Considerations When Specifying Transformers

Specifying a transformer for a particular application is no simple matter, requiring thorough training and expertise, a high level of experience, and access to well-established manufacturing capacity. 

There are a number of key variables that must be considered. While power ratings for transformers are standardised, there are many different voltage ranges around the world in terms of what each country’s energy utility provides on their medium and low voltage networks. 

In South Africa, there is typically 11 kV, 22 kV and 33 kV available in the medium voltage space for distribution transformers. On the low voltage side, we generally work with 400 V three-phase and 230 V single-phase, while 550 V,690 V and 1000V secondary networks are common in the mining industry. However, in other countries one could encounter anything between these parameters. 

The load on a transformer will also vary depending on its application. For a standard distribution-type load, the transformer is energised when it is commissioned and runs continuously with little or no switching activity. 

By contrast, a transformer in an application with renewable energy is likely to experience a full load alternating with no load at all – on a daily basis. These latter conditions would require a very different design, one that would accommodate the regular expansion and contraction of the windings as they heat and cool. 

Even the ‘standard’ distribution load on a transformer has become less consistent in recent decades, as energy-saving devices and components have been introduced. The resistive loads of the past, such as incandescent lighting, have been replaced by a range of non-linear loads such as LED lights, fluorescent lamps, inverters and variable speed drives on motors, pumps and compressors. 

There are also environmental conditions to consider when deciding on a transformer’s specifications, with applications in Africa often subject to high temperatures, rainfall and humidity. Where a dry-type transformer is located outdoors, its design will be quite different from an indoor unit. 

The process of specifying a transformer to optimally suit its application therefore needs detailed information about load conditions, environmental factors and the auxiliary equipment around it.