The power transformer ratings are measured by the maximum output voltage and current they deliver. We'll often read or hear about the volt-ampere (VA) capacity for a given unit, which equals the product of the nominal output voltage and maximum deliverable current.

Table of Contents

1.What is a power transformer 

2.What is power transformer rating/ How to calculate ratings of a power transformer

3.Applying wrong power transformer rating, what will happen? 

4.Ratings of power transformer in some power plants

1. What is a power transformer

Power transformers are an electric device that transforms electrical voltage from one level to another level. In the transmission network, the power transformer step-up voltage to transfer electric power in a long-distance. Then, we have a distribution transformer to step-down voltage to a lower level to use in factories or houses.

We have many reasons for using a power transformer in electrical power systems. But one of the most important and simple explanation for using the power transformer is to reduce power losses during electric power transmission. 

About the principle of operation, power transformers work on the magnetic induction principle between coils to convert voltage (current levels) to another voltage (current levels). 

There are some differences between power transformers and distribution transformers operating at normal levels. Usually, power transformers are utilities in electric transmission networks (that use higher voltages) for either step up or step down applications (400 kV, 200 kV, 110 kV, 66 kV, 33kV), their capacity is higher than 200MVA. Distribution transformers are put in distribution networks to transform electricity to a level (11kV, 6.6 kV, 3.3 kV, 440V, 230V) that can be utilized by an end-user and usually are rated lower than 200MVA. 



Power transformer in the transmission network

2. What is power transformer ratings/ how to calculate ratings of a power transformer?

Power transformer ratings are mainly measured in kilovolt amperes (kVA) or megavolt amperes. It means the primary winding and the secondary winding are designed to withstand the kVA or MVA ratings, which is stamped on the transformer nameplate.

The power transformer ratings are measured by the maximum output voltage and current they deliver. We'll often read or hear about the volt-ampere (VA) capacity for a given unit, which equals the product of the nominal output voltage and maximum deliverable current.

We have a formula:

P = UI
P: power transformer ratings
U: nominal output voltage
I: maximum deliverable current

For example, a transformer with 12MV output, capable of providing up to 10 A of current, has an MVA capacity of 12 MV x 10 A, or 120 MVA. The power-supply filtering makes it necessary for the power-transformer MVA rating to significantly exceed the watts' actual power that the load consumes.

A high-quality power transformer is capable of providing the necessary currents or voltages, constitutes an integral and critical part of a well-engineered power supply. The power transformer is usually the most expensive power-supply component to replace if it burns out or damaged. Hence, transformer engineers always choose the transformer ratings appropriate to the projects when designing and building a power supply.

3. Applying the wrong power transformer ratings, what will happen?

Of course, power transformers may seriously injury or damage if they are installed with an improper voltage, current, or power rating. We must consider the voltage, current, and power-handling capabilities of the primary and secondary windings when using a transformer. The nominal voltage, current, and power values represent the middle point of the respective maximum and minimum rated values.


We can determine the type and thickness of the insulation used in winding to know the maximum voltage and then use it safely. A higher maximum voltage can be applied to the windings when a better (and thicker) insulation is used.


The diameter of the wire used for the winding determines the maximum current carried by a transformer winding. If the current is excessive in a winding, a higher-than-ordinary amount of power will be dissipated by the winding in the form of heat. This heat may be sufficiently high to break down the insulation around the wire. Therefore, we must set limits to both the applied voltage and the current drawn by the MVA load requirements in other to keep the transformer temperature at an acceptable level. 

4. Ratings of power transformer in some power plants


A power transformer in power plant


Depending on construction criteria, we have power plant capacity. According to their economic feasibility, they may keep 100MW - 4500MW. According to this, they will design a generator on which the transformer depends. Power plant capacity includes high side voltage level, power production, power factor, and current. 


Power plants rely on voltage in primary windings and secondary windings to choose a suitable power transformer to step-up electrical power before transfer it to the power grid.


MBT's team hope that the above information will help you understand more about power transformer ratings. If you want to find a transformer manufacturer, we always available to help you

Contact us:

Hotline: +84 913006538

Website: http://vietnamtransformer.com/

Email: [email protected]

Address: Song Cung Industrial Zone, Dong Thap Commune, Dan Phuong District, Hanoi, Vietnam. 

(+84) 0913.006.538
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