This is a good time to introduce several additional principles and common terms associated with transformers. This material will be especially helpful from a practical standpoint by helping to clarify concepts. In addition, a number of the terms are commonly used in the electrical industry. Being familiar with the nomenclature will simplify transformer discussions and selections at your work location.
Coupling CoefficientMagnetic coupling between the primary and secondary windings of a transformer was introduced earlier. Maximum coupling occurs when all the lines of flux from the primary coil cut through the secondary coil.
Coupling of Flux Lines
Although maximum coupling is desirable for efficiency of operation reasons, it is not always achievable. The amount of coupling that takes place between the two windings is the Coefficient of Coupling. Winding both coils on an iron core helps to maximize the coupling coefficient by providing a good, confined path for the lines of flux.
Although maximum coupling is desirable for efficiency of operation reasons, it is not always achievable. The amount of coupling that takes place between the two windings is the Coefficient of Coupling. Winding both coils on an iron core helps to maximize the coupling coefficient by providing a good, confined path for the lines of flux.
Transformer Ratings
Transformers are frequently rated in kilovoltamperes (kVA), although there are other rating designations. Very large transformers are often rated in megavoltamperes (MVA), and very small transformers can be rated in voltamperes (VA). The rating tells you the maximum current that a transformer can deliver to a load without overheating.
If you know the voltage and the current, the rating can be calculated. If you know the rating and the voltage, the current can be calculated. The rating of a transformer is the same for both the primary and the secondary.
Transformers are frequently rated in kilovoltamperes (kVA), although there are other rating designations. Very large transformers are often rated in megavoltamperes (MVA), and very small transformers can be rated in voltamperes (VA). The rating tells you the maximum current that a transformer can deliver to a load without overheating.
If you know the voltage and the current, the rating can be calculated. If you know the rating and the voltage, the current can be calculated. The rating of a transformer is the same for both the primary and the secondary.
Two formulas are necessary to calculate transformer ratings; one for single-phase loads, and one for three-phase loads.
Formulas for Calculating Transformer Ratings
Frequency
A transformer cannot change the frequency of the power supply. If the supply is 60 Hz, the output will also be 60 Hz. Because transformers are designed for operation at a particular frequency, frequency requirements must be known to make the proper transformer selection. The frequency of the source should be determined, and the frequency of the load should match. Transformers used in the United States and Canada are usually designed for 60 Hz. A great deal of the rest of the world uses 50 Hz.
A transformer cannot change the frequency of the power supply. If the supply is 60 Hz, the output will also be 60 Hz. Because transformers are designed for operation at a particular frequency, frequency requirements must be known to make the proper transformer selection. The frequency of the source should be determined, and the frequency of the load should match. Transformers used in the United States and Canada are usually designed for 60 Hz. A great deal of the rest of the world uses 50 Hz.
Basic Impulse Level (BIL)
Outdoor electrical distribution systems are subject to lightning surges. Even if the lightning strikes the line some distance from the transformer, voltage surges can travel down the line and into the transformer. Other electrical equipment in the system can also cause voltage surges when they are opened and closed. These surges can be very damaging to transformers and other electrical equipment.
Outdoor electrical distribution systems are subject to lightning surges. Even if the lightning strikes the line some distance from the transformer, voltage surges can travel down the line and into the transformer. Other electrical equipment in the system can also cause voltage surges when they are opened and closed. These surges can be very damaging to transformers and other electrical equipment.
The Basic Impulse Level (BIL) is a measure of the ability of the transformer's insulation system to withstand very high-voltage, short-time surges. The BIL is rated based on:
• the kinds of voltage surge stresses the equipment is likely to encounter
• the design of the electrical system
• the voltage protection provided
To familiarize you with BIL, some typical BIL levels are shown with corresponding voltage classes. These do not apply uniformly for all electrical equipment.
Sound
Although transformers are reliable static devices with no moving parts, they do produce a humming sound. The sound originates in the core. When the magnetic flux passes through the laminated core, the laminations expand and contract, generating a hum. Transformers are designed and constructed in such a manner that the noise is minimized, but not eliminated. The sound level of a transformer is measured in decibels (dB), and determined by tests conducted in accordance with NEMA standards.
Although transformers are reliable static devices with no moving parts, they do produce a humming sound. The sound originates in the core. When the magnetic flux passes through the laminated core, the laminations expand and contract, generating a hum. Transformers are designed and constructed in such a manner that the noise is minimized, but not eliminated. The sound level of a transformer is measured in decibels (dB), and determined by tests conducted in accordance with NEMA standards.
Altitude
Air is thinner at higher altitudes, which impacts transformer cooling. Transformers are designed to operate with a normal temperature rise, at a specific height in feet above sea level. If the operation is to be at a higher altitude, the transformer's nameplate rating must be reduced. This reduction is called derating.
The amount of the reduction depends on how much the standard altitude has been exceeded. Transformer Losses and Efficiency .Most of the energy provided to the primary of a transformer is transferred to the secondary. But, some energy is lost in the form of heat. Most of this heat loss is experienced in the winding or the core.
The lower the losses, the higher the efficiency of the transformer. Losses and efficiency are very important concerns in the selection of a transformer. For example, a transformer with a lower initial cost may not be the best purchasing choice. Another transformer with a higher initial cost, but which is more efficient could prove to be the best purchasing decision in the long run.
A transformer's efficiency is defined as:
Efficiency = Output Power/Input Power
Put another way, the output power equals the input power, less the internal losses of the transformer. Transformer efficiency can vary by manufacturer, transformer type and transformer size. A 20,000 kVA power transformer, for example, might have an efficiency of 99.4%, and a small 5 kVA transformer might be 94%. You can see how efficiency is an important consideration when applying a transformer.
Copper Loss
One type of loss in transformers is Copper Loss. The copper windings, although a good conductor of electricity, are not perfect conductors. Copper has a certain resistance to current flow, as do all materials.
One type of loss in transformers is Copper Loss. The copper windings, although a good conductor of electricity, are not perfect conductors. Copper has a certain resistance to current flow, as do all materials.
One of the factors influencing copper loss is heat. Resistance increases with an increase in temperature. To minimize this problem, large electrical power distribution transformers are often cooled by circulation of water, forced air, or oil. Cooling also helps to prevent heat damage to winding insulation. We will discuss heat issues later in this module.
Heat Dissipation (Cooling)
Heat generated by losses must be removed to prevent deterioration of the transformer's insulation system, and the actual magnetic properties of the core. The insulation system is made up of the materials wound around the primary and secondary winding coils. A transformer's Insulation System Temperature Classification states the maximum temperature permitted in the hottest spot in the winding, at a specified ambient temperature, usually 40°C.
Heat generated by losses must be removed to prevent deterioration of the transformer's insulation system, and the actual magnetic properties of the core. The insulation system is made up of the materials wound around the primary and secondary winding coils. A transformer's Insulation System Temperature Classification states the maximum temperature permitted in the hottest spot in the winding, at a specified ambient temperature, usually 40°C.
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Transformer oil testing NABL
Analysis of transformer oil, over various parameters, is the most efficient and effective way to monitor the equipment’s condition and trend its failure before any catastrophe, thus saving millions. NDL’s laboratory specialises exclusively in the analysis of dielectric fluids. Using the highest quality analytical instruments, we offer a full range of ASTM, IEC & IS insulating oil tests. Our over 30 years of experience in transformer oil analysis guarantees precise and accurate analytical results, with experienced diagnostics and fault analysis. NABL and ILAC accredited
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SF6 breaker maintenance
Analysis of transformer oil, over various parameters, is the most efficient and effective way to monitor the equipment’s condition and trend its failure before any catastrophe, thus saving millions. NDL’s laboratory specialises exclusively in the analysis of dielectric fluids. Using the highest quality analytical instruments, we offer a full range of ASTM, IEC & IS insulating oil tests. Our over 30 years of experience in transformer oil analysis guarantees precise and accurate analytical results, with experienced diagnostics and fault analysis. NABL and ILAC accredited
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Transformer life assessment
Analysis of transformer oil, over various parameters, is the most efficient and effective way to monitor the equipment’s condition and trend its failure before any catastrophe, thus saving millions. NDL’s laboratory specialises exclusively in the analysis of dielectric fluids. Using the highest quality analytical instruments, we offer a full range of ASTM, IEC & IS insulating oil tests. Our over 30 years of experience in transformer oil analysis guarantees precise and accurate analytical results, with experienced diagnostics and fault analysis. NABL and ILAC accredited
to get more - https://www.ndlpower.com/transformer-monitoring-laboratory
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