This article provides a complete guide to the operating principles, configurations, and constructions of a three-phase transformer.
An electrical transformer is a passive device that transfers electrical energy between circuits through magnetic flux, inducing an electromotive force. It can either increase (step-up) or decrease (step-down) voltage levels while maintaining the same frequency of the electric current. Additionally, transformers provide electrical isolation between circuits, which is especially beneficial in signal processing applications.
Electrical transformers operate based on Faraday‘s law of induction, which states the relationship between the rate of change of a magnetic flux and the induced electrical field. It was observed that placing a conductor near a varying magnetic field creates an electric current in that conductor. The magnetic field can be from an electromagnet with a varying electric current, such as an alternating current (AC). This electromagnetic circuit is referred to as the primary winding. As the electrical current is generated and collapsed continuously at a given frequency, a magnetic field is also created and collapsed the same way. This magnetic field with varying amplitude induces an electric current to conductors influenced by its flux. The influenced conductor is usually a coil, referred to as the secondary winding. The induced electrical current has the same frequency as the electrical current from the primary circuit.
A varying magnetic field is not the sole method for inducing a current. Instead, a magnetic field can be visualized as comprising numerous lines of induction. When a conductor moves through these magnetic field lines, it can generate an electric current. This principle is commonly observed in electrical generators.
As mentioned earlier, transformers are used to change voltage levels as needed. To understand three-phase transformers, it is best to understand first the construction of a single-phase transformer. A single-phase transformer has two electrical coils as, again, the primary and secondary winding. The primary winding is where the power supply is connected, while the secondary winding is where electricity is induced.
The two windings in a transformer do not directly contact each other; instead, they are linked by a closed magnetic iron core, which channels the magnetic field. As the magnetic field flows through this core, which is also a conductor, it generates electrical currents known as eddy currents. These eddy currents lead to energy losses and heating within the transformer. To minimize these effects, the core's resistivity is increased by using thin sheets or laminations, which reduce the formation of eddy currents.
The two windings in a transformer are not electrically connected but are linked through a magnetic field. By adjusting the number of coils in the secondary winding relative to the primary winding, the voltage can be either increased or decreased. As a linear device, a transformer’s output voltage can be predicted based on the turns ratio (TR), which is the ratio of the number of turns in the primary winding to those in the secondary winding. The voltage ratio between the primary and secondary windings is directly proportional to this turns ratio.
In simple terms, power is calculated by multiplying voltage by current. In an ideal transformer, which has no losses, power remains constant between the primary and secondary windings. This means the power output in the secondary winding equals the power input in the primary winding. Therefore, if the voltage is increased, the current must decrease proportionally, and if the voltage is decreased, the current must increase accordingly.
Transformers typically have power ratings between 88% and 99%. Power losses in transformers can be categorized into iron or core losses, copper losses, stray losses, and dielectric losses. Core losses are further divided into hysteresis and eddy currents. Hysteresis losses occur due to the energy required to reverse the magnetic field as it changes direction, which is dissipated as heat. Eddy currents are induced by the magnetic field from the primary winding within the core and do not contribute to useful work. These currents are minimized by using laminations in the core.
Copper loss in transformers results from the resistance of the copper windings. As electrical current flows through these conductors, it encounters resistance, leading to a voltage drop and energy dissipation in the form of heat. Reducing copper loss typically involves increasing the cross-sectional area of the conductors, which, in turn, necessitates a larger and more costly transformer.
Stray loss occurs due to the leakage of the magnetic field affecting other conductive parts of the transformer. Although this magnetic field is weaker than that within the iron core, it still induces eddy currents in nearby conductive materials. However, these currents have a minimal impact compared to the primary eddy currents generated in the core.
In transformers, dielectric materials serve as insulation between turns or layers of windings. Transformer oil provides additional insulation, prevents arcing, and aids in heat dissipation. Dielectric loss arises from the deterioration of these insulating materials and the transformer oil over time.
Three-phase transformers operate within a three-phase electrical system, unlike the single-phase transformers discussed earlier. Although they share the same working principle, Faraday’s Law of Induction, their wiring configurations differ. To grasp these differences fully, it’s useful to explore three-phase electrical systems in more detail.
Single-phase and three-phase electrical systems utilize alternating current (AC), which continuously varies in amplitude and direction, typically following a sine wave. Other waveforms, like complex, triangular, and square waves, can also be generated. AC signals are defined by three primary properties: period, frequency, and amplitude. The period and frequency describe the wave’s time component, while amplitude indicates the strength or magnitude of the current.
An AC waveform completes a cycle with a peak and a trough, occurring at 90° and 270° within a 360° cycle. In a single-phase electrical system, the current has one peak and one trough, flowing through a single conductor, with the amplitude reaching its maximum in opposite directions. In contrast, a three-phase system features three separate conductors, each with its own peak and trough. These currents are staggered by 120° from one another. Consequently, three-phase systems achieve peak amplitudes more frequently within a given period, providing a more consistent power delivery.
A three-phase transformer includes six windings: three for the primary side and three for the secondary side. Each set of windings can be configured in either a delta or star arrangement. Essentially, these windings function as individual single-phase windings. In principle, a three-phase transformer can be constructed by connecting three single-phase transformers together.
The two primary configurations for three-phase connections are delta and star. The delta connection, also known as mesh connection, involves connecting three windings end-to-end to form a closed loop. The ends are connected to terminals, with no neutral point present. Instead, grounding connections are utilized. Additionally, delta connections can be configured as high-leg systems by grounding the midpoint of one phase. In this setup, the voltage measured across the line opposite the center-tapped phase and ground is higher compared to the voltage measured across the terminals.
The star connection, also referred to as the wye connection, features three windings and four terminals. In this configuration, one end of each winding is connected to a common neutral point, while the other ends create the three phases of the circuit.
Using either delta or star connections comes with its own set of advantages and disadvantages. To fully understand these, it’s essential to distinguish between phase and line voltages and currents. Phase voltage and current are measured across a single component, while line voltage and current are measured between two terminals. Here’s a summary of the relationships between phase and line voltages and currents for delta and star connections:
| Delta | Star | |
| Voltage | VL = VP | VL = VP x √3 |
| Current | IL = IP x √3 | IL = IP |
Where: VL = Line Voltage
VP = Phase Voltage
IL = Line Current
IP = Phase Current
In star connections, the line and phase currents are identical, while the voltages differ by a factor of √3. This setup allows for the supply of different voltages without requiring additional transformers. For instance, a 230V star connection can also provide 400V. The 230V supply is obtained by connecting between a phase terminal and the neutral point, whereas the 400V supply is accessed by connecting between two phase terminals.
In delta connections, the line and phase voltages are identical, but the currents differ by a factor of √3. Specifically, the line current in a delta connection is higher than the phase current. As a result, to handle the increased current, thicker insulation is necessary. Because of this, star connections are generally preferred for power transmission and distribution networks, as they require less insulation material.
One advantage of delta connections is their reliability. If one winding on the primary side fails, the secondary side can continue to supply voltage as long as the remaining two phases can handle the load. This feature allows the system to remain operational even in the event of a partial failure, thereby enhancing overall reliability.
In a three-phase transformer, the primary and secondary sides can have either the same or different configurations. The four possible permutations are: Star-Star (Y-Y),Star-Delta (Y-Δ),Delta-Star (Δ-Y),Delta-Delta (Δ-Δ)
This configuration features star windings on both the primary and secondary sides. In this setup, the line voltages on each side are √3 times the phase voltage. A key advantage of the star-star connection is that it provides access to a neutral terminal on both sides of the transformer, which can be grounded if desired. Grounding the star neutral helps eliminate waveform distortion. However, without grounding, the star-star configuration operates satisfactorily only if the loads on all three phases are balanced.
As the name suggests, this configuration has a star connection on the primary side and a delta connection on the secondary side. The star connection on the primary allows the neutral point to be grounded, which helps prevent distortion. In this setup, the phase voltage on the primary side is equal to the line voltage divided by √3, which is approximately 58% of the line voltage. Consequently, with a 1:1 turns ratio, a star-delta connection reduces the voltage on the secondary side by a factor of 0.58. This makes the star-delta configuration suitable for step-down transformers.
The main disadvantage of the star-delta configuration is that the primary side voltages are measured from line to neutral, while the secondary side voltages are measured from line to line. This results in a phase difference between the primary and secondary sides, making it challenging to parallel star-delta transformers with other types of winding connections.
On the primary side of a delta-star transformer, the line and phase voltages are the same. On the secondary side, however, the line voltage is √3 times the phase voltage. Therefore, with a 1:1 turns ratio, the secondary line voltage is increased by a factor of √3. Delta-star transformers are commonly used for step-up applications. Additionally, they are useful in distribution systems, as the secondary side provides a neutral point that can be used for single-phase power supply in addition to delivering full three-phase power.
Similar to the star-delta configuration, the delta-star setup also results in a phase difference between the primary and secondary sides due to the different winding configurations. This phase shift can complicate the paralleling of delta-star transformers with other types of transformers that have different winding arrangements.
In the delta-delta configuration, the line-to-line voltages on both the primary and secondary sides are equal to the phase voltage. One of the main advantages of this setup is that it maintains equal three-phase voltages even under unbalanced loads. Additionally, since both the primary and secondary sides use the same type of windings, there is no phase shift between them.
The disadvantages of delta-delta connections include the lack of a neutral point on both the primary and secondary sides and the higher cost of the required coils. Transformers with delta connections need to be wound to handle the full line voltage, which makes this configuration more suitable for low voltage applications.
In addition to the four standard permutations, there are other types of three-phase transformers that arise from modifications of the basic star and delta windings. These additional configurations include
The open delta connection, also known as V-V, is created by removing two windings from the primary and secondary sides of a delta-delta transformer circuit. This configuration provides three equal three-phase voltages at the secondary terminals under no load. It is typically used when the three-phase load is too small to justify the installation of a full three-phase transformer. With an open delta transformer bank, the maximum three-phase load that can be supported without exceeding the transformers' ratings is approximately 58% of what a full delta-delta transformer could handle. If the load increases, the open delta can be converted to a delta-delta configuration by adding the missing windings.
This is also known as a T-T connection. This is accomplished by having two transformers, with one transformer having center taps, on both the primary and secondary winding. The main transformer has the center taps while the other transformer has a 0.87 tap known as the teaser transformer. The full rating of the transformer is not being utilized since the teaser transformer only operates at 87% of its rated voltage. The winding is represented by the figure below. This type of connection is done to link a three-phase system with a two-phase system. A common application of this conversion is a power supply on a two-phase system electric furnace.
A high leg delta configuration is achieved by center-tapping one leg of the delta-connected secondary side and grounding this center tap. This setup provides both a three-phase, delta-connected supply and a single-phase supply. It is commonly used in residential and commercial distribution systems, allowing consumers to receive 240V line-to-line for larger equipment and 120V line-to-neutral for lighting and appliances, all without the need for an additional transformer.
High leg delta connection transformers are also known as red leg, wild leg, or orange leg connections. This configuration is commonly used in North America, particularly in the United States. The term "orange leg" refers to the 208V supply, which is typically marked with an orange outer finish.
Normal transformers are loud and lack energy efficiency. Toroidal transformers break with tradition, operate very quietly, and produce minimal heat. The core of a toroidal transformer is shaped like a donut with primary and secondary windings that are separated by insulation, a design that minimizes magnetic leakage that causes noise.
The voltage of a toroidal transformer is stepped up or down based on the magnetic fields interacting with the secondary coil. The output voltage is determined by the number of windings on the secondary coil; for example, doubling the number of windings results in a doubling of the voltage. The primary coil generates a positive magnetic field that facilitates the flow of electrons in either direction.
In a toroidal transformer with an alternating current (AC) signal, the magnetic field first reaches a peak and then decreases to zero voltage, completing the first half-cycle. As the cycle progresses, the magnetic field reverses direction, causing the electrons to flow in the opposite direction through the coil, creating a negative magnetic field. This reversal of the magnetic field brings the voltage back to zero, thereby completing a full cycle.
The noise level of a toroidal transformer is significantly lower than that of other transformers, primarily due to its design. The toroidal core's donut shape allows for very compact windings, which minimizes vibrations. This reduced vibration eliminates the possibility of noise, making toroidal transformers quieter and more secure.
The previous chapter covered the different types of three-phase transformers based on their phase windings. Three-phase transformers can also be classified according to their construction. They can be built using either a single core with combined primary and secondary windings or by connecting three separate single-phase transformers.
In a core-type transformer, the windings are evenly split and wound on the limbs of the core. The core consists of three limbs on the same plane. Each of these limbs contains both the primary and secondary windings. These windings may be better referred to as the high voltage and low voltage windings. The low voltage windings are wound closest to the core since it is easier to insulate. The high voltage coil is then wrapped around the low voltage winding with insulation between them. In this construction, the windings are magnetically coupled with each other, where one winding uses the other two limbs as a return path for its magnetic flux.
The shell-type transformer can be considered as three separate single-phase transformers due to the almost independent magnetic fields of the three phases. This transformer features a core with five limbs. The high voltage and low voltage windings are positioned around the three main limbs, with the low voltage winding closest to the core, similar to the core-type transformer. The two outer limbs provide additional return paths for the magnetic flux. As the magnetic field reaches the yoke, the flux splits into two paths. Consequently, the yoke and outer limbs can be sized to half of the main limbs. This reduction in yoke size helps decrease the overall height of the transformer.
The previous chapter provided a detailed discussion on the windings and cores of three-phase transformers. However, other components are equally important. This chapter will cover these additional parts of a three-phase transformer.
Insulations serve as a barrier system, separating the windings from the core and the two windings from each other. Transformers utilize various types of insulation, including oil, paper, tape, pressboard, and laminated wood.
Insulations act as a barrier system, providing separation between the windings and the core, as well as between the two windings themselves. Transformers use a range of insulation materials, including oil, paper, tape, pressboard, and laminated wood.
The tank safeguards the cores and windings from external environmental factors and also serves as a container for the oil. It is evacuated of air and other substances that could contaminate the oil and affect the dielectric properties of the insulation.
Heating the oil causes it to expand, so a separate vessel, called the conservator, is installed alongside the tank.
As the oil absorbs heat from the transformer, it transfers this heat to the cooling system. The cooling system collects the hot oil, cools it through air- or water-cooled tubes, and then returns it to the windings and core. The designations for transformer cooling systems, as standardized by IEC, are listed below.
Tap changers are used to adjust the output voltage of a transformer. Under load conditions, the transformer's output voltage may drop, necessitating an adjustment of the voltage ratio by modifying the tapping turns. This adjustment is achieved using a tap changer. The type of tap changer used depends on how frequently the output voltage needs to be adjusted.
Also known as the Buchholz relay, this component collects free gas bubbles from the transformer tank. The presence of these gas bubbles can indicate a fault within the transformer.
These devices are used to monitor the oil temperature in the transformer.
These devices are used to remove moisture from the air space above the oil level in the conservator, helping to maintain the dryness of the transformer oil.
These are safety devices designed to relieve overpressure in the event of oil flashing caused by short circuits.
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