In the operation of distribution transformers, instances occur where units run under-loaded due to excessive capacity, or overheat-and even burn out-due to overloading or overcurrent conditions. Such improper capacity selection compromises the reliability and economic efficiency of the power supply system.
Transformer capacity is typically selected based on load statistics. Since accurate load forecasting is difficult, selection is usually based on the projected peak load. This often results in oversized capacity, which adversely affects power system operation. An alternative approach is to select capacity based on economic operation criteria-specifically, by deriving the optimal economic load factor and the ratio of rated capacity to peak load based on the condition where copper losses equal iron losses. However, achieving these economic benefits in practice is challenging because actual operating loads fluctuate randomly and differ from statistical peak loads, causing operating efficiency to vary.
Currently, high-energy-consumption distribution transformers are being replaced with new, low-loss models, reducing iron losses alone by approximately 40%. Given the large number of distribution transformers and the significant load fluctuations involved, the resulting economic benefits are substantial.
The primary criterion for selecting distribution transformer capacity should be how to fully utilize the installed capacity without compromising the transformer's normal service life.
The recommended approach is to select the transformer capacity based on the projected peak load ($S_{max}$) and typical daily load curves, in accordance with the International Electrotechnical Commission (IEC) standard (1972)-specifically, the loading guide for oil-immersed transformers. This standard has been adopted in China. The advantage of this method is that it accounts for the transformer's normal overload capability, allowing for full utilization of the installed capacity without shortening its lifespan. This approach yields significant economic benefits by reducing investment costs and improving the operating conditions of the distribution network.
Computer programs based on this method have been used to generate capacity selection tables for six types of typical daily load curves.
- Irrigation and threshing grounds;
- IVillage sideline industries; lighting and threshing grounds;
- Sideline industries; lighting, irrigation, and threshing grounds;
- IRegional and county-level industrial use;
- Village composite load including industrial loads;
- Urban/town composite industrial load.
Instructions for using the attached table are as follows:
① Determine the load type and select the corresponding typical daily load curve.
② Determine the equivalent air temperature (θδ). The ambient temperature referred to in IEC standards is not the simple average ambient temperature but the equivalent air temperature; this implies that if the temperature were maintained at θδ under the given load conditions over a specific time interval, the resulting insulation degradation would be equivalent to that caused by the natural fluctuation of ambient air temperature. For practical purposes, the following values are recommended: 22°C or 24°C for the region south of the Yangtze River; 20°C for the region north of the Yangtze River; and 16°C or 18°C for the Northwest and Northeast regions.
③ Based on the projected peak load (in kVA), consult the table to determine the rated capacity (Sn) of the transformer to be selected.
Example: For load curve I with an annual equivalent air temperature of 9°C and a peak load of 1000 kVA, an 800 kVA distribution transformer should be selected.
④ Determine the normal overload capacity of the operating transformer based on environmental conditions and load type.
Example: For load curve VI with an annual equivalent air temperature of 22°C and a transformer rated capacity of 315 kVA, the maximum load the transformer can carry is 340 kVA.
It should be noted that when selecting transformer capacity using the method above, actual operation is also subject to constraints regarding the permissible duration of peak load operation to ensure safety; exceeding this permissible duration carries a risk of transformer burnout. This permissible duration can be calculated using the formula for the maximum winding temperature of an oil-immersed transformer with natural oil circulation. For convenience, the attached tables list the calculated maximum permissible operating times (τmaxe) under peak load conditions, based on an ambient temperature of 35°C and a maximum winding hotspot temperature not exceeding 140°C. For example, according to Table IV, if the annual equivalent air temperature is 20°C, the maximum permissible operating time at peak load is 17 hours. The tables indicate that when the ratio of peak load to rated capacity is 1.17 or lower, the load is considered permissible, meaning there is no risk of overheating.
