Integrating energy and material efficiency in public distribution transformers

There is growing recognition that the energy transition can only be sustainable if material use is part of the equation, an aspect reflected in recent EU regulatory initiatives. In electrical systems, successfully coupling energy efficiency with resource efficiency can be challenging, but the sustainable peak load concept for power transformers in public distribution networks is an elegant solution which succeeds in combining both objectives.

Avoiding inherent conflict

The ambition set out in the European ‘Fit for 55%’ package from July 2021 requires that all technically and economically feasible avenues to decarbonisation be rolled out rigorously. Electrical distribution networks can contribute by maximizing their energy performance. In this context, the Ecodesign regulation on transformers stipulates maximum values for both load losses and no-load losses. The original requirements came into force on 1 July 2015 and more stringent values have applied since 1 July 2021.

In the meantime, the European Green Deal outlined ambitions to make the economy more material efficient, culminating in a new Circular Economy Action Plan in March 2020. This ambition was reflected in Article 7 of the latest edition of the Transformer Ecodesign Regulation, which lists a number of issues to be addressed in the next revision of the regulation. These include ‘the possibility and appropriateness of covering environmental impacts other than energy in the use phase, such as (…) material efficiency’.

Sensible as the ambitions may sound, when it comes to electricity there is an inherent conflict between energy efficiency and material efficiency. A key measure for improving the energy efficiency of electrical systems is increasing the amount of conductor material. Only by making smart use of electrical systems, can this trade-off be mitigated or avoided in some cases. The sustainable peak load concept deployed in distribution transformers represents such a smart solution. It does not change the transformers themselves, but instead maximizes their output for material efficiency without compromising on their energy performance.

Peak loads that do not compromise reliability, lifetime, or energy efficiency

At the origin of the sustainable peak load concept lies the fact that many public distribution transformers, as currently rated, are underexploited. This has historical antecedents. Stringent rules on loss reduction, compactness, and absence of toxic substances have prompted various technological innovations, including the use of highly conductive winding material, magnetic steel with reduced losses, thermally upgraded paper, and natural esters as liquid insulation.

As a result, many transformers can now withstand higher temperatures in the windings – up to 95°C instead of just 65°C, and can handle higher peak demand without compromising unit reliability or lifetime. This peak load potential is not usually exploited, as operators continue to pursue keeping power losses below the stipulated values.

At low load levels, however, the relative importance of load losses diminishes, and the relative importance of no-load losses increases. As a result, choosing a smaller transformer for the same job has little influence on the total annual energy losses of the unit. Public distribution networks typically have such low load levels. Until recently, their loadings have been estimated only, not measured. With the introduction of smart meters, extensive measurement campaigns have now recorded full-year kWh data, which show that loadings tend lower than initially thought. The average load factors are around 15% of nameplate capacity.

This low load factor combined with the technical overload capacity leads directly to the concept of the sustainable peak load transformer. The ‘rated nameplate capacity’ will be the value by which the transformer will meet the requirements of the energy performance regulations. The ‘sustainable peak capacity’ of the transformer will be set at a higher value. As long as the transformer operates in a network with low average loadings, as is the case in public distribution networks, allowing this kind of higher peak capacity will not increase the unit’s total annual energy losses.

A modelling exercise assessing the potential benefits

A group of experts, under the European Copper Institute’s direction, conducted a modelling exercise to assess the impact of selecting sustainable peak load units for all transformer replacements in public distribution networks in the EU.

The model took the ubiquitous 400 kVA – 24 kV/0.4 kV transformer as a starting point and calculated the difference between replacing all the end-of-life 400 kVA units in the EU with conventional 540 kVA units, or with sustainable peak load 400 kVA/540 kVA units.

On energy performance, a distinction should be made between the transformer’s nameplate power losses, expressed in Watts, and its annual energy losses, expressed in kWh. The former is subject to regulation, while the latter must be taken into account when evaluating the unit’s environmental performance.

The 400 kVA / 540 kVA sustainable peak load transformer is designed according to the prevailing minimum energy performance standards for a 400 kVA unit, which means that its load losses will exceed the nameplate value during the short periods of peak load up to 540 kVA. However, its no-load losses are fixed at a lower value than that of a conventional 540 kVA unit. For load profiles with short peaks and a low average loading — as is the case in distribution networks — the increase in annual load losses will be compensated by the decrease in annual no-load losses. This was confirmed by the results of the modelling exercise: the total annual energy losses of the sustainable peak load units were calculated to be very similar to those of a conventional unit.

Figure 1 – The sustainable peak load transformer concept (substation transformer by ing.mixa from the Noun Project)

While no compromise was made on the annual energy losses, the sustainable peak load transformer’s material efficiency increased substantially, with reductions in total weight of between 11 and 15%.

This efficiency gain in material use was achieved without increasing the unit purchase cost. The modelling exercise demonstrated that the cost of the sustainable peak load model is comparable to that of a conventional transformer if all other parameters are kept the same.

Facilitating accelerated network upgrades

The expert assessment has led to the conclusion that widespread application of the sustainable peak load concept in the EU public distribution networks would be a welcome exercise. It would make sound economic sense and make a significant contribution to the twin policy goals of energy efficiency and material efficiency.

Figure 2 – The sustainable peak load transformer provides the opportunity to upgrade peak power while keeping the same unit dimensions (source: Copper Alliance)

A major economic advantage of the sustainable peak load transformer is its compactness. With the transition away from fossil fuels, substantial growth in electricity consumption is expected in some sectors supplied by distribution networks. The sustainable peak load transformer provides the opportunity to upgrade transformer peak power while keeping the same unit dimensions. This is a critical aspect in urban environments where space may be restricted, allowing cheaper installation and earlier upgrades, making the distribution grid more robust and secure.

Source: Maximizing distribution transformer resource-efficiency - Potential contribution to EU Green Deal objectives, Leonardo Energy , October 2021.