Power transformer-A comprehensive overview
Power Transformer traditionally every country in the developed world has a network of electricity transmission and distribution, the purpose of which is to transport electricity from the generating station to the places where it is needed. The devices that consume electrical energy are called loads. These loads are numerous in number but sometimes small in size. In practice, these loads are not designed for voltages higher than about 750 volts, given their individual power requirements.
In most countries, the loads are spread over very remote geographical areas. And the economic sources of energy are located at a considerable distance from the loads. In fact, this is the basis of the need for an electrical transmission system and the main problem in this is to transmit energy efficiently within acceptable limits.
but the line losses are directly proportional to the length of the lines and thus the power sequence is ensured by minimizing the losses. In practice, maintaining a specified voltage at the terminals of each low-voltage device is another basic requirement in this regard. In the power system, it has been observed that each consumer keeps turning on or off the electrical load according to his needs and hence the total load also keeps changing over time.
The required voltages are reduced during heavy loads due to voltage drop on the line and return to their normal value when the load is reduced. Basic circuit theory also shows that the voltage on the distribution system decreases as the distance between the generator and the load increases. No device (load) can function properly unless the required voltage for it differs by more than about 10% from the voltage it was designed for. Thus, it becomes necessary to determine the distance required for energy transmission.
Changing the value of DC voltage economically and efficiently is a difficult problem but transformers can easily change the level of AC voltage and this is the main purpose of transformers in power systems. And that is why DC power systems are less used than AC power systems. Considering the current development, high voltage DC transmission systems have been made possible but due to economic reasons, DC transmission is probably happening in fewer places anywhere in the world.
A power transformer’s primary purpose is to change power from one voltage level to another, but it also offers additional advantages. For instance, it can supply low voltage when a motor is starting. Circuits that are not connected to the primary power source can also be operated by it. Shifted voltage from the primary power source can also be obtained using it. In addition, it can be utilized for impedance matching and power system voltage control management.
An all-purpose tool for adjusting the voltage in the transmission and distribution system is a transformer. It is evident from all of these factors that transformers are essential to any distribution and transmission system. Because the system voltage determines the maximum allowable line drops, they are likewise limited. Thus, running the transmission system at the highest voltage is the proper course of action.
A three-phase transformer is made to control the balanced load of a three-phase system, offering a more reliable and continuous power source than a single-phase transformer, which draws electricity from a single-phase system. Three distinct conductors, each carrying alternating current (AC) augmented by two to one-third of the other conductors’ cycles, provide power to a three-phase system.
Three-phase systems are generally utilized for high-load applications because of their ability to transmit power more smoothly and efficiently than single-phase systems. Each of the three alternating magnetic fields produced by the three-phase input current in a three-phase transformer is 120 degrees out of phase. As a result, there is no loss or interruption in the power supply between phases, and the load receives power continuously.
The backbone of effective energy generation, transmission, and distribution is provided by three-phase transformers, which are crucial parts of contemporary electrical networks. They are essential in commercial and industrial settings because of their capacity to deliver balanced power and step up or down voltage levels, which guarantees that electrical power is used as efficiently as possible. Engineers, technicians, and anybody else involved in the administration and operation of electrical power systems must comprehend the types, applications, and operating principles of three-phase transformers.
Working principle of Power transformer
As is well known, a transformer is a device that allows electrical power to be transferred across circuits without altering the frequency. By raising the current, it can raise the circuit’s voltage, and by increasing the current, it can lower the voltage.
A transformer operates by means of mutual induction between two circuits that are coupled by a common magnetic flux, as is well known. The most basic of these is made up of two inductive coils that are magnetically coupled to one another by a magnetic field but electrically isolated from one another.
The two coils have a strong mutual connection. An alternating flux, which always spreads outward from its center and then back towards it, is created in a coil when it is linked to a source of alternating voltages. The majority of this flux links with another coil.
According to Faraday’s law, an EMF is produced in the other coil as a result of the flux expanding and contracting from one coil to the other. This is based on the mutual induction principle. A significant amount of current begins to flow in this circuit if the second coil’s circuit is closed.
This initiates the magnetic transfer of electrical energy from the first coil to the second coil. The primary winding is the first coil that receives electrical energy from the AC supply mains, while the secondary winding is the second coil that receives energy from the load. Without altering the frequency, a transformer moves electrical power between circuits.
It uses electromagnetic induction to carry out its function and complete its mission. Transformers are utilized in motors, generators, and control systems in a variety of ways. A step-up transformer is a type of transformer that transforms low voltage into high voltage. Additionally, a step-down transformer is a transformer that lowers high voltage. A transformer is sometimes known as a stationary machine because it lacks any moving elements.
The transformer needs minimal upkeep and repair because it has no moving parts. Furthermore, out of all the machinery, the transformer has the highest efficiency. The transformer may be readily separated up to the maximum voltage because of a number of features. The primary winding of the transformer is the one that receives input, and the secondary winding is the one that receives output. Any transformer winding can be referred to as the main winding, while the other winding can be referred to as the secondary winding.
High power transformers are those that have a rating more than 500 KVA. Since these power transformers use all cooling techniques, the selection of these techniques is based on factors like cost, operating expenses, and space requirements, among others. Power transformers typically have insulated windings and are of the standard type. We should also make sure that platform-type power transformers are always used. Platform style transformer tanks have a 200 KVA capacity. Additionally, very few lugs or brackets are needed for mounting straight to the pole.
Components of Power transformer
Structurally the transformer consists of the following parts:
1-Steel Tank
This is a tank made of steel or box of suitable shape inside which the core windings and other auxiliary and necessary equipment required for the transformer are installed. This tank is almost full of insulating oil or mineral oil Depending on the structure of the transformer core the tank is cylindrical or cubic in shape. To protect the tank from the effects of various unwanted elements the inner and outer surfaces are cleaned of rust etc. and a layer of paint is applied to it.
2-Core
In all types of transformers the core is always made of galvanized steel sheets to provide a minimum air gap with a continuous magnetic path. Steel containing a high amount of silicon is used for this purpose. Sometimes heat treatment is also performed on steel which greatly increases the permeability of the steel. And with increased permeability hysteresis loss are reduced. Making the core laminated reduces eddy current losses.
Each core layer is insulated from the other layers by a light coating of varnish or by coating the surface with a layer of oxide. The thickness of these layers ranges from 0.35 mm at a frequency of 50 C/S to 0.5 mm at a frequency of 25 C/S. In standard practice the core of a single phase transformer consists of two cores and the core of a three phase transformer consists of three cores. All cores are made from high quality silicon steel sheets.
And these cores are low loss and high permeability (meaning if a piece of iron is placed in a magnetic field the lines of force will gather around the piece to pass through it the measurement of these accumulated lines is called permeability of the material) that contains them. These cores are made in layers to fit the core perfectly inside the coil. Depending on the capacity of the transformer the cores are made round and rectangular. These cores are tightened with bolts to protect them from vibration and to avoid noise.
3-Windings
A single phase transformer has one primary and one secondary winding while a three phase transformer has three primary and three secondary windings which are properly connected. The low voltage winding is after the core and the high voltage winding is wrapped on top of the low voltage winding. Both windings are electrically insulated from each other. There is also adequate distance between the windings for the circulation of the cooling medium.
Each winding is made up of multiple thin discs to improve cooling. Windings is also made in the form of spring coils and such windings are used in the secondary’s of low voltage and high current transformers. Depending on the capacity of the transformer the coil can be square wound or continuous and can also be disk wound. And conductors can be rectangular wire or sheet wire wrapped in polyvinyl wire paper.
To increase the mechanical strength of the coil the windings are heat bonded with special varnish paper, The standard coil in a core type transformer is a concentric arrangement which has a high voltage winding on the outside and a low voltage winding on the inside. There is insulation between both windings.
4-Conservator Tank
This is a small tank that is installed above the transformer main tank. This tank is usually cylindrical in shape. The main tank of the transformer and the conservation tank are connected by a pipe. In transformers with a capacity of more than 1 MVA a buchholz relay is installed between the main tank and the conservator tank. The conservator performs the following three tasks:
a)-Provide space for the oil to expand as it heats up and supplies oil to the main tank when the oil cools down.
b)-Reduces oxidation by reducing the surface area of the oil in contact with air.
c)-The oxidized oil is confined to the conservator tank. A glass tube is attached to one side of the conservator which indicates the level of oil in the transformer. The level of cold oil must be up to the yellow mark on the gauge on this glass tube.
since the expansion co-efficient of insulating oil is high therefore its level of ascent and descent changes considerably with temperature changes. However if a lot of free space is provided at the top of the tank the contact area between the air and the oil increases which causes oil to spoil quickly. To reduce this effect a separate tank containing much less oil than the main tank is installed above the transformer through a pipe which is called a conservator. The conservator reduces the area of contact between the oil and air thus preventing the oil from spoiling quickly.
5-Breather
This is a device through which air enters and exits the transformer. When the oil expands due to heat air is expelled and when the oil cools and contracts air is drawn in. One end of breather is connected to a conservator tank. A glass container is placed inside the breather filled with silica gel. When air enter the transformer tank it also contains moisture vapor.
The silica gel absorbs this moisture and allows the cool air to enter the transformer tank. The color of silica gel is changed from blue to pink after some time due to continuous moisture absorption. This granular silica gel can be heated to make it reusable.
6-Thermometer
All transformers rated above 50 kva are fitted with a thermometer that indicates the oil temperature. In very large transformers a temperature sensor is installed inside the windings which records the temperature of the windings. Whenever the temperature rises to a dangerous level it warn by giving a signal to the alarm circuit.
A dial type thermometer is usually used for this purpose. This thermometer gives a direct reading through a sensor that is placed on the surface of the oil. Whenever the oil temperature rises above a certain temperature the thermometer sends a signal to the alarm circuit.
The thermometer is usually placed near the nameplate of the transformer so that it can be easily seen. If the oil temperature exceeds 105°c the thermometer trips the transformer.
7-Pressure Relief Pipe
A curved pipe with a glass cover is installed above the transformer tank which provides protection against excessive pressure generated inside the transformer due to the evaporation of oil or other insulating material this is called a pressure relief pipe.
The absence of this device can cause the transformer tank to burst. During transformer operation high gas pressure is generated inside the tank due to temperature increase or any other reason. There is a Micarta plate on the top of the vent that ruptures at a pressure of 1.01kg/cm².
Bursting plate breaking device is also used with relief vent with large transformers. This valve is made in such a way that it senses the process of increasing pressure in the relief valve through the adjusting spring and Bellows and the spring driven shaft mechanically breaks the bursting plate releasing the pressure.
8-Valves
Valves are used in each transformer for oil addition, oil extraction, filtering and sampling.
9-Buchholz Relay
This relay is installed in the pipe leading from the conservator tank to the main tank of transformer. This is a gas operated relay that consists of two floats. One float is connected to the trip circuit in case of emergency while the other float only give an alarm signal.
Whenever the oil level is low or any other such fault occurs gases are produced due to the decomposition of the insulating material. These gases being light rise upwards and pass through the Buchholz relay and overturn the float. Due to which the alarm or trip circuit is completed and the transformer is automatically disconnected from the main supply.
10-Bushings
Suitable bushings are used to insulated the windings terminals from the tank. These bushings are porcelain type. oil field type and capacitor type bushings and they are used on both HT and LT sides. To protect against lightning arcing horns are also installed above and below the bushings.
Fully sealed condenser type oil filled bushings are used for transformers rated above 34.5kv and porcelain type bushings are used for transformers rated below 25kv. The condenser type bushing wound like a power capacitor and uses the best quality graphite paper also its vacuum is filled with high quality condenser oil so that its dielectric strength becomes very high. In addition the main element is enclosed inside the porcelain from above and below and then it is sealed tightly with oil.
11-Tap Changing Switch)Off Load Tap Changer:
This is a device that is used to change the voltage ratio of the winding. Tap points are brought out from between the windings to maintain magnetic balance. If the primary voltage is high more turns are added and if the voltage is low a few turns are shorted. No load or off load tap changer performs quickly and efficiently.
The standard models is mounted on the tank cover but floor mounted, hand operated or motorized models are also used. Since they depend on the current voltage and number of taps they vary in terms of structure. These tap changer switches are the off-load type meaning they are operated after the transformer is turned off.
Where the voltage needs to be changed without shutting down the power supply an On load tap changing system is installed with the transformer. Internally operated switches are used for small transformers and externally operated switches are used for large transformers.
b)-On Load Tap Changer:
Voltage regulation is usually changed using a de-energized tap changer installed on the primary or secondary winding of the transformer but this method i.e. the off load tap changing switch has many drawbacks for example the transformer has to be disconnected from the line before operating the tab changing switch.
Considering the load characteristics the transformer voltage must be constant so turning off the power supply every time often leads to considerable difficulties. The on load tap changer solves these problems and is widely used due to its excellent service.
12-Oil Gauge
As the name suggests an oil gauge is a measuring instrument that indicates the level of oil. The oil gauge is usually a dial type with a pointer attached to it to indicate the oil level. The advantage of this is that the oil level can be easily seen from a distance.
Oil gauges are generally used with transformers rated above medium capacity. This gauge has a float that moves up and down according to the oil level. The gear converts this vertical motion into axial motion.
This motion is transmitted to the pointer by a pair of permanent magnets mounted inside and outside the converter. The gauge also has a connection to the alarm on the switchboard. Whenever the oil level drops the oil gauge alarm contacts activate the alarm circuit in the control room.
13-Radiator
To cool transformers with a capacity greater than 50KVA, radiators are positioned along the transformer’s main tank. These take the shape of tubes or pipes and expand the transformer tank’s surface area. In this manner, cooling is accomplished more effectively.
ONAN is the name of this transformer cooling technique. ‘Oil natural air natural’ is what ONAN signifies. With this procedure, the cold transformer oil descends from the top while the heated oil rises to the top. The heated oil cools the radiator fins and main tank as air flows through them after transferring their heat to their exterior surfaces.
14-Cooling Fans
Generally in addition to the radiator cooling fans are also installed on transformer radiators of 1 MVA to 26 MVA. The contacts of the oil temperature gauge in the transformer are set in such a way that when the temperature of the transformer oil reaches 75°c this gauge turns on the cooling fans. This method of providing cooling is called ONAF, which stand for Oil Natural Air Force.
15-Oil Pumps
In addition to radiators and cooling fans oil pumps are also installed on transformer with rating above 26 MVA. Which work to circulate the transformer oil rapidly in the main tank. Such a cooling system is called OFAF, which stands for Oil Force Air Force. The contacts are also present in the oil gauge to turn on the oil pumps at a specific temperature.
16-Transformer Insulation
Typically, a transformer’s primary tank uses mineral insulating oil. Apart from providing insulation, this oil also serves as a cooling agent. These days, transformers also use SF6 gas as an insulating medium. In addition to lowering the transformer’s weight, using SF6 gas shields it from potential fire in the case of a malfunction.
Uses of power transformer
Large Power Type
High-power transformers are those that have a rating more than 500 KVA. These power transformers are cooled using all available techniques. The cost, operational expenses, space needed, etc., all influence the choosing of various approaches.
Power transformers typically have insulated windings and are of the standard type. Platforms are always used for power transformers. Platform-type transformer tanks have a 200 KVA capacity. Additionally, very few lugs or brackets are needed for mounting a straight pole.
Distribution Type
Transformers having a constant potential and a rating below 500 KVA are known as distribution transformers. These transformers are usually oil-immersed, self-cooling transformers. These are intended for mounting on poles and platforms.
In order to provide the low voltage network, they have a special construction and are outfitted with switches and junction boxes as auxiliary devices for grounding and isolating high-voltage cables. These transformers are also protected against overload and lightning.
Small Power Type
This type is self air-cooled constant potential transformer for industrial purposes. Their windings are completely enclosed in a metal box. These transformer are available in the form of autotransformers and in additional single-phase and three-phase configurations.
Single-phase units are available in sizes above 50 KVA while three-phase units are available up to 150 KVA. Their primary voltage ranges from 125 volts to 2400 volts. Standard secondary voltages are 115/230 or 230/460 volts. Their standard sizes are 75, 150, 225, 300 volts amperes (VA).
Sign Lighting Type
These transformers are single-phase constant potential self air-cooled transformer. And these transformers provide 155 or 230 volts by stepping down the low voltage in the signs for the incandescent lamps. Their windings are also completely enclosed in metal box. And these are made up to 5 KVA in size.
Neon Sign Transformers (NSTs):
Purpose: Step up standard mains voltage (120V/230V) to the high voltage (e.g. 3kV–15kV) required to ionize neon gas in glass tubes.
Applications: Used in neon signboards, decorative lighting, and artistic displays.
LED Sign Transformers (LED Drivers):
Purpose: Convert AC mains voltage to low DC voltage (e.g. 12V or 24V) for LED signboards.
Applications: Found in modern digital billboards, backlit signs, and commercial LED displays.
Fluorescent Sign Ballasts:
Purpose: Regulate voltage and current for fluorescent lamps used in signage.
Applications: Used in older illuminated signs and gas station ceilings.
Low Voltage Landscape and Architectural Sign Transformers:
Purpose: Step down mains voltage to 12V or 24V for safe outdoor lighting.
Applications: Used in restaurant signs, pathway lighting, and outdoor advertising boards.
Control and Signal Type
This type of transformer are constant potential and self air-cooled transformer whose job is to control electrically powered switches or other devices and signal circuits by providing the appropriate voltage. These transformers can also be open type meaning that in such a case there is no metal box over their windings and there can also be enclose type.
A signal-type transformer is a specialized transformer designed to transmit, separate, or modify electrical signals in communications, audio, and electronic circuits. Unlike power transformers, which handle high voltages and currents, signal transformers operate at low power levels and are suitable for signal integrity, impedance matching, and isolation.
Audio transformers: Used in audio circuits to match impedance between amplifiers and speakers. Helps eliminate ground loops and interference in audio systems. Found in microphones, professional sound equipment, and PA systems.
Pulse transformers: Transmit digital pulses while isolating circuits, ensuring clean signal transmission. Used in telecommunications, radar, and power electronics for switching applications.
RF (Radio Frequency) Transformers: Designed for high-frequency applications such as radio transmitters, receivers, and antennas. Used in impedance matching, signal coupling, and filtering in RF circuits.
Isolation Transformers: Electrically separate circuits to prevent noise and interference.
Common in medical equipment, test equipment, and sensitive communication systems.
Matching Transformers: Ensure proper impedance matching between different electronic components. Used in radio antennas, transmission lines, and audio equipment to prevent signal loss.
Gaseous Discharge lamp Type
This type of transformer are variable voltage type transformer. These transformers are designed in such a way that the secondary voltage drops as the load increases. The voltage characteristics are achieved by a three-phase transformer with a magnetic leakage paths between the primary and each secondary.
As the load increases the primary leakage flux creates more volt drop in the primary winding in this way secondary voltage is reduced. Some such transformers also have a capacitor inside the metal box which improve the power factor. These transformers are self air-cooled type transformers.
Bell Ringing Type
This type of transformer are specially made with low capacitance and constant potential and they are used for doorbells, buzzers and door openers etc. These are self-contained air-cooled units enclosed in a metal box. From these 120 to 240 volts primary and single secondary 10 volts or three secondary voltages 6 volt, 12 volt and 18 volt can be obtained.
Step-down function: Converts high voltage to a low voltage suitable for doorbell operation.
Intermittent duty cycle: Designed for short periods of use rather than continuous operation.
Isolation: Provides electrical isolation between the mains supply and the low voltage circuit.
Encapsulated or Open Frame: Comes in enclosed or open-core designs for easy installation. Wall or Panel Mounting: Often mounted in junction boxes or near electrical panels.
Most commonly used in residential and commercial doorbells. Used in security systems as electrical buzzers or alarms that signal unauthorized access. Often found in warehouse, office, and school security systems where a low voltage signal triggers a warning sound.
Instrument Type
This type of transformer are used to reduce the voltage or current output of various application such as ampere meters, volt meters, watt meters and various protective relays. In practice it is difficult to make these transformers for high voltages. Instrument transformers are of two types.
a)-Current type
b)-Potential type
The different types of these transformers depend on the voltage installation requirement of the circuit and the accuracy required. There is also a possibility of some error in the readings obtained from the instruments that instrument transformers are suppling.
This type of transformer are not rated for the thermal basis of the safe load applied rather these transformers are rated based on accuracy. Volt-ampere rating is the load that a transformer can handle without exceeding the specified accuracy limit. If accuracy is ignored the transformers can be used at a value much higher than its rated value.
Instrument transformer are made of indoor. outdoor and portable type. Potential transformers are usually rated for 120 secondary volts while current transformers are usually rated for secondary currents of 5 amperes.
Constant Current Type
Running low voltage lamps in parallel on a constant potential system to illuminate a very large area increases costs significantly therefore costs are controlled by connecting the lamps in series and running them through a constant current transformer.
A constant current transformer is a special type transformer that converts alternating current into constant current at a constant potential whose voltage keeps changing with the load. It has a primary coil to which a constant voltage is applied and a secondary coil that moves relative to the primary and consists of a core with low magnetic resistance.
Its regulation depends on the magnetic leakage between the primary and secondary coils .However in a constant current transformer the mutual or relative states of the primary and secondary coils are adjusted through a repelling action. As the current increases the repelling force also increases causing the coils to separate.
When the current value is less than normal the repelling force ends and secondary coils become closer to each other and the normal state of current is restored. The primary winding can be wound for any desired sub-potential for example up to a maximum of 10 KV volts while the secondary can be wound to the voltage required for multiple lamps in the circuit from 15 to 200 or more lamps. This type of transformer are cooled by natural air or oil.
Series Type
This type of transformer are used with the series of incandescent street lamps and depending on the main series circuit they operate the lamps at different values of current. Their working principle is similar to that of instrument current transformer. Their primary is connected in series with the main series circuit while the lamps are connected to the secondary.
A series-type transformer is a type of transformer that is connected in series with a circuit to affect voltage, current, or impedance. Unlike conventional transformers, which are usually connected in parallel to step up or step down voltage, series-type transformers primarily function in power flow control, protection, or voltage regulation.
They are used in power measurement and protection by stepping down high currents to a measurable level. Found in high-voltage testing equipment, they resonate at a specific frequency to produce high voltages.
They are used in flexible AC transmission systems (FACTS) to control power flow and improve stability. They are used in power distribution to regulate voltage by injecting or absorbing voltage in series with the line.
