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How to choose transformer: capacity calculation method

power transformer is the key equipment in the power supply system. Its main function is to boost or reduce voltage in order to facilitate the reasonable transmission, distribution and use of electric energy, which has an important impact on the form of the main wiring of the substation and its reliability and economy. Therefore, the correct and reasonable selection of transformer type, number and capacity is a major problem in the design of main wiring

how to choose a transformer

when selecting distribution transformer, if the capacity is too large, it will form the phenomenon of "big horse pulling small car". It not only increases the equipment investment, but also makes the transformer in the no-load state for a long time, which increases the reactive power loss

if the transformer capacity is too small, the transformer will be in the overload state for a long time. Transformer is easy to burn. According to the principle of "small capacity, dense points", the distribution transformer should be located in the load center as far as possible, and the power supply radius should not exceed 0.5 km

the load rate of distribution transformer is between 0.5-0.6, and the efficiency is the highest. At this time, the capacity of transformer is called economic capacity. If the load is relatively stable, the transformer capacity can be selected according to the economic capacity in the case of continuous production

for special transformers that only supply power to dynamic loads such as drainage and irrigation, the capacity of the transformer can generally be selected as 1.2 times the nameplate power of the asynchronous motor

generally, the starting current of the motor is 4-7 times of the rated current, and the transformer should be able to withstand this impact. The capacity of the largest motor directly started should generally not exceed about 30% of the transformer capacity

it should be pointed out that the special transformer for drainage and irrigation should not be connected to other loads in general, so as to shut down in time in non drainage and irrigation period and reduce power loss

for the selection of comprehensive power transformer capacity for power supply lighting, agricultural and sideline products processing, the simultaneous power of electrical equipment should be considered, and the transformer capacity can be selected according to 1.25 times of the actual maximum load that may occur

according to the characteristics of scattered rural power users, small load density, seasonal load and strong gap, adjustable capacity transformers can be used. Capacity regulating transformer is a kind of transformer that can adjust the capacity without load according to the size of load. It is suitable for use in places with obvious seasonal changes in load

for substations or industrial and mining enterprises with large power load, the power supply mode of mother and child transformers is generally adopted. One (mother transformer) is configured according to the maximum load, and the other (sub transformer) is selected according to the low load state, which can greatly improve the utilization rate of distribution transformer and reduce the no-load loss of distribution transformer

in view of the actual situation that some distribution transformers in rural areas are in low load operation for a long time in a year, in addition to a small amount of peak power load, the power supply mode of parallel operation of parent-child transformers or transformers can also be adopted for conditional users. When the load changes greatly, transformers with different capacities shall be put into operation according to the principle of minimum power loss

the capacity of the transformer is a power unit (apparent power), expressed in AV (volt ampere) or KVA (kilovolt Ampere)

it is the product of the effective value of AC voltage and AC current, and the calculation formula is s=ui. The rated capacity of the transformer will be indicated on its nameplate

when selecting a transformer, it is necessary to know how much capacity transformer to use, which is usually considered according to the load of the actual power system

for a power supply system, after calculation, select the capacity of the transformer according to the calculated load s. For a single transformer with temporary power (on the construction site) and stable load power supply, the load rate is generally about 85%

the transformer nameplate will identify the capacity:

the transformer capacity is about 1.15 times the calculated load. For permanent power supply system, the load rate of transformer is generally 60% - 70%. However, the rated capacity of the transformer is manufactured according to a certain grade, so when selecting, the selected capacity is similar and greater than the calculated grade specification

for example, the calculated power load of a construction site is 86.06kva. Therefore, the plan that the calculated capacity of the transformer is 100 KVA is also a difficult point. According to the capacity level, 100kVA transformer can be selected

incidentally, the capacity of a single transformer should not be greater than 1000KVA. When the load is large, several transformers can be selected for parallel power supply. The parallel operation should meet the conditions of equal transformation ratio, the same connection group and the same short-circuit voltage; Secondly, pay attention to the problem of load distribution. Generally, the ratio of maximum capacity to minimum capacity does not exceed 3:1

calculate the maximum power of each phase of the load:

add the load power of each phase of phase a, phase B and phase C independently. For example, the total load power of phase A is 10kW, the total load power of phase B is 9kw, and the total load power of phase C is 11kw, taking the maximum value of 11kw

(Note: the power of each single-phase equipment is calculated according to the maximum value on the nameplate, and the power of three-phase equipment divided by 3 is equal to the power of each phase of this equipment.)

for example, the total power of phase C load = (300W x 10 computers) + (2kW x 4 air conditioners) =11kw

calculate the total power of three phases:

11kw x 3 phases =33kw (total power of three phases of transformer) total power of three phases/0.8, which is the most important step. At present, the power factor of more than 90% of transformers sold on the market is only 0.8, so it needs to be divided by the power factor of 0.8

33kw/0.8=41.25kw (total power of transformer) total power of transformer/0.85. According to the electric power engineering design manual, the transformer is committed to the development of non combustible composite polyurethane rigid foam insulation board, high imitation elastic decorative panel, and prefabricated colorful decorative polyurethane insulation board. The capacity should be selected according to the calculated load. For a single transformer with stable load power supply, the load rate is generally about 85%. If it is unbalanced, it is easy to cause premature wear of mechanical parts and data error

41.25kw/0.85=48.529kw (transformer power to be purchased), so it is OK to choose a 50KVA transformer when purchasing the regular experimental machine experimental method

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