11/07/2025
Construction of Distribution Transformer
The design of a distribution transformer is similar to that of smaller transformers. The main components of a distribution transformer include the oil tank, conservator, Buchholz relay, breather unit, oil indicator, temperature detector, pressure relief device, thermal relay, radiator, and bushing.
Oil Tank: This tank is used to immerse the windings, providing insulation and cooling.
Conservator: Positioned above the oil tank outside the transformer frame, it is connected to the main tank via a metallic tube. The oil within the conservator expands and contracts with loading, causing the temperature of the oil to rise and fall.
Buchholz Relay: Used in transformers with a conservator tank, this relay detects faults such as oil loss, and improper oil flow between the tank and transformer, and provides an early warning system.
Breather Unit: Contains silica gel that absorbs moisture from the oil. The gel changes color from blue to pink when it can no longer absorb moisture.
Oil Indicator: Shows the level of oil within the conservator unit.
Temperature Detector: Monitors the oil temperature and disconnects the transformer from service if the temperature rises beyond a certain threshold.
Pressure Relief Device: Reduces internal pressure to prevent transformer explosions.
Thermal Relay: Indicates the winding temperature.
Radiator: Enhances the cooling efficiency of the transformer.
Bushing: Connects the internal windings of the transformer to the external electrical network.
These components work together to ensure the efficient operation, safety, and reliability of the distribution transformer.
Distribution transformers consist of a magnetic core made from laminations of sheet silicon steel (transformer steel), which are either glued together with resin or banded with steel straps. The primary and secondary wire windings are wrapped around this core. This core construction is specifically designed to minimize core losses, which are the dissipation of magnetic energy as heat within the core. These losses are a significant cause of power loss in utility grids and are economically important.
Core losses are attributed to two main effects: hysteresis loss in the steel and eddy currents. Silicon steel is chosen for its low hysteresis loss properties, and the laminated construction prevents the formation of eddy currents in the core, which would otherwise dissipate power due to the resistance of the steel. The efficiency of typical distribution transformers ranges between approximately 98% and 99%.
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Types of Distribution Transformer
Distribution transformers can be broadly classified based on mounting location, insulation type, number of phases, etc.
Pole-mounted: Installed on utility poles for overhead distribution lines in residential areas.
Pad-mounted: Bolted onto outdoor concrete pads for underground systems with a locked metal housing.
Single-phase: Connected to a single phase of the three-phase supply, used widely in North America.
Three-phase: Have three separate primary coils, one for each phase, used commonly in Europe and Asia.
Liquid-filled: Filled with insulating mineral oil providing cooling and insulation.
Dry-type: Use polymeric insulation not requiring any liquid, suitable for indoor use.
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Efficiency of Distribution Transformer
With distribution transformers energized continuously, reducing no-load losses is important. They are designed with maximum efficiency at typical partial loads rather than at full-rated capacity. Core losses are minimized through laminated silicon steel and eddy current reductions. Copper loss is lowered by using thicker conductors appropriately sized.
Typical distribution transformer efficiency ranges from 98-99%, with amorphous metal core type achieving higher 99.5+% efficiency. Regular maintenance like dissolved gas analysis and infrared scanning helps sustain optimal efficiency.
Applications of Distribution Transformer
Besides supplying power to residences through overhead lines or underground cables, distribution transformers also find use in
Commercial buildings to step down utility primary voltages to required utilization voltages.
Pumping stations, farms, and isolated houses are located far from the distribution network.
Railway AC electrification systems to power the overhead contact wires.
Offshore oil rigs and remote meteorological stations/communication towers.
Distributed power generation from renewable sources like roof solar and wind turbines.
Difference between Power Transformer and Distribution Transformer
The key difference between these transformers lies in their power ratings and applications
Feature
Power Transformer
Distribution Transformer
Voltage Levels
Above 33 kV
Below 30 kV
Location
Transmission networks
Distribution networks
Efficiency
Maximum at full load
Maximum at varying loads
Size and Weight
Larger and heavier
Smaller and lighter
Power transformers have much higher power ratings above 200kVA/200kW and are used in transmission and distribution substations for bulk energy transfer over long distances.
Distribution transformers have lower ratings under 200kVA and are installed locally all over neighborhoods for final step-down of transmission voltages and distribution to customers.
Conclusion
Distribution transformers are important equipment in the power distribution sector, which brings power reliably to our homes and offices. With continuous improvements in materials, designs, and cooling methods, their efficient voltage transformation helps in minimizing transmission losses.
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FAQ Related to Distribution Transformer
What is the rating of a typical residential distribution transformer?
Distribution transformers catering to residential neighborhoods are usually rated between 25kVA to 200kVA. A smaller 15-25kVA unit may supply power to a few houses while larger ones of 150-200kVA rating feed medium sized apartment complexes.
What voltage levels do distribution transformers operate at?
Is it safe to climb utility poles with live distribution transformers?
What does an amorphous metal core offer over silicon steel?
