What a Three Phase Transformer Does in a Power System

A three phase transformer transfers electrical energy between two voltage levels on a three-phase network. Rather than ganging three single-phase units together, it places all three phases on one shared magnetic core inside a single tank or enclosure. That shared core is the reason the three phase transformer is smaller, lighter, cheaper per kVA and more efficient than the single-phase alternative, and why it dominates transmission, distribution and industrial power systems worldwide.

In practice most three phase transformers sit at one of two points in the network. At the utility distribution level they step 10 kV, 20 kV or 35 kV down to 0.4 kV for end users. Inside industrial plants they step an incoming 35 kV or 10 kV feeder down to the 400/230 V that drives motors, variable frequency drives and building services. Distribution ratings typically run from 30 kVA up to 6300 kVA.

Inside the Unit: Core, Windings and Insulation

Three elements decide how a three phase transformer performs and how long it survives in service.

• Magnetic core. Cold-rolled grain-oriented silicon steel laminations stacked with mitered step-lap joints give the lowest practical no-load loss. Wound-core and three-dimensional wound core designs shorten the magnetic path and reduce joint losses; amorphous alloy cores cut no-load loss dramatically, which is why amorphous alloy units are attractive on networks with high energy cost or long no-load hours.
• Windings. Copper or aluminium conductors, chosen for current density, short-circuit withstand and loss targets. Round wire suits small ratings, foil is common for low-voltage windings, and continuously transposed conductor is used on larger units to control eddy losses.
• Insulation system. Oil-immersed units combine kraft paper with transformer oil in a sealed tank; dry-type units use cast epoxy resin or vacuum pressure impregnated insulation. The insulation class, not the kVA rating, usually decides where the unit may be installed.

Oil-Immersed or Dry-Type: The First Real Decision

Both technologies are mature. The choice is driven by location, fire risk and budget rather than by performance alone.

• Oil-immersed transformer. Lower cost per kVA at larger ratings, better overload capability and excellent heat dissipation through ONAN or ONAF cooling. Best suited to outdoor yards, pole-mounted and pad-mounted installations, industrial substations and renewable energy step-up duty. Requires oil containment and periodic oil testing.
• Dry-type transformer. No liquid, so no fire load and no oil bund. Preferred indoors: high-rise buildings, hospitals, data centres, tunnels, metro stations and food or pharmaceutical plants where leakage is unacceptable. Cast resin units tolerate humid and polluted environments well but cost more per kVA and are less forgiving of sustained overload.

Reading the Nameplate Like a Buyer

• Rated power and voltage ratio, for example 1000 kVA, 10/0.4 kV.
• Vector group. Dyn11 is the standard distribution connection; Yyn0 and YNd11 appear in other duties. The vector group fixes phase displacement and how zero-sequence and harmonic currents behave, and it must match the existing plant before any parallel operation.
• Short-circuit impedance, typically 4 to 6 percent for distribution units. Higher impedance lowers fault current but increases voltage regulation.
• No-load loss and load loss, quoted at a defined reference temperature, plus the efficiency tier the unit is designed to meet.
• Cooling class ONAN, ONAF, AN or AF, which defines the permissible continuous load.
• Temperature rise, commonly 65 K for oil-immersed units under IEC practice.
• Tap arrangement, either off-circuit taps such as plus and minus 2 x 2.5 percent, or a load tap changing transformer where voltage must be regulated under load.
• Insulation levels expressed as lightning impulse and power-frequency withstand values.

Losses Decide the Real Cost, Not the Invoice

No-load loss is paid for 8760 hours a year whether the plant is running or not; load loss is paid in proportion to the square of the load current. On a heavily loaded industrial unit, twenty years of loss cost can exceed the purchase price several times over. This is why comparing two quotations purely on capital cost is misleading. Capitalising no-load and load loss at the local tariff gives a far more honest comparison, and it often justifies a higher-efficiency design, an amorphous alloy core or a larger rating running at 60 to 80 percent load.

Where Three Phase Transformers Are Used

• Utility and industrial distribution substations feeding factories, campuses and commercial buildings
• Renewable energy projects, including wind and solar step-up duty
• Mining, where robust tanks and high ingress protection are essential
• Rectifier and electrolysis duty, where harmonic content demands special winding design
• Preassembled and compact substations combining the transformer with high and low voltage switchgear

Five Specification Traps That Cost Money Later

• Sizing on present load only, with no allowance for expansion, then running the unit above 90 percent loading.
• Ignoring harmonic-generating loads such as drives and rectifiers, which overheat windings and demand derating or a purpose-built unit.
• Forgetting altitude and ambient correction, since standard ratings assume an ambient around 40 degrees Celsius and altitudes up to 1000 m.
• Specifying a vector group that cannot be paralleled with the existing installation.
• Buying on price alone and receiving a unit with no verified loss figures or no type-test evidence.

Tests and Paperwork to Demand Before Shipment

Routine tests should cover winding resistance, voltage ratio, short-circuit impedance and load loss, no-load loss and current, applied voltage, induced voltage and insulation resistance, plus oil dielectric strength on oil-immersed units. Type tests such as temperature rise, lightning impulse and short-circuit withstand should be traceable to the design being supplied. Ask for the routine test report, inspection certificate, manual and a written quality-tracking commitment; a manufacturer that supplies these without prompting is usually the one worth keeping.

Why Factory-Direct Supply Changes the Price Equation

Distribution margins on a three phase transformer are thin, and every trading layer adds cost without adding a single lamination. Buying from the production line shortens the chain, keeps the specification conversation with the people who build the unit, and makes custom requirements such as special impedance, non-standard voltages, rectifier duty or overload ratings far easier to accommodate. Runsheng Transformer Co., Ltd. manufactures S11, S13, S20 and S22 oil-immersed transformers, SCB10, SCB13, SCB14 and SCB18 dry-type transformers, SH15 amorphous alloy units, mining and rectifier transformers, and 35 kV and below preassembled and compact substations with high and low voltage switchgear, covering 30 to 6300 kVA with factory-direct pricing and full documentation.

Quick Selection Checklist

• Confirm the load in kVA, the load profile and the harmonic environment
• Fix primary and secondary voltages, vector group and tap range
• Decide oil-immersed or dry-type based on indoor or outdoor location and fire rules
• Agree loss limits and the efficiency tier in writing
• Confirm short-circuit impedance and protection coordination with the switchgear
• Require routine test reports, certificates and a manual with every unit

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