The generator source, the transmission lines and the loads make the quality of supply bad by contributing Harmonics. While several equipments adjust with harmonics without any major damage to itself, other sophisticated equipment may cease to function due to these harmonic voltages. For better efficiency and life span of the equipment it is important to eliminate dangerous harmonics in power supply. Electronic circuits and equipment may damage of malfunction due to excessive harmonics present in the power supply. Active harmonic filters are largely used to solve the harmonic problems. I have found an elaborate and interesting write-up at http://engineering.electrical-equipment.org/forum/general-discussion/harmonic-filter-for-better-power-quality . please go through this article and discussions for clearing your doubts on Active Harmonic filters and power conditioning.