What is a Distribution Transformer?

Introduction

A distribution transformer is a static electrical device used in power distribution systems that transfers AC power by transforming voltage and current based on the law of electromagnetic induction. It typically operates at voltage levels of 10–35kV (mostly 10kV and below) with a capacity of 6,300 kVA or less, supplying power directly to end users. As a core component of the “last mile” of power delivery, distribution transformers are found in residential areas, industrial facilities, commercial buildings, and rural power grids around the world.

In this article, we cover everything you need to know about distribution transformers — from how they work and their key components, to the different types available and how to choose the right one for your project.

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What is a Distribution Transformer?

1. Definition

A distribution transformer is a static electrical device in a power distribution system that transforms AC voltage and current to transmit AC power, based on the law of electromagnetic induction. In some regions, power transformers operating below 35kV (mostly 10kV and below) are referred to as “distribution transformers.” Locations where distribution transformers are installed are considered substations. Distribution transformers are typically mounted on poles or installed outdoors at ground level.

2. Difference from Power Transformers

The main difference between power transformers and distribution transformers lies in the voltage levels they handle.

Power transformers are used in transmission systems, operating at high voltage levels. Their primary role is to step down the high voltage delivered by the grid to levels suitable for power companies or industrial users. They generally do not have load regulation capability.

Distribution transformers operate in distribution systems at medium-to-low voltage levels. They step down the voltage from high-voltage lines to the lower voltages required by homes and businesses, and typically have some degree of load regulation capability.

Compared to power transformers, distribution transformers are smaller, lighter, and easier to relocate, making them better suited for installation in urban and residential areas.

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Basic Structure of a Distribution Transformer

A distribution transformer consists of three main components: the iron core, the windings, and the insulating oil. The windings form the electrical circuit, while the iron core forms the magnetic circuit. Together, they make up the core electromagnetic section of the transformer.

1. Iron Core

The iron core is one of the fundamental components of a transformer. It serves as the main magnetic circuit, providing both the primary magnetic flux path and the mechanical framework for the transformer body. It is typically made by stacking hot-rolled or cold-rolled silicon steel sheets with a thickness of 0.35 or 0.5mm and an insulating coating on the surface. The iron core is divided into two parts: the core limb (which holds the windings) and the yoke (which closes the magnetic circuit).

By structure:

• Core type — Simple structure, easy to manufacture, widely used.
• Shell type — More complex structure, used in small-capacity transformers and furnace transformers.

By material:

• Silicon steel sheets
• Amorphous alloy — Produced through ultra-rapid solidification, where atoms do not arrange into a crystalline structure. Transformers with amorphous alloy cores reduce no-load losses by around 75% and no-load current by around 80% compared to silicon steel core units. They are currently one of the most energy-efficient distribution transformer options available.

By assembly process:

• Stacked type
• Wound type

2. Windings

The windings are the electrical circuit part of the transformer, generally wound on a winding form using insulated flat copper wire or round copper wire. The windings are fitted onto the transformer core limbs, with the low-voltage winding on the inside and the high-voltage winding on the outside. Insulating sleeves are placed between the low-voltage winding and the core, and between the high-voltage and low-voltage windings. There are two basic winding arrangements:

(1) Interleaved (Disc) Winding

Both high- and low-voltage windings are divided into several disc coils arranged alternately along the height of the core limb. Commonly used in shell-type transformers.

(2) Concentric Winding

Both windings are made into cylindrical shapes with different diameters and placed coaxially on the core limb. Concentric windings can be further divided into:

• Cylindrical winding — The simplest type, wound continuously along the core height with interlayer insulation paper between layers. Generally used for low-voltage windings in small-capacity transformers.

• Helical winding — Multiple insulated flat conductors arranged in parallel radially, wound helically along the core axis, with each turn resembling a disc coil.

• Continuous winding — Multiple disc coils wound continuously with flat conductors, connected alternately at the inner and outer sides without any joints.

• Interleaved winding — Similar in appearance to continuous winding, but electrically adjacent turns are interleaved with other turns from the winding.

3. Bushings

Bushings are the insulating supports for the transformer’s lead-out conductors. They not only provide insulation between the conductors and ground, but also physically secure the lead-out conductors in place. Transformer bushings must have high electrical strength, mechanical strength, and good thermal stability.

4. Tap Changer

The tap changer is the voltage regulation component of a distribution transformer, used to keep the output voltage within a specified range. It works by changing the turns ratio of the primary and secondary windings to adjust the output voltage. There are two types:

• No-load tap changer (off-circuit) — Switches winding taps with the transformer de-energized. Typically operated manually.
• On-load tap changer — Allows voltage adjustment while the transformer remains in service without interruption.

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Common Types and Applications

Based on cooling method, distribution transformers are divided into oil-immersed and dry-type. Oil-immersed transformers use oil as the cooling medium — including oil-immersed self-cooled, air-cooled, water-cooled, and forced oil circulation types. Dry-type transformers use natural air convection or forced fan cooling, and are commonly used in high-rise buildings, highway toll stations, local lighting, and small-capacity electronic applications.

1. Oil-Immersed Distribution Transformer

Oil-immersed distribution transformers are key equipment in power supply and distribution systems for industrial, mining, and civil construction applications. They step down network voltages of 10(6)kV or 35kV to the 230/400V busbar voltage used by end users. These products are suitable for 50(60)Hz AC, with a maximum three-phase rated capacity of 2,500 kVA. They can be used indoors or outdoors, and units of 315 kVA or below can be pole-mounted.

Operating conditions:

• Ambient temperature: −25°C to +40°C
• Relative humidity: not exceeding 90% (at 25°C)
• Altitude: not exceeding 1,000m

2. Dry-Type Distribution Transformer

Dry-type transformers are widely used in local lighting, high-rise buildings, airports, docks, CNC machinery, and similar applications. The core and windings are not immersed in insulating oil. Cooling methods include:

• Natural air cooling (AN) — The transformer can run continuously at rated capacity for extended periods.
• Forced air cooling (AF) — Output capacity can be increased by 50%. Prolonged continuous overload should be avoided as it drives the unit into an uneconomical operating state.

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Technical Parameters

The main technical parameters of a distribution transformer include:

• Number of phases
• Rated frequency
• Rated capacity (kVA)
• Rated voltage
• Rated current
• Impedance voltage
• Load loss
• No-load current
• No-load loss
• Connection group

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How to Get a High-Quality Distribution Transformer?

When it comes to sourcing a reliable distribution transformer, choosing the right manufacturer makes all the difference. At CHBEB, we have been designing and producing distribution transformers for decades, supplying both China’s State Grid and customers across the globe.

Our transformers are built to meet the strict procurement standards of State Grid Corporation of China — one of the world’s largest and most demanding power networks. This long-term partnership is a direct reflection of our product quality, manufacturing consistency, and engineering capability.

Over the years, we have also expanded our reach to international markets, delivering distribution transformers to utilities, industrial facilities, and infrastructure projects in Asia, the Middle East, Africa, Europe, and beyond. Every unit we produce goes through rigorous testing and quality control before it leaves our factory.

Whether you need an oil-immersed transformer for outdoor substation use, a dry-type unit for indoor installation, or a custom solution built to your exact specifications, CHBEB has the experience and capacity to deliver.

Explore our distribution transformer product range:

• Oil-Immersed Distribution Transformer
• Dry-Type Transformer
• Pad-Mounted Transformer
• Substation Transformer
• Isolation Transformer
• Pole-Mounted Transformer

Contact us today to discuss your project requirements or request a free quote.