How to interpret dual rating transformers?
You might have seen transformer data with two different values of apparent power, such as e.g. 20/25 MVA or 75/100 MVA. What does it mean and how to deal with dual rating transformers?
Transformer rated power
A transformer is characterized by its nominal output power. Unlike motors, the power is not expressed in active power (kW, MW) but in apparent power (kVA, MVA). In case of a motor we are interested in the delivered shaft power. That is a mechanical quantity corresponding to the active power (thus in kW). For transformer, the relevant loading is in kVA. How much of it is active power and how is reactive power, depends on the character of the load.
There are transformers with dual rating stated in the technical specification, data sheet or rating plate. What does it mean? The dual rating is typically linked with the cooling method. The more efficient cooling the higher the possible output power. Some transformers are designed to operate with natural cooling and forced cooling. Such transformers have two power ratings: lower value for natural cooling and higher value for forced cooling.
Dry type transformers with dual rating are typically those that can operate either with air natural (AN) or air forced (AF) cooling. For example, a transformer rated 4.5/6.0 MVA can be loaded with 4.5 MVA in case of natural air cooling. When the cooling fans are used, the same transformer can provide 6.0 MVA under air forced condition. Note that both power ratings 4.5/6 MVA refer to continuous output.
Liquid immersed transformers with dual power rating are often oil natural – air natural (ONAN) and oil natural – air forced (ONAF). As an example, transformer rated 20/25 MVA can deliver 20 MVA continuously with ONAN cooling while the rated output increases to 25 MVA under ONAF cooling.
What are the benefits of dual power rating?
What is the benefit of dual rating? It can be used in two ways:
a) At lower power loading the forced cooling is not needed → extended lifetime of cooling fans, reduced auxiliary power consumption and reduced sound level with natural cooling
b) In case of failure of forced cooling system, the transformer can still be operated with reduced power.
What is the typical power output with natural cooling as percentage of power rating with forced cooling?
Typical rating with natural cooling is 60…80% of the rating with forced cooling. There is an economical design but the ratio can also be defined as user requirement.
Can converter transformers supplying variable frequency drives have dual power rating?
Principally yes. As explained above, it can be utilized to operate the system with reduced power when cooling system fails or it can be used to enhance the power rating in case of unusual high loading (e.g. test stand with different power requirements depending on test object). The increased power rating can also be used as “future reserve”.
How to interpret the short-circuit impedance of the transformer?
In this case we have two power ratings but only one value of short circuit impedance in %. For example 10/12 MVA, zk = 11.5%. If not stated differently, the short circuit impedance refers to the lower power rating, i.e. the rating with natural cooling. In this case 11.5% @ 10 MVA rating.
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References
[1] How do ONAN and ONAN/ONAF transformers compare, MB Drive Services, April 2023, available online, https://mb-drive-services.com/how-do-onan-and-onan-onaf-transformers-compare/
[2] What is the definition of short-circuit impedance, MB Drive Services, December 2024, available online, https://mb-drive-services.com/what-is-the-definition-of-short-circuit-impedance/
