Clarification on AC Power Concepts in Industry

Discussion in 'General Electronics Chat' started by ElectronicGuru, Dec 1, 2014.

  1. ElectronicGuru

    Thread Starter New Member

    Sep 26, 2014
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    I recently learned through my textbook about AC circuits, particularly the concepts of average power, reactive power, effective/RMS values, PF factor, complex power, etc.

    The book started to delve into real-world applications of these concepts in industry and I'd like some clarification.

    1. Why do we need reactive power in circuits in general and particularly in the power industry? What use do capacitors/inductors play?

    2. When a company wants to increase its PF factor for reduced cost, one way to do so is by installing capacitors in parallel. Mathematically, I see the effect of this is to add a negative reactive power component to the total power, thus making the PF factor closer to unity (since circuits tend to be inductive in nature; see question 3). But what is the intuitive explanation for this? I've read online (but not in my book) about inductors producing reactive power and capacitors consuming it, but what does this really mean? What is the capacitor doing in the power grid that makes the average power closer to the apparent power?

    2. Why is it that the power grid is favoured toward inductive loads (i.e., positive or lagging PF)? My textbook and Wikipedia simply state that it is so without providing examples.

    Thanks for any help in answering these questions!
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Interesting how conventions change. I was always of the opinion (conventional? ) that inductive loads "absorbed" reactive power while capacitive loads "supplied" reactive power - notwithstanding the fact that neither ideal inductors or capacitors absorb or supply real power.
    Free spinning AC synchronous machines can be configured by excitation control to control system reactive power - sometimes referred to as synchronous condensers when "behaving" as a capacitor.

    Reactive power is not "needed" in the sense that it serves any useful purpose. It is an unavoidable consequence of the nature of certain electrical loads.
    The normally lagging power load (by my convention) produces negative Vars. To reduce the effective Vars at the load one has to produce positive or leading Vars which are displaced 180 degrees in phase relative to load Vars. A capacitor in parallel with the load produces leading Vars.There is either partial or complete cancelation between positive and negative Vars whilst the effective real power remains unchaged.

    Irrespective of convention, the power conductors conveying real and reactive power to the load must carry the current commensurate with the effective load apparent power. Reducing the apparent power by Var cancelation reduces the current demand and hence the Joule heating losses in the overall power supply network.
     
    Last edited: Dec 1, 2014
  3. WBahn

    Moderator

    Mar 31, 2012
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    You answered your first question in passing while asking your second: "since circuits tend to be inductive in nature". Many large industrial loads, such as motors, are inductive. As a result, when you power them from an AC source you have to supply both the real part of the load (which gets turned into useful work) and the reactive part of the load (which doesn't). The reactive component is essentially energy that is shuttles back and forth between the load (e.g., the motor) and the grid as the inductance is energized and de-energized twice in each cycle (once in one direction and once in the other). But the current associated with the reactive part of the load has to travel through wires that have resistance and, hence, some real power is dissipated as heat and has to be made up by the generators. Also, the generators have to be sized to supply the total current required for both the real and the reactive parts of the load.

    By placing a capacitor bank near the inductive load (or, if you had a large capacitive load you would place an inductor near it) you localize the reactive power so that now it is shuttled back and forth between the load and the compensation capacitors. Since they are much closer to the load there is far less resistive heat loss and the power company only has to supply/absorb the reactive component at power up and shut down of the load.

    As for "absorbing" and "producing" reactive power, these are just terms to capture the notion that whatever inductors due that capacitors due the opposite. You could use either label for either one as long as you are consistent. Since reactive power in an inductive load is positive most texts that I've seen call this "absorbing" to be consistent with the notion that a resistive load that has positive power is absorbing power. It also makes intuitive sense if we consider that most industrial loads are inductive and we think of loads as absorbing power.

    Note that PF is always positive as long as the real part of the load is positive (in other words, we aren't talking about a load that is powering the generator, which CAN happen). That is why we have leading PF and lagging PF.
     
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