Transformers For The Electric Power Industry Pdf
Sinikka Curz
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.
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:
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).
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:
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.
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 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.
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.
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.
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.
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.
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.
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