I monitored a commercial building for one week and the harmonics (neutral) result as:
THDv(N)=31.2% && 3rd harmonic=21.89%
THDi (N)= 194.4% && 3rd harmonic= 186.8%
could anyone tell me what are they mean and how to mitigate them..
That means that the third harmonics combine in the neutral. It also means that you have lots of computers or other non-linear loads connected from L to N.
The percentage is not a problem. The question is how many amps you measure in the neutral. Of course, if you got 32% THDv, it doesn't look good.
As Skogs is alluding to, what matters is the size of the cable. If you've got 150A running in a 16mm cable then you've got a problem. If the same current is running in a 70mm cable, then it is not an issue; with the caveat that as long as your voltage harmonics on the phase voltages are not excessive.