Comprehensive Guide to Distribution Transformers

This comprehensive guide is your go-to resource for understanding distribution transformers, vital components in the power transmission and distribution network. We delve into the working principle, different types like the pole mounted and 3-phase distribution transformer, their ratings, diagrams, key manufacturers, and the essential parts making up these complex devices. As we explore these various facets of distribution transformers, you’ll gain a deep appreciation for their critical role in stepping down high voltage power to usable levels in commercial and residential applications.

ultimate distribution transformer guide

Liquid-Filled Radial Feed Pad Mounted Transformer

Rated Power:75~2500 kva No-load Loss:180~2680W On-load Loss:1250~27786W

Liquid-Filled Loop Feed Pad Mounted Transformer

Primary Voltage Ratings: 34.5-19.92/13.8-7.957/13.2-7.62/12.47-7.2 or others

Pole Transformer

TypeCSP type Frequency: 50/60Hz; Rated Power: 5~167kva

Single Phase Pad mounted transformer

Rated Power:15~167 kva No-load Loss:50~350W On-load Loss:195~1410W

If you have been looking for a transformer, chances are, you might have heard of distribution transformers, which is one of the most widely used transformers in the world for many reasons such as easy installation of distribution transformer, and their useful distribution transformer working principle that is supported by the distribution transformer parts.

However, with so many distribution transformers available in the market, it is hard to find which one is suitable for you. For that specific reason, DAELIM, one of the top distribution transformer manufacturers in the world, will help you make the right purchase decision by assisting you with what you need to know about this electrical device.

For starters, it is vital to know what are distribution transformers first before going through their mechanism, functions, parts, installation, and many more.

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-The calculation formula of transformer transformation ratio, the voltage at both ends of the loaded primary winding must be the rated value.

What is a Distribution Transformer?

Basically, a distribution transformer is also known as a common kind of isolation transformer, and its main function is to alter the high voltage of the transformer from normal voltages like 240/120 volts to use in electric power distribution/s.

When it comes to the distribution system, there are several types of transformers out there. Including but not limited to single-phase transformers, three-phase transformers, pad-mounted transformers, pole-mounted transformers, and many more.

In general, these types of transformers are usually available in various sizes with different levels of efficiencies, and there is a difference in their insulating oil as well. The selection of these transformers relies on the equipment and also your budget.

Basically, there are four kinds of distribution transformer connections out there. These are the star-star connection, delta-delta connection, star-delta connection, delta-star connection, and ZigZag or delta Zigzag.

rural distribution transformers
rural distribution transformers

How are Distribution Transformers Designed?

In terms of the design of the transformer, the distribution transformer is closely manufactured the same way small-size transformers are done.

This means that the main distribution transformer parts mainly include an oil tank, oil indicator, breather unit, Buchholz relay, conservator, temperature detector, thermal relay, bushings, pressure relief device, and several more.

This leads us to the functions of these distribution transformer parts.

distribution tranformer designed
distribution tranformer designed

What are the Parts of a Distribution Transformer?

Just like other types of transformers, distribution transformers consist of several components such as:

Oil Tank

The oil tank’s primary function is to soak the windings by it inside.

Moreover, it is also used in oil-filled transformers.

There are even some types of high-voltage capacitors, lamp ballasts, fluorescent, and high-voltage switches and circuits breakers that’s purpose is to insulate, suppress corona arcing and discharging.

Lastly, it can also serve as a coolant.


A transformer’s conservator is formed above the oil tank, specifically outside the transformer’s frame.

Moreover, it is also connected to the main tank with the help of a metallic tube.

The oil inside the tank can be easily enlarged and contacted if there is a need to.

It is accessible throughout the loading to adjust the temperatures whether to increase or decrease.

Buchholz Relay

The Buchholz relay is primarily used when a conservator tank is being used since it will indicate errors like loss of oil as it goes low.

This also involves the improper flow of oil between the tank and the transformer itself.

In other words, in electric power distribution and transmission, a Buchholz relay is considered a safety device that can be mounted on some oil-filled power transformers and as well as reactors that are equipped with an external overhead oil reservoir or also known as a conservator.

For the generic designation of this type of device, it is known as a detector relay.

Breather Unit

A breather unit involves a silica gel that is responsible for absorbing moisture in the oil.

Moreover, it changes its color from blue to pink if it is not capable of absorbing the moisture in the oil.

In simpler terms, a transformer’s breather helps prevent atmospheric moisture from occurring.

Furthermore, this reduces maintenance costs.

For conservator-style transformers, this movement of the oil level will cause what is known as a transformer breathing cycle.

This happens as air is brought into the tank and expelled again later.

Oil Indicator

The oil indicator is basically what indicates the level of the oil within the conservatory unit.

In other words, if air bubbles remained in the transformer, then it is expected that gas will be generated due to an internal failure or if there is a leakage at the transformer tank.

Temperature Detector

As the name suggests, the temperature detector of a transformer is responsible for monitoring the temperature of the oil.

This means that if the temperature of the oil were to increase to a certain level, the transformer will be automatically disconnected from the service.

Pressure Relief Device

The function of the pressure relief device is to decrease the pressure within the transformer to avoid an explosion or fire from occurring inside the transformer.

The P.R.D is also used to avoid high oil pressure builds inside the transformer during faulty conditions.

It is fitted on top of the main tank.

Basically, the P.R.D enables the rapid release of excessive pressure that is generated in the event that there is a serious fault.

Thermal Relay

On the other hand, the thermal relay is used as an indicator for the temperature of the winding.

The thermal relay is also considered a protective device for the transformer since it protects it from overloads.

This type of protection generates the temperature of the transformer’s windings.

Moreover, the simulation is also based on the measure of the current and on the thermal time constant of the transformer.


In simple terms, a radiator of a transformer enables faster cooling rates.

This means that it plays a vital role in regulating the temperature of the transformer or increasing the loading capacity.

This is the basic function of a radiator of both power transformers and distribution transformers.

In short, the radiator is used to increase the transformer’s cooling efficiency.


The main purpose of the bushing for transformers is for connecting the internal windings of the transformer with the help or assistance of an exterior electrical network.

A transformer bushing is also an insulating structure that facilitates the passage of an energized, conductor that is also carrying a current through the grounded tank of the transformer.

The two principal types of bushing constructions are solid or bulk type and capacitance-graded, and they are also sometimes called condenser type.

Parts of a Distribution Transformer
Parts of a Distribution Transformer

What is an Isolation Transformer?

As mentioned, isolation transformers like distribution transformers are used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device in front of the power source for safety reasons.

What type of transformer is used in distribution?

According to the way it is being used or its application, these transformers are categorized into several kinds such as single-phase transformer, three-phase transformer, pad-mounted transformer, pole-mounted transformer, and underground transformer just to name a few.

Single-phase Distribution Transformers

Single-phase distribution transformers are specifically designed for networks wherever a three-phase supply is not available or required.

In a normal situation, these are specially used for repairing overhead distribution loads in residential.

These are also applicable in industrial lighting, light commercial loads, and as well as power applications.

pole-mounted transformer and single phase transformer
pole-mounted transformer and single phase transformer

Three-phase Distribution Transformers

When it comes to three-phase distribution transformers, this kind of transformer is commonly used for holding electrical energy from the main distribution circuit to a minor distribution circuit. Moreover, this type of transformer transmits currents to a secondary distribution circuit that also reduces the voltage of the primary distribution circuit.

These transformers also reduce the voltage supply for the primary circuit based on the consumer requirement. It is expected that the voltage always changes and it is dissimilar for the users of commercial, residential, and also in the light industry.

These transformers work on different levels of voltage and frequency that is based on the standards of existing countries. These transformers are also widely available in both single-phase and three-phase ones.

Single-phase transformers are commonly found in residential applications. On the other hand, three-phase distribution transformers are used with pad underground primary circuits.

Pad-mounted Transformers

Aside from single-phase and three-phase pad-mounted distribution transformers, this type includes a locked steel cupboard that is arranged on a concrete pad.

This type of transformer is also installed in places where they do not have space for a fenced enclosure.

Moreover, this transformer is also used with an electric power distribution line at an overhead electrical line for reducing the primary voltage to supply for the customers.

A single transformer of this type can without a doubt serve many homes and even a large building.

The usual power rating of this transformer usually ranges from 75 kVA to 5000 kVA, and this also includes fixed switches and fuses.

Single Phase and Three Phase Padmounted Transformer From Daelim
Single Phase and Three Phase Padmounted Transformer From Daelim

Pole-mounted Transformers

Aside from pad-mounted transformers, there is also a pole-mounted transformer, which is the most common transformer that can be seen in the public since it is safe for displaying outdoors but above-ground only.

This type of distribution transformer can be seen mounted on an electrical service pole at the height of overhead cables.

These are used for changing high distribution voltages to low, usually around 120/240-volt power.

These types of transformers are also used in a wide rural area that usually ranges from 16 kVA to 100kVA.

They can be seen available in small sizes and are easy to fit on single-pole structures.

These transformers are very reliable in harsh climates since they have a strong build that is specifically designed to withstand various climates.

The tanks of the transformer can be shaped and protected from corrosion-resistant materials.

The design is also to avoid the accumulation of water.

For coastal areas, the tanks can still be protected from zinc spray.

On the other hand, high corrosive regions will require stainless steel tanks for maximum protection.

Pole-mounted Transformers
Pole-mounted Transformers

How are Pole-mounted Transformers Used?

Pole-mounted distribution transformers have several applications, such as their ability to change voltages from high to low (for consumer use).

But its main or primary function is for stepping down the voltage to provide isolation between the two windings (primary and secondary).

Moreover, this type of transformer also has the ability to distribute power even in the most remote areas which are generated from power plants.

In general, this distribution transformer has the ability to distribute electricity to industries with less voltage that are under 33kV and 440 volts to 220 volts for domestic purposes.

What is the Working Principle of Distribution Transformers?

When it comes to the distribution transformer working principle or the process of how they work, a distribution transformer is also known as a service transformer that is known for providing the final voltage transformation in the electric power distribution system, stepping down the voltage that is being used in distribution lines to the level being used by the customer.

How to Install Distribution Transformers?

When it comes to the installation of distribution transformers they are fairly easy to install.

However, it is highly recommended that you let the professionals do the work.

For professional help, advice, and assistance, contact DAELIM’s team for immediate accommodation.

This is to fully inspect and secure the transformer to see if it is ready to operate or not.

Moreover, the installation costs are only low since distribution transformers are much easier to install compared to other types of transformers.

What is a distribution transformer and what are its uses?

A distribution transformer, also known as a distribution power transformer, is a crucial component in the electrical power distribution system. The main role of these transformers is to lower the voltage supplied from a transmission system to a level that is usable in residential and commercial applications.

The typical application of a distribution transformer is seen in the final phase of power distribution. Here, it helps in stepping down the voltage to the desired level which is safe for use in homes, businesses, and other places. This process happens in multiple steps. Initially, power from power plants is transmitted at a very high voltage to minimize power loss. This high voltage is then brought down in stages through a series of transformers before it finally reaches the end consumer.

Distribution transformers are crucial for maintaining the reliability and efficiency of the power grid. Their ability to step down voltages to safe and usable levels is critical for electrical appliances and devices. Without these transformers, the power supplied would be too high, leading to damage and potential safety hazards.

There are various designs of distribution transformers, including pole-mounted, pad-mounted, and underground transformers. Pole-mounted transformers are common in residential areas, while pad-mounted and underground transformers are typically used in commercial and industrial settings. Each of these designs caters to specific needs and circumstances.

Apart from residential and commercial uses, distribution transformers also play an essential role in industrial settings. They provide the required voltage for different industrial applications, ranging from manufacturing to automation.

Another essential application of a distribution transformer is in the isolation of different parts of the electrical grid. This isolation helps protect equipment and individuals from electrical faults, thereby enhancing the overall safety of the power system.

Moreover, distribution transformers also aid in load management. They can be utilized in a smart grid system for efficient load management, thereby leading to energy savings and improved grid reliability.

Lastly, distribution transformers contribute significantly to renewable energy systems. They are vital for integrating distributed energy resources like solar and wind power into the grid.

What are the different types of distribution transformers?

There are several types of distribution transformers, each serving specific needs and purposes in the electrical power system. The main types include single-phase transformers, three-phase transformers, pad-mounted transformers, pole-mounted transformers, and underground transformers.

Single-phase transformers are common in residential areas and small commercial buildings. They are typically used for lighting and heating purposes. A single-phase transformer only has two wires— the primary and secondary— and can handle voltages up to 120V.

Three-phase transformers, on the other hand, are commonly used in industrial and large commercial buildings. They handle high-voltage power and distribute it evenly across three different phases. This ensures more efficient and reliable power supply, as the power load is spread across the phases.

Pad-mounted transformers are ground-mounted units that are often found in urban and suburban areas. They are designed for high-voltage applications, typically serving commercial and industrial facilities. These transformers are enclosed in a locked steel cabinet, providing an added layer of security.

Pole-mounted transformers are installed on utility poles and are a common sight in residential areas. They serve individual homes or small groups of homes, and help in stepping down the voltage for residential use.

Underground transformers are used where overhead distribution lines are not feasible, such as in densely populated areas or places with severe weather conditions. They are placed in a vault underground, making them less prone to damage and outages.

Each type of distribution transformer is designed to meet specific electrical needs and environmental conditions. Their designs and capacities vary depending on the application, making them a versatile and essential part of the power distribution system.

How Does a Distribution Transformer Work?

Understanding the working principle of a distribution transformer involves comprehending the basic principles of electromagnetic induction. A distribution transformer operates on Faraday’s law of electromagnetic induction, which states that a change in the magnetic field within a closed loop of wire induces an electromotive force (EMF) in the wire.

In the case of a distribution transformer, the primary and secondary windings are wound around a common iron core. When alternating current (AC) is passed through the primary winding, it generates a changing magnetic field around the winding due to the nature of AC. This changing magnetic field is linked with the secondary winding through the iron core.

The secondary winding, being subject to this changing magnetic field, experiences induced EMF according to Faraday’s law. The amount of voltage induced in the secondary winding depends on the turns ratio of the primary to the secondary windings. If there are fewer turns in the secondary winding compared to the primary, the transformer steps down the voltage, which is typically the case in distribution transformers.

The distribution transformer, thus, enables the conversion of high-voltage power into a lower voltage suitable for home or commercial use without any moving parts. This conversion happens efficiently with minimum energy loss, thanks to the core materials used and the design of the transformer.

In three-phase distribution transformers, the principle remains the same, but it happens simultaneously for all three phases. Three separate but connected coils in the transformer handle this transformation. This three-phase power is crucial for heavy machinery and equipment in industries that require a steady, continuous power supply.

To ensure safety and minimize losses, these transformers are designed with insulation systems and cooling methods. These designs vary based on the size, type, and application of the distribution transformer.

How are distribution transformers rated?

Distribution transformers are rated based on a number of factors including their maximum output (kVA), voltage levels, frequency, phases, insulation type, and cooling method. The most common ratings are for power and voltage.

The power rating of a distribution transformer is expressed in kVA (kilo Volt Amperes). It signifies the maximum load that the transformer can handle without exceeding its specified temperature limit. This power rating is crucial as it defines the capacity of the transformer and helps in determining the right size of the transformer for specific applications.

Voltage rating is another vital parameter for a distribution transformer. It represents the input (primary) and output (secondary) voltages of the transformer. For instance, a common rating for a transformer in the US might be 13,200/480 V, indicating that the transformer steps down voltage from 13,200 volts to 480 volts.

In addition to power and voltage ratings, the frequency at which the transformer operates is also specified. Most countries operate at a frequency of either 50 or 60 Hertz. The transformer must be designed to operate efficiently at this frequency.

Phases are another crucial rating factor for distribution transformers. They can either be single-phase or three-phase transformers, which refers to the type of power input they can handle.

Lastly, the type of insulation and cooling methods used in the transformer also form a part of its rating. Common cooling methods include oil-filled and dry-type transformers, which influence the transformer’s efficiency and safety.

What are the key parts of a distribution transformer?

A distribution transformer is made up of several key parts, each contributing to its functionality and efficiency. Here are the main components:

  • Core: The core of a distribution transformer is typically made of laminated sheets of steel. This core provides a controlled path for the magnetic flux generated in the transformer.

  • Windings: Windings, made of copper or aluminum wires, are coiled around the core. There are two types of windings in a distribution transformer: the primary winding, which receives the input voltage, and the secondary winding, which delivers the output voltage.

  • Insulation: The windings are insulated from each other and from the core to prevent electrical short circuits and provide safety. Different materials like oil and paper are used for this purpose.

  • Cooling System: Transformers generate heat due to electrical losses. Therefore, they are equipped with cooling systems to dissipate this heat and keep the transformer temperature within safe limits. Oil-filled transformers use the oil for cooling, while dry-type transformers use air.

  • Bushings: Bushings are insulating structures that allow conductors to pass through the transformer tank (in case of oil-filled transformers). They isolate the conductive parts from the tank and prevent any leakage current from flowing to the earth.

  • Conservator and Breather: In oil-filled transformers, a conservator and breather are used to compensate for oil level variations due to temperature changes. The breather also keeps the moisture out of the transformer.

  • Tap Changer: A tap changer is used to regulate the output voltage of the transformer by changing the turns ratio of the windings. This allows the transformer to adjust to varying loads and maintain a constant voltage output.

  • Transformer Oil: In oil-filled transformers, transformer oil serves multiple purposes. It provides insulation, aids in cooling, and prevents oxidation of the core and windings.

Each of these parts plays a crucial role in the functioning and efficiency of a distribution transformer.

What are the top manufacturers of distribution transformers?

The manufacturing landscape of distribution transformers is vast and varied, with several key players dominating the market. These manufacturers are known for their technological prowess, product quality, and innovative designs. Among the leading manufacturers of distribution transformers are ABB, Siemens, General Electric, Schneider Electric, and Eaton Corporation.

  • ABB: ABB is a global leader in power and automation technologies. They offer a wide range of distribution transformers, including liquid-filled and dry-type transformers. ABB transformers are known for their efficiency, reliability, and environmental friendliness.

  • Siemens: Siemens provides a broad portfolio of distribution transformers suited to various applications. Their transformers are designed for reliability, durability, and efficiency, ensuring optimal performance and longevity.

  • General Electric: GE offers an array of transformer solutions, including single-phase, three-phase, pad-mounted, and pole-mounted transformers. Their products are designed for energy efficiency and cost-effectiveness.

  • Schneider Electric: Schneider Electric’s offer includes oil-immersed and resin transformers. Their transformers are designed for safety, reliability, and efficiency, catering to the diverse needs of industrial, commercial, and utility customers.

  • Eaton Corporation: Eaton provides a variety of transformers, including low voltage, medium voltage, and specialty transformers. Their products are designed for high performance, safety, and longevity.

These manufacturers continually invest in research and development to innovate and enhance their product offerings, ensuring they meet the changing needs and demands of the power distribution sector.

How is a distribution transformer diagram used in understanding its working?

A diagram of a distribution transformer provides a visual representation of its internal structure and functioning. It typically includes the core, windings, and other components like the cooling system, insulation, bushings, and tap changer.

To better understand, here’s a basic breakdown of a distribution transformer diagram:

[Primary Winding]--------[Secondary Winding]
        |                           |
       ( ) Core                    ( ) Core
        |                           |
        | Insulation                | Insulation
        |                           |
    [Tap Changer]               [Output Terminals]
  1. The Primary Winding receives the input high-voltage from the power grid.
  2. The Secondary Winding delivers the output low-voltage to the households or businesses.
  3. The Core provides a path for the magnetic flux generated in the transformer.
  4. The Insulation prevents electrical short circuits between the windings and the core.
  5. The Tap Changer adjusts the turns ratio of the windings to regulate the output voltage.
  6. The Output Terminals are where the transformed power is taken to be distributed to the end users.

Remember, this is a simplified representation of a distribution transformer. The actual components and their arrangements can be more complex, and there can be additional parts like the cooling system and protective devices depending on the transformer type and its applications.

Closing Thoughts

The role of a distribution transformer in the power transmission and distribution network is of paramount importance. Its function to step down high-voltage power to a level suitable for commercial or residential use is integral to our daily lives. By understanding the types, components, working principles, and manufacturers of distribution transformers, we can better appreciate their significance and the innovation behind these essential devices. Remember that safety and efficiency are key in the operation of these transformers, and they have been designed and built keeping these factors at the forefront. With the continuous innovation in the power distribution sector, the evolution of distribution transformers continues to meet the changing needs of the industry.

Final Thoughts

Whether you have a small or big project for your purpose of needing a distribution transformer, there is without a doubt that this type of transformer is perfect for you since it has the ability to supply electricity to multiple houses, and does not have a problem of supplying electricity to a single building.

If you have any questions or concerns you would want to raise, do not hesitate to contact DAELIM’s team of professionals for immediate assistance.

DAELIM aims to help you make the best purchase decision and establish a successful project for you.

About Daelim

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About Bin Dong

Hello, I am Bin, General manager of Daelim which is a leading transformer manufacturer. If you have problems when you are looking for the equipment, what you need to do is tell us.

Custom Distribution Transformer

If you find that the existing distribution transformer types or power cannot meet your requirements. You can choose to tell Daelim. Daelim has a team that has always had a wealth of design transformers, and can give you a specific design plan in the shortest time.

Daelim’s distribution transformers comply with IEEE, ANSI, CSA, IEC certification, and are used in North America (such as Canada, the United States, Mexico), South America (such as Ecuador, Chile), Europe (such as Spain, Lithuania) and some Asian countries. Daelim even has a professional installation team that can provide you with installation services.