Distribution Transformers: Working Principles, Types and Parts

Distribution Transformers: Working Principles, Types and Parts

Transformers are indispensable devices, facilitating the transfer of electrical energy between circuits through a process known as electromagnetic induction. Distribution transformers are used extensively in the primary distribution of electricity to secondary distribution circuits. They act as isolating devices to prevent power surges and circuit overloads. With features like bushings, fuses, and insulation, distribution transformers enhance the reliability of the distribution network, serving as vital components in the power distribution system. This article delves into the operational principles, various types, and crucial components of distribution transformers, highlighting their significance and the technical nuances in their functionality in modern power systems.

Working Principles of Distribution Transformers

The working principle of a distribution transformer is based on electromagnetic induction, a fundamental concept first discovered by Michael Faraday. Here’s how it works:

  1. Electromagnetic Induction: Distribution transformers operate on the principle of electromagnetic induction, which states that a voltage can be induced in a conductor by a changing magnetic field. The transformer consists of two or more coils—known as windings—wound around a common magnetic core.
  2. Core and Windings: The core of the transformer is made from high-permeability silicon steel, which helps to efficiently channel the magnetic field. The primary winding of the transformer is connected to the high-voltage power source and carries the input power, creating a magnetic flux. The secondary winding encircles the same core and is exposed to this alternating magnetic flux generated by the primary winding.
  3. Inducing Voltage: As the alternating current (AC) flows through the primary winding, it creates a changing magnetic field around it. According to Faraday’s Law of Electromagnetic Induction, this changing magnetic field induces an electromotive force (EMF) or voltage in the secondary winding. The amount of voltage induced in the secondary winding is proportional to the number of turns in the windings and the intensity of the magnetic flux.
  4. Voltage Transformation: The key to transforming voltage levels in a transformer lies in the ratio of the number of turns of wire in the primary winding to the number of turns in the secondary winding. If the secondary winding has fewer turns than the primary, the transformer reduces the voltage and increases the current, making it a step-down transformer. Conversely, if the secondary has more turns, the transformer increases the voltage and decreases the current, functioning as a step-up transformer. Distribution transformers typically function as step-down units, reducing high transmission voltages to lower levels suitable for residential, commercial, or industrial use.
  5. Energy Transfer Efficiency: Transformers are generally very efficient, achieving maximum efficiency between 95% to 99%. This high efficiency is achieved through careful design to minimize losses due to heat in the windings, eddy currents in the core, and magnetic hysteresis (energy lost due to the magnetic properties of the core).

This principle allows for the widespread distribution of electricity over long distances, minimizing energy loss and maximizing delivery efficiency.

Types of distribution transformers

Distribution transformers can be categorized based on several factors:

Based on Cooling Methods

Dry-Type Transformer These transformers do not use liquid for cooling and are instead cooled by air circulation. Dry-type transformers are highly favored in environments where fire safety is a major concern, such as indoors, in underground installations, or near residential areas. They are environmentally safer and easier to maintain than oil-filled transformers, but generally have a lower overload capability and are more costly.

Oil-Immersed Transformer: Oil-immersed transformers use oil both as an insulator and coolant. This type provides better insulation and effective cooling, allowing the transformer to be smaller in size and cost-effective compared to dry-type transformers. However, they require stringent maintenance to handle potential oil leaks and fire hazards, making them less ideal for indoor or environmentally sensitive installations.

Based on Installation Environment

Pole-Mounted Transformer: These transformers are installed on utility poles, typically at a height to keep them out of reach. They are designed for rural and suburban overhead distribution where space is not constrained. The elevation helps in cooling by air and protects against vandalism. These are generally single-phase transformers due to their application in less densely populated areas.

Pad-Mounted Transformer: Suited for urban areas, pad-mounted transformers are placed on a concrete pad and enclosed in tamper-proof metal cabinets. They are commonly used where underground power distribution is prevalent, offering both safety and aesthetic benefits by hiding the transformer components from public view. These are often three-phase transformers to cater to higher load demands in commercial zones.

Submersible Transformer: These are specially designed for operation in conditions where water exposure is possible, such as flood-prone areas or within underground networks. Submersible transformers are robust, completely sealed, and capable of operating while submerged, addressing unique environmental challenges without compromising the reliability of power distribution.

Based on Phase Configuration

Single-Phase Transformer: These are typically used in residential settings where the demand for electricity is relatively low and distributed over a wider area. Single-phase transformers are simpler in design and cost-effective, suitable for stepping down the voltage for home use from the nearest distribution line.

Three-Phase Transformer: In commercial and industrial settings where large amounts of electrical power are required, three-phase transformers are essential. They distribute power more smoothly and efficiently, providing a continuous power flow that is ideal for heavy-duty machinery and commercial systems. These transformers are more complex and expensive but necessary for balancing and handling high power demands.

Each transformer type is tailored for specific operational needs, balancing factors like cost, safety, environmental concerns, and power handling capabilities. These variations allow for flexibility in addressing the unique demands and constraints of different electrical distribution networks.

Parts of a Distribution Transformer

To understand how a distribution transformer functions, it is essential to know its key components:


Windings in a distribution transformer are usually made of copper or aluminum. Copper is often preferred for its higher efficiency and greater conductivity, although it is more expensive than aluminum. These conductive coils are crucial for the transformer’s function, tightly wound around the core to facilitate the induction process required for voltage transformation.

The primary windings are connected to the high voltage input lines and are typically made of thicker wire or strip to handle higher currents and electrical stresses. The secondary windings, which are connected to the output that delivers electricity at a usable voltage, may use thinner wire, as they carry the electric current at reduced voltages. The ratio of turns between the primary and secondary windings determines the voltage transformation ratio of the transformer, crucial for ensuring the voltage is adjusted correctly to meet the needs of the power distribution system.


The core is generally made from layers of laminated silicon steel, which helps in reducing eddy currents—undesired localized currents that can cause additional heating. This lamination and the particular grain orientation of the steel minimize hysteresis losses (energy lost due to the magnetic properties of the material). The shape of the core (rectangular, shell type) also influences the efficiency and performance of the transformer.


Insulation in transformers must be able to withstand thermal, electrical, and mechanical stresses. Materials like oil-impregnated paper and resin are commonly used because they have good dielectric properties and durability. Proper insulation is vital for preventing short circuits between windings or between windings and the core.

Tap Changer

The tap changer adjusts the voltage ratio between the primary and secondary windings to regulate the output voltage, accommodating changes in load or input voltage. This is crucial in maintaining consistent voltage levels despite fluctuations in demand or supply. Mechanical wear and electrical erosion can affect the tap changer’s performance, hence the need for regular inspections and maintenance.


The outer tank provides physical protection and containment for the transformer’s internal components. It is designed to be completely sealed or to have controlled breathing capabilities to manage internal pressures. The tank is also grounded to prevent electrical hazards and is usually painted to withstand environmental impacts like rust and UV degradation.


Bushings must handle the high voltages and environmental conditions they are exposed to. They are made from porcelain or composite materials and are carefully designed to manage the electric field distribution. Failure of a bushing can lead to catastrophic damage, so they are built to strict standards and are regularly tested.


The breather’s role in maintaining the moisture balance in the oil is critical to preserving the oil’s insulating properties. As the transformer heats and cools, the breather acts to keep the internal atmosphere dry. The silica gel in the breather turns from blue to pink as it absorbs moisture, indicating when it needs to be replaced.


The conservator tank adjusts its volume with oil temperature changes to accommodate expansion or contraction and prevent pressure buildup. This helps in maintaining a stable oil level in the main tank, which is crucial for optimal insulation and cooling. Some modern transformers use bladder-type conservators to completely isolate the oil from the external air.

Cooling Fins or Radiators

These metal fins or radiators increase the surface area of the tank, enhancing the heat dissipation process. They are particularly vital in larger transformers, where heat generation is significant. In some designs, fans or pumps are added to force the circulation of air or oil, respectively, to improve cooling efficiency further.

For a comprehensive guide, check out our article about all transformer parts.

Installation of Distribution Transformers

  1. Site Selection: Choosing the right location for a distribution transformer is crucial for efficiency and accessibility. It should be near the load to minimize energy losses in distribution lines and accessible for maintenance. Consideration of environmental factors like flooding or pollution is also essential, especially for transformers in rural or urban areas.
  2. Foundation and Mounting: Distribution transformers can be mounted on poles or placed on concrete pads (pad-mounted). Pole-mounted transformers are elevated, which aids cooling and reduces tampering risks. Pad-mounted transformers require a solid, level concrete foundation to ensure stability and protect against ground moisture and physical damage.
  3. Connection and Commissioning: Ensuring correct and safe electrical connections is vital. This involves connecting high-voltage lines to the primary winding and low-voltage lines to the secondary winding, along with proper grounding to prevent electrical hazards. Commissioning tests check for correct phasing, voltage output, and safety compliance.
  4. Safety and Compliance: Compliance with local electrical safety codes and standards is mandatory. This includes maintaining proper clearance from structures and ensuring all safety devices are installed, such as fault circuit indicators and surge arresters.

Feel free to check out our installation article for a comprehensive guide.

Maintenance of Distribution Transformers

  1. Routine Inspections: Inspectors look for signs of wear, oil leaks (in oil-immersed transformers), and damage to external components such as bushings and casing. Regular cleaning to remove dirt and debris that could impede cooling efficiency is also important.
  2. Oil Management (for oil-immersed types): Regular testing of the transformer oil for dielectric strength and contamination is critical. Oil should be sampled and tested at least annually, and if necessary, oil filtering, degassing, or replacement should be carried out to maintain insulation properties and heat transfer efficiency.
  3. Thermal Imaging: This is particularly useful for identifying overheating components within the transformer. Overheating can be an indicator of internal faults, overloading, or insulation failure. Thermal imaging helps in taking preemptive maintenance actions before major failures occur.
  4. Electrical Testing: Electrical tests such as insulation resistance, turn ratio testing, and power factor testing help ensure the internal components of the transformer function correctly. These tests are often performed during scheduled maintenance shutdowns to minimize disruption.
  5. Load Monitoring and Analysis: Monitoring the load on distribution transformers helps in understanding their performance and efficiency under actual operating conditions. This data is crucial for assessing whether a transformer is appropriately sized for its current or future load requirements, potentially preventing overloading and promoting energy efficiency.
  6. Preventive and Predictive Maintenance: Based on inspection and test data, preventive maintenance schedules can be established to replace parts or perform repairs before failures occur. Advanced predictive maintenance techniques can utilize sensors and data analytics to predict transformer failures, optimizing maintenance resources and scheduling.


Distribution transformers are critical for the seamless operation of electrical distribution networks, adapting high-voltage electricity from power stations to usable levels for residential and commercial use. Understanding the different types, components, and installation and maintenance practices of these transformers is essential for ensuring their optimal performance and longevity. As the demand for electricity grows and technologies evolve, the role of distribution transformers will continue to expand, necessitating ongoing advancements in their design and application to meet future energy needs effectively and sustainably.

At TTES, we have over 100 combined years of experience manufacturing, maintaining, and repairing large distribution transformers. We can take care of all concerns related to large distribution transformers. If you’re looking for industry-leading lead times of just 20 weeks on average, don’t hesitate and reach out to us for a free quote!

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