loss and therefore higher energy efficiency grades. Suffixes such as M indicate a sealed tank, MR indicates a sealed type with conservator features, and RL indicates a wound-core three-dimensional magnetic circuit that shortens the flux path and further reduces no-load loss. SH15 and SCBH15 use amorphous alloy cores, whose very thin metallic ribbon and low flux density give the lowest no-load loss of all, making them attractive where the transformer runs lightly loaded for long hours, such as in rural distribution or renewable energy collection.

For dry-type units, SCB means a three-phase cast-resin transformer with a laminated core, and the number again denotes the loss level: SCB10, SCB13, SCB14 and SCB18 step up in efficiency, while SGB10 refers to a dry-type design with a different cooling and construction arrangement. In practice, S13, S20, S22, SCB13, SCB14 and SCB18 designs are now the mainstream choices because they already meet modern efficiency limits, while older S9 or SCB9 designs are being phased out of new projects.

Parameters to confirm before ordering

• Rated capacity. Distribution units commonly span 30 to 6300 kVA. Undersizing causes overheating and short life; oversizing wastes capital and raises no-load loss, so load calculations with a sensible diversity factor matter more than a round number.
• Voltage ratio and tapping. For example 10/0.4 kV or 35/0.4 kV. An off-circuit tap changer of plus or minus 5 percent, or plus or minus 2 by 2.5 percent, compensates for supply voltage variation; an on-load tap changing transformer is used where voltage must be regulated while energised.
• Vector group. Dyn11 is the standard for distribution duty because it limits harmonic effects and balances single-phase loads; Yyn0 and YNd11 appear in specific applications.
• Impedance voltage. Usually around 4 to 6 percent for distribution transformers. It must match the upstream protection settings, or short-circuit coordination and voltage regulation will both suffer.
• Cooling class and temperature rise. ONAN for natural oil cooling, ONAF with fans for extra capacity, AN and AF for dry-type. Cooling class determines the sustained overload the unit can tolerate.
• Insulation level and altitude or ambient derating. Units installed above 1000 m or in hot climates must be derated or specially designed.

Where three phase power transformers are used

Applications range from utility distribution feeders and industrial plant rooms to renewable energy projects, where transformers step up inverter or turbine output, and to heavy industry, where rectifier transformers feed electrolysis and smelting lines, mining transformers supply underground workings, and purpose-built units serve lightning arrester testing, capacity regulation and other special duties. Many projects are best served by a preassembled substation that combines the transformer, high-voltage switchgear and low-voltage switchgear in a single tested enclosure, cutting site work and footprint.

Common purchasing mistakes worth avoiding

• Comparing only the purchase price. A cheaper, lower-efficiency unit can cost far more in no-load loss over its lifetime.
• Accepting a nameplate without type test reports, routine test certificates or a loss measurement record.
• Overlooking the matching of impedance and vector group with existing switchgear and protection.
• Buying through several layers of trading companies, which adds markup without adding engineering support.

Buying from the manufacturing source is the simplest way to control both cost and specification. A factory that builds the core, winds the coils and performs its own routine tests can confirm loss figures, adjust tapping ranges and supply documentation that matches the delivered unit.

Where Runsheng Transformer fits in

Runsheng Transformer Co., Ltd. manufactures oil-immersed transformers, dry-type transformers, amorphous alloy transformers, mining and rectifier transformers, plus 35 kV and below preassembled substations with high and low voltage switchgear. Units are produced to national standards and shipped with manuals, inspection reports and test certificates, backed by lifelong quality tracking. Because the company sells at factory-direct prices with a comprehensive inventory, buyers can compare loss levels, dimensions and lead times directly with the people who build the transformer.

If you are specifying a three phase power transformer for a new project or replacing an aging unit, send the capacity, voltage ratio, vector group, installation environment and expected load profile, and request a quotation with both standard and high-efficiency options. Comparing those two figures side by side is usually the fastest way to see the real cost of ownership.

Contact: phone +86 13406394988 | email runsheng@7813transformer.com | WhatsApp 008613508930968