The basics of what causes vibration:
When a conductor carries current, there is a magnetic field set up around this conductor. The attraction or repulsion between them being directly related to the distance between them and the current each is carrying.
And there is one added factor. This current, in each conductor, rises from zero to a peak and falls back to zero 100 times a second - polarity becomes a non-issue as no magnets are involved - and we are ignoring any attraction to the earth's magnetic field. In 3 phase systems this could reduce to 50 times a second as one conductor can be affected by two conductors carrying the other phases.
The final requirement is the conductor has 'give' and can move. Worst still, it has the correct pendulum weight and it can swing back and forth in time to the magnetic field undulations.
Vibration is born.
The danger of vibration is simple - it can shake things loose. The vibration itself will not impact power quality in any way. If the vibration causes a conductor to clash against another, it is not the vibration that is the cause of the power quality, but the fact the conductors were not spaced correctly. Similarly, if a connection bolt shakes loose, it is not the vibration that is the cause, but the wrong bolt for the job. As shown above, vibration will exist where there are AC carrying conductors.
This vibration may not necessarily be related to the fundamental, but rather to a harmonic. But one must be very careful here. Splitting the harmonics into individual frequencies removes the fact that they are all part of a distorted fundamental waveform. Exceptions are the Neutral of a 3-phase systems which could have a strong 150Hz component (3rd harmonic) if carrying a high degree of direct rectified load across the three phases.
What will need to be determined is what frequency the conductors are vibrating at. This, in itself, proves to be a feat. We cover measuring vibration in the section on Measuring Techniques.