ADE7880 problems with THD readings on utility

Hi,

We have set-up the ADE7880 through I2C and everything is functioning correctly when connected to 3-phase calibrator.

However when we connect the ADE7880 to the utility the THDv value rises and falls from 1% to 8% and 8% to 1%, respectively.

We have verified that the utility do not go over 3%. (verified with other power meters).

Something is very weird!

Why when connected to calibrator it gives correct values but not when connected to Utiltity (power provider).

Thanks