3-Phase system Homework Question

Discussion in 'Homework Help' started by sanka, Jun 5, 2010.

  1. sanka

    Thread Starter New Member

    Jun 5, 2010

    I have been given a homework question that im abit unsure where to start and how to go about it. Question below:

    Three houses are connected to a symmetrical three phase 400V, 50Hz supply, fed from the local LV substation distribution transformer. The transformer is a three-phase, 4 wire, unbalanced star, with the out of balance current flowing down the neutral as normal. The houses have loads with the following resistances:

    Brown Phase house load = 20Ω
    Black phase house load = 15Ω
    Grey phase house load = 10Ω

    If a fault occurs so that the neutral breaks at the transformer, calculate the individual house voltages.

    Any help or direction would be much appreciated!
  2. mik3

    Senior Member

    Feb 4, 2008
    You can assume a non-zero voltage at the neutral point (e.g. Va) and calculate the phase currents with respect to the line voltages. You should have a simultaneous equations to solve and find the value of Va.
  3. sanka

    Thread Starter New Member

    Jun 5, 2010
    Thanks, I found a way of getting around it.

    Firstly I converted to unbalanced delta, then converted the new loads to polar form to allow me to find the phase current from the 400v/R. Converted the phase current to cartesian for when I converted delta back to star so I could subtract the relevant phase currents from eachother, giving me the line current.

    I then simply multiplied the line currents by the relevant loads to give my my house voltages.