WAZIPOINT Engineering Science & Technology: Why not 400KV to 11KV Step Down Transformer Directly Used in a Transmission System?

Thursday, May 18, 2023

Why not 400KV to 11KV Step Down Transformer Directly Used in a Transmission System?


Electrical Power Transmission and Distribution Network System
Fig- Typical Electricity Network System

The electricity network system mainly works under three major sects- Power Generation, Power Transmission, and Power Distribution.

Why can't we step down 400KV direct to 11KV in a transmission system?

To find the answer to why we cannot use the high-voltage or extra-high voltage step down directly to the distribution level voltage, let us know the transmission and distribution voltage level from the generation station to the user connection point.

Electricity Generation Voltage

Generating Voltage: 11kV

Normally the electricity generating voltage is 11000Volt or 11kV (11-kilo volt). Electricity generating at 11000 V or 11 kV at the generating stations is much more economical than generating at a higher transmission voltage level. It is cheaper to generate electricity at a relatively lower voltage and then step it up for transmission to distribute to the customer.

Electricity Transmission Voltage

Transmission Voltage: 132kV, 230kV, 400kV, 765kV

In a power transmission system, electricity is transmitted at very high voltages or extra high voltages depending on the amount of power, and length of the transmission line.  

Actually, the transmission system uses high voltage (132kV, 230kV) or extra high voltage (400kV, 765kV) to minimize the energy losses that occur due to resistance in the power lines. 

This high voltage or extra high voltage is stepped up from the generating voltage of 11kV. Because of low voltage transmission to long-distance not physible in economically and technically.

Electricity Distribution Voltage

Distribution Voltage: 33kV, 11kV

The electricity distribution voltage is 33kV and 11kV, somewhere it may 66kV. These low voltages are stepped down from higher voltages (such as 132kV, and 230kV), and the grid substation is used to reduce the voltage levels from extra high voltage to high voltage and high voltage to distribution medium voltages.  

Electricity Customer Voltage

Customer Voltage: 0.4kV, 11kV, 33kV, 132kV or 230kV

Substations are located near the end-users, where the electricity is distributed to homes, factories, and other consumers step-down the voltage level as required considering the category of customer.

  1. Secondary Customer, 120-volt single phase, 400-volt three phase;
  2. Primary Customer, 11kV;
  3. Subtransmission Customer, 33kV;
  4. Transmission Customer, 132kV.

You must not confuse to see the voltage level is different from above in the other article, both are correct. Voltage levels are not the same in all countries, it is different from country to country.

Voltage Levels in Electricity Transmission Network System

In the above figure, the voltage levels in a transmission line are shown as 132kV, 230kV, or 400kV, but it is not limited, many countries use different levels of transmission voltage. 

Also, in a country, they use different voltages in transmission lines according to the amount of load transfer and their distances.

The transmission line grid system uses a step-down transformer to down the voltage level from Extra-high voltage to high-voltage, like- 400kV step-down to 230kV, again 230kV step-down to 132kV voltage.

All equipment details for your further reference

The Necessity to Use Electrical Transformers in Power System

According to the above discussion, we found that generally, customers use low voltage like- 230 volts for single-phase loads and 400 volts for two or three-phase loads. 

High voltage is actually not used to connect the consumers' pieces of equipment. So, a transformer is necessary to step down or step up the voltage levels.

A step-up transformer is used at the generation level to the transmission level, on the other side, a step-gown transformer uses at the customer end to make the high voltage into usable low voltages.

Hope, you already understand the necessity of a transformer in the electricity transmission and distribution system. Now, we are near to the answer to our question-

Why not use it directly 400KV to 11KV Step Down Transformer?  


However, it is not possible to step down 400 kV directly to 11 kV because the step-down transformer required to do so would be too large and impractical. 

The size of a transformer is proportional to the ratio of the input voltage to the output voltage, so stepping down from 400 kV to 11 kV would require a transformer with a ratio of approximately 45:1. 

Such a transformer would be extremely large, heavy, and expensive, making it impractical to use in a power system.

Instead, the voltage is stepped down in stages, with intermediate substations reducing the voltage gradually from 400 kV to 230 kV, then to 132 kV, and finally to 11 kV or 33 kV. This allows for the use of smaller and more practical transformers at each stage, making the system more efficient and cost-effective.

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