Actually, in a  building continuously indicate  neutral to earth voltage  as 10 to 11 V. Main cause for this is harmonics of VFDs.  Hence , I want to know is there recommended maximum voltage levels in-between neutral and earth of TT system (with relevant standard reference). (In above mistakenly I wrote a minimum now I'll correct same.) 

There are no recommendations of N-PE voltage from safety standards.

However, there are recommendations for EMC (although not from standards) - which have, in some cases, been as low as 5 to 10 V.

There is a solution to this ... although not without cost ... which is to use transformers to permit the filters to perform as they would in a TN system. This is recommended in the standards for VSDs, and also in BS EN 60204-1. The transformer secondary neutral is earthed, thus forming TN-S circuits in the TT installation.

There are no recommendations of N-PE voltage from safety standards.

However, there are recommendations for EMC (although not from standards) - which have, in some cases, been as low as 5 to 10 V.

There is a solution to this ... although not without cost ... which is to use transformers to permit the filters to perform as they would in a TN system. This is recommended in the standards for VSDs, and also in BS EN 60204-1. The transformer secondary neutral is earthed, thus forming TN-S circuits in the TT installation.