How to choose？
When selecting distribution transformer, if the capacity is too large, it will not only increase the equipment cost, but also make the transformer in no-load state for a long time, so that the reactive power loss will increase.If the transformer capacity is too small, it will make the transformer in a long-term and overload state, the transformer will be easy to burn out .Distribution transformers should be located as far as possible in the load center, the supply radius should not exceed 0.5 km.The load rate of distribution transformer is between 0.5 and 0.6, and the efficiency is the highest. At this time, the transformer capacity is called economic capacity.
For the lighting, agricultural and sideline products processing use of power supply and other comprehensive transformer capacity selection, to consider the power of electrical equipment at the same time, according to the actual may appear 1.25 times the maximum load transformer capacity.
According to the characteristics of scattered users, low load density, seasonal load and strong gap, the transformer with adjustable capacity can be used.Adjustable capacity transformer is a kind of transformer which can adjust its capacity without load according to load size.
For transformer substation or industrial and mining enterprises with large power load, the main and branch transformer power supply mode is generally adopted. One (main transformer) is configured according to the maximum load, and the other (branch transformer) is selected according to the state of low load, which can greatly improve the utilization rate of distribution transformer and reduce the no-load loss of distribution transformer.
The capacity of a transformer is a unit of power (apparent power) expressed in terms of AV (volts ampere) or KVA (kilovolts ampere).It is the product of RMS of ac voltage and ac current, and the calculation formula S=UI. A power supply system, after calculation, according to the calculated load S to choose the capacity of the transformer.For a single transformer that temporarily USES electricity (on the construction site) and provides stable load supply, the load rate is generally about 85%.
Capacity marked in the nameplate:
The transformer capacity is about 1.15 times of the calculated load.For permanent power supply system, the load rate of transformer is usually 60% ~ 70%.However, the rated capacity of the transformer is manufactured according to a certain grade, so the selected capacity is similar to that of the calculated grade specification.
For example, the calculated load of a construction site is 86.06kVA.Then the transformer capacity is 100kVA, and 100kAV transformer can be selected according to the capacity grade.
By the way, the capacity of a single transformer should not be more than 1000kVA.When the load is large, several transformers can be used for parallel power supply.Parallel operation should meet the same transformer ratio, the same connection group, the same short-circuit voltage and other conditions;Second, pay attention to the problem of load distribution, generally the ratio of maximum capacity to minimum capacity is no more than 3:1.
The maximum power of each phase of the load:Add the load power of A phase, B phase and C phase independently, such as the total load power of A phase is 10kW, the total load power of B phase is 9kW, and the total load power of C phase is 11kW, with the maximum value of 11kW.(note: the power of 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 this equipment in each phase.)
For example: c-phase load total power = (300W X 10 PCS) + (2kW X 4 PCS) =11kW
Calculate the total three-phase power:
11kW X 3 phase =33kW (total three-phase power of transformer) total three-phase power /0.8, which is the most important step. Currently, the power factor of transformers sold in the market is only 0.8 for more than 90%, so the power factor needs to be divided by 0.8.The total power of the transformer 33kw /0.85. According to the "power engineering design manual", the transformer capacity should be selected according to the calculated load. For the single transformer supplied with stable load, the load rate is generally about 85%.
41.25kW/0.85=48.529kW (transformer power to be purchased), so it is ok to choose 50KVA transformer when purchasing.