Technology Literature

How to Choose the Capacity and Quantity of Transformers?

The power transformer is the key equipment in the power supply system. Its main function is to step up or step down to facilitate the reasonable transmission, distribution and use of electric energy. Therefore, the correct and reasonable selection of the type, number and capacity of the transformer is an important issue in the design of the main wiring.

(1) Selection of the number of transformers
The number of transformers is generally determined based on comprehensive consideration of conditions such as load level, power consumption and economic operation. When one of the following conditions is met, two or more transformers should be installed:
1. There are a large number of primary or secondary loads. When the transformer fails or is repaired, multiple transformers can ensure the reliability of power supply for primary and secondary loads. When there is only a small number of secondary loads, a transformer can also be installed, but the low-voltage side of the substation must have a tie power supply with sufficient capacity as a backup.
2. The seasonal load changes greatly. According to the size of the actual load and the number of transformers put into it, economical operation and energy saving can be achieved.
3. Large centralized load capacity Although it is a three-level load, but the power supply capacity of one transformer is not enough, two or more transformers should be installed at this time.
When the capacity of the backup power supply is limited, the important loads should be concentrated and the non-important loads should be powered by different transformers, so as to facilitate the switching of the backup power supply.

(2) The overload capacity of the transformer
The rated capacity of the transformer (nameplate capacity) is the maximum visual temperature continuously output within the specified service life (20 years) under the specified ambient temperature (the maximum daily average temperature is 300C, the maximum annual average temperature is 200C, and the maximum annual temperature is 400C). in power.
Since the actual load changes, and most of them are under load, the insulation aging speed is slower than the rated time, which is equivalent to extending the service life and reserves a certain overload capacity.
1. Normal overload
Day and night and seasonal loads are not uniform, and overload is allowed.
(1) Day and night look up the table to determine the overload multiple
(2) Season 1% rule The total cannot exceed 15% sbn
Considering the above at the same time, the outdoor transformer should not exceed 30% sbn
Indoor transformer does not exceed 20% sbn
2. Accident overload

In the event of an accident, a relatively large overload in a short time is allowed, but the running time cannot exceed the specified value. (see table)

(3) Transformer capacity selection
The choice of transformer capacity should be determined according to the calculated load of the equipment it carries, as well as the type and characteristics of the load. First of all, it is necessary to accurately calculate the load, which is the basic basis for the calculation of power supply design. One of the most commonly used methods to determine the calculated load is the demand coefficient method. The basic formula for determining the calculated load of a three-phase electrical equipment group according to the demand coefficient method is:

1. A single main transformer

SN =(1.15~1.4) Sc

2. Two transformers

SN =(0.6~0.7) Sc

SN >Sc (I + II)

When any transformer runs alone, it should meet the needs of primary and secondary loads.
3. Capacity upper limit
Workshop transformer, transformer should not exceed 1000 KVA.
The capacity of dry-type transformers above the second floor should not be greater than 630KVA

A single distribution transformer in a residential substation should not exceed 630KVA.

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