Residual value of the Phase Period records in the absence of a signal. 180 Hz, Electric Noise

I am developing a PQM system for a three-phase voltage with three more wires (neutral - ground) to the outputs of a VFD. The voltage and current values ​​reach almost zero when the VFD is turned off (as it should be), but the frequency value begins to be read at a value close to 60 Hz, if the lines are disconnected by means of a contactor and measures 180 Hz with the lines connected but with the VFD output at zero.

When restarting the ADE7880 the first reading goes to 0, but then the following measurements are 60 Hz or 180 Hz, depending on whether I have the lines open or closed through the main contactor.

Someone could tell me why this is due.