How can we determine the rated power of a transformer?
This question is frequently posed by electricity distribution companies worldwide, reflecting their significant concern over whether they receive the exact product outlined in tender documents. Let's delve into this matter.
When discussing the rated power of a transformer (kVA), we are referring to a transformer designed to handle the specified load (as indicated on the nameplate):
1- Continuously,
2- Without any mechanical or electrical failures,
3- Without exceeding temperature rise limits,
4- Without deviating from specified no-load and load losses (within tolerance).
Essentially, if a transformer can function successfully under the declared load while meeting these criteria, we can confirm that its actual rated power matches the value stated on the nameplate.
Now, let's consider a 200 kVA distribution transformer designed according to the following parameters:
1- Ambient temperature of 40°C,
2- Oil temperature rise of 60°C,
3- Winding temperature rise of 65°C.
Technically, this transformer should be operable in an ambient temperature of up to 40°C throughout its lifespan (typically 20-25 years).
But what if the transformer is used in a location where the ambient temperature is 50°C instead of 40°C?
Under these conditions, the winding and oil temperature rise of the transformer under 100% load would likely exceed the specified values, drastically reducing the transformer's lifespan and potentially leading to early failure (depending on manufacturer tolerances).
And what if the transformer is used in a location where the ambient temperature is 30°C instead of 40°C?
In this scenario, under 100% load, the winding and oil temperature rise of the transformer would likely be lower than the specified values, resulting in an expected longer lifespan or the possibility of loading the transformer beyond the specified value (for example, loading it to 215 kVA instead of 200 kVA in our example). Of course, in this case, losses and impedance would change accordingly and must be taken into consideration.
As a result we can say that if you want to be sure wheter you buy a transformer exactly with specified rated power or not, then you should subject the transformer to the temperature rise test. If temperature test result is comply with the declerad value, then you may consider what you buy is matching what you have ordered (from rated power prespective).
In conclusion, it can be stated that to ensure whether you are purchasing a transformer with the specified rated power, it is advisable to conduct a temperature rise test on the transformer. If the temperature rise test results align with the declared value, then it can be concluded that the purchased transformer matches the ordered specifications, particularly in terms of rated power.
