In an unbalanced 3-ph (solidly grounded) because the grounded points are at the same potential, how would current flow in the neutral?

Thread Starter

skyline1397

Joined Sep 13, 2017
13
In a Four-Wire Wye (Star) system with both source and load are solidly grounded. If the load impedances are not equal (loads are unequally distributed) which is the case most of my time, this load unbalance cause the line/phase currents to be different in magnitude and displaced from one another by unequal angles, where their vectorial/phasor sum would result in nonzero current to flow from the load neutral back to the supply neutral. Assuming a negligible resistance in the neutral wire and since it's a solidly grounded system shouldn't the supply neutral and the load neutral be at the same potential? If yes then how would current flow in the neutral? and if they are not at the same potential why?
 

crutschow

Joined Mar 14, 2008
25,247
Assuming a negligible resistance in the neutral wire and since it's a solidly grounded system shouldn't the supply neutral and the load neutral be at the same potential? If yes then how would current flow in the neutral? and if they are not at the same potential why?
The current flows because there is enough voltage difference between the wire ends (however small) to cause the current to flow in the "negligible resistance"
Certainly if the wire would have no resistance then it would require no voltage for the current to flow.
 

Thread Starter

skyline1397

Joined Sep 13, 2017
13
In a badly unbalanced load, the neutral current is much greater than the wire resistance current. How come? what is the cause of this huge current flow, although both neutral points are at equal potentials?
 

crutschow

Joined Mar 14, 2008
25,247
the neutral current is much greater than the wire resistance current.
What do you mean "wire resistance current"?
The current through a wire always obeys Ohm's law.
The current though it equals the voltage across the wire divided by the wire resistance.
although both neutral points are at equal potentials?
The are not at exactly equal potential.
How many times do I have to say that?
 

Thread Starter

skyline1397

Joined Sep 13, 2017
13
I meant the neutral current due to the resistance of the neutral wire, say it's 5 A (which is constant since the resistance is constant). In real life, if the loads are unbalanced, the neutral current would be much more than 5 Amps.
Why would there be a current more than the 5 Amps?
 

nsaspook

Joined Aug 27, 2009
7,467
Assuming a negligible resistance in the neutral wire and since it's a solidly grounded system shouldn't the supply neutral and the load neutral be at the same potential? If yes then how would current flow in the neutral? and if they are not at the same potential why?
Being solidly grounded doesn't change the circuit operation.

The total circuit loop is NOT between a point connection on the supply neutral and a connection point on the load neutral. You must see the circuit as a whole.

 
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