What is Reactive Power?
First, let us consider what reactive power is mathematically and then we will see what it means practically.
Active power = Voltage * component of current in phase with the voltage i.e. V*I cos(theta)
Reactive power = Voltage * component of current 90 degrees out of phase with voltage i.e. V*I sin(theta)
Where theta = angle between Voltage and Current.
This was mathematical explanation which we find in most textbooks but
that doesn’t tell a thing about what actually reactive power is.
So, let us now jump into practical considerations and i’ll try to explain what reactive power is and why it is said that it does not do any useful work with practical examples.
Let us consider a Transformer.
As you might be knowing, both the windings of a transformer are not connected electrically (they have insulation between them) but still electric power flows from one winding to another.
Electric power cannot flow through air or any insulation under normal conditions, then how the hell this happens ?
This is the point where reactive power comes into play.
Before explaining the role of reactive power, let me explain about transformer working in short.
In transformer, current flows in primary winding and sets up magnetic flux. This magnetic flux links with secondary winding and then current is induced in secondary winding and this is how we get current on secondary side.
Now let us comeback to the role of reactive power.
So, from where do you think this magnetic flux responsible for transformer operation came from ? and the answer is REACTIVE COMPONENT of current.
Reactive power acts as a BRIDGE between primary and secondary windings.
It creates a constant bridge over which active power travels and move on to do some useful work.
I think you still might not have got what actually reactive power.
So let me explain it one more time in layman’s terms using simple analogy.
Consider there is a small river dividing two cities and you need to build bridge over it to connect both the cities. After building bridge, you also need to construct a house on the other side of the river.
For the whole work including bridge and house, you’ve got only 100 wooden planks of which 10 are used to create bridge over the river. The workers will use this bridge to cross the river and transport items and construct a house on the other side. Here the main work is to build the house on the other side for which we’ve created a bridge to move from one point to another. In this case, are the wooden planks used in constructing the bridge directly contributing to the building of house ? NO. We’ve used 10 wooden planks for bridge and that cannot be used for building the house.
And this – THE BRIDGE is what reactive power does.
100 wooden planks is the total complex power of which some amount is used to create bridge (10 planks) and remaining active power (90 planks) does the useful work. This is why we say reactive power does not do useful work.
Now you might be thinking OK, THIS WAS THE CASE WITH A TRANSFORMER BUT WHAT ABOUT OTHER THINGS? EVERYTHING DOES NOT HAVE ELECTRICAL ISOLATION LIKE A TRANSFORMER BUT STILL REACTIVE POWER FLOWS ?
Yes, you are 100% correct.
Reactive power assists the flow of electromagnetic energy.
To explain it further it will require some technical details and will take another long post.
I think you’ve got a basic idea of what a reactive power does.
If you have any other question you can comment blow.