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Power system operation

  1. Oct 14, 2016 #1

    cnh1995

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    I recently read a chapter on power system analysis in a book and I have some questions.
    1) The three phase power at the secondary distribution level is unbalanced i.e. not equal for each phase but it is nearly balanced at the HV transmission level.
    Is this true? Also, is the power balanced at the generator end? Basically, is there a difference in degree of balancing between transmission and distribution?

    2) In the distribution network, there are a lot of power electronic and electronic loads which lead to distorting current waveforms and hence, harmonic content of the distribution network current is very high. Are these harmonics mitigated at the transmission level using filters? Is the transmission current almost sinusoidal? If no, then where is mitigation of the harmonics carried out?

    Thanks a lot in advance!
     
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  3. Oct 15, 2016 #2

    anorlunda

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    Good questions.

    Neither unbalances nor harmonics are 100% eliminated, but at the power plants, they are much less than at the distribution level.

    The transmission system feeds many distribution circuits, the unbalances in those circuits tend to cancel each other. So the first answer is diversity. Distribution engineers worry about unbalances all the way up to the substation, as they assign small loads to phases in single phase branches.

    Transmission lines can introduce their own imbalances because of asymmetric geometry. The phases are transposed at intervals to counter that. Look it up.

    The reactive impedance is lines and transformers is higher for harmonics. ##X=j\omega L##. For Nth harmonic X is N times larger. That is the main filter.

    Edit: I forgot to mention that in extreme cases, harmonics are filtered using shunt capacitors. But that is rare.
     
  4. Oct 15, 2016 #3

    cnh1995

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    Thanks a lot! This is very helpful. I'll read more about this and post again if anything is unclear.
     
  5. Oct 15, 2016 #4
    One of the ways they can mitigate harmonics is to use condensers. These are synchronous motors which draw little power. They act as "flywheels" for the power grid. The inertia of the rotor helps maintain a sinusoidal current. The excitation of the motor can be tuned to make power factor corrections as well.
     
  6. Oct 17, 2016 #5
    IMO - it really come from the averaging of many distribution level systems connected to one Transmission level feed. Not all ( none) of the Distribution level feeds would be imbalanced in the same way.

    Sometimes large industrial users - that receive a large MV feed (distribution level by Definition) are forced by the utility to correct the imbalance, PF or harmonics, or they pay a hefty surcharge. I have a customer working with a battery factory with 30MW load - they have 2 36KV feeds - and 95% of their load are basic 6 pulse rectifiers (with the two transformer they "look" like a 12 pulse) - it is really a mess, and they pay a lot in surcharge AND additional Transformer losses (additional real power they pay for that yields no benefit).
     
  7. Oct 17, 2016 #6

    dlgoff

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    Here's a schematic of a transmission line circuit. compliments of above link
    PLC.jpg
     
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