It also be called harmonic interference. harmonics is the odd wave exists in the sine wave (3,5,7, ... ...), the changes have nothing to do with frequency, but have absolute relationship with the changes of voltage and current. Harmonics have no use in electronic circuits, it will only lead to the over-temperature of electronic devices or the damage of electronic components, therefore, a lot of UPS are equipped with filters in the input and output side to filter out the useless harmonic interference and other noise,and protect the equipment.