Unbalanced star transformer

[Electrix123456]

Hello

In a wye secondary transformer with the star point connected to ground, if phases are unbalanced the star point remains at ground potential. Does a current flow into to ground if phases are unbalanced? If so, how can it if it has no return path to the transformer itself?

 

[LarryFine]

There should be no "ground" current as long as the grounded conductor pathway (from service neutral back to the transformers) is intact.

And welcome.

 

[GoldDigger]

Hello

In a wye secondary transformer with the star point connected to ground, if phases are unbalanced the star point remains at ground potential. Does a current flow into to ground if phases are unbalanced? If so, how can it if it has no return path to the transformer itself?

With a wye secondary no current will flow anywhere unless some sort of load connection is made to at least one outer point and the star point or to two outer points. With no connections, no current will flow no matter how unbalanced the voltages may be. It is that simple.

 

[Electrix123456]

What if the wye transformer is supplying a 3 phase unbalanced load?

If the star point is not grounded, there will be a voltage present which would be the vecotral sum of the unbalanced phases.

If the star point is grounded, this is now at ground potential. Wouldn't some current flow to ground with the imbalance?

 

[Sahib]

If the star point is grounded, this is now at ground potential. Wouldn't some current flow to ground with the imbalance?

If some current flows to ground with the imbalance, the potential of grounded star point would rise with respect to remote ground which may be readily checked by you!

 

[oldsparky52]

What if the wye transformer is supplying a 3 phase unbalanced load?

If the star point is not grounded, there will be a voltage present which would be the vecotral sum of the unbalanced phases.

If the star point is grounded, this is now at ground potential. Wouldn't some current flow to ground with the imbalance?

Current will take all paths, more flows through the least resistive path.

This is why you ground the neutral at only one point, to try to prevent parallel paths for unbalanced current.

Are you thinking of transformers in the same building or where the PoCo is delivering power?

 

[Besoeker3]

Hello

In a wye secondary transformer with the star point connected to ground, if phases are unbalanced the star point remains at ground potential. Does a current flow into to ground if phases are unbalanced? If so, how can it if it has no return path to the transformer itself?

You may find it easier to think of the connection to the star point as neutral whether grounded or not.

 

[Sahib]

The OP's confusion, it seems, is how there could be ground current flow while the transformer star point is at ground potential. I think I clarified it in my last post.

 

[kwired]

Grounding only puts the point connected to earth at same potential as earth.

Current leaving other points of the source is trying to get back to the source, in a three phase wye source there is three other possible points in the source it is seeking. Current will travel through ground to get back to the source, only because one point of that source is connected to ground. Current won't seek ground if there isn't a path to the source through said ground.

If you take same three phase wye system and ground an outer point instead of the neutral - you now have a difference ground reference and the neutral point will have voltage to ground. We don't normally see this because NEC requires the neutral in that case to be the conductor that gets grounded, but you certainly could ground any other point of the system - you just can't ground two points -that results undesirable current between those two points.

 

[mivey]

Hello

In a wye secondary transformer with the star point connected to ground, if phases are unbalanced the star point remains at ground potential. Does a current flow into to ground if phases are unbalanced? If so, how can it if it has no return path to the transformer itself?

Depends on how you bond it. If the star point can freely float to its natural neutral point (the lowest energy state), no ground current will occur. If the star point is bonded and is held to a fixed or semi-fixed neutral point (a higher energy state), ground current will occur.

 

[mivey]

If some current flows to ground with the imbalance, the potential of grounded star point would rise with respect to remote ground which may be readily checked by you!

The OP's confusion, it seems, is how there could be ground current flow while the transformer star point is at ground potential. I think I clarified it in my last post.

A potential difference is not required in all current paths.

 

[winnie]

If the star point is not grounded, there will be a voltage present which would be the vecotral sum of the unbalanced phases.

If the star point is grounded, this is now at ground potential. Wouldn't some current flow to ground with the imbalance?

Keep in mind that there are various levels of approximation when you are trying to understand a system. Here are some useful questions to consider:

If the star point is not grounded, why would there be any particular voltage present?

If you approximate the wye system as totally not connected to 'ground' then the voltage between the star point and ground is _undefined_, and could be just about anything. It might be 0V, it might be thousands of voltage DC, or it might be something else. With no connection there is no current flow and nothing driving the star point voltage.

If you consider that there is actually some amount of capacitive coupling and insulation leakage current between the phases and ground, then it is very likely that the star point will be at roughly the vector sum of the phase voltages, as you suggest. However once you start considering this 'leakage' current you know see the closed circuit for current to flow when you ground the wye.

-Jon