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Calculation of transformer capacity

Power transformer is the key equipment in power supply system. Its main function is to boost or depressurize to facilitate the reasonable transportation, distribution and use of electric energy. It has an important influence on the form of main wiring and its reliability and economy of substation. Therefore, the correct and reasonable choice of transformer type, number and capacity of the main wiring design is a key issue.

Selection of distribution transformers, if the capacity is too large, not only increased investment in equipment, but also make the transformer in a long-term no-load state, so reactive loss increased. If the transformer capacity is too small, will make the transformer long-term and overload conditions. Easily burn the transformer.


Based on the principle of "small capacity, dense point", distribution transformers should be located at the load center as far as possible, and the power supply radius should not exceed 0.5 km.
Distribution transformer load rate of 0.5 to 0.6 between the highest efficiency, then the capacity of the transformer called the economic capacity. If the load is relatively stable, continuous production of the economic capacity of the transformer capacity can be.

For only dedicated power supply such as irrigation and drainage transformers, generally according to the induction motor power rating of 1.2 times the capacity of the transformer.
General motor starting current is 4 to 7 times the rated current, the transformer should be able to withstand this impact, the largest direct start motor capacity, generally should not exceed the transformer capacity of about 30%.

It should be noted that: Drainage and irrigation transformers generally should not access other loads in order to stop the drain in a non-irrigation and timely manner to reduce power loss.

For power supply lighting, agricultural and sideline products processing and other comprehensive selection of power transformer capacity, power equipment to be considered at the same time, according to the actual maximum load may be 1.25 times the selection of transformer capacity.

According to the dispersion of rural power grid users, load density, load seasonality and gap characteristics, can be used to adjust the capacity of transformers. Adjustable capacity transformer is a load capacity can be adjusted according to the size of the load capacity of the transformer, which is suitable for load changes significantly seasonal use

For substations or large load-consuming industrial and mining enterprises, the general use of maternal and sub-transformer power supply, one (mother transformer) according to the maximum load configuration, the other (sub-transformer) at low load conditions can be greatly increased Distribution transformer utilization, reduce the no-load loss of distribution transformers. For some distribution in rural areas in addition to a small amount of peak load in a year, a long time in the low-load operation of the actual situation, the conditional users, you can also use mother or transformer parallel operation of the power supply. When the load changes greatly, according to the principle of the lowest power loss, put different sizes of transformers.

Transformer capacity is a power unit (apparent power), with AV (voltammetry) or KVA (KVA) said.
It is the product of the AC voltage and the RMS rms, calculated as S = UI. The size of the transformer rated capacity will be indicated on the nameplate (see below).
Select the transformer need to know how much capacity of the transformer, this is usually based on the actual load of the power system to consider.
A power supply system, after calculation, according to the calculation of the load S to select the capacity of the transformer. For a single transformer that temporarily supplies electricity (on the construction site) and operates at a steady load, the load factor is generally about 85%.