What are the differences in structure and application scenarios between three-phase transformers and single-phase transformers?

In power systems, transformers are the core equipment for voltage conversion. According to the number of phases, transformers are mainly divided into two categories: single-phase transformers and three-phase transformers.

Many people will have questions when selecting: Why do most industrial and commercial power distribution use three-phase transformers? Have single-phase transformers been eliminated? Under what circumstances must a single-phase transformer be selected?

II. In-depth analysis of structural differences

1. Iron core structure

The single-phase transformer uses a single-phase double-column core (two core columns + upper and lower iron yokes), and the magnetic flux flows back and forth in a closed loop. The structure is simple, the manufacturing process is mature, and the requirements for the magnetic properties of silicon steel sheets are relatively loose.

Three-phase transformers have two mainstream core structures:

• Three-phase three-pillar type: three iron core pillars + upper and lower iron yokes, with compact structure and high material utilization rate. It is the standard structure of most distribution transformers. However, when the three-phase magnetic flux is unbalanced, the zero-sequence magnetic flux needs to be closed through the tank wall, causing additional losses and local overheating.

• Three-phase five-column type: Two side yokes are added to the three-phase three-column type to provide a low reluctance circuit for the zero-sequence magnetic flux and effectively reduce the zero-sequence impedance. It is often used in large-capacity transformers and situations where zero-sequence current needs to be limited (such as grounding transformers, rectifier transformers).

Key difference: The side yoke of the three-phase five-column iron core provides a closed path for the third harmonic magnetic flux, which can reduce harmonic losses and noise in the iron core. This is a structural advantage that single-phase transformers do not have.

2. Winding configuration

Single-phase transformer: Each set of core columns is equipped with a set of primary windings and a set of secondary windings, a total of 2 sets of windings. The winding connection method is simple and the insulation design is relatively easy.

Three-phase transformer: Each set of core columns is equipped with a set of primary windings and a set of secondary windings, a total of 6 sets of windings. Windings need to be connected according to specific connection groups. Common ones are:

Dyn11 is currently the most mainstream connection group in 10kV/0.4kV power distribution systems. Its advantages are:

• The high-voltage side delta connection provides a circulation path for the third harmonic current and suppresses the harmonic voltage

• Low voltage side star connection for output 220V single phase and 380V three phase

• Strong ability to withstand single-phase unbalanced loads and small neutral point deviation

3. Capacity and efficiency

The capacity calculation formula of single-phase transformer is:   

S = U \times I

Where U is the voltage and I is the current. Single-phase transformers are usually suitable for small-capacity scenarios. After exceeding 1000 kVA, the material utilization rate and volume economy of single-phase structures drop sharply.

The total capacity of a three-phase transformer is the sum of the three phases:

S = \sqrt{3} \times U_L \times I_L

At the same rated capacity, a three-phase transformer is better than three independent single-phase transformers:

• Save about 20%~30% of iron core material (three phases and three columns share an iron yoke)

• Save about 15%~20% of winding copper material (the vector sum of the three-phase currents is zero, and the neutral line current is small)

• The floor space is reduced by approximately 40%~50%

• Loss is reduced by about 10%~15% (material reduction → simultaneous reduction of resistance loss and iron loss)

III. Comparison of application scenarios

Typical applications of single-phase transformers

1. Residential terminal power distribution

The low-voltage power distribution system in urban residential areas will eventually be broken down into single-phase 220V power supply. Although a three-phase transformer is used from the substation to the building, the last link from the building distribution box to each household is actually realized through a single-phase branch circuit. In some rural areas, due to low load density and scattered distribution, single-phase pole-mounted transformers (10kV/0.22kV) will be directly used to supply power to a few to more than a dozen households.

2. Low power electronic equipment

• Control transformer (such as 220V/36V transformer for machine tool control circuit)

• Instrumentation power transformer

• Isolation transformers for access control, security and communication equipment

• Power transformers in household appliances (some still in use)

3. Railway single-phase power supply system

The electrified railway uses 25kV single-phase power frequency alternating current (50Hz), and the catenary is single-phase power supply. The traction substation converts the three-phase grid voltage into single phase through a single-phase transformer or a V/v connection transformer and supplies it to the catenary network. This is a typical application of single-phase transformers in high-power scenarios.

4. Single-phase grid connection in new energy power generation

In distributed photovoltaic systems, household photovoltaics (3~10kW) usually use single-phase grid-connected inverters, and the supporting single-phase step-up transformer connects the inverter output to the low-voltage distribution network.

Typical applications of three-phase transformers

1. Industrial manufacturing

Core equipment such as three-phase asynchronous motors, frequency converters, and rectifiers in the factory require three-phase power. The three-phase transformer is the "heart" of the factory power distribution room, reducing 10kV or 35kV high voltage to 380V/220V low voltage for use in the production line.

Typical configuration: SCB14-2000/10 Dyn11 dry-type transformer, 2000kVA capacity, 10kV high voltage side, 0.4kV low voltage side.

2. Commercial buildings

Large buildings such as shopping malls, office buildings, hospitals, and data centers have large electrical loads and contain a large number of three-phase equipment (central air conditioners, elevators, and water pumps). Three-phase transformers are standard equipment in power distribution systems.

Special requirements:

• Hospital: IT system using Dyn11 + isolation transformer to ensure continuity of power supply in key areas such as operating rooms

• Data center: adopts SCB18 dry-type transformer, first-class energy efficiency, low loss, and cooperates with UPS system

3. Grid transmission

The entire power transmission from the power plant to the user relies on three-phase transformers:

• Generator step-up transformer: Increase the generator outlet voltage (10~27kV) to 110kV/220kV/500kV

• Substation main transformer: realizes conversion between different voltage levels

• Distribution transformer: reduces 10kV to 0.4kV for consumer use

4. New energy power station

• Wind power generation: A three-phase box transformer (0.69kV/35kV) is installed in each wind turbine cabin to boost the power generated by the wind turbine and send it to the collector line.

• Photovoltaic power generation: centralized power stations use three-phase step-up transformers (inverter output 0.4kV/0.8kV → 35kV/110kV)

• Energy storage power station: The DC side of PCS (energy storage converter) is connected to the AC grid through a three-phase isolation transformer

IV. Special wiring method: use single-phase transformer to form a three-phase system

In some special scenarios, multiple single-phase transformers can be combined into a three-phase transformer group. Common wiring methods include:

* V/V wiring is a classic solution for electrified railway traction power supply: the high-voltage sides of two single-phase transformers are connected in a V shape (line voltage), and the low-voltage side supplies power to two sections of contact network respectively, which not only saves investment, but also achieves the independence of the two-phase power supply.

V. Selection Decision Guide

When faced with specific projects, decisions can be made quickly based on the following logic:

Conditions for selecting single-phase transformer

• The load is purely single-phase equipment (such as residential electricity, low-power electronic equipment)

• Capacity requirement ≤ 1000 kVA (above this value, three-phase economics is significantly dominant)

• Space is limited and three-phase equipment cannot be arranged

• Special applications: railway traction, single-phase experimental equipment, isolation transformer

Conditions for selecting three-phase transformer

• The load includes three-phase equipment (motor, frequency converter, rectifier)

• Capacity requirement ≥ 100 kVA (three-phase efficiency advantage starts to show)

• Requires both three-phase and single-phase power supply (Dyn11 coupling)

• Requirements for power supply continuity, balance, and harmonic suppression

• Most scenarios such as industry, commerce, power grid, etc.

VI. Clarification of common misunderstandings

Three common misunderstandings

1. "Single-phase transformers are simpler, so they are more reliable": The structure of single-phase transformers is indeed simple, but after a hundred years of development, the technology of three-phase transformers has become very mature. Under the same capacity, the failure rate of three-phase transformers is no higher than that of single-phase transformers, and the three-phase system has the advantage of redundancy (if one phase fails, it can operate at reduced capacity for a short period of time).

2. "Three-phase transformers cannot be used for single-phase loads": This is wrong. Dyn11 connected three-phase transformer, with star-shaped neutral point lead out on the low-voltage side, can easily output 220V single-phase power, and single-phase loads can be flexibly distributed among the three phases.

3. "The efficiency of a single-phase transformer must be lower than that of a three-phase transformer": In a small-capacity (<10kVA) scenario, the efficiency of a single-phase transformer may be higher than that of a three-phase transformer because there are fixed losses in the core and windings of the three-phase transformer. But after exceeding 50kVA, the material utilization advantage of three-phase transformers makes their efficiency comprehensively surpass that of single-phase.

VII. Summary

The relationship between single-phase transformers and three-phase transformers is not "who replaces whom", but each performs its own duties and complements each other:

• Single-phase transformers have the advantages of simple structure, low cost, and are suitable for small-capacity terminal power distribution. They are the "behind-the-scenes heroes" of residential electricity and electronic equipment.

• Three-phase transformers have high material utilization, good power supply balance, and are suitable for large-capacity industrial power distribution. They are the "mainstay" of modern power systems.

Only by understanding the structural differences and application scenarios between the two can we make the optimal choice in engineering practice - neither wasting investment nor burying hidden dangers in power supply.