properly ground a transformer, and the secondary electrical system Figure Control transformers are common in appliances and electrical equipment. This Bachelor thesis is about the basics of Power Transformers, such as . responsible for transformation action in an electrical transformer. A transformer is an electrical apparatus designed to convert alternating Taps are provided on some transformers on the high voltage winding to correct for high .
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The Transformer. The principle parts of a transformer and their functions are: path for the most lines of flux with the least loss in magnetic and electrical energy. Transformers are electrical devices consisting of two or more coils of wire used to transfer electrical energy by means of a changing magnetic field. One of the. The Transformer as an Isolation. Device. • Transformers are useful in providing electrical isolation between the primary circuit and the secondary circuit because.
When current in the primary coil is changed the flux linked to the secondary coil also changes. The transformer is based on two principles: first, that an electric current can produce a magnetic field electromagnetism , and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil electromagnetic induction. Changing the current in the primary coil changes the magnetic flux that is developed.
The changing magnetic flux induces a voltage in the secondary coil.
A simple transformer has a soft iron or silicon steel core and windings placed on it iron core. Both the core and the windings are insulated from each other. The winding connected to the main supply is called the primary and the winding connected to the load circuit is called the secondary.
Winding coil connected to higher voltage is known as high voltage winding while the winding connected to low voltage is known as low voltage winding. In case of a step up transformer, the primary coil winding is the low voltage winding, the number of turns of the windings of the secondary is more than that of the primary.
Vice versa for step down transformer. These breathers form a barrier and resist the atmospheric moisture from contact with oil. Special care must also be taken to avoid sledging.
Sledging occurs when oil decomposes due to overexposure to oxygen during heating. It results in the formation of large deposits of dark and heavy matter that clogs the cooling ducts in the transformer. The quality, durability and handling of these insulating materials decide the life of the transformer. All the transformer leads are brought out of their cases through suitable bushings. There are many designs of these, their size and construction depending on the voltage of the leads.
Porcelain bushings may be used to insulate the leads, for transformers that are used in moderate voltages. Oil-filled or capacitive-type bushings are used for high voltage transformers. The selection between the core and shell type is made by comparing the cost because similar characteristics can be obtained from both types. Most manufacturers prefer to use shell-type transformers for high-voltage applications or for multi-winding design. When compared to a core type, the shell type has a longer mean length of coil turn.
Other parameters that are compared for the selection of transformer type are voltage rating, kilo-volt ampere rating, weight, insulation stress, heat distribution and so on. Transformers can also be classified according to the type of cooling employed.
The different types according to these classifications are: Types of Transformers based on Cooling method 1. 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.
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. Power transformers are available as single-phase or three-phase apparatus. The construction of a transformer depends upon the application.
Transformers intended for indoor use are primarily of the dry type but can also be liquid immersed. For outdoor use, transformers are usually liquid immersed. This section focuses on the outdoor, liquid-immersed transformers, such as those shown in Figure 2. Insulation is known to deteriorate with increases in temperature, so the insulation chosen for use in transformers is based on how long it can be expected to last by limiting the operating temperature.
The temperature that insulation is allowed to reach under operating conditions essentially determines the output rating of the transformer, called the kVA rating. Standardization has led to temperatures within a transformer being expressed in terms of the rise above ambient temperature, since the ambient temperature can vary under operating or test conditions. To obtain absolute temperatures from these values, simply add the ambient temperature.
Standard temperature limits for liquid-immersed power transformers are listed in Table 2. The normal life expectancy of a power transformer is generally assumed to be about 30 years of service when operated within its rating. Outside the U. Based on some standards, the kVA rating can refer to the power that can be input to a transformer, the rated output being equal to the input minus the transformer losses.
Power transformers have been loosely grouped into three market segments based on size ranges. These three segments are: 1.
Small power transformers: to kVA 2. Medium power transformers: to MVA 3. Large power transformers: MVA and above Note that the upper range of small power and the lower range of medium power can vary between 2, and 10, kVA throughout the industry. Transformer insulation is rated by the BIL, or basic impulse insulation level, in conjunction with the voltage rating. Internally, a transformer is considered to be a non-self-restoring insulation system, mostly consisting of porous, cellulose material impregnated by the liquid insulating medium.
Standard insulation classes have been established by standards organizations stating the parameters by which tests are to be performed. Wye-connected windings in a three-phase power transformer will typically have the common point brought out of the tank through a neutral bushing. See Section 2. Depending on the application — for example in the case of a solidly grounded neutral versus a neutral grounded through a resistor or reactor or even an ungrounded neutral — the neutral may have a lower insulation class than the line terminals.
There are standard guidelines for rating the neutral based on the situation. Methods of removing this heat can depend on the application, the size of the unit, and the amount of heat that needs to be dissipated. The windings and core are the primary sources of heat, although internal metallic structures can act as a heat source as well.
The heat is carried by the insulating medium until it is transferred through the transformer tank wall to the external environment. Radiators, typically detachable, provide an increase in the surface area available for heat transfer by convection without increasing the size of the tank.
Fans can be installed to increase the volume of air moving across the cooling surfaces, thus increasing the rate of heat dissipation.
Larger transformers that cannot be effectively cooled using radiators and fans rely on pumps that circulate oil through the transformer and through external heat exchangers, or coolers, which can use air or water as a secondary cooling medium. Ratings are determined based on the temperature of the unit as it coordinates with the cooling equipment that is operating. Usually, a transformer will have multiple ratings corresponding to multiple stages of cooling, as the supplemental cooling equipment can be set to run only at increased loads.
Increases in current result in increases in the magnitude of the forces proportional to the square of the current. Since the fault current is a transient event, it will have the asymmetrical sinusoidal waveshape decaying with time based on the time constant of the equivalent circuit that is characteristic of switching events.
This offset factor is derived from the equivalent transient circuit. K typically varies in the range of 1. As indicated by Equation 2.
The current will experience two peaks per cycle, so the forces will pulsate at Hz, twice the power frequency, acting as a dynamic load. Magnitudes of forces during these situations can range from several hundred kilograms to hundreds of thousands of kilograms in large power transformers.
For analysis, the forces acting on the windings are generally broken up into two subsets, radial and axial forces, based on their apparent effect on the windings.