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.

Before the invention of transformers, in the initial days of the electrical industry,power was distributed through direct current (DC) at low voltages. Voltage drops in electrical lines limited the use of electricity to only urban areas where consumers were served using distribution circuits of small length. All the electrical equipment had to be designed for 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.

Transformers classified according to location and broad function

1.Generator transformers

The power generated at a generating station (usually at a voltage in the range of 11 to 25 kV) is stepped up by a generator transformer to a higher voltage (220, 345, 400 or 765 kV) for its transmission over long distances. Generator transformers are important and critical components of any power system. They usually have uniform loads and are designed with higher losses since the cost of supplying power is cheapest at the generating station.Lower noise levels are usually not specified since the generators supplying the transformers are even noisier.
A tap-changing mechanism with off-circuit taps, suitable for a small variation in the HV voltage (e.g., r 5%), is preferred since the voltage can be easily controlled through the field excitation of the generator. Generator transformers having an on-load tap changing mechanism are used for reactive power control.
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 are required to supply power to auxiliary equipment during the setting-up operation of generating stations and subsequently during each start-up operation. The rating of these transformers is small, and their primary winding is connected to a high voltage transmission line.This may result in a smaller conductor size for the HV winding,necessitating special measures for increasing the short-circuit strength. A split secondary winding arrangement is often employed to achieve economical circuit breaker ratings.

4.Interconnecting transformers or autotransformers

These transformers are used to interconnect two systems operating at different system voltages (e.g.,400 kV and 220 kV, 345 kV and 138 kV). There is no electrical isolation between their primary and secondary windings; some volt-amperes are conductively transformed and the remaining are inductively transformed.
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

Using distribution transformers, the primary feeder voltage is reduced to an actual utilization voltage (~ 415 or 460 V) for domestic/industrial use. Several types of transformers fall into this category due to many different arrangements and connections. The load on these transformers varies widely, and they are often overloaded.
A lower value of no-load loss is desirable to improve their all-day efficiency. Hence, the no-load loss is usually capitalized with a high rate at the tendering stage. Since very little supervision is possible, users expect a minimum level of maintenance on these transformers.The cost of supplying losses and reactive power is highest for these transformers.

Transformers classified according to specific applications

1.Phase shifting transformers

These are used to control power flow over transmission lines by varying the phase angle between the input and output voltages of the transformer. Through a proper tap-change, the output voltage can be made either to lead or lag the input voltage. The total phase-shift required directly affects the rating and size of the transformer.
Presently, two distinct types of core construction are used, viz. a single-core design and a two-core design.The single-core design is used for small phase-shifts and lower MVA /voltage ratings, while the two-core design is employed for bulk power transfer with higher ratings of phase-shifting transformers.The design consists of two transformers,one associated with the line terminals and the other with the tap changer.

2.Earthing or grounding transformers

These are used to provide a neutral point that facilitates grounding and detection of earth faults in an ungrounded part of the network (e.g., delta-connected systems).Their windings are usually connected in a zigzag manner, which helps in eliminating third harmonic voltages in the lines.These types of transformers have an additional advantage since they are not affected by the DC magnetization problems normally associated with power electronic converters.

3.Transformers for rectifier and inverter circuits

These are otherwise normal transformers except for their special design and manufacturing features which enable them to counter harmonic effects. Due to extra harmonic losses, the operating flux density in their core is kept lower (around 1.6 Tesla). The winding conductor dimensions need to be smaller to reduce eddy losses, and a proper de-rating factor has to be applied depending upon the magnitudes of various harmonic components.
Thermal design aspects need to be carefully looked at for eliminating hot spots. For the transformers used in high voltage direct current (HVDC) systems, the design of their insulation system is a challenging task because of combined AC-DC voltage stresses.

4.Furnace duty transformers

These transformers are used to feed the arc or induction furnaces, which are characterized by a low secondary voltage (80 to 1000 V) and a high current (10 to 60 kA) depending upon their MVA rating. A non-magnetic steel material is invariably used for the termination of LV leads and for the tank portion in their vicinity to eliminate hot spots and minimize stray losses.
For applications involving very high currents, LV terminals in the form of U-shaped copper tubes having suitable inside and outside diameters are designed so that they can be cooled by oil/water circulation from inside. In many cases, a booster transformer is used along with the main transformer to reduce the rating of the tap-changer.

5.Freight loco transformers

These are mounted in the engine compartments of locomotives, and their primary winding is connected to an overhead line. The primary voltage is stepped down to an appropriate level for feeding to a rectifier;the output DC voltage of the rectifier drives the locomotive. The structural
design of the transformers has to be good enough to take care of locomotive vibrations. The mechanical natural frequencies of the whole structure need to be analyzed to eliminate resonant conditions.

6.Hermetically-sealed transformers

The construction of these transformers is such that it does not permit any outside atmospheric air to get into the tank. It is completely sealed without any breathing arrangement, obviating the need for periodic filtration and related maintenance activities.
These transformers are filled with mineral oil or synthetic liquid as a cooling and dielectric medium,and they are sealed completely with an inert gas (nitrogen) layer between the medium and the top tank plate. A welded cover construction is used for the tank to eliminate bolted joints and related leakage problems.
Variations in the medium volume are absorbed by the inert gas layer. The tank is designed for high-pressure conditions at elevated temperatures. In another type of sealed construction, the use of inert gas is avoided; the expansion of the medium is absorbed by deformations of the structure used for the cooling arrangement,which can be an integral part of the tank.

7.Outdoor and indoor transformers

Most transformers are suitable for outdoor duty and are designed to withstand atmospheric pollutants. The creepage distance of the bushing insulators is decided according to the pollution level.The higher the pollution level, the greater the creepage distance that is required between the live terminal and ground.
On the other hand, transformers for indoor applications are kept in a weatherproof and properly ventilated room.Standards define the minimum level of ventilation required for effective cooling.Adequate clearances should be kept between the walls and the transformer to eliminate the possibility of higher noise levels due to reverberations.

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.