This article takes an in-depth look at power transformers.
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Power transformers are electrical instruments used in transmitting electrical power from one circuit to another without changing the frequency. They operate by the principle of electromagnetic induction. They are used in transmitting electrical power between generators and distribution primary circuits. Power transformers are used to step up or step down the voltage in distribution networks. Since they have no rotating or moving parts, these instruments are considered static devices. These instruments work based on an alternating current (AC) electrical system.
A power transformer is a mere classification of transformers with a voltage range varying between 33 kV-400 kV and a rating above 200 MVA. The voltage ratings of power transformers available in the market include 400 kV, 200 kV, 110 kV, 66 kV, and 33 kV. The other types of transformers include distribution (230 V-11kV) and instrument transformers.
Power transformers are essential in minimizing substantial energy losses, due to Joule’s effect, in the transmission of large amounts of electrical power over long distances by converting it into high-voltage current then stepping it down to a safer low-voltage current. They are commonly found in power plants, industrial plants, and electric utility companies.
Power transformers operate based on Faraday’s law of electromagnetic induction. This law is the working principle of all transformers, inductors, motors, generators, and solenoids.
Faraday’s law states that when a closed-loop is brought near a fluctuating magnetic field, an electromotive force (emf) will be induced across it.
When alternating current is allowed to flow through a coil, an alternating or fluctuating magnetic flux surrounds the coil (primary winding). The magnetic flux produced by the primary winding passes through a ferromagnetic core to be transmitted effectively to a secondary winding. The magnetic flux will then induce an emf in the secondary winding due to electromagnetic induction. The induced emf will stimulate the flow of current in the secondary winding.
The total voltage in a winding is equal to the voltage per turn of the coil multiplied by the number of turns. Since the voltage per turn of the primary and secondary windings are the same, the induced voltage in the secondary winding can be related to the input voltage on the primary winding. This relationship is expressed by the equation:
Vs = Vp/Np x Ns
Where V represents the total voltage in the winding, N represents the number of turns of a winding, and the subscripts p and s refer to the primary and secondary windings, respectively. The ratio of the number of turns in the secondary winding to that of the primary winding (Ns/Np) is called the turns ratio.
If the number of turns in the secondary winding is fewer than the number of turns in the primary winding, the voltage output is lower than the input voltage (step-down transformer). On the other hand, if the number of turns in the secondary winding is more than the number of turns in the primary winding, the voltage output is higher than the input voltage (step-up transformer).
Since energy is conserved, the relationship between the alternating current in the primary and secondary windings is represented by the below equation:
Vp Ip = Vs Is
Where I represents the current.
The basic parts of transformers are the core and the primary and secondary windings, which are discussed in more detail in this chapter.
The core supports the windings and provides a low reluctance path for the magnetic flux. It is made by stacking and laminating thin steel sheets. The sheets are insulated from each other by a coating. To reduce eddy current losses and hysteresis losses, the iron or steel sheets are less than one millimeter thick, and their carbon content is maintained below 0.1%. Eddy current is further reduced by alloying the steel with silicon. The vertical sections of the core in which the windings are carried are referred to as the limbs, while the horizontal sections of the core that couples the limbs are referred to as the yokes.
The windings are made up of copper or aluminum conductor coil with a specific number of turns. Copper is the preferred material since it offers high electrical conductivity and high ductility; these properties reduce the amount of winding and make the material easier to wrap around the core.
A transformer consists of at least two windings- the primary and the secondary windings. The primary winding is the winding in which the input voltage is applied, while the secondary winding is the winding that receives the output voltage. The primary and the secondary windings in a phase of a transformer can play as the high voltage (HV) winding or the low voltage (LV) winding:
Other parts of power transformers include the following:
Insulating materials are used to isolate the windings from the core, the primary and the secondary windings, and each turn of the windings. These materials protect the transformer from damage. Transformer insulators should have high dielectric strength, good mechanical properties, and can withstand high temperatures.
Paper and pressboard can be used as an insulator (i.e., dry-type transformers); however, they have limited service lives and require frequent replacement as these materials can degrade. Hence, transformer oils are more common compared to solid insulating materials. They provide enhanced insulation between conducting parts, act as a coolant for the coil and windings assembly, and have fault detection features. Hydrocarbon mineral oils consisting of aromatics, paraffin, naphthene, and olefins are used as transformer oils. Oil contamination must be prevented to preserve the oil’s dielectric properties and insulating features.
Tap changers are devices that regulate the transformer’s output voltage as it responds accordingly to the varying input voltage and load by adjusting the number of turns in one winding. This adjustment, therefore, changes the turn ratio. During offloading conditions, the output voltage increases, whereas during loaded conditions, the output voltage decreases. Tap changers are typically connected in the HV winding to make fine voltage regulations and minimize core losses of the transformer. The current is also lower in the HV winding, which minimizes the risk of sparking and igniting the transformer oil.
There are two types of tap changers. Onload tap changers are designed to tap the voltage without disrupting the current flow to the load. Whereas offload tap changers require disconnecting the load of the transformer before operating.
Bushings are insulated barriers that contain the terminal that connects the current-carrying conductor from an electrical network to the ends of the transformer windings. The bushing insulation is typically made from porcelain or epoxy resin. The bushings are mounted over the main tank.
The transformer tank (or the main tank) houses and protects the core, windings, and other components from the external environment. It serves as the container for the transformer oil. It is constructed from rolled steel plates or aluminum sheets.
The following are present in large transformers insulated with hydrocarbon mineral oil:
The conservator is a tank that serves as the reservoir of the transformer oil and is located above the main tank and bushings. Transformer oil from the conservator is supplied to the main oil tank inside the transformer through a pipeline. The conservator has a flexible bladder that allows the expansion and contraction of the oil. It has an adequate space to allow the expansion of the oil during high ambient temperatures. The conservator is vented to the atmosphere to balance the pressure changes during the expansion and contraction of the oil by intaking or releasing air.
The breather delivers moisture-free air to the conservator by passing air through a small bed of silica gel inside a cylindrical container. The silica gel acts as an air filter that strips and controls the moisture level inside the conservator and the main tank. The breather is connected by a pipeline to the conservator.
Moisture can degrade the insulating properties of the transformer oil or may even lead to internal faults. Therefore, it is necessary to remove the moisture.
The cooling system is a critical component of transformers regardless of the insulating material utilized. Power losses occurring in the transformers are in the form of heat increasing the temperature of the windings and the core. Consequently, the temperature of the insulating material will also increase. Without a cooling system, these components may be damaged or decomposed if heated continually. The cooling system of transformers consists of fans, radiators, and cooling tubes. Heat transfer mechanism occurs by natural and/or forced convection and radiation.
For dry-type transformers, cooling may be accomplished by the following methods:
For oil-immersed type transformers, cooling may be accomplished by the following methods:
The explosion vent is a metallic pipe with a diaphragm at its free end located slightly above the conservator tank. It releases gases, transformer oil, and energy during internal faults to relieve the excessive pressure inside the transformer, thus preventing the explosion of the transformer. Faults elevate the internal pressure of the transformer to dangerous levels. When such circumstances occur, energy will be released into the atmosphere, destroying the diaphragm at relatively low pressure.
The Buchholz relay is a device installed along the pipeline connecting the conservator and the main tank. It detects faults in the transformer by sensing the emitted gases to activate the trip and alarm circuits. Once the trip circuit is activated, the circuit breaker will then disrupt the current flow to the primary winding. Emitted gases are generated by the heat released induced by faults.
Power transformers can be classified based on core and winding construction, turns ratio, phases, and core material.
1. Core and Winding Construction and Arrangement
Berry-type transformers have the core arranged like spokes of a wheel. They have distributed magnetic circuits and contain more than two independent magnetic circuits.
In core-type transformers, the primary and secondary windings surround the core. The core of these transformers is constructed by joining two L-shaped steel strips and stacking them to form the layer. To avoid high reluctance at the joints, the strips are arranged such that continuous joints are eliminated. The limbs and the yoke carry the whole of the flux.
In shell-type transformers, the core surrounds the primary and secondary windings. The core of these transformers is constructed by joining E-shaped and I-shaped steel strips and stacking them to form the layer. The central limb carries the whole of the magnetic flux, and the side limbs carry half of the flux.
2. Turns Ratio (Ns/Np)
Isolation transformers have a turns ratio equal to 1, which means the number of turns in the primary and secondary windings is equal. They are used to isolate the load from the power source while supplying alternating currents. They protect the electrical device, operation, and persons from electrical noise, shock, and damage. They are commonly used in computers, measurement devices, industrial machinery, laboratory and medical equipment, and other sensitive equipment.
Step-down transformers have a turns ratio of less than 1, which means that the primary winding has more turns. These transformers convert the high voltage and low current input from the primary winding to a low voltage and high current output on the secondary winding.
In the electricity distribution, the step-up transformers are situated in the power generating station while the step-down transformers are installed in the substations. This application can be further visualized by looking at the image below:
Step-up transformers have a turns ratio greater than 1, which means that the secondary winding has more turns. These transformers convert the low voltage and high current input from the primary winding to a high voltage and low current output on the secondary winding.
3. Phases
Autotransformers consist of a single winding tapped at certain points across its length to supply a fraction of the primary voltage. The primary and secondary windings are linked to each other, which are wounded on a single core. Autotransformers have a more compact size and are cheaper than the conventional double winding transformer, which can deliver the same VA rating. However, they do not have electrical isolation between the primary and secondary windings. They are widely used in induction motors, railways, audio systems, and lighting systems.
Single-phase transformers consist of a single pair of windings arranged in a core and generate a single alternating voltage which is represented by a single sine wave. They have four terminals: each winding has two terminals. No star (wye) or delta connections are present in this type of transformer.
Single-phase transformers have a simple construction and are used in residential and light commercial power supply. They are more popular in rural areas where the demand for electrical power is low, making a single-phase transformer the most cost-effective option.
Three-phase transformers are composed of three pairs of primary and secondary windings. They can be constructed by connecting three single-phase transformers to form a transformer bank or by assembling three pairs of windings into a single laminated core. Three-phase transformers generate three-phase alternating current flowing in separate conductors. Three sine waves represent this, and the waves are separated by 120 degrees from one another. The amplitude is reached more frequently which makes three-phase transformers supply power at an almost constant rate.
The windings of the primary and secondary sides are linked in either delta or star connections. The primary and secondary windings can either have the same or different connections. Hence, several three-phase transformer configurations are possible:
Three-phase transformers are popular due to their efficiency in heavy-duty applications since they effectively utilize winding connections. They are used in large motors, electric power distribution grids, and other large loads. They are also less expensive than three single-phase transformers which provide the same VA rating.
4. Core Material
Air core transformers have no physical transformer core. Their primary and secondary windings are wound in a solid insulating material. They are used in transmitting radio-frequency currents.
Ferrite core transformers have a ferrite core. Ferrites are ceramic, which is made up of iron oxides, zinc, nickel, and manganese. The commonly used ferrites in transformers are manganese zinc ferrite and nickel-zinc ferrite.
Ferrites have high magnetic permeability, the property of a material to allow magnetic flux to flow through it. They also have high current resistivity and low eddy current losses for a wide frequency range, making them ideal for high-frequency applications. Ferrite core transformers are widely used in wideband transformers and electronics applications.
Iron core transformers possess an electromagnetic core made up of laminated iron sheets. They are the most common type of transformer under this category. Iron cores have high flux linkage, which is attributed to their excellent magnetic properties.
Toroidal core transformers are transformers with torus or donut-shaped cores made from iron or ferrite. Their primary and secondary windings are wound on the toroidal core. Due to their ring shape, their toroidal cores have low magnetic flux leakage and high inductance and Q factors. Hence, their efficiency is high. Toroidal core transformers are used in telecommunications, power distribution, and industrial control systems.
There are four main types of transformer losses that affect the efficiency of power transformers:
Copper losses, sometimes called resistive or I2R losses, are the energy losses caused by the electrical resistance of the windings to current flow. The electrical resistance of material measures the opposition to the current flow; it depends on the length, nature, cross-sectional area, and temperature of the material. Copper losses are also influenced by the amount of current flowing through the circuit. Copper losses are quantified by calculating the value of I2R.
Hysteresis losses are caused by the friction encountered by the ferromagnetic molecules in the core due to magnetization and demagnetization, as the magnetizing force flows in forward and reverse directions. The internal friction developed causes heat to develop within the transformer.
Eddy current is produced in the core’s cross-section as a result of the fluctuating magnetic field. It is minimized by laminating thin metal sheets (laminas) together to construct the transformer core. The laminas are insulated by a special coating. Through lamination, the eddy current is produced and flows separately in all laminas, and the path for the eddy current is drastically reduced.
Flux losses occur when some of the magnetic flux lines from the primary winding flow through the air instead of passing through the secondary winding. This loss can be caused by the magnetic saturation of the core. In this circumstance, the core cannot accept flux lines anymore. The smaller ratio of the reluctance of the air and the core also contributes to flux loss.
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