Different types of transformer

Introduction

Electrical energy travels through a complex network before reaching homes, industries, and transportation systems. Different types of transformer play a central role in this process because they adjust voltage levels and allow electricity to move safely through each stage of the power network. Different types of transformer operate at generating stations, transmission systems, substations, and distribution networks where voltage conditions must change to support efficient energy transfer. Power stations generate electricity at moderate voltage levels that suit generators and turbines, yet long distance transmission requires much higher voltage to reduce losses. Transformers use electromagnetic induction to change voltage without altering frequency or interrupting energy flow. Through these devices engineers maintain stable voltage, reduce energy losses, and protect electrical equipment. Reliable transformer operation allows electrical power systems to deliver steady electricity to communities and industries.

Different types of transformer in a typical power system

Electrical power systems contain many interconnected elements that must operate together with precision and stability. Transformers appear at multiple stages across this network because each stage requires different voltage conditions. Engineers design transformers according to their location and operational purpose within the system. Power generation equipment produces electricity at moderate voltages such as 11 kV or 25 kV because these levels match generator design limits. Transmission networks require higher voltages so that electricity travels long distances with reduced current and minimal energy loss. Later stages of the network reduce voltage gradually until it reaches safe utilization levels for homes and industries. Through these changes transformers support efficient power transfer and protect electrical equipment across every section of the grid.

Transformers classified according to location and broad function

Engineers classify transformers according to their role inside the electrical network. Each transformer performs a particular task that supports voltage control and energy transfer across power systems. Some transformers operate near generating stations while others function at substations or near consumer loads. Classification helps engineers understand where each transformer should operate and how it should perform. Design parameters such as voltage rating, insulation strength, cooling arrangement, and mechanical construction vary according to application. Through proper classification engineers ensure that transformers operate safely while supporting efficient energy delivery. The following categories describe major transformer types used across power generation and transmission infrastructure.

1. Generator transformers

Generator transformers operate directly at generating stations where electrical energy first enters the transmission network. Their purpose involves increasing generator output voltage to high transmission levels suitable for long distance energy transfer. Power stations normally generate electricity at moderate voltage levels because generator insulation limits prevent extremely high voltages. Generator transformers step up this voltage so that current decreases while power remains constant. Reduced current lowers resistive losses in transmission lines and allows efficient power delivery across large distances. These transformers handle large power ratings and must operate reliably under continuous load conditions. Strong insulation, cooling systems, and mechanical design ensure stable operation under demanding electrical conditions.

• Generating stations produce power (typically at voltages between 11 and 25 kV) and step it up to higher transmission voltages (220, 345, 400, or 765 kV) using generator transformers. These transformers play a vital and critical role in every power system. Designers assign them uniform loads and allow higher losses because supplying power at the generating station costs the least. Since the generators themselves generate significant noise, specifications rarely demand low noise levels for the transformers.
• Engineers prefer a tap-changing mechanism with off-circuit taps for small variations in high-voltage levels (e.g., ±5%) because they can easily control the voltage through generator field excitation. For reactive power control, they use generator transformers equipped with on-load tap-changing mechanisms.
• They may be provided with a compact unit-cooler arrangement for want of space in the generating stations; such transformers have only one rating with oil-forced and air-forced cooling. Alternatively, oil-to-water heat exchangers can be used for the same reason. It may be economical to design the tap winding as a part of the main HV winding and not as a separate winding.
• This may be permissible since axial short-circuit forces are generally lower due to a small tapping range. Special care has to be taken while designing high current LV lead terminations to eliminate hot-spots in the structural parts in their vicinity. A CTC conductor with epoxy bonding is commonly used for the LV winding to minimize eddy losses and to provide higher short-circuit strength.The overexcitation conditions specified by the users have to be considered while designing generator transformers.

2. Unit auxiliary transformers

Unit auxiliary transformers supply electrical power to equipment that supports generating station operation. Power plants contain many systems such as pumps, fans, conveyors, cooling systems, and control equipment that require electricity to function. During normal operation these transformers receive power from the generator output and step down voltage to levels suitable for internal plant machinery. Reliable auxiliary supply ensures safe plant operation and continuous electricity generation. Engineers design these transformers to handle varying loads because plant equipment operates at different times during the generation process. Proper coordination between generator transformers and auxiliary transformers ensures uninterrupted energy supply for internal systems.

• These are step-down transformers with their primary winding connected to the generator output directly. The secondary voltage is of the order of 6.9 kV for supplying power to various auxiliary equipment in the generating station.

3. Station transformers

Station transformers provide power during startup conditions and maintenance activities when generators are not yet producing electricity. At these times generating equipment requires external electrical supply to operate auxiliary systems that prepare the plant for generation. Station transformers connect directly to transmission lines or nearby substations and supply the necessary startup energy. These transformers usually operate at lower ratings compared with generator transformers because their main purpose involves temporary support rather than continuous high power transfer. Reliable station transformers ensure safe plant startup procedures and support maintenance operations without interruption.

• These transformers supply power to auxiliary equipment during the setting-up phase of generating stations and during each subsequent start-up. They operate at a small rating and connect their primary winding to a high-voltage transmission line. This setup may require a smaller conductor size for the HV winding, so engineers implement special measures to increase short-circuit strength. To achieve more economical circuit breaker ratings, they often use a split secondary winding arrangement.

4. Interconnecting transformers or autotransformers

Large electrical networks often operate at multiple voltage levels for efficiency and reliability. Interconnecting transformers link these different voltage networks and allow controlled energy transfer between them. Autotransformers provide an economical solution for this purpose because their design shares common windings between primary and secondary circuits. This arrangement reduces material requirements and improves efficiency. Engineers install these transformers at major substations where transmission systems with different voltage classes connect. Through these devices power operators balance energy flow across networks and maintain stable grid operation.

• Engineers use these transformers to interconnect two systems that operate at different voltages, such as 400 kV and 220 kV or 345 kV and 138 kV. Their primary and secondary windings share a direct electrical connection, so the transformer conductively transfers part of the volt-amperes and inductively transfers the rest.
• The design of an autotransformer becomes more cost-effective as the ratio of the secondary winding voltage to the primary winding voltage approaches unity.Autotransformers are characterized by a wide tapping range and a loaded or unloaded delta-connected tertiary winding.
• The unloaded tertiary winding acts as a stabilizing winding by providing a path for third-harmonic currents.Synchronous condensers or shunt reactors are connected to the tertiary winding,if required, for reactive power compensation. An adequate conductor area and a proper supporting arrangement should be provided to the unloaded tertiary winding to help it withstand short-circuit forces under asymmetrical fault conditions.

5. Receiving station transformers

Receiving station transformers operate at substations located near major load centers. These transformers reduce transmission voltage to levels suitable for regional distribution systems. Electrical energy arriving from transmission networks may carry voltages such as 220 kV or 132 kV which remain too high for direct use by consumers. Receiving station transformers step down this voltage to feeder levels such as 33 kV or 11 kV. From these substations electricity travels through distribution feeders that deliver power to cities and industrial areas. These transformers must accommodate fluctuating load conditions because demand changes throughout the day.

• These are step-down transformers that reduce a transmission or sub-transmission voltage to a primary feeder level voltage (e.g., 220 kV/33 kV transformers).They can be used to feed industrial plants directly. Loads on them vary in a wider range, and it is expensive for the generator to supply the power lost in them in the form of no-load and load losses.
• The farther the location of transformers from the generating station, the higher the cost of supplying the losses is. Automatic tap changing on load is usually necessary, and the tapping range is generally higher to account for a wide variation in the voltage. A lower noise level is usually specified for those transformers that are close to residential areas.

6. Distribution transformers

Distribution transformers represent the final stage of voltage conversion before electricity reaches consumers. These transformers reduce feeder voltage to safe utilization levels used by homes, businesses, and small industries. Typical output voltage may range around 415 V or 230 V depending on system design. Distribution transformers often operate near consumer locations and must maintain high efficiency because they run continuously throughout the day. Engineers design these transformers to minimize no load losses and maintain stable voltage during changing load conditions. Reliable distribution transformers ensure that electricity reaches consumers safely and efficiently.

• Distribution transformers reduce the primary feeder voltage to the actual utilization level, typically around 415 or 460 V, for domestic and industrial applications. This category includes several transformer types with various arrangements and connection methods. These transformers experience widely varying loads and often operate under overloaded conditions.
• To improve all-day efficiency, engineers aim to achieve a lower value of no-load loss. Utilities typically capitalize no-load loss at a high rate during the tendering stage. Because these transformers operate with minimal supervision, users expect them to require very little maintenance. Among all transformer types, these units incur the highest cost for supplying losses and reactive power.

Transformers classified according to specific applications

Beyond location based classification, engineers also categorize transformers according to specialized applications. Certain industries and power systems require transformers designed for unique electrical conditions. These transformers handle tasks such as power flow control, grounding support, harmonic reduction, and heavy industrial loads. Special construction techniques ensure reliable performance under demanding conditions. Application based classification allows engineers to design transformers that address specific operational requirements within electrical networks.

1. Phase shifting transformers

Phase shifting transformers regulate the direction and magnitude of power flow between transmission lines. By adjusting the phase angle between input and output voltages these transformers control how power moves across interconnected networks. Grid operators use them to balance load between parallel transmission lines and prevent overload conditions. These transformers often include tap changing mechanisms that adjust phase shift values during operation. Proper control of power flow improves system reliability and prevents congestion in transmission corridors.

• Engineers use these transformers to control power flow over transmission lines by varying the phase angle between the input and output voltages. By adjusting the tap changer, they can make the output voltage lead or lag the input voltage.The total required phase shift directly influences the transformer's rating and size.
• Currently, engineers use two distinct types of core construction: single-core and two-core designs. They select the single-core design for small phase shifts and lower MVA or voltage ratings. For bulk power transfer and higher-rated phase-shifting transformers, they employ the two-core design. This configuration consists of two transformers—one connected to the line terminals and the other to the tap changer.

2. Earthing or grounding transformers

• Engineers use these transformers to provide a neutral point that enables grounding and earth fault detection in ungrounded parts of the network, such as delta-connected systems. They typically connect the windings in a zigzag configuration to eliminate third harmonic voltages in the lines. These transformers offer an additional advantage by avoiding the DC magnetization issues that commonly occur with power electronic converters.

3. Transformers for rectifier and inverter circuits

• These transformers function like standard units but include special design and manufacturing features to counter harmonic effects. To manage additional harmonic losses, designers maintain a lower operating flux density in the core, typically around 1.6 Tesla. They reduce the dimensions of the winding conductors to minimize eddy losses and apply an appropriate de-rating factor based on the magnitudes of the various harmonic components.
• Engineers must carefully address thermal design aspects to eliminate hot spots. When designing transformers for high voltage direct current (HVDC) systems, they face the additional challenge of creating insulation systems that can withstand combined AC-DC voltage stresses.

4. Furnace duty transformers

• Engineers use these transformers to supply power to arc or induction furnaces, which require low secondary voltages (80 to 1000 V) and high currents (10 to 60 kA), depending on their MVA rating. To eliminate hot spots and reduce stray losses, designers consistently use non-magnetic steel for terminating the LV leads and for the nearby sections of the tank.
• For applications involving very high currents, engineers design LV terminals as U-shaped copper tubes with appropriate inner and outer diameters to allow oil or water to circulate through them for cooling. In many cases, engineers also include a booster transformer alongside the main transformer to reduce the load on the tap-changer.

5. Freight loco transformers

• Engineers mount these transformers in the engine compartments of locomotives and connect their primary winding to the overhead line. The transformer steps down the primary voltage to a suitable level for feeding into a rectifier, which then produces a DC output to drive the locomotive. Designers must ensure the structural design of the transformer can withstand locomotive vibrations. They also analyze the mechanical natural frequencies of the entire structure to avoid resonant conditions.

6. Hermetically-sealed transformers

• Engineers construct these transformers to prevent any outside atmospheric air from entering the tank. They completely seal the unit without including a breathing arrangement, which eliminates the need for periodic filtration and related maintenance tasks.
• Engineers fill these transformers with mineral oil or synthetic liquid to serve as both a cooling and dielectric medium. They seal the tank completely and add an inert gas layer—typically nitrogen—between the liquid and the top tank plate. To prevent leakage problems, they use a welded cover construction instead of bolted joints.
• The inert gas layer absorbs variations in the volume of the cooling medium. Designers build the tank to withstand high-pressure conditions at elevated temperatures. In another type of sealed construction, engineers eliminate the use of inert gas and instead allow the structure of the cooling arrangement—often integrated into the tank—to deform and absorb the medium’s expansion.

7. Outdoor and indoor transformers

• Manufacturers design most transformers for outdoor use and ensure they can withstand atmospheric pollutants. Engineers determine the creepage distance of the bushing insulators based on the pollution level. As the pollution level increases, they increase the required creepage distance between the live terminal and ground.
• On the other hand, engineers install transformers for indoor applications in weatherproof and properly ventilated rooms. Industry standards define the minimum ventilation levels necessary for effective cooling. To prevent excessive noise caused by reverberations, installers must maintain adequate clearances between the transformer and the surrounding walls.

Conclusion

Different types of transformer form the backbone of modern electrical power systems because they manage voltage levels, regulate energy flow, and ensure safe delivery of electricity to consumers. These devices operate at every stage of the power network including generating stations, transmission corridors, substations, and distribution systems. Engineers design transformers according to location, load behavior, and application requirements so that each device performs its specific role effectively. Understanding different types of transformer helps engineers design reliable electrical infrastructure capable of supporting growing energy demand. Through continuous improvement in transformer technology power systems continue delivering efficient and stable electricity across expanding global networks.