The consistent transmission of electricity is facilitated by the use of power transformers. Their placement within power grids is calculated to maximize the effectiveness of distribution.
In the case of a power transformer, it raises or lowers the voltage to optimal values to guarantee efficient usage. This is required both for the safe long distance transmission of energy, as well as for safer reduced voltages to be used in households and industries.
For professionals in the electric power industry, understanding how these devices work is just as crucial as knowing the multifunctional roles they serve.
What is a Power Transformer?
A power transformer is a static type of electric device. It transfers electric power from one circuit to another. This device does not need any moving components because it operates through electromagnetic induction.
It employs the electromagnetic induction principle to change voltage levels efficiently due to power distribution or transmission. There are two or more coils of wire attached to a singular magnetic core, also known as a coil.
A power transformer is made up of two coils of wire, more precisely, the primary and secondary windings. The windings are positioned around a central laminated iron core, which is made of stacked steel laminations.
This component serves the dual purpose of focusing and directing the magnetic flux lines, which are created by the current that flows through the coils. Individually, each magnetic and electrical part is enveloped within a steel container filled with insulating oil.
Indeed, this liquid transforms to serve the insulating and cooling needs of large power transformers while they are in operation. Moreover, utility transformers can be larger in size, therefore, they can also have other internal peripheral parts, like bushings, protection circuits, and cooling ducts.
How Does a Power Transformer Work?
The working principle of a power transformer is built on electromagnetic induction. It is the process of a magnetic field inducing a voltage into a circuit that is placed nearby.
Specifically, a varying magnetic field, in this case the one generated in the primary wire coil due to the alternating current passing through it, induces voltage in a secondary coil placed on the same core.
The steps involved in a power transformer’s transformation process are rather intriguing. These steps include:
- The primary winding is connected to an alternating current source (AC). This produces a magnetic field that changes around the core because of electricity running through its coils.
- The core also acts as a powerful magnet, and the strength of its magnetic field increases during one half of the AC cycle and collapses during the reverse half.
- The changing magnetic flux fills the inner core and cuts the secondary coil placed over the core unit.
- In the opinion of Faraday’s induction law, each winding EMF is induced as the changing field in the magnetic coil passes through the wire coil turns.
- The rate of change of the flux, the count of turns on the winding, and certain other details of the transformer influence the value of generated EMF in the secondary branch of the transformer.
- The transformer’s turn ratio allows setting the output voltage to the required level by changing a number of turns on the two coils.
- After going through the isolated secondary winding, this transformed voltage can be used for secondary power transmission or distribution purposes.
Parts Of Power Transformers
Each part of a power transformer is a specific detail and works independently to influence the performance of the transformer, exhibiting a different effect.
Primary components include the core, windings, tap changer, insulating materials, transformer oil, conservator, breather, Buchholz Relay, cooling tubes, and explosion vent.
A transformer usually has a core, bottom windings, top insulating materials, and oil, while those rated above 50 KVA come with more parts. In this publication, we will explore the basic parts of power transformers.
Windings
The most important part of a power transformer is its windings, and we can compare it to the heart of the human body. These windings use copper, or in some rare instances, aluminum coils, insulated with several layers of paper between each turn. They are usually handmade.
Over the years, core and shell types of main winding designs and technologies have emerged, along with different variants. Although both designs share the same electromagnetic basis, their mechanical construction differs.
In core-type design, the winding is placed around the legs of the magnetic core, whereas in shell-type design, the core surrounds and traverses the windings.
Every company has its own distinct approach to these winding techniques, none of which can be fully mechanized.
The process of manufacturing the windings is very labor-intensive, necessitating a great deal of skill, experience, and continuous compliance with a rigorously defined quality control standard.
This is because in the case of a winding conductor, the conductor is insulated with varnish or insulating paper, which cannot withstand excessive heat or mechanical stress.
That stated, the paper insulation will guard against partial thermal damage, extreme current flow, severe voltage, and considerable mechanical forces without risk of compromising the strength of the insulation paper.
It should be noted that during the lifetime of the transformer, the incorporated and removed components cannot be easily altered during normal operation, and the unit must only be disassembled at designated facilities.
Core
The core is the most important component of a transformer and is usually the heaviest one. It is made of steel with a high value of magnetic permeability and low resistance to magnetic flux.
To reduce losses and magnetizing currents, the core is made from thin sheets of steel that are mere millimeters thick.
The common technique for a core involves stacking the sheets to the needed size on automated machines and proceeding afterwards with manual assembly to construct a core.
For single phase small distribution transformers, wound cores provide higher productivity and efficiency.
These cores are made up of two principal segments and horizontal yokes, and vertical legs. Usually, the legs of the core are in the same plane, however, with three phase transformers, the legs could also be placed in a triangular spaced arrangement known as a hexaformer.
Distribution transformers of small size are sometimes built using the Hexaformer core design, but their market share is quite limited.
Despite the fact that hexaformers produce fewer losses, a ‘traditional’ core would still be more efficient and productive in other aspects.
Tap Changer
Various transformers include supplementary turns in the high voltage (HV) winding, with some of the turns being connected to a device known as a “Tap Changer.” This device allows a definite range of voltage variation during the transformer’s entire service life.
Both the windings and the tap changer include movable contacts in their electrical circuits. Two dominant subdivisions of tap changers are De-Energized Tap Changer (DETC), which is mechanically simple, alters voltage while the transformer is unloaded, and On-Load Tap Changer (OLTC), which is a more sophisticated, complex type that functions under load.
It is important to point out that the presence of a wide range of different designs worsens the situation, especially On-load Tap Changers (OLTC), which increases the failure rate of transformers because of the automobile type over time, caused mainly by the movable contacts of the threads.
Insulating materials
The three main types of insulating material used in power transformers are: mineral oil, paper, and pressboard in their different forms. The condition of the transformer and possible issues can be determined from the mineral-insulating oil contained in the tank.
The paper material for insulation is also used for the Winding Wires, while the pressboard helps in reinforcing the electrical insulation and in providing dielectric spacings in some areas, for example, the primary duct in between the windings.
Due to the presence of organic insulating materials such as paper, pressboard, and even mineral oil, a transformer’s life span is limited due to aging.
Unlike other transformer components, the insulation cannot be easily removed, repaired, or replaced, so its lifetime defines the overall lifetime of the transformer.
Bushings
Bushings are the enabling parts that provide the connection to the power system via the grounded tank. The transformer is highly susceptible to explosion if there is the failure of the bushing. Designing high voltage bushings is a technically complex task.
The primary cause of this is the colossal voltage gradient created by the full potential of the HV bushing’s middle section and the grounded tank that is just a few centimeters away.
Any burn that is created from the bushing can release a significant amount of energy because of excessive sparking that is caused by the bushing, which results in the tank opening slightly and igniting the oi,l which causes a great explosion.
That is why HV bushings are designed to endure exceptionally high voltage in an extremely limited space filled with oil and paper insulation.
Buchholz Relay
For transformers with a power rating exceeding 500kVA, Buchholz relays become mandatory equipment, protected by oil.
The conservator is designed to permit access for internal faults, oil outflow, gas accumulation, and rapid outflow of oil. Contactless switches are incorporated into the releaser for alarm signaling of normal working conditions in production.
Tank
The transformer tank contains the oil for housing it, gives physical support, and protects the various parts of the transformer. Furthermore, the rest of the magnetic circuit and different metal components are grounded by the tank.
The body of the tank is formed by molding rolled steel plates into containers and fitting elevating hooks and cooling tubes. More often than not, steel plates will be substituted with aluminum sheets in an attempt to lower the weight of the transformer and prevent stray losses.
Oil Temperature/Pressure gauges
These gauges are used to check the internal features of the transformer and especially its windings. These devices help in finding the temperature and pressure gradients in the transformer’s oil and windings.
In addition, they act as alarms when the temperature and pressure go to levels that are likely to damage the windings and transformer.
Cooling system
In large power transformers, oil and water or air coolers are used to lower the temperature through forced circulation. The type of cooling change is determined based on the dynamic and static circulating medium and the type of cooling circulation.
For example, a transformer using air as the external medium and mineral oil as the internal cooling medium is denoted by the code ONAN; meaning both circulations are natural.
Usually, internal cooling uses forced cooling to push mineral oil, represented by O, through the radiators towards the D windings, while external cooling uses “A” air or “W” water.
Natural air convection is used for external circulation, N and F is for forced circulation. A single transformer can have multiple active cooling types. Depending on the temperature and power a transformer is operated, it will use fans or pumps.
Air Breather
An air breather functions to control the silicone oil level when the oil volume in a transformer’s conservator unit changes due to temperature changes. Air breathers have moisture absorbing silica gel.
Delivery silica gel is blue colored and, more importantly, it is pink when moisture is absorbed. Silica gel can be reused when heated to 120 degrees Celsius until it turns blue again.
Oil Conservator
The primary role of the oil conservator tank is to enable the transformer oil to expand and contract without being restricted due to surrounding temperature fluctuations encountered in the main transformer tank for transformer oil.
This drum shaped device is located above the transformer tank. It contains a Buchholz relay on the conduit pipe connecting the tank as well as a level gauge indicating the oil level in the guard tank.
Explosion Vent
The transformer’s explosion vent functions as a release port for oil and air gas emissions during an emergency. Ordinarily, this part consists of a pipe, usually metallic, which has a diaphragm at one end and is mounted just above the conservator tank.
Too much oil leakage could result in oil being forced into the tank, which leads to increased pressure and more danger. In this case, the diaphragm breaks at a small pressure, which enables the energy in the transformer to vent to the surroundings.
Drain Valve
The valve placed at the lower part of the transformer tank is known as a drain valve, and its primary purpose is to facilitate oil substitution. The transformer can be easily serviced by a refill through this opening, which functions much like a faucet.
Types Of Power Transformers
There are various classifications of power transformers, including their core and winding structure, turns ratio, number of phases, and core material.
Shell-Type Transformers
In shell-type transformers, the primary and secondary windings are located inside the core. The core is made from E and I-shaped steel strips, which are stacked to form the layers.
In this type of construction, the central limb of the core bears all the magnetic flux while the side limbs share the flux, each carrying half.
Berry-Type Transformers
Berry-type transformers have cores shaped like the spokes of a wheel. They exploit distributed magnetic circuits and have several independent magnetic circuits, typically over two.
Core-Type Transformers
In core-type transformers, the primary and secondary windings are placed around the core. The core is fabricated from two L-shaped steel strips that are joined and stacked to form the core layers.
To minimize the reluctance at the joints, the strips are arranged to avoid having continuous joints. The limbs and yoke of the core carry the flux.
Autotransformers
An autotransformer contains one winding arranged in such a way that it can tap off a portion of the primary voltage. The primary and secondary windings are interlinked with each other, and both are mounted on a single core.
Compared to the conventional double-winding transformer, which can deliver the same VA rating, the autotransformer is smaller in size and more economical.
However, they do not provide electrical separation between the primary and secondary windings. They are commonly found in use with induction motors, railways, audio equipment, and lighting systems.
Isolation Transformers
Isolation transformers have a turns ratio of 1, which indicates that the primary and secondary windings have the same number of turns. They serve to disconnect the load from the power source while providing it with an alternating current.
Isolation transformers also protect the device, its operation, and users from electrical noise, shocks, and damage. They find application in computers, measurement devices, industrial machinery, laboratory and medical equipment, and other devices that are sensitive.
Step-Down Transformers
Step-down transformers have a turns ratio of less than 1, which indicates that the primary winding has more turns. In easier terms, these transformers change the high voltage and low current input from the primary winding into a low voltage and high current output on the secondary winding.
In electricity distribution, step-up transformers are located at power generating stations, while step-down transformers are stationed at substations.
Step-Up Transformers
Step-up transformers have a turns ratio greater than 1, which means that the secondary winding has more turns. These transformers change the low voltage and high current input from the primary to a high voltage and low current output on the secondary winding.
Three-Phase Transformers
A three-phase transformer consists of three sets of primary and secondary coil windings. It is possible to build these by either connecting three single-phase transformers together in a transformer bank or placing three sets of windings in one laminated core.
Three-phase transformers provide three-phase AC electricity, which circulates in distinct conductors. The three-phase system is represented by three sine waves that are 120 degrees apart. Thanks to the frequent attainment of amplitude, three-phase transformers can provide power almost nonstop.
The windings on the three-phase transformers’ primary and secondary sides may be star or delta connected. On the primary and secondary sides, these configurations can be the same or different. This brings about various configurations of three-phase transformers:
- Star-Star Connection
- Star-Delta Connection
- Delta-Star Connection
- Delta-Delta Connection
- Open-Delta Connection
- Scott-T Connection
- High Leg Delta Connection
Unlike single-phase transformers, three-phase transformers are more suitable for heavy-duty operations due to the effective use of winding connections.
They are common in large motors, electric power distribution systems, and other major loads. In addition, it is more cost-effective to utilize a three-phase transformer than three single-phase transformers with the same VA rating.
Single-Phase Transformers
Single-phase transformers have one coil of wire wrapped around a magnet core and produce a single alternating voltage, which is represented by one sine wave.
They have four terminals since each of the two windings has two terminals, rather than one terminal per winding. These transformers do not have star (wye) or delta connections.
Because single-phase transformers can be built easily, they are widely used to supply electricity to households and small commercial enterprises.
In addition, they are more common in rural areas where there is less demand for electricity, making these transformers economical for such applications.
Iron Core Transformers
Iron core transformers have an electromagnetic core made of laminated iron sheets. This type is the most commonly used in this category. The use of iron cores enables high flux linkage because of their very good magnetic properties. This makes them useful in a large number of applications.
Ferrite Core Transformers
Ferrite core transformers use ferrite as the core material. Ferrite is a type of ceramic comprised of iron oxides, zinc, nickel, and manganese. Examples of ferrites used in transformers are manganese-zinc ferrites and nickel-zinc ferrites.
This class of materials is characterized by high magnetic permeability, or the ability to conduct magnetic flux lines in it.
Additionally, they exhibit high electrical resistivity and low eddy current losses at high frequencies, which makes them advantageous in high frequency applications. Therefore, ferrite core transformers are widely utilized in broadband transformers and other electronics applications.
Air Core Transformers
Air core transformers do not have a core, they have primary and secondary windings placed on a solid cylindrical insulator, which acts as a transformer core. These types of transformers find application in radio frequency current transmission.
Toroidal Core Transformers
Toroidal core transformers are constructed from iron or ferrite. The primary and secondary windings are wrapped around ring shaped cores, or toruses, giving them a donut like shape.
Their toroidal design minimizes magnetic flux leakage and improves inductance and Q factors, which increases overall efficiency. These transformers are commonly found in telecommunications, power distribution, and industrial control systems.
Why are Power Transformers Used?
Power transformers have numerous applications in the electrical power system. Some include:
- Voltage Level Adjustment: For different tasks such as lighting, heating, and communication, power transformers provide tailored voltage levels. A three-phase transformer, for example, can serve single phase power for home needs and three-phase power for industrial use.
- Reduction of losses in electricity transmission: Relatively inexpensive electricity is generated at low voltage levels, which results in a high current. This high current gives rise to large line losses due to ohmic heating. A step-up transformer at the generating station increases the voltage level, which allows the current to be reduced. This is critical, as it improves line losses, points of decrement on the power factor, and overall efficiency. The same situation can also be approached at the receiving side, where a step-down transformer can reduce the voltage level to a suitable value for distribution and consumption.
- Impedance Matching: Power transformers align the load and source impedances by adapting the voltage and current levels, improving power transfer and circuit efficacy.
- Galvanic Isolation Provided: Power transformers provide electrical isolation between circuits of diverse potentials or frequencies. Their use prevents short circuits, ground faults, and significant damage to equipment by electrical interferences.
Power Transformer Applications
Power transformers find application in various sectors, including:
- Power generation: In power plants, power transformers are utilized to increase the voltage at the output terminals (To step up).
- Power transmission: Power transformers are used in stepping up and stepping down voltage in various locations within the transmission network for effective power delivery.
- Power distribution: In power distribution, power transformers step down the voltage to be usable by domestic and commercial consumers. Depending on demand, it functions at varying loads and maintains a high voltage regulation.
- Lighting System: Power transformers are capable of providing both low voltage and high current simultaneously for lighting systems, including fluorescent lamps and neon signs.
- Audio Devices: In speakers, amplifiers, and microphones, power transformers are utilized to isolate and amplify the audio signal.
- Electronic devices: A power transformer provides a low voltage and a regulated power supply for computers, televisions, and other electronic devices.
What Are The Transformer Losses?
The efficiency of power transformers can be affected by four specific losses:
Eddy Current Loss
Eddy currents form in the core’s cross-section because of the change in magnetic fields. In order to reduce losses, the cores of transformers are made of thin, laminated metal sheets.
Each lamina is covered with a special coating that insulates the surfaces, thus reducing the course available for eddy currents, and therefore, their magnitude.
Copper Loss
Also known as resistive or I²R losses, copper or winding losses develop due to the resistive nature of the windings, causing current to pass through them.
The resistance of a winding’s material, which will give rise to these losses, is determined from the length and cross section area of the material and its nature and temperature.
Copper losses are also affected by the resistance of the windings. These losses are quantified using the formula I²R.
Flux Loss
Some of the magnetic flux lines emanating from the primary winding do not go through the secondary winding and instead go through air. These losses are known as flux losses.
The reason for these could be the magnetic saturation of the core, where the core tends to choke on excess flux lines. Also, these losses can be caused by the difference in the reluctance value of air through which the core flows and the core itself.
Hysteresis Loss
The friction of the ferromagnetic molecules of the core during the processes of magnetizing and demagnetizing leads to hysteresis losses.
With each magnetizing force moving in a forward and reverse direction, it generates friction within the transformer, which produces heat.