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Homework Help: Transformers in parallel

  1. May 11, 2015 #1
    1. The problem statement, all variables and given/known data
    A load of 1000 kVA at 0.866 p.f. lagging is supplied by two 3 phase transformers of 800 kVA capacity operating in parallel. Ratio of transformation is same : 6600/400 V, delta/ star. If the equivalent impedances referred to secondary are (0.005 + j.015) ohm and (0.012 + j 0.030) ohm per phase respectively. Calculate load and power factor of each transformer.
    In the solution of given question while calculating the secondary current for star connection , they found it as I2=1000 *1000/√3*400 .. but generally in questions , the voltages we are given are line voltages , but according to this it seems that the given voltage 400 is phase voltage ..!! So this become confusing that what should we consider the given data whether line or phase??
    Also , for calculating the load by formula load = 3 VpIp , they took phase voltage as 254..!! How this voltage came up???

    2. Relevant equations
    Considering the 2 transformers in parallel as A & B,
    I2A=ZB/ZB+ZA * I2 I2B=ZA+ZA+ZB *I2

    3. The attempt at a solution
    secondary current I2 = 1000*1000*√3 /400 , this is the secondary current according to me as I am considering the voltage given as line , but in textbook solution the voltage is √3*400..!!
    Last edited: May 11, 2015
  2. jcsd
  3. May 13, 2015 #2


    User Avatar

    Staff: Mentor

    If 400v is the line voltage, then for a Y-connected load the current would be found using:
    power per phase = 1000/3 kVA
    current = power per phase / (400/√3)

    Is that leading us to what your textbook shows?

    Would you like to reconsider this? How would you determine the voltage across one load phase?

    Your formulae under Relevant Equations are in dire need of some parentheses.

    BTW, avoid all bold type next time, when over-used it becomes ineffective
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