What is a Power Transformer?
Power Transformers are critical components in our electrical systems that play a crucial role in efficient power transfer across the grid. They step up the voltage while reducing the current or step down the voltage while increasing the current, thereby ensuring seamless power transmission and distribution.
The core principle behind this transformation lies in electromagnetic induction, which facilitates energy transfer between the transformer’s windings without any direct electrical connection. They are magnetically coupled through mutual induction instead of physical wiring linking the windings.
Power transformers usually have two or three windings around a single steel core. We refer to a transformer with a third winding as a tertiary power transformer. This third winding supplies power to auxiliary circuits or a reactor in most cases. Typically, manufacturers craft the windings from copper or aluminum. To ensure proper insulation and protection, they enclose the steel core and windings in a steel tank filled with insulating oil, such as mineral oil.
The windings are brought out from the tank via bushings. A power transformer’s low voltage (LV) side has smaller bushings with larger bus bars, whereas the high voltage (HV) side has a larger bushing with smaller conductors. This is because, at the LV side, the current will be larger comparable to the HV side.
In this brief article, we shall focus on some fundamentals of power transformers.
Figure 1 illustrates a power transformer with some parts labeled. The size and design depend on its application.
Table of Contents
Why do we Use Power Transformers in Electrical Systems?
The importance of power transformers in electrical systems is undeniable because they play a crucial role in efficiently transferring power over long distances. Power transformers are highly efficient because the power received on one side is nearly the same as that delivered on the other. For such a scenario, the transformer would be considered an ideal transformer. Figure 1 below is an illustration of a two-winding transformer with primary and secondary parameters.
An ideal transformer is assumed to have:
- No winding losses, that’s zero winding resistance
- Permeability of the is infinite (μ = ∞), and the reluctance of the core is zero
- Flux is completely confined to the core, that’s no leakage flux and
- No core core losses, that’s no eddy current or hysteresis losses
Let’s perform some simple calculations to support the equivalence of the power transfer in both windings, assuming an ideal power transformer.
Input and Output Power of an Ideal Transformer is Equal
From transformer current, voltage, and turns ratio relationships:
Let
N1 = number of turns on the primary winding
N2 = number of turns on the secondary winding
V1 = primary side voltage
I1 = primary side current
V2 = secondary side voltage
I2 = secondary side current
Turns Ratio
Turns ratio (a) = primary winding number of turns divided by the secondary winding number of turns
a = N1/N2 ———————– (1)
Turns ratio in terms of voltages and number of winding turns:
V1 /V2 = N1/N2 ——————– (2)
Primary voltage:
V1 = aV2 ————————— (3)
V2 = V1/a ————————- (4)
Turns ratio in terms of currents and number of winding turns:
I1 /I2 = N2/N1 ——————– (5)
I1 /I2 = 1/a ———————— (6)
Input current (primary):
I1 =I2 /a ————————– (7)
Output current (secondary):
I2 = aI1—————————-(8)
Complex Power
The primary side of the transformer receives complex power, expressed as:
S1 =V1I1* ————————-(9) and the
While the secondary side of the transformer delivers complex power, expressed as:
S2 = V2I2* ———————– (10)
Using the substitution method, substitute primary voltage (V1) and current (I1) into equation (9).
S1 = aV2 × I2/a = V2I2*,
Therefore, S1 = V2I2*
* conjugate (considering voltage as reference).
Hence, S1 = S2 for an ideal transformer. For an ideal transformer, the total apparent power remains equal on both the primary and secondary sides of a two-winding transformer.
This is not the case in practice because there will eventually be some losses across the windings. Refer to this lecture notes for more details. The efficiency of single and three-phase power transformers varies, with typical values within the range of 97% to 99.9% [1].
Where do we use power transformers?
The transformer’s use largely depends on its location within the electrical grid. Will it serve at a power generating plant, a sub-transmission, or a distribution station? Or will engineers and project developers deploy this transformer for a specialized purpose?
Common use of power transformers:
Step-Up Transformers: These transformers increase the voltage from a low voltage (LV) to a high voltage (HV). We use step-up transformers to increase generation voltage to transmission and sub-transmission levels, minimizing power losses over long distances. Generators (synchronous) produce electricity at lower voltages (for instance, 15 kV). Due to technical losses and voltage drops, transmitting the power at low voltage to customers over a long distance is uneconomical. As such, the system steps up the 15 kV to a higher voltage (for example, 138 kV) for transmission. There is a significant reduction in power loss (heat) and voltage drop for the delivered power since the current output at 138 kV is far less than the current at the 15 kV side of the transformer.
Step-Down Transformers: Step-down transformers lower a high voltage for local distribution, making it safe for use in homes and industries. The power produced at the power plant goes through several transformations before reaching our power outlet at 110 V or 120 V for the 60 Hz systems or (230 V for the 50 Hz system).
Check here for Several applications of a power transformer
How does a Power Transformer Work?
The transformer works based on the principle of electromagnetic induction. Whenever a current flows through a conductor, a magnetic field is produced. So, from Figure 2 above, separate wires (one at the primary side or source and the other at the secondary side or load) are wound around the transformer’s core.
The flow of current (I1) induces an electromotive force (emf or voltage, V1) at the primary side of the transformer. The built-up electromagnetic field cuts through the transformer core, thereby inducing voltage on the secondary side. The current that flows in the secondary winding is indirectly proportional to the current at the primary side for an unequal number of winding turns.
Emerging Technologies in Power Transformers
Transformers have evolved over the past century. The Edison Tech Center provides the “The History of the Transformer” in greater detail.
For the past 138 years, transformers have served the world in the best possible ways. Power transformers have significantly been improved. They are efficient and could last over 45 years on average.
Nonetheless, power transformers continue to emerge with fascinating improvements due to an evolving grid coupled with high penetrations of renewables, digitalization, smart grids, and challenges such as cyberattacks. There have been improvements in core materials, insulation composition, cooling techniques, diagnostic, and advanced monitoring systems.
Despite all the good stuff, the transformer market faces a few challenges regarding increasing efficiency and supply chain.
Moreover, it is crucial to delve into recent developments and innovations in power transformers.
Potential Advances and Expected Emerging Trends
Transformers are advancing in digitalization and innovative capabilities. Sensors and Internet of Things (IoT) tools enable real-time monitoring of performance metrics. These metrics include moisture content in oil, loading profiles, winding and oil temperatures, and overall health of the transformer.
- Through a program initiated by the United States Office of Electricity, the Transformer Resilience and Advanced Components (TRAC) aims to accelerate the grid’s modernization by resolving critical challenges power transformers face. Large transformers are envisioned to achieve objectives such as.
- “Cost comparable to conventional units
- Efficiency > 99 % at all levels of loading
- 25 % size/weight reduction
- Controllable impedance range 5 – 21″ [2]
- Innovations in new steel and superior magnetic materials, such as amorphous steel cores, are aimed at boosting the efficiency and ecological features of transformers [3]
- Solid-state Transformers (SSTs) are emerging as a significant alternative to traditional power transformers. SSTs utilize power electronics to provide more efficient and flexible voltage regulation than conventional transformers. They are particularly beneficial in integrating renewable energy sources and managing bidirectional power flows in smart grids. SSTs are considered energy efficient and superior technology for the future of power systems. One important thing to note is that SSTs operate at a higher frequency than conventional power transformers. Also, the input and output frequencies differ from traditional transformers, which maintain primary and secondary frequencies while voltages are converted [4].
- An innovative study under the U.S. Department of Energy’s Office of Electricity TRAC Program explores developing, testing, and deploying a groundbreaking, flexible large power transformers. Titled “Design, Deployment, and Characterization of the World’s First Flexible Large Power Transformer,” this prototype seeks to revolutionize transformer technology by enabling online adjustment of short-circuit impedance from 4% to 18%.
Summary
Could one imagine a power grid without a transformer? Operating the entire grid without a power transformer would be impractical, infeasible, and uneconomical.
Before procuring a transformer, one should first identify the project or facility’s needs, conditions of location for installation, and specific industry requirements for appropriate sizing/selection.
Moreover, it is important to note that conventional transformers are not flexible despite serving us in extensive ways. A power transformer failure leads to a wide-area electricity interruption, and as a result, a long downtime is expected, thereby impacting our health and safety as well as businesses. Additionally, procuring a transformer typically takes over 52 weeks because each transformer is specifically made to meet the needs of each project.
Furthermore, innovations and emerging technologies such as solid-state transformers (SSTs) and online adjustable short circuit impedance of large power transformers (LPTs) promise the next generation of power transformers with adaptability and flexibility.
Lastly, future transformers will emerge capable of handling the next-generation evolution in grid hardware advancement, being more adaptive, reliable, economical, and flexible relative to present technologies.
Below is a reference document (Courtesy of WECC) is provided for download.
- Title: Transformers
- Format: PDF
- File Size: 2.92 MB
- Number of Pages: 84
- Source: Western Electricity Coordination Council (WECC)
References
[1] Gulick B, Spiller M. Stenzel N. (2023). Maddox. Transformer DOC Efficiency Standards. Retrieved December 28, 2024, from https://www.maddox.com/resources/articles/transformer-doe-efficiency-standards
[2] Megger. (2023). A global view of power transformer technology present and future. Retrieved December 28, 2024, from https://www.megger.com/en/et-online/april-2023/a-global-view-of-power-transformer-technology-pres
[3] Plant Automation Technology. (n.d.). Emerging Trends in Transformers Design and Technology. Retrieved December 28, 2024, from https://www.plantautomation-technology.com/articles/emerging-trends-in-transformer-design-and-technology
[4] Khan, S., Rahman, K., Tariq, M., Hameed, S., Alamri, B., & Babu, T. S. (2021). Solid-state transformers: Fundamentals, topologies, applications, and future challenges. Sustainability, 14(1), 319.
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