Overview of Transformer Station Selection, Voltage Level Selection, Power Supply Diagram.

+ Workshop transformer station: This station receives electricity from the intermediate transformer station and converts it into appropriate voltage levels to serve the loads of factories, workshops, or consumers. The primary side usually has voltage levels: 6kV, 10kV, 15kV, .... The secondary side usually has voltage levels: 380/220V, 220/127V., or 660V.

-In terms of structure, people divide it into indoor stations and outdoor stations.

+ Outdoor BA substation: In this substation, the high voltage equipment is placed outdoors, while the low voltage distribution part is placed indoors or in prefabricated iron cabinets specifically used for distribution to the low voltage side. Small capacity transformer stations (  300 kVA) are placed on pillars, while large capacity stations are placed on concrete or wooden foundations. Building an outdoor substation will save costs compared to an indoor substation.

+ Indoor BA station: At this station, all electrical equipment is placed.

in the house

- Select location, quantity and capacity of transformer stations:

In general, the location of the transformer station needs to meet the following requirements:

- Near load center, convenient for power supply.

- Convenient for operation and management.

- Save investment costs and operating costs, etc.

However, the final location chosen also depends on other conditions such as: Ensuring the space does not obstruct other activities, aesthetics, etc.

Select voltage level: Because the factory is supplied with electricity from a 15kV line, and the factory's load only uses 220V and 380V voltage. Therefore, we will install a 15/0.4kV step-down transformer station to supply electricity to the factory's load.

 Power supply diagram:

With low voltage power grids supplying factories and households, people usually follow two main wiring diagrams:

-Ray diagram:

M

MM

Figure 3.1 Circuit diagram and ray diagram

This diagram has the advantages of: high reliability, easy to implement protection and automation plans, easy to operate, etc. But the disadvantage is high investment capital.

-Branch diagram:

M

M

M

Figure 3.2 Branch circuit diagram

For this scheme, the cost is lower, the flexibility is higher when it is necessary to change the technological process, rearrange the machines, but the disadvantage is that the reliability of the power supply is not high.

The ray diagram is used when there are concentrated consumers at the distribution point. The branch diagram is used in long rooms where consumers are spread out next to each other.

 For the power supply network for Tien Tan plastic factory, we will use a combination of the two diagrams above. Large capacity devices will be wired separately (ray diagram), while medium and small capacity devices can be connected together (branch diagram).

3.1.2 Select quantity and capacity of MBA :

Regarding the number of MBAs, there are usually options: 1 MBA, 2 MBAs, 3MBAs.

- Option 1 MBA: For type 2 and type 3 consumers, we can choose the option of using only 1 MBA. This option has the advantage of low cost, simple operation, but the reliability of power supply is not high.

- Option 2 MBA: This option has the advantage of high reliability of power supply but the cost is quite high so it is often only used for households with large or important capacity (type 1 households).

- Option 3 MBA: Power supply reliability is very high but the cost is also very high so it is rarely used, usually only used for especially important consumers.

Therefore, depending on the importance of the consumer, as well as economic criteria, we choose the appropriate option.

3.1.2.1 Concept of MBA overload:

When calculating the selection of an MBA, often the simple selection method is based on the allowable overload conditions of the MBA.

Systemic overload or normal overload of the transformer

pressure:

This rule applies when in normal daily mode there are

when the transformer operates under load (K 1 < 1) and sometimes under overload (K 2 > 1).

The calculation sequence is as follows:

- Based on the load graph through the transformer, choose a transformer with a capacity less than S max and greater than S min (S max > S b > S min )

- The load graph through the transformer is converted into a load graph with only

two levels K 1 and K 2 with overload time T 2 .

- From the load capacity curve of the transformer (MBA) with corresponding capacity and ambient temperature, determine the allowable overload capacity K 2cp corresponding to K 1 , K 2 and T 2 .

- If K 2cp > K 2 means that the selected MBA is capable of operating with the diagram.

given load where the transformer's hottest point temperature (  cd ) is never

>140 0 C and the transformer life is still guaranteed.

-If K 2cp < K 2 , the selected transformer does not have protection capability.

To ensure the above two conditions, a transformer with a larger capacity must be selected.

When choosing an MBA with a capacity greater than S max, there is no need to re-check this capability.

Multi-step load curve isometric approach to two-step load curve:

Ki =​

S i S dmB

K i > 1: overload (3.1)

Ki < 1: under load

- Determine K 2 , T 2 by isotropy of the region with K i > 1 according to the formula: