A global view of power transformer technology present and future

Author: Dr Diego Robalino

Electrical energy, a fundamental component of human life today, has not yet become available to all societies. The need for technological growth to ensure safe and reliable energy provision is the subject of discussions globally but there are also major concerns about climate change and the effect of global warming. Certainly, it is almost impossible to envisage a perfect balance between technological growth and environmental protection, but all those involved in energy generation, transmission, distribution, and consumption have an active part to play in making life as sustainable as possible. The net effect of electrification depends most on future advances in the cost and efficiency of electric end-use technologies and their social impact.

The reliability of power systems is another global concern. The North American Electric Reliability Corporation (NERC) defines a reliable bulk power system as one that is “able to meet the electricity needs of end-use customers even when unexpected equipment failures or other factors reduce the amount of available electricity.” NERC relies on a set of policies designed to support adequate operation of the grid to maintain a constant balance between supply and demand, as well as security to respond to and withstand sudden, unexpected disturbances, or unanticipated loss of system elements due to natural causes, as well as disturbances caused by man-made physical or cyber attacks.

The power grid is meant not only to be reliable but also safe and efficient. The grid is evolving to provide a more resilient and cleaner energy future where the methods of energy generation and distribution change and, therefore, electrical asset design and manufacturing evolve to match the current technological demand, thereby reducing losses and improving performance. Research and development, testing, and global co-operation are needed to encourage the assessment and adoption of new designs, technologies, and approaches that support this continuous evolution.

Power and distribution transformer technology

In the United States of America, the Office of Electricity manages the Transformer Resilience and Advanced Components (TRAC) Program to accelerate the modernisation of the grid by addressing challenges with large power transformers and other critical grid hardware. Interested readers are encouraged to visit the Office of Electricity website for more information.

The TRAC program looks after coordinated efforts to increase energy efficiency, improve operations, enhance asset utilisation and management, increase system resilience, and  support increased domestic manufacturing.

TRAC envisions power transformers being flexible and adaptable for advanced applications in the future power grid. Objectivies include, but are not limited to:

• Cost comparable to conventional units
• Efficiency > 99 % at all levels of loading
• 25 % size/weight reduction
• Controllable impedance range 5 – 21 %

A flexible transformer can adapt to a range of voltage ratios and impedance levels, which leads to reduced manufacturing times and costs compared with today’s transformers. One important benefit is that flexible transformers will be available to replace damaged transformers in days rather than months as it is at present.

The US Department of Energy (DOE) has regulated the energy efficiency level of low voltage dry-type distribution transformers since 2007 and has issued a new ruling on efficiency levels for low-voltage dry-type distribution transformers. The new efficiency levels, which came into effect on 1 January, 2016,  are commonly referred to as the DOE 2016 Efficiency levels. Because of the new regulations, manufacturers have had to redesign their products to increase efficiency.

On 14 September, 2021, a new Federal Register was published by the DOE: 10 CFR Part 431 “Energy Conservation Program: Test Procedure for Distribution Transformers”. This reports the technical analyses and results that support the evaluation of energy conservation standards for distribution transformers. Changes in test procedures are in-line with the changes in updated IEEE standards including C57.12.00-2015; C57.12.01-2020; C57.12.90-2015; C57.12.91-2020.

Population without access to electricity (source IEA [1])

Transformer efficiency is not only a current topic in the North American region. In July 2015, the minimum energy performance standard produced by CENELEC (the European Committee for Electrotechnical Standardization) specified maximum losses for both the core and the windings of distribution transformers and the minimum peak efficiency for power transformers.

Increases in distribution transformer efficiency are based on a reduction of losses, of which there are two principal varieties: no-load losses and load losses. No-load losses occur mostly in the transformer core, and for that reason, the terms ‘no-load loss’ and ‘core loss’ are sometimes interchanged. ‘Load loss’ arises mainly in the windings. Measures taken to reduce one type of loss typically increase the other type. Some examples of options to improve efficiency include: higher grade electrical core steels, different conductor types and materials, and adjustments to core and coil configurations.

Changes in design and construction are not easily implemented. For example, the use of amorphous steel presents a number of challenges. First, there are few suppliers: only one in the US, with international production in China, Japan, Germany, and South Korea. Second, the cost per pound of amorphous electrical steel is approximately 1.5 times that of a typical M3 grain-oriented electrical steel. As a result, amorphous cores have a very small penetration in the current market, with grain-oriented steel predominating in the manufacture of distribution transformers.

The application of distribution transformers varies significantly by type – liquid-immersed or dry – and ownership. Electric utilities own approximately 95 % of liquid-immersed distribution transformers, whereas commercial/industrial entities use mainly dry transformers.

The renewable energy market

The National Renewable Energy Laboratory (NREL) provides an analysis of the grid integration opportunities, challenges, and implications of renewable electricity generation for the US electric system. The NREL reports point to major factors in the energy consumption forecasts, which include: 

• Vehicle electrification dominates incremental growth in annual electricity demand with the average electric vehicle being driven 12 to 14 thousand miles per year
• Addition of solar PV, supplying power to commercial and residential buildings, as well as to transportation systems
• Changes in global climate are tending to increase the use of air conditioning and space heating

As reported in the World Energy Outlook 2021 published by the International Energy Agency (IEA [1]), a new energy economy is emerging. It is not quite clear how the emerging process is going to evolve, but it will be different in many ways. PV and electric vehicle sales reached new records in 2020. Some of the studies presented in the IEEE Transformers Committee show loading is likely to increase by between 10 and 40 %. It is therefore important to consider a scenario where the average equivalent load is close to 50 % of the transformer’s nameplate capacity, but the peak load may exceed 100 % of this capacity. One way of dealing with this potential load increase is by adopting an upgraded insulation system consisting of natural or synthetic ester fluids used in conjunction with thermally upgraded kraft paper.

To this evolution of the power grid and the integration of renewable sources, distributed generation, and microgrids, developments in power electronics are creating the possibility of solid-state transformers (SSTs). These promise to manage the highly variable, two- way flow of electricity between, say, a microgrid and the main grid. SSTs can be significantly smaller than an equivalent conventional transformer, about half the weight and a third of the volume, but there are limitations relating to cost and to voltage levels. Future research can be confidently expected to reveal more about SSTs.

Improved testing and diagnostics technologies

From time to time, new terminology appears which may sound quite daunting. For example, digitalised power transformers. In this context, digitalisation implies that sensors are embedded in the power transformer to continuously monitor its performance or condition. The sensors may support Dissolved Gas Analysis (DGA), temperature and moisture measurement, loading profile cooling control, and more. The objectives are to facilitate predictive asset management, minimise losses, and enhance efficiency.

The life of a power transformer is in reality the life of its insulation system. Due to their affordability and beneficial properties, cellulose-based materials are by far the most common type of solid insulation used in power transformers, often used in conjunction with insulating fluids. Made from pure cellulose, these materials have excellent electrical and oil impregnation characteristics, as well as good mechanical properties.

In relation to insulation materials, research objectives established by groups such as TRAC include:

• Dielectric strength > 300 V/mil
• Dielectric loss angle (tan delta) < 0.05 % at 60 Hz
• Enhanced material properties that remain stable over the useful life of assets (20 to 40 years)
• Temperature withstand > 130 ºC in continuous operation

Testing is fundamental. More materials are now in the research and development pipeline and their behaviour must be well understood, not only by researchers, but also by end-users. In the last two decades, we have heard more about the use of ester fluids in power and distribution transformers. Transformers with solid insulation immersed in mineral oil represent the most significant fire safety hazard in electrical substations. Ester liquids however are less of a fire hazard than mineral oils, as they not only have higher flash and fire points but also lower net calorific value. By using a less flammable fluid than traditional oil as a coolant and dielectric insulator, the risks associated with potential transformer fires are significantly reduced.

In addition, synthetic and natural ester fluids are readily biodegradable, they show very low oral toxicity, and they are not classified as toxic to aquatic life. These factors may permit easier use in installations in sensitive environments such as water catchment areas and offshore wind farms.

Turning to solid insulation, high-temperature transformers are now quite common around the world. High-temperature insulation, including enamel and tape wrap for conductors, winding spacers, and mechanical support materials, is commonly used in mobile, locomotive, and rectifier transformers. These applications benefit from the lighter weight, improved reliability, and longer life offered by the use of high-temperature materials. For many years, these materials have also allowed manufacturers to provide solutions for repair applications and mobile transformers.

High-temperature transformers for traction applications have been produced for many years, but more recently, this technology has become increasingly common in pole-type distribution transformers and wind-turbine transformers. Those interested in the use of high-temperature insulating materials in power transformers are recommended to read IEC 60076-14.

Summary

Demand in emerging and developing economies remains on the growth trajectory that resumed in the second half of 2020, and it is likely that the projected strong economic recovery for China and India will further accelerate this trajectory. This means that reliability of supply and affordability of electricity are set to become even more critical in every aspects of people’s lives.

Solar PV and wind already represent rapidly evolving sources of new electricity generation. The renewable energy market, if it follows the plan towards the 2050 net-zero emissions scenario (NZE), will be much larger than today’s oil industry.

Digitalisation, monitoring, and control of transformer performance are becoming more available and affordable. Predictive maintenance based on advanced data processing algorithms is enthusiastically progressing and the key concern is no longer how to handle the volume of data involved, but how to be confident in the quality of the data.

The introduction of new types of insulating fluids will help with the development of transformers to meet future requirements, but it can also be a challenge for the transformer industry when the behaviour of the new fluids is not fully understood. The performance of an insulating fluid is highly dependent on its chemistry and alternative insulating fluids such as esters behave differently from the well-known mineral oil.

Whatever the challenges, however, and irrespective of how the power grid evolves, one thing is certain: power transformers will continue to play a crucial role in transmission and distribution for years to come. As we have seen, even though transformers have been with us for almost a century and a half, progress in their design and construction continues apace, which means that the future is sure to bring developments that are both interesting and exciting.

References

[1] IEA, Population without access to electricity in the Stated Policies and Net Zero by 2050 scenarios, 2000-2030, IEA, Paris https://www. iea.org/data-and-statistics/charts/population-without-access-to-electricity-in-the-stated-policies-and-net-zero-by-2050-scenarios-2000-2030