# 3-Phase system Homework Question

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

1. ### sanka Thread Starter New Member

Jun 5, 2010
3
0
Hi,

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

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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.
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Any help or direction would be much appreciated!
Thanks

2. ### mik3 Senior Member

Feb 4, 2008
4,846
67
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
3
0
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.