Unbalanced load in 3 phases system

[hisham1986]

Dear All,

Could someone please inform me about what happens to the unbalanced current that returns in the neutral conductor in a three phases transformer??does it get dumped in the grounding loop connected at the star point of the transformer or it stays trapped inside the circuit?

Thank you all in advance

 

[waross]

Example;
A three phase transformer bank with the secondary connected in star.
The A phase winding is supplying 100 amps to the load.
The B phase winding is supplying 100 amps to the load.
The C phase winding is supplying 150 amps to the load.
There Amps of unbalance on the neutral. This will return to the C phase winding.
100 Amps from each phase will cancel.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wareagle]

hisham
It sounds like you are caught up in that old example the flow of
electrical current is like water flowing through a hose. Nothing
could be further from the truth. If you take a simple circuit like
a lamp and connect the two wires from the lamp to 120 volts the lamp
lights. At the time the voltage is applied the electrons in the conductor
begin vibrate and bounce around in both conductors. There is actually little
movement in the wire. The electrons that move are considered drifting. This
drift can be calculated but not here. The actual movement is about one millimeter
per second depending on the actual amperage and conductor size. The flow of the
current moves back to the source so that everything is like it was before
the voltage was applied. The electrons do not just disappear. I have a site
that does a good job with your question. I hope this helps.

http://amasci.com/miscon/whatis.html

 

[abrahamJP]

Unbalanced load cause currents to flow in neutral and so cause neutral to be shifted.
So there is a limit that unbalance should be within 3%

 

[DRWeig]

Current (as we describe it mathematically in circuit analysis, not physically as wareagle described), must complete a loop. It originates at the three phase supply. In the circuit of a 208V single phase heater, the current might leave phase A and return to the source through phase B. In an unbalanced wye such as a panelboard feeding multiple loads that do not match, or a single phase load such as a 120V lamp, the current returns to the source through the neutral. If that source is a utility that only runs the three phases to a building transformer, the neutral current enters the ground through the service entrance bond and returns to the utility source through the ground.

So the answer to your question is that current (both physically and mathematically) cannot be trapped. It must complete its loop back to the source, one way or another.

Best to you,

Goober Dave

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[waross]

" the neutral current enters the ground through the service entrance bond and returns to the utility source through the ground."
That would be a code violation in North America. If loads are connected from line to neutral, a neutral conductor back to the source must be provided.
Exception: There are some areas where rural distribution systems use a ground return in place of a neutral conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

True, Bill. Thanks for cathing my generalization.

Best to you,

Goober Dave

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