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Three phase voltage drop

Derek Eccles

For a balanced 3 phase system the supply authority, distributes 400v at the supply transformer.

The supply cables are 150m in length, so care is required to ensure the voltage drop does not become excessive at the customers terminals

Assuming there is a votage drop of 20v, across each cable at a particular balanced load, what voltage would the customer see at his terminals ?

I presume 360v ?

Regards, Derek

  • 0

    well if the line voltage drops are unequal, it does not affect the relative phases of star connected loads, (they remain at 120 degrees) but it does alter the relative phasing of any delta connected load. In effect you are trying to find the length (voltage) of one side of a triangle when the other two are at exactly 120 degrees apart, but one of length 220 units and the other of length 210 units. Unlike the previous case you cannot invoke symmetty and treat as two half triangles where pythagoras applies, so no root (3) this time

    The fastest way to solve this sans computer involves an A3 sheet of squared graph paper and a propelling pencil.
    Failing that then there is a lot of rather messy O level == GCSE level trigonometry to do.

    I will not attempt that in a public forum during a tea break it is tempting fate too much.
    M.

  • 0 in reply to Derek Eccles

    I hope I have found the answer.

    For a STAR connected load with a neutral connection the line (and in this case, phase) currents I1, I2 and I3 are phasors and are related to the neutral current In by :

    In = I1 + I2 + I3.

    If the load is balanced In = 0.

    If the load is unbalanced, it is possible to calculate the neutral current (using phasors) and therefore the voltage drop in the supply cable.

    This means the star/neutral point at the load will not be zero volts - not sure what this is,

    I guess the effect will be the phase voltages will not be 230v and not equal. Likewise the line voltages at the load will not be 400v and not equal.

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