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Question
Sir/madam ! I know what is reactive power. But some say the reactive power is useless power. Others say the reactive power is important to maintain good voltage profile. Which one is true ? I am totally confused with this concept. If reactive power maintains good voltage profile. Please explain how it maintains good voltage profile with waveforms and examples.

Thanks for your previous answers.

Answer
I think you may have gotten a wrong idea about reactive power.  It is merely the fact of life when there are inductive or capacitive elements in the AC circuitry.

In some machinery systems the reactive circuitry is desirable for control and regulation of the machines.

In power delivery systems reactive power eats up power with I*2 X R losses in the power lines.  Thereby making it desirable for a low pf (power factor) to keep losses and power consumption to the lowest level.

In electronics we use reactive elements for tuning and frequency selection by properly introducing reactive elements (capacitors and inductors).

So, in a power distribution system it is common to introduce a reactive element near the load system in order to keep the voltage and current in phase.

However, since the reactive current must travel on the lines it can introduce IR voltage drop due to the current, even though it is not adding to the delivery of power to the load.

I will let you do your own Internet search on this topic as it is readily found with key words in your google or bing search engines.

Examples:

https://en.wikipedia.org/wiki/AC_power#Reactive_power

https://en.wikipedia.org/wiki/Volt-ampere_reactive

http://www.allaboutcircuits.com/textbook/alternating-current/chpt-11/true-reacti

And you can add your own search results.

Hope this helps.  

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Electronics questions about AC, DC and digital theory.

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Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

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IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

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BSEE University of North Dakota

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