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Homework Help: 3 phase AC question

  1. May 4, 2009 #1
    The system with a balanced 3 phase load of 100kW at a 0.75 power factor, the system voltage is 12500V line to line, grounded wye.

    What size of capacitor can bring the power factor to 0.90 lagging, then what will be the real power?
  2. jcsd
  3. May 8, 2009 #2
    This was my work.
    Would anyone please verify my approach is correct?

    Initial 3 phase load of Ptotal =520kW, Pf1=0.85, and Vline=31540volts

    For grounded wye,

    Pf1=Sf1 * (cosf1), where Sf1= Pf1/(cosf1)

    Sf2=Pf2/(cosf2) = 173.33kW/0.96=180.55kVA

    Xf(reactive) = Vf2/Qf=182092/Qf=331.567E6/ Qf
    Xf(before) = 331.567E6/107k=3086Ω
    Qfcap= Qf1-Qf1=107k-50.54k=56460VAR
    Xfcap= Vfcap2/Qfcap= 331.567E6/56460= 5872 Ω

    a) The size of the capacitor per phase C is defined as
    C=1/(2*p*f* Xfcap)=1/(376.99*5872)=0.416mF since it is assumed f=60Hz

    b) Real power should remain same since we are not changing anything about the resistance part of the impedance so
    Ptotal = 3* Pf =520kW
    Pf=173.33kW per phase

    C) Qtotal = 3*Qf
    So Qtotal before = 3*107kVAR=321kVAR and Qtotal after = 3*50.54kVAR=151.62kVAR
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