Transformer rated in KVA/MVA. Why? Explained

• June 24, 2020
• Gaurav J

Transformers are rated in KVA or MVA. Why we cannot rate them in kW or kVAR? In this tutorial, let’s understand this.

Before moving on in this tutorial, I strongly recommend you to go through my tutorial on active reactive and apparent power. This will help you to understand the concept more clearly.

Already done watching? Then let’s start.

Transformer rated in W/KW/MW

Let’s say, we have a transformer to which we only resistive load will be connected. Resistive load can be, lights, electric heaters, ovens, etc.
As the load is only resistive load, we know that, it is only going to draw active power. Therefore, in such case, I can rate my transformer in W, kW or MW.

The condition is load must be purely resistive, and load should not draw any reactive power. If this condition is met, then I can rate my transformer in W, KW or MW.
For instance, I can rate my transformer as 100 KW, which indicates I can take 100kW of active power from the transformer.

Transformer rated in VAR/KVAR/MVAR

Now, let’s understand the scenario 2. In this case, let’s say, I know load on my transformer will be purely inductive. Examples of inductive loads are induction motor, fans, pumps, relay, etc. Here, I know, my load is only going to consume reactive power. And no active Power will be consumed. Therefore, in such case I can rate my transformer in VAR, KVAR or MVAR. This is only possible when the load connected on transformer is going to consume only reactive power and no active power.

For instance, I can rate my transformer as 100 KVAR, which indicates I can take 100KVAR of Reactive power from the transformer.

Transformer rated in VA/KVA or MVA

In practical, we do not know what type of load will be connected across the transformer. The load could be resistive or inductive or capacitive. Or maybe the combination of all.
In this case, the load is not only consuming active power or reactive power. The load is consuming both the powers. And as we know when the load consumes both active and reactive power, we call it as apparent power. And the unit of apparent power is VA/KVA/MVA.
Therefore, as the load is consuming apparent power we must rate our transformer in VA/KVA/MVA.

And that’s the reason why transformers are rated in VA/KVA/MVA. Yes it is a simple as that.