During operation, distribution transformers can sometimes operate underloaded due to excessive capacity, or overheat or even burn out due to overload or overcurrent. Improper capacity selection compromises the reliability and economic efficiency of power system supply.
Transformer capacity is selected based on load statistics. Since accurate load forecasts are difficult to make, the capacity is typically selected based on the projected maximum load. This often results in over-capacity, adversely impacting power system operation. If selected based on economic operation, the transformer's maximum economic load factor and the ratio of rated capacity to maximum load are derived, assuming copper and iron losses are equal. However, since actual operating loads are not necessarily the maximum loads calculated from load statistics, and loads are random and operating efficiency fluctuates, achieving economic benefits is difficult.
Currently, new low-loss transformers are being used in distribution systems to replace high-energy-consuming transformers, reducing iron losses alone by approximately 40%. Given the large number of distribution transformers and the large load fluctuations, the economic benefits are significant.
Making full use of the transformer's capacity without compromising its service life should be the primary consideration in selecting distribution transformer capacity. The recommended approach is to select distribution transformer capacity based on the predicted maximum load (Smax) and the typical daily load curve, in accordance with the International Electrotechnical Commission (IEC) standard, Guidelines for Oil-Immersed Transformers. This standard has been adopted in China. The advantage of this approach is that it takes into account the transformer's normal overload capacity, fully utilizing the transformer's rated capacity without shortening its lifespan. This reduces investment and improves the operating conditions of the distribution network, resulting in significant economic benefits.