Transformers and Substations Handbook 2014

We should never forget that copper remains an important and expensive component of a transformer. However, losses within the machine have an associated cost and it is useful to understand the trade-off between the initial cost of the copper versus the cost of the losses over the lifetime of the machine.

Proper transformer sizing and copper windings

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By E Swanepoel, Copper Development Association Africa (CDAA)

of the transformer and the core steel selected; hence the emphasis on proper sizing. Coil losses, or load losses, originate in the primary and secondary coils of the transformer and are a result of the resistance of the winding ma- terial. This is where the selection of copper wind- ings can make a difference. Proper sizing Transformers are sometimes installed in advance of occupancy, so the engineer does not necessar- ily know the load that will be placed on the unit. As the installer is often not the party paying the electricity bill, there can be a tendency to oversize the transformer capacity relative to the load it will see. Since the no-load loss is a function of the kVA capacity of the transformer, careful selection of transformer capacity, appropriate to its intended task, will ensure the lowest core loss. Energy Star (TP-1) transformers may not be efficient enough Energy Star, an international standard for energy efficient consumer products, originated in the USA where it was created in 1992 by the Environmen- tal Protection Agency and the Department of En- ergy. Since then, Australia, Canada, Japan, New Zealand, Taiwan and the European Union have adopted the programme. Devices carrying the Energy Star service mark generally use 20 to 30% less energy than required by federal standards.

As the electrification of Africa continues, choosing the right component is critical if the result is to be cost effective and efficient.

Transformers are essential for the transmission, distribution and utili- sation of electrical energy. They are used in virtually every commercial and industrial building, from the service transformer that reduces dis- tribution voltage to a more usable voltage for buildings to step-down transformers that serve individual floors, to small transformers for in- dividual equipment. Transformers can be expected to operate for 20 to 30 years or more. Over such a long life span, the operating cost of a transformer can greatly exceed its initial price, so selection of the right transformer for economic performance involves examining the unit’s capacity (size) and efficiency. In this context, efficiency means looking at the core steel and the winding material. Transformer losses In simplest terms, transformer losses comprise core losses (also called no-load losses) and coil losses (called load losses). Core losses originate in the steel core of the transformer and are caused by the magnetising current needed to energise the core. They are constant, irrespective of the load on the transformer, hence the term ‘no-load’. They continue to waste energy as long as the trans- former is energised. No-load losses vary depending on the size (kVA)

Copper Development Association Africa

The Energy Star label is applied to transformers that meet a certain minimum standard for efficiency, known as the National Electrical Manufacturers Association (NEMA) TP-1 [1]. This standard is intended to promote the manufacture and use of energy efficient transformers by establishing minimum efficiency standards, albeit with certain built- in assumptions. It contains a simplified method for evaluating the initial cost of transformers along with the costs of core and load losses. It also presents tables of minimum transformer efficiencies based on kVA size, voltages and liquid or dry-type. Unfortunately, there is nothing particularly efficient or cutting-edge about transformers that meet TP-1. Yes, they are an improvement on so-called ‘standard’ transformers, which are still made and sold widely. However, many transformers are available from various manufacturers that exceed the efficiency levels of TP-1, and can provide a faster payback of their purchase price.

The Copper Development Association Africa (CDAA) has represented the local copper industry in southern Africa since 1962. Its head office is based in Johannesburg and, on behalf of its members, the organisation is committed to promoting and expanding the use of copper and copper alloys throughout Africa.

Visit www.copperalliance.org.za

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Transformers + Substations Handbook: 2014