Ground harmonic currents

[charlierod]

I work for at a production facility for a petroleum company. We have an uninterruptible power supply UPS feeding an offices block (computers, printers, communication systems and so on). The UPS is supplied through a 480/208 D/y transformer (secondary solidly grounded plus neutral conductor). I measured neutral and ground currents and found that the values are almost equal (2 Amperes). Furthermore, those currents were found to be triplen harmonics (180 Hz) althogh their values were negligible when compared to phase currents (65 Amperes,60 Hz). The question is, Why neutral and grounding conductors carry similar currents?. Could be that situation dangerous despite the low values of the currents?. I think that a bond between neutral and ground might be downstream the transformer, What's your opinion?

All comments and suggestions are welcome

 

[GOTWW]

My first thought is that
Equal currents = parallel paths.
That is there is a neutral-ground bond
somewhere downstream. It should not be difficult to track
down, but will probably require an outage.

 

[jbartos]

Comment: Typically, the neutral current returns are due to the load unbalance. The ground fault currents tend to be common mode currents caused by current leakages to ground, using the ground return path to the power supply source.

 

[busbar]

One value the doesn’t seem to be mentioned is the current on the transformer-secondary bonding jumper between the grounded-circuit conductor {“neutral”} and (“green”) equipment-grounding conductor. If 2 amperes is not causing any unsafe or inefficient operation, particularly if the loads are substantially 1ø switchmode power supplies, it's probably best forgotten, or checked again in a month or a year.

 

[wheelerTi]

You can use Zero sequence fiter transformer to attenuate the triplen harmonics(3rd, 9th etc) causes by unbalance distribution on loads.