Best Practice Manual For Transformers

Background

Distribution transformers are very efficient, with losses of less than 0.5% in large units. Smaller units have efficiencies of 97% or above. It is estimated that transformer losses in power distribution networks can exceed 3% of the total electrical power generated. In India, for an annual electricity consumption of about 500 billion kWh, this would come to around 15 billion kWh.

Reducing losses can increase transformer efficiency. There are two components that make up transformer losses. The first is “core” loss (also called no-load loss), which is the result of the magnetizing and de-magnetizing of the core during normal operation. Core loss occurs whenever the transformer is energized; core loss does not vary with load.

The second component of loss is called coil or load loss, because the efficiency losses occur in the primary and secondary coils of the transformer. Coil loss is a function of the resistance of the winding materials and varies with the load on the transformer.

In selecting equipments, one often conveniently avoids the concept of life cycle costing. But the truth is that even the most efficient energy transfer equipment like a transformer, concept of life cycle cost is very much relevant.

The load profile of electronic equipment from the computer in the office to the variable speed drive in the factory-drives both additional losses and unwanted distortion.

Since transformer manufacturers test only under ideal (linear) conditions, a substantial gap exists between published loss data and actual losses incurred after installation. In fact, test results published in a 1996 IEEE Transaction paper documented an almost tripling of transformer losses when feeding 60kW of computer load rather than linear load.

Slightly different practices are followed in USA and UK to account for harmonics while selecting transformers.

A guide to this guide

This Best Practice Manual for Electric Transformers summarise the approach for energy conservation measures pertaining to selection, application and operation of electric distribution transformers.

The details of design methodology and the varied approaches for materials, construction are not in the scope of this manual. However, some theoretical aspects are discussed where ever deemed fit.