Forums » Electrical Engineering

Reactive power

    • 42 posts
    January 12, 2020 12:14 PM PST

    Dear all,

    I am missing Alex in this forum. Thanks all for supporting and answering some of my questions.

    what will the losses because of reactive power in power system ?

    • 156 posts
    January 12, 2020 1:13 PM PST

    There is no reactive power losses. Reactive power can cause or contribute to the real power loss. 

    • 71 posts
    January 12, 2020 3:01 PM PST
    Hameedullah Ekhlas said:

    There is no reactive power losses. Reactive power can cause or contribute to the real power loss. 

    if current through a device lags the voltage, then the device consumes reactive power.

    • 103 posts
    January 13, 2020 6:48 AM PST

    Reactive power results in reduced PF, increased current, & more heat for the same amount of work. 

    I miss Alex too !


    This post was edited by Steve Ward at January 13, 2020 6:50 AM PST
    • 75 posts
    January 13, 2020 12:36 PM PST

    Adding to the other posts:

    Normal Industrial Reactive power is generally inductive, lagging - motor loads.

    Yes, it can be capacitive reactive, but uncommon - discuss that one later.

    For an induction motor to work, the stator magnetic field has to be charged.  This takes energy.  So, on part of the cycle, the magnetic field is charged, energy provided from the generator.  On other parts of the cycle, the magnetic field collapses, and sends the energy all the way back to the generator.  The current supplying this power comes from the generator, goes through the transmission lines and transformers, and goes through the motor stator windings.  This current heats the generator stator, transmission lines, transformers, and motor stator, but does not produce any real power, (as is torque out the end of the motor shaft.)  However, that reactive current does produce heat, which is real power, in all the components it goes through.

    The power company only gets paid for the real power they deliver.  So, they are paying for all the losses in the generator and transmission equipment.  And that is capacity that is being taken up by non-revenue producing current.

    The generator gets a double hit.  The inductive reactive current heats the stator.  So does the rest of the current, but no money from the reactive current.  The inductive, lagging power pulls the generator voltage down.  The Voltage Regulator responds by increasing the DC current to the field.  This can be X00A to X000A.  Not only is there increased stator heating, there is also increased rotating field heating.  If one examines a generator capability curve,  the lagging side is trimmed back due to the rotor and stator heating.

    The power company insists on being paid to run their machinery.  However they are only being paid to the real power delivered.  And they are not getting paid for all the heat pumped into the atmosphere by the reactive current losses.

    For example:

    For a 480V, .86pf, 1000KW load, the apparent power is 1176KVA

    Current supplying real power is 1203A.  At .85pf, current is 1415A.

    One could say the current is 1415/1203  = 1.176 

    Power is (I^2)R, so the transmission losses will be 1.176^2 = 1.38 higher.

    So, what ever losses they have with the power factor at 1.0, the losses are 38% higher at .85pf

    Their solution is to measure the reactive power and charge a penalty if the reactive power goes over a scheduled amount - commonly pf lower than .85pf lagging.  They may not be able to charge you for the VARS,  but they can charge a penalty if you have too many VARS.

    Usually reactive vars are not a problem inside of the plant.  The distribution losses are minor and inconsequential.  The money is in looking at any reactive power penalty.  That is the usual driver to do any power factor correction.

     

    • 103 posts
    January 13, 2020 4:03 PM PST

    Just like to add something that has been implied, but not stated directly.

    Transformers & AC motors cannot function without reactive power.

    Excellent job Carl !