Different types of transformer

Introduction

Transformers, the quiet laborers in the electrical industry, are available in different sizes and forms to cater to the various requirements of power systems. Their primary function is to increase or decrease voltages, enabling the smooth transfer of electrical power between different locations on the grid. This article discusses the diverse varieties of transformers designed for specific uses and demands in the ever-changing field of electrical engineering.

Illustration of various transformers featuring different designs and structures. Includes cooling fins, insulators, and utility box details. Text: "Different types of transformer."

Before inventing transformers, engineers in the early electrical industry distributed power using direct current (DC) at low voltages. Voltage drops in electrical lines limited electricity use to urban areas, where short distribution circuits served consumers. Designers had to ensure all electrical equipment operated at the same voltage.

Development of the first transformer around 1885 dramatically changed transmission and distribution systems. The power generated with alternating currents at low voltages could be stepped up for transmission purposes to higher voltages (with lower currents), reducing voltage drops and transmission losses. The use of transformers made it possible to transmit power economically to areas hundreds of kilometers away from generating stations.

Step-down transformers then reduced the voltage at the receiving stations for the distribution of power at various standardized voltage levels for its use by end-consumers. Transformers have made AC systems flexible because the various parts and equipment of the power system can be operated at economical voltage levels with suitable voltage ratios.

A single-line diagram of a typical power system is shown in Figure 1.1.Voltage levels are different in different countries. Transformers can be broadly classified into several types, some of which are based on location and broad function, and others according to their applications.

Diagram illustrating various transformers in a power system: station, generator, converter, rectifier, furnace, step-down, and distribution transformers.

Transformers classified according to location and broad function

1.Generator transformers

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

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

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

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

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 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

1.Phase shifting transformers

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.

There are other types of transformers, which have applications in electronics, electric heaters, traction, etc. Some of the applications have a significant impact on the design of the transformers. The duty (load) can be very onerous. For example, the winding current densities in transformers with frequent motor-starting duty have to be lower due to the high starting currents of the motors (which can be of the order of 6 to 8 times the full load current).

Conclusion

Transformers in electrical power systems play a crucial role, serving different purposes and adapting to various requirements of modern society. Engineers and stakeholders need to comprehend the nuances of these transformers to effectively build and improve the infrastructure supporting our interconnected world.