What are the exact meanings of reactive and apparent powers? Do we say they are supplied or consumed?

Question

(The following doubt is from the topic of Sinusoidal Steady-State Analysis of electrical circuits)

I understand the meaning of real power - it is the average power consumed (or delivered) by an element in a circuit. But I do not get what reactive power is? From talking to people, what I have understood is - it has no real significance, but it is some quantity defined to ease the process of circuital analysis. Is this understanding of mine wrong? If it is wrong, can I please know the EXACT significance of it? Having understood the meaning and significance of the reactive power, can I also know the meaning and significance of apparent power?

Also, please go through the below picture:

In the following picture, "Element" can be anything - a voltage source, current source, resistor, impedance etc.

So I am also not able to comfortably use the terms supplied and consumed in case of apparent and reactive powers. If someone can help me understand my doubts, it would be really helpful! It would be also helpful if links to some useful reading material are provided. Thank you!

Answer 1

Power triangle: -

Real power supplied to the load is on the lower axis but, because not all loads are purely resistive, the apparent power may be larger. This is because (in a simple sinusoidal situation): -

$$\text{Apparent power} = V_{RMS}\times I_{RMS}$$

Whereas as actual power consumed (Real power) is: -

$$\text{Real power} = V_{RMS}\times I_{RMS}\times \cos(\Phi)$$

Where \$\Phi\$ is the phase angle between voltage and current.

For a simple resistor load, \$\Phi\$ is zero hence: -

$$\boxed{\text{Apparent power equals Real power and Reactive power equals zero}}$$

For a simple inductive load, \$\Phi\$ is 90° hence: -

$$\boxed{\text{Apparent power equals Reactive power and Real power equals zero}}$$

It's a case of using Pythagoras to show how these quantities inter-relate.

As far as supply/consumption is concerned, the electricity network supplies volts and amps at a certain phase angle and that phase angle is load dependent. This is a supply of electricity but, implied in that is reactive power because, you can calculate reactive power from apparent power and real power.

The load will consume real power because real power produces heat and kinetic energy. The load doesn't consume apparent power and neither does it consume reactive power. Consumption is a word reserved for real power.

Answer 2

To answer your questions, it require a little bit of basic theory.

When working with ac quantities, 2 things were observed:

• When load is connected to the supply source, some power is consumed by circuit and electrical energy is converted to heat, rotating and other types of energy
• But on closer inspection, it was seen that voltage and current profile were higher than the load required(resistive part of the load)

So in short, real power is actual energy being consumed by the load resistive part and transferred to another form where as, reactive energy is the power flow between energy storing elements(inductor, capacitor) and supply source, that means, in a cycle, some energy flows from the supply and gets stored in inductor/capacitor and then during rest of the part of the cycle, energy is given back to the source. So the net balance arrives at 0 energy in average sense.

Relevance of reactive power: Since both power flows in the circuit, the rms quantity actually represents in a correct way of the rating of voltage and current being supplied from source to load(Although reactive power does not contribute to anything). That is why we want the reactive power requirement of the load to be as low as possible(or some workaround). Higher the reactive power, higher the in-between components ratings(Cost increases).

Apparant power gives you a total of energy being supplied from source(Real + Reactive).

I think by looking into this theory, you will be able to answer your questions

A book on Basic Electrical engineering(not many people may know of). But it is a good book to look on fundamentals. Book Link

Youtube Lecture Series(Prof. L. Umanand): Youtube Lecture-15. The whole playlist is good for understanding