The Star Point

[La Poste]

Greetings.

I am hoping someone could help me understand the star point in a three phase star configured transformer.
It's something that is mentioned a lot in electrical discussions.

I know the star point is attached to the ground and held at 0 Volts.
I also know that the star point is always at a difference of 230 Volts RMS from the line conductor but I am not sure why.

The star point is right in the middle of the three lines that are 120 degrees out of phase in a star wound transformer.
Does the fact it is right in the middle account for the reason it is always at 0 volts, is it some sort of balancing act that is done by the three phases that results in it being at 0 volts?

My knowledge is very patchy on this subject, I mean I have seen the diagrams, I learned to do the calculations at college but that was a long time ago.

I also know that when electricity is flowing in the line it is always trying to get back to the star point or does the return part of the electrical current actually cross the star point and then flow in one of the other phases sort of treating the star point like a cross roads for traffic between the three phases?

So could someone just explain what exactly is the star point and why it is at 0 volts.

I sure would appreciate it.

Thanks very much.

 

[ackbarthestar]

Theoretically, with a perfectly balanced load/loading it would be equal to the sum of the instantaneous currents which would be zero.
It is very unlikely that this would be the case since it would rise and fall according to the imbalances pulled by all the various single/three phase loads.

 

[SirKit Breaker]

I’ve got some drawings of the two main types of distribution transformers (DYn11 and DYn1) and the vector diagrams, earthing arrangements, etc. Also the effect of an unearthed neutral, which can be quite devastating. It blew every PC in an office where I worked.
Give me an hour to find them in my wonderful filing system, it’s a total shambles!

Tony is the man, i will keep my gob shut, so as to not complicate it.:wink_smile:

 

[La Poste]

I am thinking if the earth connection is removed from the star point the voltage at the star point will rise above 0 volts, I also think the line voltage will be 230 volts with respect to the star point but this 230 volts may be higher with respect to the ground.

I am not sure how much the star point voltage will float about though.

 

[Knobhead]

The amount the voltage floats depends on the current being drawn. See the vector diagrams in my post.