Nonlinear loads and neutral current

[mbrooke]

191003-0811 EDT

mbrooke:

Assuming ideal transformers, then the current at the input of the transformer is a replication of the output current, but scaled by the turns ratio.

If on the output side the wye mid point only has internal current flow, that is, no outward neutral current (all currents balance at the midpoint), then on the input side there will be no external midpoint current flow because of the scaled replication of the currents. Whatever balancing was necessary for this to occur took place on the secondary side.

In the real world this won't be true because of magnetizing current, and slight differences in the transformers.

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But differences aside, there absolutely no load of combination of loads that can or will produce primary current? In other words its physically impossible? I'm trying to think a case that could or can produce current.

 

[winnie]

I think that Gar makes a very good point. Imagine for a moment that you had the maximum possible unbalanced system: only a single phase load connected from A to B, with nothing connected to C. Additionally this single phase load is so amazingly non-linear that no fundamental current flows, only 3rd harmonic current. There _must_ be 3rd harmonic current flowing on both the A-N coil and the B-N coil.

Since there is no neutral connection there is no neutral current on the secondary side. The 3rd harmonic current on the A-N coil has to exactly balance and match that on the B-N coil, because there is no other place for it to go. Since the current on the primary pretty well matches the current on the secondary, the primary currents will also have to match.

-Jon

 

[mbrooke]

I think that Gar makes a very good point. Imagine for a moment that you had the maximum possible unbalanced system: only a single phase load connected from A to B, with nothing connected to C. Additionally this single phase load is so amazingly non-linear that no fundamental current flows, only 3rd harmonic current. There _must_ be 3rd harmonic current flowing on both the A-N coil and the B-N coil.

Since there is no neutral connection there is no neutral current on the secondary side. The 3rd harmonic current on the A-N coil has to exactly balance and match that on the B-N coil, because there is no other place for it to go. Since the current on the primary pretty well matches the current on the secondary, the primary currents will also have to match.

-Jon

Hard for me to fathom, but it makes sense. Also comforting to know that Y-Y with all its advantages can be used in the real world.