Electrical Engineering ⇒ Topic : All Day Efficiency

The load on a transformer over a day or over a season will not remain constant at the rated value, but will average at a lower value. For example, the distribution transformers which supply the lighting and domestic loads, etc.have their primaries energised all through 24 hours of the day, although their secondaries supply little or no-load most of the time during the day. This results in the iron losses occurring throughout the day while the copper losses occur only when the transformer is on-load. Hence to obtain maximum efficiency, a transformer should be designed such that the core losses are very low. Therefore, the efficiency of such transformers should be based on their performance throughout the day. This is done by comparing the energies.Energy efficiency or all-day efficiency is defined as the ratio of the total output energy of the transformer expressed in kilowatt hours to the energy input to the transformer, both energies evaluated over a 24 hour period of a complete load cycle. That is

ALL DAY EFFICIENCY

The commercial efficiency of a transformer is expressed as:

This is not valid for distribution transformers. The primaries of distribution transformers are energized for all the twenty four hours. Their secondary supply is less during the day. Therefore, core losses occur throughout the day whereas the copper losses occur when the transformer is loaded. Therefore, the distribution transformers are designed so that the core loss is minimum.Since copper loss depends on the load, it is less important. The performance of such a transformer is designed by the all day efficiency. It is defined by

This efficiency is always less than the commercial efficiency of a transformer. The load cycle of the transformer must be known to calculate the η_aII day . Load factor is used to facilitate practical calculations