Dear Mr. Mansour, there is a vast of problems and solutions regarding this subject and each method addreses one point. More general the problem is, the higher harmonics occured by the conversion from dc to ac. Higher harmonics are produced by the transformer and the electronic converter. For their elimination several measures are taken (special transformer design, converter with small THD, distortion rate). Usually the system is accompanied by a voltage regulator and/or a filter. Also important is that the input voltage needs to be fixed and stable as possible. Input voltage fluctuations have an impact to output voltage.  

Many thanks