Selection Principle of Chosing The Capacity and Number of Power Transformer(一)

The capacity and number of transformers in substations are important factors affecting power grid structure, power supply safety, reliability and economy. The selection of capacity and number of transformers often depends on the current situation and growth speed of regional loads, the size of one-off construction investment, the capacity of the surrounding power grid or power plant to provide load, and the distribution devices connected with them. Technical and performance indicators depend on the nature of load itself and the level of reliability requirements for power supply, on the cost of transformer unit capacity, system short-circuit capacity and transportation and installation conditions, etc.

In recent years, with the continuous improvement of transformer manufacturing technology, the quality and safe operation level of transformer itself has been greatly improved; the no-load loss of transformer has been greatly reduced, and the economy of transformer has been greatly improved. The load rate of operation has been continuously reduced; the national energy saving and emission reduction policy encourages enterprises to carry out economic operation; the number and capacity selection of construction, expansion and transformer capacity increase has not yet been clearly specified in China, and it is also a systematic project constantly improved with the improvement of technical level, generally based on conventional experience and the viewpoint of planners; combined with relevant regulations and systems, In the author’s opinion, the following factors should generally be considered:

(1) The rated capacity of transformer should be able to meet the demand of power load in the power supply area, that is, to meet the need of total calculation load of all electrical equipment, and to avoid long-term overload operation of transformer. The transformer capacity of newly-built substations should meet the needs of planned load for 5-10 years, prevent unnecessary expansion and capacity increase, and reduce the large area and long-term power outages caused by capacity expansion. For substations with higher reliability power supply requirements, it is better to put two transformers in one time, and the normal load rate of transformers is not more than 50%.
(2) For substations with important users in the power supply area, one transformer should be considered in the condition of failure or outage maintenance, other transformers should ensure the first and second loads of users within the allowable time after taking into account the overload capacity. For substations with general loads, any transformer should be able to ensure that 70% to 80% of the total load of the power supply is not affected, and urban transformers should be transformed. The number and capacity of transformers in power plants should meet the requirements of N-1.
(3) Although the unit capacity cost of large capacity transformer is low, the construction of large capacity substation in high load density power supply area can save investment, and the larger the capacity, the more obvious the effect will be; but in order to ensure the flexible operation mode of power supply, it is necessary to consider the use of multiple transformers, the selection of single transformer capacity should not be too large or too small, and the possibility of expansion should be reserved for load development to realize the capacity of substation. From small to large, the number of transformers is from small to large, and the operation mode of three transformers in urban substations is the most flexible and reliable.


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