Copper: The material of choice in transformer

Copper is known to improve transformer energy performance

Transformers are used in virtually every commercial and industrial building, from the service transformer reducing the distribution voltage to a more usable voltage for the building, to step-down transformers serving individual floors, to small transformers for individual apparatus or functions. Typically, a transformer is a long-lived device that can be in service for decades.

Design and manufacturing practice
Of the many elements that go into the design and manufacture of power transformers in order to improve their short-circuit withstand capabilities, the choice of conductor material is the most important, as its mechanical properties, such as yield strength and modulus of elasticity, are critical to performance.

For this reason, good design practices, such as those of ABB, use copper with a minimum yield strength of 90 N/mm2 at 0.2 per cent offset (in other words it would require a stress greater than 90 N/mm2 to cause a permanent strain of 0.2 per cent). This figure goes as high as 280 N/mm2 and beyond for heavy-duty transformers with frequent short-circuits such as those used for arc furnaces. The use of the right grade of copper is considered by good designers as the best way of ensuring high short-circuit withstand capability in power transformers.

Improving transformer energy performance with copper Higher copper content in transformers improves energy performance and consequently lowers lifecycle costs in most cases.
The various costs incurred during the long life of a transformer can be broadly categorised into purchase, operating and end-of-life costs. Of these, the operating costs – principally comprising the cost of energy losses in the transformer – are dominant. Therefore the astute purchaser will give a high weightage to the transformer’s energy performance in his decisions and will not base these on direct costs alone.

Transformer designers have a mix of options available for improving energy performance – principally the use of larger core and conductor cross-sections, or a lower loss core material, or a better conductor, i.e. copper. The optimisation of this mix is done on an ongoing basis by designers and manufacturers, based on relative material costs at the given time and the specific conditions of procurement on energy performance. This is the reason for the variety of transformer designs seen in the market.

European Commission study
In preparing for the implementation of the Energy Related Products (ERP) Directive (2009/125/EC) related to power and distribution transformers, the European Commission commissioned a study by VITO and BIOIS to investigate, among others factors, the improvement potential in the design of various types and sizes of transformers in terms of least lifecycle costs. For each base case, the study worked out various design options and compared these for their improvement potential over the respective base case. The results show that in all cases investigated, the transformer design option that gives the least lifecycle cost has lower energy losses and uses substantially more copper than the respective base case.

KEMA study
An independent study across the European Union carried out by KEMA came to a similar conclusion – power transformers use more copper wherever a higher financial weightage (loss factor) is given to energy performance by the purchaser.

Active Repair Pilot
Currently, the DT failure rate in India is high in the order of 12-15 per cent (in State Utilities), as against global average of less than 1 per cent. So far many business models are being tried out for reducing commercial losses, however, technical loss reduction is still dependent on infrastructure changes and requires capex infusion from utility and hence are pursued at O&M level but within limits.

The feasibility of reducing technical losses in Distribution Transformers (DT) through Active Repairs over the currently prevalent Reactive repair methods is being explored. The proposed DT Active Repair concept is to bring down no-load and load losses proactively, rather than waiting for DT to breakdown and then getting repaired. This is a first-of-its-kind concept in the country and there is a strong business case for Indian DISCOMs to consider this approach, on account of following reasons:

• Reduces technical losses, thereby saving power procurement costs
• Allows for further improvement in energy efficiency of transformers as mandated by BEE in PAT-2 cycle
• Improves transformer reliability, thereby reducing downtime
• Increases kVA capacity of the transformer

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Creates an opportunity for the DT OEM or repairer to become a stakeholder in network O&M.

The field results, through the pilot, reveal that the proposed active repair helps in technical loss reduction. In some cases, the measured value of technical losses was better compared to estimated value. In addition to technical loss reduction, there was also kVA capacity enhancement for the transformer. Thus, proposed Active Repair concept has the potential for improving the Energy Efficiency (EE) and performance besides life extension of distribution transformer.

There are different models in copper helped to improve efficiency:
Full Down-payment Model: In this model, for active repair of selected DTs, utility can pay full payment (normal ‘As-Is’ repair cost & investment for active repair including material costs) to the empaneled repairer. As the active repair will be done by replacing Al winding with Cu winding for technical loss reduction, utility will fully retain the Return on Investment (RoI) from efficiency improvement and Cu scrap value (75-80 per cent) whenever this DT is scrapped.

Pay for Performance Repair Model: In this model, for active repair utility will pay normal ‘As-Is’ repair cost to the repairer. The repairer will invest money for active repair of selected DTs (selection will be done by repairer). As the active repair involves use Cu winding, repairer will receive Return on Investment (RoI) from Cu scrap value (75-80 per cent) whenever this DT is scrapped.

Managed Service Provider (Management contract): In this model, a Managed Service Provider (or MSP) who is a dedicated third party company (it can be a DT manufacturer, repairer or service provider), will procure and manage Distribution Transformers for a utility based on agreed Service Level Agreements (SLAs). This MSP can help restore the technical losses to near Ideal levels and add huge value in terms of loss reduction savings and manage DT assets with best practices.

Conclusion
Thus, it can be concluded that the higher copper content in transformers results in an improved energy performance and consequently in lower lifecycle costs in most cases.

Compiled by_
Manas Kundu, Director Energy Solutions,
International Copper Association India

References

• VITO and BIOIS study for preparing the implementation of the new Ecodesign or Energy Related Products (ERP) Directive (2009/125/EC) related to power and distribution transformers, on behalf of the European Commission DG ENTR unit B1 OT2: Distribution and power transformers Tasks 1 – 7 Contact VITO: Paul Van Tichelen, Contact BIOIS: Shailendra Mudgal, www.ecotransformer.org 2010/ETE/R/106, January 2011
• Drivers for Copper use in Power Transformers, S.A.A. Houtepen, J. Bloem KEMA, Arnhem 20 December 2011 , 74101182-ETD/SUP 11-2903
• Loss Reduction through Active Repair of Distribution Transformer: Results from the field, paper submitted at TRAFOSEM India 2017. Authors: Samir Jadhav, Owner, High-Rise Transformers (HRT), Rahul Bagdia, Director, pManifold Business Solutions & Manas Kundu, International Copper Association India