Harmonics on three phase balanced load

[danwinsal]

When you have a dedicated service installed for a balanced three phase load then would there be harmonics on the service ground? The service is for a 480 volt 3 phase motor load with vfd and other electronic controls. I assume this would make it non-linear. If there are no harmonics on the service ground then could the service ground be derated?

 

[kwired]

When you have a dedicated service installed for a balanced three phase load then would there be harmonics on the service ground? The service is for a 480 volt 3 phase motor load with vfd and other electronic controls. I assume this would make it non-linear. If there are no harmonics on the service ground then could the service ground be derated?

VFD that only runs on the three phase conductors still has harmonic content - but doesn't put any harmonics on the neutral conductor.

Add a bunch of 277 volt electronic ballast lighting units and you place harmonic currents on the neutral. Problem with wye connected systems is those harmonics are additive on the neutral conductor even if the neutral is otherwise balanced across phases. Average facility lighting usually doesn't produce enough harmonics to be to much trouble in general. Data processing facility however will have higher harmonic levels on the neutral.

 

[danwinsal]

VFD that only runs on the three phase conductors still has harmonic content - but doesn't put any harmonics on the neutral conductor.

Add a bunch of 277 volt electronic ballast lighting units and you place harmonic currents on the neutral. Problem with wye connected systems is those harmonics are additive on the neutral conductor even if the neutral is otherwise balanced across phases. Average facility lighting usually doesn't produce enough harmonics to be to much trouble in general. Data processing facility however will have higher harmonic levels on the neutral.

This service would have only the three phase VFD with no neutral loads. In saying that then the service grounded conductor size could be reduced correct?

 

[kwired]

This service would have only the three phase VFD with no neutral loads. In saying that then the service grounded conductor size could be reduced correct?

No neutral - no current on neutral - no harmonics on neutral.

The drive still has harmonic distortion it just isn't placing any of it on the neutral conductor of the system.

It is when we have non linear line to neutral loads on a wye system when we get the additive effect of harmonics on the neutral and need to be concerned about the loading conditions of the neutral. Single phase systems don't have the same problem - harmonic currents are still there but don't add up the same way because there is a 180 degree phase angle instead of the 120 degree phase angle.