High 3rd harmonic on the neutral

The thing that is going around in my head is trying to balance three single phase supplies connected to a three phase supply is chasing unicorns.


A large care home I did work at had a three phase supply, but three single phase meters and installations around the building. You had to be careful what you connected to what, but any neutral current issues are in the DNO network, not in the installations.


With three single phase installations connected to a three phase distribution board within the installation then presumably from what has been said when all the single phase installations are in use the neutral bar in the distribution board becomes a star point and because it’s AC with a positive and minus cycle current neutral flows back through the phases.


We all assume when we open up a single phase consumer unit the neutral bar is tied to earth and has negligible voltage on it although it could a lot of current flowing through it, you can touch it but try removing a conductor under load and sparks will fly with the end of the conductor immediately coming up to 240 volts.


With the three phase board as well as being tied down to earth the voltages should cancel as they are on different parts of the cycle? In addition the current should cancel in the neutral conductor by feeding back into one or more phases? But it will only cancel completely if the single phase loads are perfectly matched?


That’s three questions ?


Then the harmonics throw it all out of balance.


Andy B.

The thing that is going around in my head is trying to balance three single phase supplies connected to a three phase supply is chasing unicorns.


A large care home I did work at had a three phase supply, but three single phase meters and installations around the building. You had to be careful what you connected to what, but any neutral current issues are in the DNO network, not in the installations.


With three single phase installations connected to a three phase distribution board within the installation then presumably from what has been said when all the single phase installations are in use the neutral bar in the distribution board becomes a star point and because it’s AC with a positive and minus cycle current neutral flows back through the phases.


We all assume when we open up a single phase consumer unit the neutral bar is tied to earth and has negligible voltage on it although it could a lot of current flowing through it, you can touch it but try removing a conductor under load and sparks will fly with the end of the conductor immediately coming up to 240 volts.


With the three phase board as well as being tied down to earth the voltages should cancel as they are on different parts of the cycle? In addition the current should cancel in the neutral conductor by feeding back into one or more phases? But it will only cancel completely if the single phase loads are perfectly matched?


That’s three questions ?


Then the harmonics throw it all out of balance.


Andy B.