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Reactive Power

  1. Nov 6, 2011 #1
    Hi
    I am studying instrumentation and during my internship at a power plant i have come across many basic Power engineering stuff. My concept in the field are not very clear and a term i fail to understand is Reactive power..
    I had studied that reactive power just travels back and forth in a circuit and is not good, it just damages the transmission lines and production machinery but during my work i have seen turbines producing reactive power and real power. When i asked what was that for i was told that the electric supply company requires power in watts and VARs and upon further inquiry i was told that a normal appliance (say a fan )cannot run until it gets a certain amount of reactive power to overcome its reactance......
    I can not understand this concept completely can anyone explain it to me in physical terms or recommend some good material to read so i can understand the physics of it.
    Thanks
     
  2. jcsd
  3. Nov 6, 2011 #2

    NUCENG

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    Basically the transmission lines from the utility to the customer have more than just resistance. A long wire has inductance and the wire to ground has capacitance. The longer the transmission the larger are the values of these "reactive" equivalent component.

    At the load end, motor loads have fairly large inductance to build the magnetic fields needed for the motor to work.

    If you have ever studied RC and LC circuitswith AC supplies, you will know that reactive loads currents are 90 degrees out of phase with the true power to resistive loads. They do not produce real work. The net effect of these reactive loads is that generator currents have to include sufficient reactive power to charge those capacitances and to build the magnetic fields in those inductances. The term VAR is volt-ampere-reactive and is a description of the reactive load on the generator.
     
  4. Nov 6, 2011 #3

    Averagesupernova

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    What I took the liberty of putting in bold and italics sounds like alot of mumbo-jumbo to me. Consider an inductor with virtually no resistance. In other words wound with a very very good conductor like gold. When this inductor is hooked to an AC source a specific amount of current will flow. How much current flows is determined by the inductance of the wound coil. Of course the inductive reactance is determined by the inductance of the coil and the frequency of the source that is driving it. This load is virtually 100% reactive since there is little to no series resistance because of the gold conductor. The current is 90 degrees out of phase with the driving voltage. Virtually NO power (actual power, in watts) is dissipated. There is nothing special about the generator, the inductive load is determining the reactive power. If we disconnect the inductor and replace it with a resistor that draws the same amount of current then the generator will supply real power to the load. Nothing has changed about the generator, it is the resistor that will determine the real power.
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    The reason the power company likes you better if you only draw real power and not reactive is that reactive power is drawing current down the transmission line but not actually doing anything useful for the end user. Reactive power never does. If the end user can compensate for this with power factor correction capacitors then the inductive reactance is cancelled with capacitive reactance and no reactive power is delivered from the power company. The total current running in the transmission lines is reduced.
     
  5. Nov 6, 2011 #4
    @Averagesupernova i think i was not clear in my question the confusion for me is that the turbine HMI shows the power produced..it reads Real Power 16.5 MW and Reactive Power 2.2MVAR...If it is just a waste of power then why is it not compensated for immediately after production..why is it send to the grid station? i am told its not a wastage in fact it is charged from the distributor company at a separate rate than real power.
    and it is provided for domestic use with no separate mention in the bill as the domestic consumption of reactive power is almost negligible compared to real power but large industrial setups are charged for the total power delivered i.e Real+Reactive.. So i guess its not useless as i thought.

    @NUCENG
    so u mean that the reactive power is not entirely useless it is used to charge the capacitors and inductors initially?? But why transmit the reactive power over such large distance i mean that it is automatically created whenever there is a phase shift between voltage and current.
    So even if i have a perfectly in phase supply of current and voltage(neglecting the wire inductance etc) my circuit(inductive or capacitive) will automatically produce the waveform shift and hence the required reactive power and if it is a perfectly resistive load no shift will be produced and hence no reactive power..

    Please tell me if i my question is not clear..
     
  6. Nov 7, 2011 #5

    NUCENG

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    With a single generator and load that would be possible. But utilities serve thousands of customers and loads have on/off switches. Capacitor banks can compensate for some reactance to compensate for the generally inductive loads. But in the real world some reactive loads will remain. Once you start saying things like "neglecting the wire inductance etc." you pass outside the real world.
     
  7. Nov 7, 2011 #6
    Thanks a lot NUCENG..
     
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