Transformers: Types, Parts, Operation

123 Oil and Gas Equipments

A transformer is an electrical device which, by the principles of electromagnetic induction, transfers electrical energy from one electric circuit to another, without changing the frequency. The energy transfer usually takes place with a change of voltage and current. Transformers either increases or decreases AC voltage.

 

Transformers are used to meet a wide variety of needs. Some transformers can be several stories high, like the type found at a generating station or small enough to hold in your hand, which might be used with the charging cradle for a video camera. No matter what the shape or size, a transformers purpose remains the same: transforming electrical power from one type to another.

 

In Very Simple words.

Transformer is a device which:

1. Transfer Electrical power from one electrical circuit to another Electrical circuit.
2. It’s working without changing the frequency.
3. Work through on electric induction.
4. When, both circuits take effect of mutual induction.
5. Can’t step up or step down the level of DC voltage or DC Current.
6. Can step up or step down the level of AC voltage or AC Current.

 

BASIC PARTS OF TRANSFORMER

Any transformer consists of the following three basic parts in it.

 

1. Primary coil.
The primary coil is the coil to which the source is connected. It may be the high voltage side or low voltage side of the transformer. An alternating flux is produced in the primary coil.

 

2. Secondary coil
The output is taken from the secondary coil. The alternating flux produced in the primary coil passes through the core and links with their coil and hence emf is induced in this coil.

 

3. Magnetic coreThe flux produced in the primary passes through this magnetic core. It is made up of laminated soft iron core. It provides support to the coil and also provides a low reluctance path for the flux.

 

COMPONENTS OF TRANSFORMER

These are the basic components of a transformer.

1. Core
The core acts as support to the winding in the transformer. It also provides a low reluctance path to the flow of magnetic flux. It is made of laminated soft iron core in order to reduce eddy current loss and Hysteresis loss. The composition of a transformer core depends on such as factors voltage, current, and frequency. The diameter of the transformer core is directly proportional to copper loss and is inversely proportional to iron loss. If the diameter of the core is decreased, the weight of the steel in the core is reduced, which leads to less core loss of the transformer and the copper loss increase. When the diameter of the core is increased, the vise versa occurs.

 

Why Are Windings Made of Copper?
• Copper has high conductivity. This minimizes losses as well as the amount of copper needed for the winding (volume & weight of winding).
• Copper has high ductility. This means it is easy to bend conductors into tight windings around the transformer’s core, thus minimizing the amount of copper needed as well as the overall volume of the winding.

 

2. Winding
Two sets of winding are made over the transformer core and are insulated from each other. Winding consists of several turns of copper conductors bundled together, and connected connected in series.

 

Winding can be classified in two different ways:
1. Based on the input and output supply
2. Based on the voltage range

 

Within the input/output supply classification, winding are further categorized:
1. Primary winding – These are the winding to which the input voltage is applied.
2. Secondary winding – These are the winding to which the output voltage is applied.

 

Within the voltage range classification, winding are further categorized:
1. High voltage winding – It is made of copper conductor. The number of turns made shall be the multiple of the number of turns in the low voltage winding. The conductor used will be thinner than that of the low voltage winding.
2. Low voltage winding – It consists of fewer number of turns than the high voltage winding. It is made of thick copper conductors. This is because the current in the low voltage winding is higher than that of high voltage winding.

 

According to capacity of transformer three types of coils are usually designed:
• Square wound
• continuous
• Disk wound

Input supply to the transformers can be applied from either low voltage (LV) or high voltage (HV) winding based on the requirement.

 

3. Insulating Materials
Insulating paper and cardboard are used in transformers to isolate primary and secondary winding from each other and from the transformer core.
Transformer oil is another insulating material. Transformer oil performs two important functions: in addition to insulating function, it can also cool the core and coil assembly. The transformer’s core and winding must be completely immersed in the oil. Normally, hydrocarbon mineral oils are used as transformer oil. Oil contamination is a serious problem because contamination robs the oil of its dielectric properties and renders it useless as an insulating medium.

 

4. Transformer oil
Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

 

5. Conservator Tank
It is a small tank which used in high power transformers. It is connected above the main tank of transformer. It has cylindrical shape.Main tank and conservator tank connected to each other through a pipe. Buchholz relay is used between conservator tank and main tank in transformers having capacity more than one MVA. Conservator tank have following functions in transformer :

 

• It provide place for hot transformer oil to expand. It also provide oil in transformer after oil become cool.
• It also use to decrease oxidation by reducing area of oil around air.
• Oxidized oil remain in conservator tank. Mirror tube is also connected with conservator tank to read oil level in transformers. A pre marked gauge is also present in mirror tube. It is necessary to have cool oil level up to mark of gauge.

 

6. Breather
The breather controls the moisture level in the transformer. Moisture can arise when temperature variations cause expansion and contraction of the insulating oil, which then causes the pressure to change inside the conservator. Pressure changes are balanced by a flow of atmospheric air in and out of the conservator, which is how moisture can enter the system.

If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to internal faults. Therefore, it is necessary that the air entering the tank is moisture-free.
The transformer’s breather is a cylindrical container that is filled with silica gel. When the atmospheric air passes through the silica gel of the breather, the air’s moisture is absorbed by the silica crystals. The breather acts like an air filter for the transformer and controls the moisture level inside a transformer. It is connected to the end of breather pipe.

 

7. Tap Changer
The output voltage of transformers varies according to its input voltage and the load. During loaded conditions, the voltage on the output terminal decreases, whereas during off-load conditions the output voltage increases. In order to balance the voltage variations, tap changers are used. Tap changers can be either on-load tap changers or off-load tap changers. In an on-load tap changer, the tapping can be changed without isolating the transformer from the supply. In an off-load tap changer, it is done after disconnecting the transformer. Automatic tap changers are also available.
Tap changer switch is used to regulate secondary voltage in case of low voltage in primary side of transformer. Two type of tap changing switches are used:

1. Off load switch : It is used to change winding voltage ratio. Tap changing switch are connected with high voltage side of transformer. As it name suggest off load tap changing switch used only in transformer off condition.
2. On Load Switch : On load tap changer switch can be used with on load transformer.

 

8. Cooling Tubes
Cooling tubes are used to cool the transformer oil. The transformer oil is circulated through the cooling tubes. The circulation of the oil may either be natural or forced. In natural circulation, when the temperature of the oil rises the hot oil naturally rises to the top and the cold oil sinks downward. Thus the oil naturally circulates through the tubes. In forced circulation, an external pump is used to circulate the oil.

 

9. Buchholz Relay
The Buchholz Relay is a protective device container housed over the connecting pipe from the main tank to the conservator tank. It is used to sense the faults occurring inside the transformer. It is a simple relay that is operated by the gases emitted during the decomposition of transformer oil during internal faults. It helps in sensing and protecting the transformer from internal faults.

 

10. Explosion Vent
The explosion vent is used to expel boiling oil in the transformer during heavy internal faults in order to avoid the explosion of the transformer. During heavy faults, the oil rushes out of the vent. The level of the explosion vent is normally maintained above the level of the conservatory tank.

 

11. Radiator :
In 50KVA above transformers, radiators are used with main tank of transformer for cooling purpose. It is like a pipes or tubes. It increases the surface area of transformer. Radiator makes cooling in transformer more effective. This method of cooling is called ONAN ( oil natural air natural).

 

12. Cooling fans :
In 26MVA and above transformers, cooling fans are also used on radiator. Oil temperature gauge provide on or off signal for cooling fans. When temperature becomes greater than 75º, temperature oil gauge turn on cooling fans. This method of cooling is called ONAF ( oil natural and air forced ).

 

13. Oil pumps :
In 26 MVA above transformers oil pumps are also used along with cooling fans and radiator.oil pumps used to rotate oil in transformer. This method of cooling is called OFAF ( oil forced and air forced) .

 

14. Oil gauge :
Oil gauge is used for the measurement of oil in transformer. It displays oil level. Oil gauge is usually of dial type. Pointer on dial type gauge used to measure oil level. It is used with medium level to high voltage transformers.

 

15. Bushings :
Bushings are used to bring windings terminals out of tank and also use for insulation. For example porcelain, oil filled and capacitor type bushings. Arching horns are also connected to bushings to provide protection from lightning. In above 34 KV transformer, completely sealed condenser type bushings are used. In less than 25KV transformer plain bushings are used.

 

16. Thermometer :
Thermometer is also used in above 50KVA transformers. It is used to measure temperature of oil. In high power transformers, thermometer is also used inside windings which measure temperature of windings. Whenever temperature increase up to dangerous level, it activates alarum signal.

 

HOW TRANSFORMERS WORK

It is important to remember that transformers do not generate electrical power; they transfer electrical power from one AC circuit to another using magnetic coupling. The core of the transformer is used to provide a controlled path for the magnetic flux generated in the transformer by the current flowing through the windings, which are also known as coils.
There are four primary parts to the basic transformer. The parts include the Input Connection, the Output Connection, the Windings or Coils and the Core.

 

* Input Connections – The input side of a transformer is called the primary side because the main electrical power to be changed is connected at this point.

* Output Connections – The output side or secondary side of the transformer is where the electrical power is sent to the load. Depending on the requirement of the load, the incoming electric power is either increased or decreased.

 

* Winding – Transformers have two windings, being the primary winding and the secondary winding. The primary winding is the coil that draws power from the source. The secondary winding is the coil that delivers the energy at the transformed or changed voltage to the load. Usually, these two coils are subdivided into several coils in order to reduce the creation of flux.

 

* Core – The transformer core is used to provide a controlled path for the magnetic flux generated in the transformer. The core is generally not a solid bar of steel, rather a construction of many thin laminated steel sheets or layers. This construction is used to help eliminate and reduce heating.

 

When an input voltage is applied to the primary winding, alternating current starts to flow in the primary winding. As the current flows, a changing magnetic field is set up in the transformer core. As this magnetic field cuts across the secondary winding, alternating voltage is produced in the secondary winding.

 

The ratio between the number of actual turns of wire in each coil is the key in determining the type of transformer and what the output voltage will be. The ratio between output voltage and input voltage is the same as the ratio of the number of turns between the two windings.
A transformers output voltage is greater than the input voltage if the secondary winding has more turns of wire than the primary winding. The output voltage is stepped up, and considered to be a “step-up transformer”. If the secondary winding has fewer turns than the primary winding, the output voltage is lower. This is a “step-down transformer”.

 

In short, a transformer carries the operations shown below:
1. Transfer of electric power from one circuit to another.
2. Transfer of electric power without any change in frequency.
3. Transfer with the principle of electromagnetic induction.
4. The two electrical circuits are linked by mutual induction.

 

IDEAL TRANSFORMER CHARACTERISTICS

An ideal transformer is characterized by the following:

1. There is NO flux leakage which means that the fluxes associated with the primary and secondary currents are restricted within the core.
2. Primary and Secondary windings have no resistances which mean that the applied voltage (source voltage) v1 is same as the induced primary voltage e1; that is, v1 = e1. Similarly, v2 = e2.
3. The magnetic core has an infinite permeability which means that the reluctance of the core is zero. Hence, a very small amount of current is needed to set up the magnetic flux.
4. The magnetic core is lossless which means that hysteresis, as well as eddy current losses, are insignificant.

 

TRANSFORMER CONFIGURATIONS

There are different configurations for both single-phase and three-phase systems.
• Single-phase Power – Single-phase transformers are often used to supply power for residential lighting, receptacle, air-conditioning, and heating needs. Single phase transformers can be made even more versatile by having both the primary winding and secondary winding made in two equal parts. The two parts of either winding can then be reconnected in series or parallel configurations.

 

• Three-phase Power – Power may be supplied through a three-phase circuit containing transformers in which a set of three single-phase transformers is used, or on three-phase transformer is used. When a considerable amount of power is involved in the transformation of three-phase power, it is more economical to use a three-phase transformer. The unique arrangement of the windings and core saves a lot of iron.

 

• Delta and Wye Defined – There are two connection configurations for three-phase power: Delta and Wye. Delta and Wye are Greek letters that represent the way the conductors on the transformers are configured. In a delta connection, the three conductors are connected end to end in a triangle or delta shape. For a wye, all the conductors radiate from the center, meaning they are connected at one common point.

 

• Three-phase Transformers – Three-phase transformers have six windings; three primary and three secondary. The six windings are connected by the manufacturer as either delta or wye. As previously stated, the primary windings and secondary windings may each be connected in a delta or wye configuration. They do not have to be connected in the same configuration in the same transformer. The actual connection configurations used depend upon the application.

 

TYPES OF TRANSFORMERS

Transformers can be classified on different basis, like types of construction, types of cooling etc.

 

(A) BASED ON CONSTRUCTION

1. Core Type Transformer
It has a single magnetic circuit. The core rectangular having two limbs. The winding encircles the core. The coils used are of cylindricaltype. As mentioned earlier, the coils are wound in helical layers with different layers insulated from each other by paper or mica. Both the coils are placed on both the limbs. The low voltage coil is placed inside near the core while high voltage coil surrounds the low voltage coil.Core is made up of large number of thin laminations. As The windings are uniformly distributed over the two limbs, the natural cooling is more effective. The coils can be easily removed by removing the laminations of the top yoke, for maintenance.

 

2. Shell Type Transformer
It has a double magnetic circuit. The core has three limbs. Both the windings are placed on the central limb. The core encircles most partof the windings. The coils used are generally multilayer disc type or sandwich coils. As mentioned earlier, each high voltage coil is in
between tow low voltage coils and low voltage coils are nearest to top and bottom of the yokes.The core is laminated. While arranging the laminations of the core, the care is taken that all the joints at alternate layers are staggered.This is done to avoid narrow air gap at the joint, right through the cross-section of the core. Such joints are called over lapped or imbricated joint. Generally for very high voltage transformers, the shell type construction is preferred. As the windings are surrounded by the core, thenatural cooling does not exist. For removing any winding for maintenance, large number of laimnations are required to be removed.

 

(3) Berry type transformer
The core looks like spokes of wheels. Tightly fitted metal sheet tanks are used for housing this type of transformer with transformer oil filled inside.

(B) BASED ON PURPOSE

1. Step up transformer: Voltage increases (with subsequent decrease in current) at secondary.
2. Step down transformer: Voltage decreases (with subsequent increase in current) at secondary.

 

(C) BASED ON USE

1. Power transformer: Used in transmission network, high rating
2. Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.
3. Instrument transformer: Used in relay and protection purpose in different instruments in industries
* Current transformer (CT)
* Potential transformer (PT)

 

(D) BASED ON COOLING

1. Oil Filled Self-Cooled Type
Oil filled self-cooled type uses small and medium-sized distribution transformers. The assembled windings and core of such transformers are mounted in a welded, oil-tight steel tanks provided with a steel cover. The tank is filled with purified, high quality insulating oil as soon as the core is put back at its proper place. The oil helps in transferring the heat from the core and the windings to the case from where it is radiated out to the surroundings.
For smaller sized transformers the tanks are usually smooth surfaced, but for large size transformers a greater heat radiation area is needed, and that too without disturbing the cubical capacity of the tank. This is achieved by frequently corrugating the cases. Still larger sizes are provided with radiation or pipes.

 

2. Oil Filled Water Cooled Type
This type is used for much more economic construction of large transformers, as the above-told self-cooled method is very expensive. The same method is used here as well- the windings and the core are immersed in the oil. The only difference is that a cooling coil is mounted near the surface of the oil, through which cold water keeps circulating. This water carries the heat from the device. This design is usually implemented on transformers that are used in high voltage transmission lines. The biggest advantage of such a design is that such transformers do not require housing other than their own. This reduces the costs by a huge amount. Another advantage is that the maintenance and inspection of this type is only needed once or twice in a year.

 

3. Air Blast Type
This type is used for transformers that use voltages below 25,000 volts. The transformer is housed in a thin sheet metal box open at both ends through which air is blown from the bottom to the top.

 

(E) BASED ON WINDING

1. Two Winding Transformer
The two-winding transformer is one in which two windings are linked by a common time-varying magnetic flux. One of these windings, known as the primary, receives power at a given voltage from a source; the other winding, known as the secondary, delivers power, usually at a value of voltage different from that of the source, to the load. The roles of the primary and secondary windings can be interchanged. However, in iron-core transformers a given winding must operate at a voltage that does not exceed its rated value at rated frequency – otherwise the exciting current becomes excessive.

 

2. Auto Transformer
The autotransformer is a special type of power transformer. It consists of a single, continuous winding that is tapped on one side to provide either a step-up or a step-down function. This is different from a conventional two-winding transformer, which has the primary and secondary completely isolated from each other, but magnetically linked by a common core. The autotransformer’s windings are both electrically and magnetically interconnected.

 

USES AND APPLICATION OF TRANSFORMER

The most important uses and application of Transformer are:
• It can rise or lower the level of level of Voltage or Current (when voltage increases, current decreases and vice versa because P =V x I, and Power is same) in an AC Circuit.
• It can increase or decrease the value of capacitor, an inductor or resistance in an AC circuit. It can thus act as an impedance transferring device.
• It can be used to prevent DC from passing from one circuit to the other.
• The transformer used for impedance matching.
• The transformer used for isolate two circuits electrically.
• The transformer used in voltmeter, ammeters, protective relay etc.
• The transformer used in rectifier.
• It is used in voltage regulators, voltage stabilizers, power supplies etc.

 

Transformer is the main reason to transmit and distribute power in AC instead of DC, because Transformer not work on DC so there are too difficulties to transmit power in DC. in the DC Transition and distribution, the level of voltage Step up by Buck and Boost Converter but it is too costly and not suitable economically. The main application of Transformer is to Step up (Increase) or Step down (Decrease) the level of Voltage. in other words, Increase or decries the level of Current, while Power must be same.

 

Other Uses and application of Transformer:

It step up the level of voltage at generation side before transmission and distribution.
in distribution side, for commercial or domestic use of electricity, transformer step down ( decries) the level of voltage for example form 11kV to 220 V single phase and 440 V three phase.

The Current Transformer and Potential Transformer also used power system and in the industry. Also, it is used for impedance matching. So these were the simple uses and application of transformer.