Improving Outcomes

Technologies to make energy efficient transformers

Transformers often waste significant amounts of electricity due to their poor design and improper use. As a result, consumers and businesses have to pay high electricity bills, utilities struggle to meet the excessive demand for power, governments are burdened with additional economic development challenges, and the planet suffers from pollution and greenhouse gas emissions. The transition to energy-efficient transformers is well under way in most developed countries. However, many developing and emerging economies are just beginning to explore such opportunities.

Technology basics

Transformers with high voltage of 230 kV and above, and self-cooled power ratings that exceed 60 MVA are referred to as large power transformers. They can be found at generating stations and electrical substations. Medium power transformers generally have voltage ratings of 36-230 kV. These are three-phase units with power ratings between 2.5 MVA and 60 MVA. These transformers are most often used to transfer power to a sub-transmission circuit. The voltage is further reduced by medium voltage distribution transformers through circuits in order to distribute electricity to residential, commercial, and industrial consumers. Transformers can be liquid-filled (cooled with mineral oil or other insulating liquid), or dry-type (cooled with air). In some markets, there is a special subgroup of low-voltage distribution transformers of up to 1 kV.

There are different types of transformers catering to specific needs. The most common transformer is liquid filled with windings that are insulated and cooled with a liquid. These can be found at all stages of the electricity network, from generation to transmission and distribution. They are usually filled with mineral oil, which is flammable and may be prohibited for use inside buildings, but fire-resistant liquids are also available.

Liquid-filled transformers are housed in a sealed tank that facilitates fluid circulation through winding ducts and around wire coils. The heat removed by the fluid from the core-coil assembly is transferred to the environment through the tank walls that have fins to enhance the cooling effectiveness of the system, or through external radiators, such as pumps and cooling fans, with passive or active fluid circulation. Some countries have environmental protection laws requiring containment troughs around the perimeter of liquidfilled transformers to guard against leaks. Liquid-filled transformers tend to be more efficient than dry-type transformers for the same rated power (kVA5). They also tend to have greater overload capability and a longer service life.

A dry-type transformer is insulated and cooled by the air circulating through the coils. These are found in certain distribution networks and are typically used by commercial and industrial customers, rather than electric utilities. High capacity transformers that are used outdoors are almost always liquid filled. Low capacity transformers used indoors are often dry-type since the fire risk is lower than those that use mineral oil. Dry-type transformers are typically housed in enclosures, and vacuum pressure impregnated varnish and epoxy resin is used to insulate the windings. Dry-type insulation is typically designed to withstand operating temperatures up to 220°C.

In some capital-constrained markets, businesses and utilities may choose to install refurbished transformers to reduce overall costs. These tend to be less reliable and have already lost some of their useful life. They can experience higher losses than new units either because of the old and inefficient materials used in their construction and/or the repair work done to revive them. Refurbished transformers do not offer the same durability and reliability as new units and yet incur the same installation and commissioning costs and even higher operating costs. Capital-constrained electric utilities often procure less efficient transformers since the purchase price is lower. They also procure fewer units with higher loading to offset the high purchase price.

Liquid-filled transformers have their electrical windings immersed in a dielectric fluid to reduce electrical clearances and greatly improve cooling performance. Mineral oil has long been seen as the fluid of choice for electrical transformers due to its favourable cooling and electrical performance. It does have some shortcomings including flammability, poor environmental performance, low moisture tolerance and corrosive sulphur.

An alternative cooling fluid is ester-based fluid. It is fire safe, readily biodegradable, free from corrosive sulphur compounds and has excellent moisture tolerance. These attributes are important from the environmental perspective, particularly in areas such as Africa with pole- and ground-mounted transformers installed in remote areas without oil containment facilities. Ester fluids have also been shown to extend the life of electrical insulation, which prolongs the service life of the transformer.

Focus on efficiency

Although most transformers have efficiency levels greater than 98 per cent, a study conducted for the European Commission found that energy consumed during a transformer’s service life is still the dominant factor contributing to environmental damage. Therefore, it is critical to adopt cost-effective measures to alleviate the environmental impact of transformers.

Transformer losses occur either in a no-load condition where an energised transformer is ready to convert the voltage or a load condition where the energised transformer is actively converting the voltage. The losses are manifested in the transformer as excess heat, which occur in the transformer core and/or windings. The following sections discuss how losses occur and what can be done to minimise them.

A transformer can be made energy efficient by improving the construction materials such as better-quality core steel and winding material, and by modifying the geometric configuration of the core and winding assemblies. Energy-efficient transformers often require a tradeoff between high costs and lower-loss materials and designs. For a given efficiency level, the no-load and load losses are inversely related and reducing one usually increases the other. The accompanying table lists the five approaches to reducing no-load losses. One of these is a material substitution option and four are transformer design options.


When comparing like designs, the one with the lowest total cost of ownership should be selected as it would be the most economical in terms of the purchase price, revenue loss and capital investment. On a lifecycle cost basis, an energy-efficient transformer is very appealing given its non-stop operation and 25-year service life. These savings translate into a reduction in peakloading, lower electricity bills and improved supply reliability. Payback periods vary with equipment and electricity costs and can be as short as one year or as long as six years or more. For transformers, a six-year payback on a product that typically lasts more than 25 years is still very attractive.

To conclude, there are a host of technologies available in the transformer space that can help address the challenges facing the utilities. However, for optimal outcome, it is necessary to identify the system requirements and select the ideal technology solution through a cost-benefit analysis.

Based on a UN report, “Accelerating the Global Adoption of Energy- Efficient Transformers”


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