Low-voltage (LV) harmonics and their properties have been known since early in the 20th century. Even harmonics where for example already mentioned in 1910  and the blocking of zerosequence harmonics by transformers in 1914 . Early papers about harmonics addressed transformers as the source of emission. Already in 1944 it was written “it is well known that mercury-arc power rectifiers produce harmonics in the ac and dc systems” . The introduction of HVDC, television and, more recently, computers and other types of consumer electronics, has resulted in a broader interest for harmonics.